Pronounced Intermittent Hesitation and Unresponsiveness

I've always noticed a little hesitation when The Lorelei isn't warmed up to full operating temperature.

I've also always seen a low idle. It tends to settle in at 600rpms +50. I talked to a former Toyota Tech and he told me that I ought to be able to adjust idle by opening up the set screw at the front of the throttle body where the air intake is. Here:

After opening it up a turn and a quarter, the idle was set at 800rpm. I then took it on a 125 mile drive. Halfway in, I got some rather extreme hesitation. Not stalling, but not accelerating either. But then it would kick in, intermittently, creating a herky jerky condition. I pulled over, turned the car off, turned it back on, and then drove home. I was gentle because I could feel that it just didn't have the vigor. I drove 60 miles home at speeds up to 110km/h, so it isn't like there's a gross mechanical problem.

Once home, I reverted the set screw back to the original position, disconnected the battery, touched the positive and negative terminals to each other as an effort to drain the ECU capacitors. I also disconnected the EFI and Hazard fuses and the EFI Main Relay. My thought was that perhaps changing that set screw had messed up the computer's sense of reality.

The next morning I hooked everything up, took it to get coffee, and everything was fine. Got it on the expressway and gave it the beans and it was responsive. I took it on a 100 mile drive, and performance was flawless--good responsiveness, and everything you would ask for. Temperatures were cool.

I drove it to work today, no problems on the way in. But on the way home I got the hesitation again. Temperatures were brisk. This is what it looks like:

https://youtu.be/z4WyUioWGIo

Symptom--Engine Hesitates And/Or Poor Acceleration is one of the longer troubleshooting chains in the Big Green Book:

Clutch or Brakes? Unlikely.

Vacuum Leaks in Air Intake Line? No. Checked recently with Throttle Body Cleaner.

Air Filter? Extremely unlikely. New K&N installed a couple of weeks ago.

Check Diagnosis System? No "check engine light," continuous flashing of engine light when TE1 and E1 connected and ignition switch to on but engine not started.

Check Ignition Spark? Possible. Is it possible that the distributor is malfunctioning intermittently?

Check Fuel Pressure? Possible and somewhat more likely. Air-Fuel mixture goes full lean when the problem manifests.

Check Injectors? Unlikely. Car performs well when problem not manifesting.

Check Spark Plugs? Unlikely. Car performs well when problem not manifesting.

Check T-VIS Valve? Possible. I don't have the equipment to test this nor the knowledge.

Check EFI Electronic Circuit Using Volt/OhmMeter. Wiring connections, power to ECU, Air Flow Meter, Water Temp Sensor, Intake Air Temp Sensor, Throttle Position Sensor, Injection Signal Circuit.

My sense is that there's something wrong with the fuel system--like the engine is not getting the fuel it needs. The Air-Fuel Ratio meter is handy for this, because I can see that it goes lean. I have not changed the fuel filter. The fuel pump is the original fuel pump. I do not have a fuel smell in the engine compartment or about the undercarriage that suggests a leak. Been running 93 octane. Alternately, it might be the Throttle Position Sensor. I tried to test it, but I couldn't get the screws to loosen up and I'm not stripping the cross-tip heads. Alternately, if the Air Flow Meter is malfunctioning, it could be telling the motor not to work.

If you got this far you are awesome in my book. Thoughts?

It sounds like you're losing fuel, my thinking is that if you were losing ignition it'd be going super rich rather than super lean. The first thing I would do is get your trusty paperclip or whatever you use to pull codes and use it to jump B+ and FP in the diagnostic box in the engine bay to bypass most of the stuff that turns the fuel pump on and off. Note that this will disable some of the safety features that shut off the fuel pump in an accident so obviously be careful. What this also does is switches on the pump as soon as the key is turned to ON, not just during starting/running, so you can actually hear the fuel running around the system with the engine off. With the jumper in I'd drive around and see if the problem comes back. If it doesn't, something you bypassed is the problem (AFM or COR or wiring). If it does, the stuff you bypassed is fine and the problem is elsewhere.

If it does come back I'd shut the engine off, turn the key back to ON and listen to see if the fuel keeps flowing. If it's cutting out then it should be down to the ignition switch, the EFI main relay (I had a flakey one of those once, very annoying), the pump or the wiring. If the fuel keeps flowing no problem then my best guess would be the injectors aren't firing. The caps in the ECU for the CEL systems like to leak onto the injector circuit and cause problems so I'd be pulling the ECU (easy job, just pull back the carpet) and taking a look in there. On a bit of a tangent given the age of these cars it's not a bad idea to preemptively replace the caps in the ECU anyways.

