Low Compression?

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Low Compression?

Postby Aaron » Mon Jul 06, 2020 3:29 am

I got 140/140/148/148. The BGB says 128 is the minimum, but just curious if this seems low to anyone. Having issues with rough idle and no power. The tach barely gets up to 2k, but I can't tell if it's just the tach that's off. Buying a gauge soon to check.
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Re: Low Compression?

Postby 93celicaconv » Mon Jul 06, 2020 9:59 am

Service limit minimum is 128 psi. And a deviation of 14 psi from lowest to highest. Normal on a well used engine is 164 psi. I would say yours is low overall, but within the deviation range.

Questions regarding how you checked your compression:
- was engine cold, warm, or hot
- was pressure gauge calibrated for accuracy
- was throttle body air plate wide open or closed
- was battery on a charger during the testing, or at a minimum of 12.6V throughout the process
- was the engine turned over long enough to ensure the gauge was at the peak pressure
- was the compression tester seated tightly in the spark plug hole with a good o-ring to prevent pressure loss

Because your compression valves were relatively close across all 4, I'm thinking there were non-engine issues causing the low overall pressure readings.
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Re: Low Compression?

Postby underscore » Mon Jul 06, 2020 6:49 pm

My car had similar numbers when I bought it and it ran just fine. All the stuff 93celicaconv listed (and more) can cause variations in the results. Unless the numbers are excessively low I think the main thing to watch for with a compression test is variations between the cylinders. ie 140/140/148/148 looks good to me, but if it was 140/200/199/200 I'd be concerned.

Are all the cluster warning lights working properly (coming on briefly at startup) and do any come on while driving?
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Re: Low Compression?

Postby Aaron » Mon Jul 06, 2020 9:29 pm

Thanks for the replies. See below:

- was engine cold, warm, or hot
I warmed up the car to operating temp for a few minutes and shut it off.

- was pressure gauge calibrated for accuracy
This was bought new at Harbor Freight Tools. The instructions do not mention any kind of calibration procedure.

- was throttle body air plate wide open or closed
Wide open

- was battery on a charger during the testing, or at a minimum of 12.6V throughout the process
Brand new battery, has had the car running for about 15 minutes total. I neglected to test the voltage or keep it charged.

- was the engine turned over long enough to ensure the gauge was at the peak pressure
Yes, I believe so. It was about 6 seconds per cylinder, I would guess if I had to. I waited for the needle to maintain the highest number achieved for a couple cranks or so to ensure it would not go any higher before signaling to disengage the starter.

- was the compression tester seated tightly in the spark plug hole with a good o-ring to prevent pressure loss
I would say yes to this as well. Brand new tool and o-ring and I tightened it by hand but pretty good.

-Are all the cluster warning lights working properly (coming on briefly at startup) and do any come on while driving?
They work upon startup, but I haven't driven the car much yet, so it's hard to answer the second question. I still have to replace & bleed the brakes and do a coolant flush (only distilled water in there now) before I can really test drive it. My driveway is at the top of a hill, so I want to be sure it sounds like an actual car that runs before I end up getting stuck with a dead car at the bottom. It will idle sort of all right -- it was ok sitting there warming up to operating temp for the compression check -- but the gas pedal doesn't do much even though the throttle is fully opening.

I also just realized I'm missing the alternator grounding strap yesterday. Not sure how big an effect on running that has, but I'll make one up and test it out again. Also waiting on a heater hose that ended up leaking after being removed and replaced. Also going to order a boost/vac gauge to check for possible low vacuum at idle. Also open to other ideas. Thanks for the input!
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Re: Low Compression?

Postby underscore » Mon Jul 06, 2020 11:44 pm

Get the ground strap on or use jumper cables to make a temp one. My car ran like you describe when I forgot to tighten my alt belt down, having the ground missing would have a similar effect (the alt not doing what it should).
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Re: Low Compression?

Postby lalojamesliz1 » Tue Jul 07, 2020 8:39 am

I would try a different guage....
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and a 2 car garage :shoots:
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Re: Low Compression?

Postby alltracman78 » Tue Jul 07, 2020 9:06 pm

The gauge is probably fine. Unless the needle doesn't start at zero it's unlikely the gauge is off any meaningful amount (even expensive gauges have an allowable range).

Also unlikely the alternator ground is the problem. It definitely won't hurt to install one, but it grounds itself through the bracket - head - block - transmission. As long as you have a good ground from your battery to your transmission (that's where the battery negative cable grounds) you should be fine. And if your starter is working ok your ground should be fine.
Once again, nothing wrong with having a ground cable for the alternator; Toyota installed one from the factory IIRC and I have one on mine. But unlikely it's causing this problem.

If this started suddenly it's unlikely it's the engine mechanicals. Especially since all 4 cylinders tested effectively the same.

Sounds like a vacuum leak to me.

Also, since you have a post revision (92) ECU you can jump TE2 (yes TE2, not TE1) and E2 and put the ECU into a more sensitive diagnostic mode. Might help pinpoint. But I would look for a vacuum leak first.
Intake hoses and idle up hoses for the pwr steering are the 2 likeliest off the top of my head. But any vacuum line could have gotten brittle and cracked/fallen off.
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Re: Low Compression?

Postby 93celicaconv » Tue Jul 07, 2020 11:28 pm

A vacuum leak wouldn't cause the relatively low compression pressures. It could cause rough idling and difficulty getting top engine speed though.

