55 psi compression in left two cylinders, minimum in others, diagnostic ideas?

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Astral

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Contributor
Hey folks, P5 with 168K. Noticed some pinging last 1000 miles. Checked for obvious vacuum leaks/cracked hoses, replaced the cruise control hose but made little difference (must've been barely leaking). Decided to go down the service manual list of things to check: ECTs are good, well under 241F, then check compression. Been running MP3 flash since I bought it, so it's had 93 gas all the time.

Compression numbers, with all plugs out, warm engine are: ~55, ~55, 115, ~117. When I bought this car with ~145K, the compression numbers were around 140. So 55 is ridiculously low--the car seems to be down on power a little, but not crazy much. Pinging under part throttle, sometimes under full throttle. Been running 93 and octane booster last 1000 miles or so (had some ski trips to drive on, heh, didn't realize the compression was this s***).

Poured a little oil into the low cylinders and it didn't change the compression readings, so I know it's not scoured piston walls. So it must be either valves or head gasket--perhaps blown head gasket in between cylinders?

The car is not overheating or running very poorly. I do get stumbly idle now and again, but nothing like "crap EGR" idle that I've had with P5s before.

ANy suggestions for next steps? Order a head gasket right away, or go have a leakdown test done to see if it's valves or HG?

Thanks!
 
I feel like if it were the head gasket it would be overheating the engine like crazy. My dad had a head gasket go on his Chevy van and it overheated within 2 minutes of a cold start. Just my two cents.
 
Is the exhaust pipe smoking anything (burning coolant)?

Is your coolant level dropping?

Just for experimentation, try putting a plug in one of the low compression cylinders and test compression on the other low compression cylinders, any difference?
 
Good idea dill, I was thinking the same thing. If its a break in the head gasket between 1 and 2 the numbers should increase slightly with the plug in the other. It wouldn't cause coolant drop or white smoke if that's all that was happening.

Seafoam is an option too...but if you're close to 100psi off I don't think a little carbon cleaning is going to resolve it.
 
chuyler1 said:
Good idea dill, I was thinking the same thing. If its a break in the head gasket between 1 and 2 the numbers should increase slightly with the plug in the other. It wouldn't cause coolant drop or white smoke if that's all that was happening.

Seafoam is an option too...but if you're close to 100psi off I don't think a little carbon cleaning is going to resolve it.
Click to expand...

Thanks :)

I do think however that the results should be taken with a grain of salt, because the compression could be lost through the cooling jackets somehow. What I am trying to say is that maybe the compression could still leak through the cooling system somehow, even though you blocked the other cylinder from losing compression. Are the coolant jackets full of coolant mixture when the vehicle is off? I would think so, but I do not want to make assumptions I am not sure of. :p
 
I put a plug in cylinder 2 and measured 1 (leftmost) and it read something like 35psi (even lower). I'm not sure if that's because the starter had to work harder to spin the engine and I should've had it hooked up to another running car's battery or something.
 
The_big_dill said:
Is the exhaust pipe smoking anything (burning coolant)?

Is your coolant level dropping?
Click to expand...
Coolant level is good, not dropping.

I noticed that exhaust pipe had more whiteness that I'm used to last few weeks of winter, but I wasn't sure if that was just because of high humidity (I could see my hot breath too), or what. I didn't notice any smokiness yesterday.
 
Astral said:
Coolant level is good, not dropping.

I noticed that exhaust pipe had more whiteness that I'm used to last few weeks of winter, but I wasn't sure if that was just because of high humidity (I could see my hot breath too), or what. I didn't notice any smokiness yesterday.
Click to expand...

If you didn't have a booster hooked up to a healthy battery while checking compression, I would start from scratch with a booster on a healthy battery. Otherwise your readings would not be accurate.
 
The_big_dill said:
If you didn't have a booster hooked up to a healthy battery while checking compression, I would start from scratch with a booster on a healthy battery. Otherwise your readings would not be accurate.
Click to expand...
Good point, I'll retest tonight!
 
So someone suggested to measure the compression while cold. Normally that would cause the compression to read lower, but if the compression goes UP, then that means there are tight valves (since metal is contracted while cold). I ran the compression test today with a booster from a running car, while cold, and got 50-50-110-110 (basically 5 psi colder than when I read it warm). So no difference.

I put a plug in cylinder 2 and read the compression and it was 30 psi in cylinder 1 (while I was still attached to the running car's battery).
 
Astral said:
So someone suggested to measure the compression while cold. Normally that would cause the compression to read lower, but if the compression goes UP, then that means there are tight valves (since metal is contracted while cold). I ran the compression test today with a booster from a running car, while cold, and got 50-50-110-110 (basically 5 psi colder than when I read it warm). So no difference.

I put a plug in cylinder 2 and read the compression and it was 30 psi in cylinder 1 (while I was still attached to the running car's battery).
Click to expand...

Sounds like its time for a top end rebuild.

But first double check by putting more than usual engine oil in the weak cylinders just to confirm.
 
Problem found. Did a leak down test, it's definitely the head gasket between the two cylinders: you can hear air blowing out of the other cylinder. The other two cylinders are leaking air around piston rings, but not nearly as badly. Still, not really happy about 115psi compression in two cylinders.

Debating whether to replace the head gasket or do an engine replacement.*Not in the mood for a rebuild.
 
Astral said:
Problem found. Did a leak down test, it's definitely the head gasket between the two cylinders: you can hear air blowing out of the other cylinder. The other two cylinders are leaking air around piston rings, but not nearly as badly. Still, not really happy about 115psi compression in two cylinders.

Debating whether to replace the head gasket or do an engine replacement.*Not in the mood for a rebuild.
Click to expand...

Thanks for the update.

I would say you are lucky that its only the gasket and not the rings that bring your compression down to 55 ;)
 
As you've noted, you can't just replace the headgasket because of the worn rings in the 115psi cylinders (you'd still have a faulty engine), and you don't want to do a rebuild... so it seems the only option then is to install a used engine.

If New England salt isn't destroying the car, a used engine + new gaskets/seals/plugs/wires/coils/belts/water pump etc. for $1k (I'm guessing) + a weekend of your labor will give you at least a couple peaceful years of use.
 
Bummer Oleg. Let me know if you need help with a swap. I've never done one...but I'm a fast learner.
 
SO I decided I'm going to cheaply ghetto hack a head gasket in there, with bare minimum parts (gaskets + bolts), inspect for anything funky, but otherwise just stitch it back together, so I can run it w/o pinging. That gives me plenty of time to contemplate on an engine swap of some kind.
 
Astral said:
SO I decided I'm going to cheaply ghetto hack a head gasket in there, with bare minimum parts (gaskets + bolts), inspect for anything funky, but otherwise just stitch it back together, so I can run it w/o pinging. That gives me plenty of time to contemplate on an engine swap of some kind.
Click to expand...

I would have gone with the same choice, let us know the compression after you do this.
 
I'm sure the motor will be fine for driving around town once you do the head gasket. If you're trying to shave 10ths off your auto-x laps, well, that's different.
 
Yup, a break it is:

<a href="https://www.flickr.com/photos/uksi/14252496311" title="Untitled by ilp, on Flickr"><img src="https://farm3.staticflickr.com/2904/14252496311_08766740c5_z.jpg" width="480" height="640" alt="Untitled"></a>

<a href="https://www.flickr.com/photos/uksi/14069184760" title="Untitled by ilp, on Flickr"><img src="https://farm3.staticflickr.com/2899/14069184760_b6eb8871b3_z.jpg" width="640" height="480" alt="Untitled"></a>

No other issues visible in the gasket. No idea how this happened.
 
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