Also note that being JDM your car doesn't have a starter relay or clutch switch so ignore those in this diagram.

Well

If you're seeing a really lean condition my guess would be a misfire. Or your AFM is going, which is also a good possibility.

Remember, the ECU is dumb. So is the O2 sensor. All they see is O2. You could literally pour fuel down your exhaust and it would have no clue.
Misfire is going to cause no combustion = extra unburned O2 in the exhaust = lean condition from the O2.

Make sure your cap and rotor are tight and in good shape. Make sure your plug wires are tight. If it is a misfire also possible your coil is going.
Once you know everything is tight I would suggest running a can of seafoam through your intake. See if that helps.

You can check the AFM signal with a volt meter. Not a multimeter, an analog volt meter.
Hook up the positive to the AFM signal wire (you can backprobe into the ECU connector), ground negative. Drive. You should see smooth sweeps. If it's catching anywhere you have a problem.

If yours is a 92 or 93 you can jump TE2 (yes 2, not 1) and E1. Will put you in a more sensitive diagnostic mode.
Will also work on RCs because they have the post revision ECU.

That's all I have time for right now, sorry.

Thank you very much underscore and alltracman78!

I jumped +B and FB as instructed


then took the Lorelei on a test drive. The problem appeared, here:

https://youtu.be/1blMT3RcO6w?t=365

so something bypassed is the problem. Based on the wiring diagram above, it is not:

AFM
Circuit Opening Relay
Fuel Pump Relay
Starter Relay
Fuel Pump Resistor

With +B and FB jumped, I turned the ignition to ON but did not start the motor. I knelt down by the back wheel and could hear the fuel pump humming. It did not cut out, or indicate that it was faulty in any way. It was quite quiet--I had to put in my hearing aids to hear it as I have military grade hearing.

This leads me to:

Injectors Not Firing
Caps in ECU for CEL Systems Leaking Onto Injector Circuit

I didn't have the time or the wherewithal to dig into the ECU. However, I did lay eyes on it--it's there all right, buried behind a mass of wires. Pulling the ECU and getting the caps replaced is down the road--I had an old Macintosh computer motherboard that had to have the caps redone, so I know that's a thing. Is anyone doing that as a service?

Continuing on to alltracman78's advice: I am in fact seeing an intermittent lean condition with accompanying lack of acceleration--it feels like it just goes limp, then will cut back in if I leave the gas pedal depressed where it was when the fuel cut out. The Lorelei likes to idle at 600rpm and 15:1 to 15.3:1 air:fuel--so slightly leaner now than what I observed immediately after installing the air-fuel ratio gauge: 14.7 to 14.8:1.

I checked the distributor for tightness--it is firmly attached. I did not disassemble it or take it off to check inside. The leads are solidly connected, and amazingly they are Yazaki EZ-16 ES-X leads, dated 1992!


Those are some old ass spark plug wires. That leads me to consider misfire due to the extreme age of the spark plug wires. However, to see what I'm seeing, all four would have to go out at once...I'm not seeing behavior that suggests that one cylinder is dropping out.

I do not have an analog voltmeter on hand, so could not check the AFM signal.

I jumped TE2 and E1 and turned the key to on. Got the continuous flashing engine light indicating no codes.

It's hard to troubleshoot these sorts of issues on our older cars because there isn't as much diagnostic info as is available on newer stuff. Even then, it can still be a bit of a hunt. I'm always good at spending other people's money, though!

I'm leaning towards ignition on this one. Have you had a tune-up recently? Those wires are due for a refresh, too. It could also be something like a bad ground, either the (least likely) grounding strap on the back of the engine the coil uses for spark or one of the grounds between the engine and the chassis.

I'd start with the basics though. No sense in going full ham right away. This car can break the most resilient owners. Best to be pliable and go in easy at first

Ignition is my thought as well. Something between distributor, spark plug wires, and spark plugs. If I were to go full ham like you said I’d just replace the whole distributor, rotor, wires, and plugs.

I’m taking it in to ask the techs to run through the checklist in the Big Green Book for hesitation and poor acceleration. I’ll mention our suspicions about what it could be.

UPDATE: I took it in this morning and dropped it off. I left a Big Green Book on the passenger seat open to the EFI section Hesitation and Poor Acceleration.

