Lean under vacuum?

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scottjackson1209

New member
I noticed an issue pop up today where my car is running incredibly lean only under vacuum. It is in the 18s and begins to cut out like it's not getting any fuel but if I get into boost it appears to run fine. Motor is an ST205 into a 165
 
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scottjackson1209

New member
205 I've been having issues with it over fueling worse than normal lately. I think I will be purchasing a megasquirt in the coming weeks
 
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Mr Alltrac

Member
When I had a leak in my intake manifold gasket it seemed similar to this. It would max out my arm uego wideband at idle but would run normal when I was cruising or under boost.
 
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klue

New member
scott in order to better understand your situation we need some more details.
how are you measuring the AFR?
how are you controlling fueling?
also give some details on your set up

Megasquirt is a great option on a budget. As a dealer for Megasquirt let me know as much as you can and can recommend the best set up for you

Luke
 
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scottjackson1209

New member
Mr Alltrac":l1nzch5u said:
When I had a leak in my intake manifold gasket it seemed similar to this. It would max out my arm uego wideband at idle but would run normal when I was cruising or under boost.
Click to expand...

Mine is fine at idle...only when light throttle cruising with 0psi to vacuum

klue":l1nzch5u said:
scott in order to better understand your situation we need some more details.
how are you measuring the AFR?
how are you controlling fueling?
also give some details on your set up

Megasquirt is a great option on a budget. As a dealer for Megasquirt let me know as much as you can and can recommend the best set up for you

Luke
Click to expand...

AFR is being measured on an AEM uego wideband, the wideband is also doing the narrowband output to the ECU since my stock 02 sensor took a dump and i didn't want to fork out the money for it when my wideband can do the same thing.

fueling is stock as far as FPR and injectors. I do have a walbro 255LPH in the tank.

Currently running 12PSI boost and everything else is set up the same as a ST205 should be.

I pulled the EFI fuse and let it sit for a while then went on a drive and didn't notice the issue again. I have noticed it running INCREDIBLY rich lately more than it normally does and was looking into getting a megasquirt mostly just for tuning out some of the fuel...not looking to make big power....yet... :D

anyway luke I am looking into the DIYPNP from DIY Autotune for my MS. I could probably get the one they have for the ST185 but I am intriuged by building my own and saving around $300 bucks doing it lol! Let me know what sort of suggestions you have? I know in the past there was an issue with the cam gear in the distributer being an issue but according to the folks over at DIY Autotune it isn't so much of an issue with MS2 correct? any info you can give me would be great! Also what are your thoughts about MS3? is it worth it to go with MS3 vs MS2 if i'm just tuning out some fuel and controlling boost? I can always upgrade it down the road if I desire right?
 
___Scott___

___Scott___

Active member
Megasquirt and the OEM ST205 ECU are both speed/density systems. If you have an issue with your Intake Air Temp sensor, and you plan to use that same sensor on a Megasquirt setup, you'll continue to have problems.

If the IAT sensor is fine, then make sure your MAP sensor wiring is all good. Beyond that, it's down to making sure your ground connection between the engine and chassis is good and also the ground connection between the chassis and ECU is good.

It doesn't hurt to double check the vacuum hose to the MAP sensor either.

There is a root cause to the problems you've been having. Figure out what it is before throwing money and new parts at it.
 
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scottjackson1209

New member
___Scott___":1x261gcf said:
Megasquirt and the OEM ST205 ECU are both speed/density systems. If you have an issue with your Intake Air Temp sensor, and you plan to use that same sensor on a Megasquirt setup, you'll continue to have problems.

If the IAT sensor is fine, then make sure your MAP sensor wiring is all good. Beyond that, it's down to making sure your ground connection between the engine and chassis is good and also the ground connection between the chassis and ECU is good.

It doesn't hurt to double check the vacuum hose to the MAP sensor either.

There is a root cause to the problems you've been having. Figure out what it is before throwing money and new parts at it.
Click to expand...