What could cause all 3 is a timing problem involving the cams to the crank.

Have you tested your ignition timing with a timing light? Can you?

I had a timing belt change along with other engine work done at a shop. The engine idled fine, but was a bit lethargic. Did the timing test, and it was way off of 10 deg BTDC. Took it back to the shop. They set the timing wrong. Corrected that issue - everything was fine. Was 1 tooth off. Maybe yours is more than 1 tooth off. Easy to check - just jumper your diagnostic ports properly, warm engine without radiator fans on, and see what you have.
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Re: Low Compression?

Postby Aaron » Wed Jul 08, 2020 3:24 am

I did check and set the timing per the BGB instructions. It was off by 1 degree (11 instead of 10). The transmission ground is good, too; I cleaned it up, even.

I'm leaning towards vacuum leak as well. The hoses look original and not in great shape at all; going to order some silicone vacuum hose. Would be great if there was a kit. I know I need a good amount of 3mm; anyone happen to know how much of which diameters are needed altogether?

I know the intake hoses are a common problem, but they don't appear to be cracked. Possible they're still leaking even without obvious visible damage?
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Re: Low Compression?

Postby alltracman78 » Wed Jul 08, 2020 3:32 am

Ignition timing really has nothing to do with cam timing. Obviously there is some relationship but just because your ignition timing is on doesn't mean your cam timing is. I assume you didn't just do the timing belt. It is possible one tooth slipped. To check it you need to pull the upper cam cover.

To look for a vacuum leak you need to spray something like brake clean around the engine. If there is a leak when you spray near the leak the engine will suck it in and the idle will raise.
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Re: Low Compression?

Postby 93celicaconv » Wed Jul 08, 2020 10:03 am

In my case, my ignition timing wasn't on because the cam timing was off by a tooth, and the distributor was at the full extent of travel on the slots towards getting ignition timing right, but the slot limited getting there. I'm pretty sure if you are off a tooth on cam timing, you can't get ignition timing at 10 deg BTDC. How many teeth are on the crank timing sprocket?

There are 46 teeth on the cam sprockets, 23 on the crank sprocket. If you are off 1-tooth, relative to the crank sprocket, the mechanical (cam) timing will be off by (1/23 x 360 deg) = 15.6 deg relative to the crank (which is where you check timing, right?). There is not enough slot space in the distributor to recover being off 15.6 deg at the crank.

So that is the math, and this is what played out in the case with our ST185 - it is discoverable in ignition timing in that you can't get the distributor to 10 deg BTDC when your timing is off by a tooth.
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Re: Low Compression?

Postby alltracman78 » Wed Jul 08, 2020 6:22 pm

And if had been the exhaust cam? Or a slave cam like the 5SFE?

I answered quickly on my phone so I didn't have a regular keyboard to type; I paraphrased badly. Yes cam timing can cause your ignition timing to be way off.
The point is you don't check your ignition timing to see if your cam timing is off. You check your cam timing.

I'm not trying to piss in anyones wheaties. :)
Just trying to help this guy get his car figured out as simply as possible.
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Re: Low Compression?

Postby underscore » Wed Jul 08, 2020 7:29 pm

alltracman78 wrote:To look for a vacuum leak you need to spray something like brake clean around the engine. If there is a leak when you spray near the leak the engine will suck it in and the idle will raise.


With how cramped everything is I like to use a boost leak tester. It made it very obvious when the vacuum lines came off the PS pump.
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Re: Low Compression?

Postby 93celicaconv » Thu Jul 09, 2020 12:51 am

alltracman78 wrote:And if had been the exhaust cam? Or a slave cam like the 5SFE?

I answered quickly on my phone so I didn't have a regular keyboard to type; I paraphrased badly. Yes cam timing can cause your ignition timing to be way off.
The point is you don't check your ignition timing to see if your cam timing is off. You check your cam timing.

I'm not trying to piss in anyones wheaties. :)
Just trying to help this guy get his car figured out as simply as possible.

I see your point, allracman78. And you have a valid exception. I was just thinking on my experience - the exhaust and intake cams were properly positioned relative to each other - the crank was off a tooth. It is possible for the crank and the intake cam (which involves the distributor) to be timed together properly, but the exhaust cam to be off a tooth. Would display similar symptoms, but would not show an improper timing. You are right on this!!!
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Re: Low Compression?

Postby Aaron » Thu Jul 09, 2020 1:31 am

Regarding the cam timing, I did set the crank to TDC and check that the cam gears were aligned with the markings on the cover. They were. If you're saying I need to open up the valve cover and check that the gears are properly mounted to the camshafts, I have not done that. Is that what you're saying? I'm not super experienced with the timing setup, so I might not be understanding properly. Is there a page I should reference in the BGB?

I came across a new issue while inspecting vacuum hoses. See below; it looks like one of the wires going into the IAC valve is broken off right at the base. Any advice on how to repair in this case? I'm fine with splicing wires, but I'm just not sure how to get a new wire into the connector itself. And how much of a factor do you guys think this is in the overall picture?

20200708_205655.jpg


Still waiting on silicone heater hose to come in, so I probably won't be able to start it up again until the 18th. The boost leak tester looks cool, but I don't have a compressor yet. Thanks for all the input so far.
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