Shortly after arriving I got a text, to which I responded:



I got a call at like 10:30. The service writer left a voicemail saying he had a question, so I called back. He said that they don't really work on tuned vehicles, citing the boost controller, and they'd be willing to refer me to an aftermarket tuning shop. Well shit, there it is. I've been relegated to the tuner community ghetto. I knew this might be a consequence of having that dual temp gauge air-fuel ratio gauge installed. I said "Just go down the checklist in the Big Green Book." He said he would call back.

When I got off work I called them and the service writer said that they found the fuel pump was only putting out 20psi when it should be putting out 32psi. That sounded plausible. He quoted me close to $2000 to replace the fuel pump and fuel filter--$650 in parts and $1250 in labor. He said it would take a week to get the parts. I asked what brand the fuel pump would be, and he wasn't sure.

I said I'd need to think about that before I commit and I'd be in to pick it up. He said that they'd need some time to put it back together; they'd taken the hood off so they could see the fuel pressure gauge while test driving it.

I could go get a second opinion on what is causing the hesitation and poor acceleration response. I have the sense that they're right on the fuel pressure, though it might be caused by some reason other than the fuel pump being weak. Or I could check that myself. I guess it's time to up my auto skills and get a fuel pressure gauge.

How hard is it to replace a fuel pump yourself? From what I understand, the fuel pump is accessed under the seat:

https://youtu.be/p1Y1oWT8paE

The fuel filter is a bit of a process but not incredibly tough. Frankly, I'm not too sold on the need to replace the fuel filter. The service manager for that very shop told me not to fiddle with the filter, as the lines are crimped hard lines and he said it can be a hassle to get them reconnected after.

Any advice on which fuel pump should I get?

P.S.: I didn't get an offer for the Lorelei from the dealership.

hiya,
The factory 185 has a dual voltage circuit for the fuel pump. Low voltage for lower fuel flow requirements, and higher for high. Why this exists, I have no idea. But I'd make sure that isn't coming into play for your car before going any further down the pump rabbit hole. If you are confirmed for sure getting lower than expected fuel pressure, then I'd start with a fuel filter before doing the pump. Honestly, not a bad idea to do the filter anyways if you don't know when it was done last. Same with spark plugs, distributor cap & rotor, and wires. Those are just consumable items that(probably get changed more often than they need to be) do go bad eventually.

According to the service writer, the techs resorted to taking the hood off so they could drive it with the fuel pressure gauge attached. LOL

If I trust that they did this--and that's a pretty weird thing to lie about--I guess I should trust that they were seeing 20psi droops.

Now for the cause...That's interesting about the dual voltage circuit. So, what you're saying is that it's possible that the circuit might be delivering low voltage in error when the high voltage should be in effect. Changing the fuel pump if that system is faulty would not solve the problem.

Do you have a suggestion on how to test whether that dual voltage system is malfunctioning? I guess that I could try and set up a multimeter with alligator clips on the fuel pump and drive the car and try and place the multimeter where I or a passenger can see it. If I see the voltage dropping when it shouldn't that would be the sign of that malfunction. if the voltage stays right, but I get the performance drop, then I guess the fuel pump is to blame.

When I consider how egregious that $2000 estimate is--I mean, shit, that's like [X days] of wages--I am strongly inclined to just roll my sleeves up and get into it. After I retire, I'm considering getting some of that sweet veterans' transition benefits training in auto skills courses at the local community college. I watched this video and I'm encouraged:

https://youtu.be/mDIlwbx0B-s

I mean really. Much of the problem is that they're probably using a Toyota Parts Center for the fuel pump and fuel filter:


If that's the case, then like $1400 of the $2000 is labor. Ten hours? Huh.

I need some advice on what parts to get and where to get them and any special tools.

This guy? https://treperformance.com/i-133151-toy ... QkQAvD_BwE

As far as the filter, I went ahead and got that OEM filter. It wasn't too expensive. This thread: viewtopic.php?t=51523 says "Toyota considers the OEM fuel filter good for the life time of the car, and does not specify a replacement interval."

It's been a long time since I've had one of my 185's on factory fuel, so I can't really tell you about when it makes the decision to switch between high and low voltage without some time spent RTFM.

I can tell you though that the factory wiring path is loooong between the relay in the fusebox and the fuel pump itself - it's common (and I personally think required) when upgrading fueling to run bigger gauge wire, or shorten the path to minimize voltage drop, or both.

