Vacuum Lo, Compression OK=internal probs?

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lavabit
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Vacuum Lo, Compression OK=internal probs?

Post by lavabit »

I'm trying to diagnose a problem where my 2.2 engine starts intermittently missing after it has run for 20 to 40 minutes.

I did a compression test and all 4 cylinders measured between 175 and 176 PSI (this almost seems too good for an engine with 220K miles on it?).

I put a vacuum gauge on the intake manifold nipple that usually feeds the purge valve and the gauge held rock solid at 13.5" Hg while the engine ran ok and during the missing also.

From what I've read, this vacuum reading is low and should be at least 17" Hg, not the 13.5" I got.

Also, the exhaust is sooty and the plugs are covered with a thin film of carbon which I cleaned off by hand.

What on earth kind of problem could my engine have where it intermittently misses when hot, but the vacuum gauge holds steady the whole time?
Last edited by lavabit on Sun Nov 02, 2008 6:42 am, edited 1 time in total.

tmarcel
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Post by tmarcel »

I would almost certainly think that you've got a bad (incorrect) reading compression test gauge since all 4 cylinders are similar. These motors AFAIK never read that poorly even with 220k miles.

As far as your missing issues, I would start looking for something in the electrical system. First check all your grounds thoroughly. Possibly even go back through and overlay with new wires and connect to each point. Wouldn't be the first time with a Subaru that's for sure.

Can you get your hands on a wideband like an LM-1? This way you can check to see if there's any correlation to the missing vs AFR.

lavabit
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Post by lavabit »

I edited my post above. One of the things I fixed was that I originally incorrectly had listed the compression I measured as 75 to 76 psi. What I should have typed was 175 to 176 psi.

I had already done the electrical troubleshooting and there is very little chance now, due to the testing and swapping out I did, that the problem is electrical.

Something else I noticed that I may not have mentioned is that the car is idling too fast-about 1400 rpm when in park, about 1000 to 1100 rpm when in gear but with the car not moving. Sometimes the idle speed goes up and down even when the car is not also missing.

If you know of any internal engine problems that could be causing this intermittent missing after 20 to 40 minutes, sooty exhaust, and wavering idle speed even though compression test results are fine and even though vacuum gauge reading holds rock solid at 13.5" Hg, please let me know.

tmarcel
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Post by tmarcel »

Do you have a CEL on? Possibly an IACV would be my guess. Either a sticking and/or bad IACV.

555BCTurbo
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Post by 555BCTurbo »

Is this a Turbo or non-Turbo?
Nick

1987 Audi 4000CS quattro...soon to be 20VT
1994 Dodge Ram 2500 4x4 CTD, #11 plate, 30 psi, Scotty II intake, 4" exhaust

94SS_Canada
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Post by 94SS_Canada »

tmarcel wrote:Do you have a CEL on? Possibly an IACV would be my guess. Either a sticking and/or bad IACV.
I dont believe the IACV has anything to do with the engine after it is warm.
FastRust

tmarcel
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Post by tmarcel »

"Idle Air Control Valve" - It is just that. It's a valve operated by a duty cycle to achieve a certain RPM for idle and to compensate for high voltage situations and/or air conditioning drag. So if it was bad, then it could cause the situation that he/she is having.

ronemus
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Post by ronemus »

It sounds as though you have a rich mixture, which could be caused by the intake airflow sensor (MAP or MAF) or an exhaust oxygen sensor; I'd bet on the oxygen sensor on a high-mileage engine like yours.

lavabit
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Post by lavabit »

I re-measured the vacuum (the previous time I did it was the first time I've ever used a vacuum gauge). I'm inexperienced in interpreting all the nuances of the readings.

This time, I connected the vacuum gauge to the larger outlet on the intake manifold used for the brake booster.

This time I also backed off the throttle plate stop screw back to its normal position so that the idle dropped back down to about 750 rpm from the roughly 1400 rpm I had it at before in an effort to at least stop the engine from stalling when it got hot and the missing began.

This time, I got a vacuum reading of about 20.5" Hg which is within the normal spec.

The vacuum gauge needle vibrates extremely fast, looks like a blur, between about 20.2 and 20.9. Is a rapid vibration like that over a small range ok or does it mean my engine has a problem?

My engine is a non-turbo and uses an automatic transmission. It is the 2.2 engine.

Since I re-setup the vacuum test properly, I haven't yet looked to see if there is any correlation between when the engine misses and a drop in vacuum.

Just for argument's sake, what if there was a vacuum drop when the engine missed. Would that really narrow down the cause of the problem? After all, missing could be caused by a spark problem or by a fuel problem or by a valve problem and probably a few other problems too.

Since missing, regardless of its cause, suddenly drops the rpms, wouldn't the vacuum always drop just due to the suddenly lower rpms?
And therefore, wouldn't the vacuum test really not narrow down the cause of the missing at all?

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