I ran multiple new ground cables when I did my swap so I dont believe that is my issue but I will double check them all tomorrow.

I will try and test the IAT (if there is a way to do that). my first thought was the map sensor vacuum line, I checked that immediately when I shut the car off and it appears fine. I'm leaning towards an issue with the ECU since pulling the EFI fuse fixed it...for now. I've been having other issues as well (like backfiring when touching the throttle after coasting) and they have gone away for a little while after resetting the ECU which is why I am thinking that it could have something blown/corodded/shorted inside.
 
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Mafix

New member
you have a vacuum leak. that explains all of your symptoms.

do you have access to a smoke machine?
 
___Scott___

___Scott___

Active member
Still running the original ST165 EFI relay?

If so, that's the worst relay design I've ever seen. I'd recommend replacing it. That may only be a temporary fix, but will at least tell you if you have flaky power to your ECU.
 
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scottjackson1209

New member
Mafix I don't have a smoke machine but will a vacuum leak cause that on a speed density system? What is another easy way to check for vacuum leaks?

Scott I am running the 165 EFI relay. What is wrong with it? What would you recommend? I've never heard of that issue before.

Could a bad batch of gas be the cause? I did fill up the night before the issues appeared. And it was acting up again on the way from work this morning.
 
___Scott___

___Scott___

Active member
A vacuum leak would explain your problems only if you were running a metered air system, like with an AFM or MAF. Speed/density systems are very tolerant of small vacuum leaks (one of their drawbacks actually.)

The ST165 EFI relay is flaky by design. The "point" contacts inside can easily bounce while driving down the road, or one of the idiotic contacts inside that connect the coil to the external terminals could flake out on you. When that happens, at least with a Megasquirt, the computer will reset and one of the first things it will do after reset is take a pressure reading from the MAP sensor to establish elevation (under the assumption that the manifold pressure is at ambient pressure.) That data is used in the fueling calculations later. If that happens while the engine is running, the reading will be erroneous and screw up the fueling calculations. You will likely see rich/lean problems (like you have) as a result.

The stock ST165 ECU doesn´t like it either and will toss out nonsense error codes. I fixed the problem by taking the relay apart, removing the guts and replacing them with solid-state electronics. A new relay only solved the problem for a few months before getting flaky again.
 
underscore

underscore

Well-known member
If you have access to an air compressor you can easily build a vacuum leak tester. Resetting the ECU might seem like a fix only because the ECU will be in a different mode for the first bit after it has completely lost power.
 
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scottjackson1209

New member
Okay so I've determined it only does it at 0 psi and 0 vacuum and only until about 3k rpm then it jumps to 13 afr for a second then bounces between 14 and 16ish. Also occasionally at idle it has started to lope pretty bad and even died once. I'm hoping bad fuel but it is starting to seem otherwise. I'm going to try and pressure test all my intercooler piping and intake manifold tomorrow.
 
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scottjackson1209

New member
so I decided to see if the ECU was storing any codes today. Turns out it was storing one for knock and one for 02 sensor. I'm confused though because my CEL wasn't on...anyway. I didn't check but I think my 02 sensor is not connected currently....(my ECU is getting the simulated narrowband output from my AEM eugo) and I think that it being disconnected causes a code because the ECU can't see it even though it's recieving a signal from my aem (something to do with the sensor heater or something?) as far as the knock control code...I'm hoping it was bad gas....I cleared it and put some fresh gas in with some fuel stabilizer and some royal purple fuel system cleaner. Hopefully this works out! I will report out after some good driving in the next couple of days.
 
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zf_165_gt

New member
if you want to see what kind of readings and actions your ecu takes, my android toyobd1 app gets really usefull and the st205 is supported.
 
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scottjackson1209

New member
4 wire. The plug was melted and grounding out against my down pipe...oops! Fixed it and got it all connected again. Let's see how it goes today.
 
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