In factory form, there's a resistor pack under the radiator overflow tank that I think the ecu manipulates in order to change the voltage being sent to the pump. You can force the pump to stay in high-flow mode if you use a paperclip and bridge fp and b+ in the diagnostic box (it's a little grey box in the engine bay. USA cars it's near the brake booster. Yours, might be near the wiper motor). You should see full pressure with those terminals bridged. Try doing that and go drive the car and see if it makes a difference.

Personally speaking, I'm a big fan of doing the normal maintenance items before digging in deep on troubleshooting. Often times, it's just something consumable that is causing issues. cap/rotor/plugs/plug wires should still be done unless you know that they're relatively new. The fuel filter also needs to get swapped - I don't care what anyone says, it's not a lifetime part.
The difficulty in changing it just depends. If spark plugs are 1 out of 10 on our cars then I'd give the fuel filter 3-3.5. What can make it challenging is that the steel fuel feed line comes into the bottom of the filter (no visibility) and attaches via a flare nut which can sometimes seize up. Limited space and lack of care can easily cause that hardline to irreversibly twist, and then the fuel filter swap gets a lot more interesting...
I think the procedure is in the BGB and I know there's a writeup here on at.net if you want to see how some of the other users have done it. I wouldn't recommend it unless you're comfortable wrenching on the car though, just because of the risk that comes along with screwing up the fuel delivery hardline. If you do decide to take it on, make sure you use a flare wrench for that bottom flare nut.

If you are looking for brand recommendations, I really like WIX filters. It's the same as NAPA gold products. I run them on all of my cars (except my GTI, but that's bc buying an ''oil change kit'' from an online vendor that includes a filter is cheaper than going to oreillys to buy supplies, so whatever...)

I really doubt that the fuel pump itself is to blame. It's not impossible, but it's the last thing I'd go change. The tank drops out from under the car, and then you get at the pump while the tank is off. It's a bit of a process, and our old friend flare fittings show up for the party, too. If you have tried everything else and still think it's the pump, you could first do another diagnostic run: after the fuel filter swap, bridge fp/b+ then hook the fuel pressure gauge back up and go for another drive to see what pressure the line is at.

hope all this helps.
edit - i just scrolled through the thread and see that you already know about the fp b+ stuff. this is what i get for posting at midnight when i should be in bed. sorry for the duplicate info

The fuel pump for the 2WD models is under the rear seat, the pump for our cars requires dropping the tank unfortunately. Why Toyota didn't just make an access flap in the trunk I'll never understand. Even with dropping the tank, 10 hours labour to swap the fuel pump and filter sounds like a lot to me. It also seems odd to jump right to replacing the pump without doing just the filter first, like grip-addict says, those are a wear item. It's not like you're saving any work doing them together either.

The filter is pretty easy to change, maybe a little more of a squeeze for you since you've got AC lines running above it but it's not a bad job. Give the fittings on the top and bottom a spritz with something like PB Blaster once a day for a few days before doing it to help loosen things up. Pop the wheel off to get to the bottom fitting more easily. Given how old the wires are as well, and how nasty the wires on my car were 10 years ago (granted with more mileage) it's probably a good idea to do a bit of a tune up with plugs/wires/cap/rotor/fuel filter. Hopefully it fixes the problems your having, but if it doesn't it's all stuff that should be done anyways so no potential loss there.

If you do change the pump, personally I avoid Walbros simply because there are so many fakes out there. Since you're not doing anything crazy to the engine personally I'd go for a Denso pump from RockAuto, iirc Denso is an OEM supplier for Toyota and other manufacturers so the quality is good without the dealer markup on the price.

A couple of things. In no particular order. Sorry they're mostly negative, trying to be short to save time.

Don't use that wiring diagram, it's not for a 185. It's probably for a 165. 185s Dont use the switch in the AFM for the fuel pump.
I'll try to drop more info later.

How do you post pictures on here now? Been so damn long.....

Why isn't your factory boost gauge working in the video?

The shop doesn't want to mess with the car because it's old and has an intermittent problem. Could be a whole bunch of different things. Lots and lots of diag time probably required.

Best way to by pass the factory fuel pump 2 stage system is to remove and jump the relay. It's a yellow one in the main? relay block. Been many many years...
The 5SFE Celicas actually came with a factory jumper in place instead of a relay. I have one in my 93 IIRC. Much better than a paper clip.

When you jump TE2 and E1 you need to drive it to get the diagnostic benefit. Codes will flash if they come up. If not it will just flash regular.

Have you done the ignition stuff and seafoamed yet? Do that shit before you waste any more time.....

You might be on the right track with the ECU. It's actually not that hard to remove. Looks much worse than it is. Takes patience.

I doubt your fuel pump is the problem, but if you're dead set on replacing it I MIGHT know of somewhere able to do it. It's in RI though. Long drive for you.

The 2 stage system for the fuel pump is to improve fuel economy. Less voltage to pump = better economy. When you want to go fast the relay bypasses the fuel pump resistor and gives more fuel. It's possible the shop wasn't getting on it enough to go into high voltage mode?

I picked up The Lorelei.

The estimate was like this: "Fuel Pump Replacement: Inspected MAF + Turbo Pressure sensor, test good. Inspected for possible clogged cat by removing AF Sensor and visually inspecting inner sides + test drove and still bogged. Tested fuel pressures, @ idle is 32psi, during road test found during acceleration readings drop as low as 20psi @ full throttle. Recommend replacing fuel pump + fuel filter w/diagnosis. $1978.09"

Suffice to say, I did not have them order the parts and proceed. And I agree--they don't want to deal with it. I won't be back for anything. Watching them fumble around with a pre-OBD-2 vehicle is embarrassing, and their estimates are way high for both parts and labor.

I drove home, but it sure acted up most of the way:

https://youtu.be/02ZEx4rbZl0?t=720

Yes, alltracman78, you're right--the factory boost gauge sometimes doesn't register. It seems like it is stuck and then it will flick up, sometimes. Other times, like where the video above is, it registers what the aftermarket gauge shows through the vacuum range. I do not know why this is, and it was always like this, so I don't associate it with this problem.

I decided to proceed as advised: Plugs, Wires, Distributor Cap and Rotor, and Fuel Filter. I picked up a can of SeaFoam and some Denso spark plugs:



I ordered a Standard Motor Products Distributor Cap, Bosch Rotor, and Denso Spark Plug Wires from Rock Auto:

You all will totally believe this, as your own AllTracs and GT-Fours have had minds of their own, doubtless.

I drove The Lorelei to get a coffee and it didn't suffer any crippling lack of throttle response or acceleration. Same deal on the way back at full operating temperature. Perfectly normal operating range. The factory turbo gauge seemed to be working. I'd filled up with 10 Gallons of 93 octane the night before and poured the bottom half of a bottle of Royal Purple MaxClean into the tank.

https://youtu.be/bwmhhQVjzII?t=330

Once I got home I put a can of SeaFoam Spray through the motor, spraying it directly into the throttle body after the intercooler with the motor at 1700rpm. I didn't get the pronounced fog out the tailpipe like I have seen in the SeaFoam videos I've watched (ChrisFix, ProjectFarm). I got a bit toward the end.

I ran The Lorelei on a test drive, and it worked fine. I was able to take it up through wide open throttle, and got good throttle response and full boost.

https://youtu.be/gY71J8krgmY?t=320

A couple of thoughts:

1. I am getting some power drops here and there--misfires. As I get more familiar with this motor, I'm getting more confident at making these calls, which I found difficult because I didn't have a chance to register a change in performance. I will seek to get after this with new ignition components.

2. It's definitely NOT the fuel pump--I mean, never say never but I'm pretty sure. The fuel pump is delivering fuel.

3. The factory gauge cluster turbo pressure gauge sticking and flicking, I am chalking this up to it being a 30-year old gauge. What relation that might have to performance, I do not know. The throw money at it solution would be to source a new or different used turbo pressure sensor and see what happens, I guess.

4. I wonder whether there might be some relation between the fuel level and throttle response dropout. Is there some gunk in the tank that the strainer filter is catching up, or which is getting through to the motor and causing dropouts?

5. I took a video of the motor idling warm. It's got a little vibration and a little ticking--nothing I don't remember hearing even in a new Toyota motor. Maybe a little rumbly because it's idling at 600rpm versus 800 rpm. I took this as a baseline to compare it to after I change out the ignition components.

https://youtu.be/jBLAAItSRGE

Thanks for staying with me through this!

Oh they definitely have minds of their own, sometimes I swear "self healing" was a dealer option that was selected for mine.

Just to cover bases here, was the 10gal how much it took to fully fill the tank again, or just how much you put in? Did the problems start shortly after the last fillup or was it not for a while after?

Think it might be some bad gas? I started seeing big problems reasonably shortly after adding 10 Gallons of 93 octane from my regular Shell station near the house--but that's the same gas I've been using. Here's the day by day blow by blow:

28 October: I put 10 Gallons of 93 octane in, when the gauge read 1/4 tank. My standard routine is to add 10 Gallons at 1/4 tank and that takes the gauge up to 3/4 or slightly higher. I don't go to the click stop because I had fuel vent out through the charcoal canister once.

29 October: I drove it with no performance problems--a 20 mile round trip.

30 October: I experimented slightly with the Apexi S-AFC, riching the fuel mix slightly at 800rpm and 2400rpm range bands, but as this did not help with the idle--which I was hoping to get at--I zeroed these back out. I also tested the high boost range with the Blitz Twin SBC, attaining 16psi.

I checked for vacuum leaks using throttle body cleaner (no leaks) then adjusted the set screw on the front of the throttle body just above the air intake, turning it counterclockwise to achieve 800rpm warm idle.

I then went on an extended drive. About 60 miles in, I got extreme hesitation and poor acceleration. I was able to drive home, by gently accelerating to achieve road speed. 125 miles all told.

I disconnected the positive and negative terminals from the battery and then touched them together in an effort to drain the ECU's capacitors. I also returned that throttle body set screw to its former position.

31 October: I drove it 8 miles to get coffee--no irregularities, but I drove it gently and that drive was entirely off boost. I added 10 Gallons Shell 93 octane and added half a bottle of Royal Purple MaxClean. I then drove it 100 miles--mostly highway at high speed. No major issues on and off boost.

1 November: 6 miles to the shop to get the brakes checked, 6 miles back.

2 November: Drove it to and from work--a 20 mile round trip. Had some stumbling and poor acceleration on the way home.

4 November: Took it in to the shop. It was fine on the way there--a six mile trip. Their recommendation is a new fuel pump.

5 November: Drove it home from the shop. It stumbled repeatedly over the six mile trip. I put 10 Gallons Shell 93 octane in it, and the bottom half of a bottle of Royal Purple MaxClean in it.

6 November (Today): It drove fine to and from coffee, and I put a can of SeaFoam Spray through it, then test drove it--all was well.

I changed the installed NGK R BKR6EP spark plugs, which now that I had a chance to really look at them in comparison with new plugs, looked quite fouled. That #2 plug shows evidence of a bad valve cover seal by the oil staining. The previous owner included that spark plug wrench in the tool bag, which I take to be a sign of somewhat more involved ownership.


I installed Denso SK20PR-A8 Iridium plugs. Comparing the new Denso plugs with the old NGK plugs, the Densos are heat range 20, middle of the curve, while the NGKs were heat range 8, in the performance/tuner range. I might think of Densos in the 22 or 24 range at some point, but these are what's in it for now.


The plugs changed, I drove it to go get some burgers. Had some minor stumbling on the way back.

On the off chance the fuel is a factor I'll change up my fuel brand next fill up. Standing by for distributor cap, rotor, and spark plug wires.

I was thinking either bad gas, or maybe your fuel gauge is off and "1/4 tank" is actually much lower than that. The two times I've gotten bad gas though my car ran like absolute garbage the entire time, not just intermittently, so I'm leaning away from that.

We have troubleshot this problem--much thanks to everyone for your thoughts!

We took The Lorelei out on a long drive today on the Blue Ridge Mountain Road, which I wholeheartedly recommend if you reside in Northern Virginia. I have to give my wife props for riding shotgun in a suspect vehicle that might take a dump any moment!

Performance was flawless until we got just under half a tank. We were going downhill and The Lorelei entered a fuel starved condition, which then resolved itself as we pointed uphill. We concluded that this performance dropoff is entirely related to how much fuel is in the tank, and whether The Lorelei is nose down or nose up.

We got to a gas station, I put 8 Gallons of 93 octane into the tank. I could hear it sloshing in the tank but did not reach the click stop. The Lorelei drove flawlessly the 50 miles home.

Reviewing the blow by blow above, it's clear that the stuttering and lack of acceleration response happened when the fuel gauge was below 1/2 tank.

The sensation while driving when the problem condition was manifesting is best characterized as "microstuttering," as if the fuel was not quite reaching the intake tube, or the fuel pump was cutting out (perhaps it is not fully immersed?). Is the weight of the fuel in the tank pressing down on the fuel entering the pump and assisting in creating sufficient pressure? I would not have expected the condition to disappear when The Lorelei is nose up, as it's my understanding that the fuel pump is toward the front of the tank.

In any case, I seem to have found the conditions under which this troubling behavior occurs, and while correcting it is on my to do list, at least I'm not chasing phantoms anymore.

So, given these findings, what could it be? Weak fuel pump? Badly positioned fuel pump?