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Old 02-06-2012, 02:32 PM   #1
Sneaker Pimp
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Default Cylinder #3 very low compression. Please advise on options. New car time?

Hello all,

I've had an intermittent SES light/P0303 code (cylinder #3 misfire) for a few months. Messing with the plugs/wires/coils always made it go away so I figured it was the ignition. I put in new wires and plugs a week ago, but the code came back after 5 days so I finally checked the compression. I took the data OldNuc has recommended in other threads. It's really bad:

COLD test:
Cyl PSI #times engine turned over to get to max PSI
#1 210 (5)
#2 210 (5)
#3 60 (4)
#4 195 (5)

I assume/hope things will be better warm, and I'll run a warm test tonight, but I wanted to get some feedback ASAP.

The car runs well though! A -little- rough at startup sometimes but smooths out quickly. Does burn a lot of oil, about a quart every 400 miles. Plugs pulled last week showed 1,2 and even #3 looked OK, a little tan, but #4 had black deposits. I can put up a pic if you'd like.

What are my options? Is "do nothing" short term even an option? How about medium/long term? (this is my only car and I've been driving the car for months like this) How do I figure out the exact cause, and what repair options do I have?


Thanks for your time and expertise.
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Old 02-06-2012, 02:45 PM   #2
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

I could describe a process to determine if it is valves or rings but with the oil consumption you report, and the fact these engines are known to have an issue with oil control rings AND very little head problems, it is most likely rings.

But with that said, you can take an oil squirt can and squirt a couple of good shots of regular engine oil into the cylinder giving you problems. Let it set for a minute then re-do the test. If the compression improves significantly, it is rings. The oil you squirted in goes down around the rings and "seals" it up for the duration of your test. If it does NOT improve at all, you are likely looking at a valve issue.

If you have a way to put compressed air into the cyl, you can listen for air comming out at key locations. The cylinder in question has to be at top center with both intake and exhaust valves closed, (firing position). Put a little compressed air, not a lot in through the spark plug hole. BE SURE you are in park or first gear and the parking break is set, this can cause the engine to turn.

While the air is going in, listen at the exhaust pipe and the breather intake. Also via the PCV valve. If you note air comming from any of these areas you can determine the cause. Air from the exhaust pipe is a bad exhaust valve, from the breather is an intake valve, PCV valve would be rings.
...
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Old 02-06-2012, 02:50 PM   #3
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

The leak down test (pressurizing the cylinders) is also best performed hot. When cold you can expect to hear compressed air going by the valves. Everything doesn't fit perfectly when the engine is cold.

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Old 02-06-2012, 03:05 PM   #4
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

most likely a burned exhaust valve due to chronically low fuel pressure. unplug the injector on cylinder #3 and drive it as-is until you figure out the next step (fix or replace).

considering the issues (low compression and high oil consumption), if you want to keep the car your best options are to have the current motor rebuilt or find a good used motor. with either of these options you'll have to fix whatever burned the valve, likely either a weak fuel pump, plugged fuel filter, faulty fuel pressure regulator or a combination of the three.

Last edited by magtec; 02-06-2012 at 03:10 PM.
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Old 02-06-2012, 05:24 PM   #5
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

Burned valve on #3 + DOHC engine = known issue.

it's due to the extra heat from the EGR valve + constant WOT + constant lean fuel mixtures.

As said, you'd have to replace/rebuild the engine or get another car.
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Old 02-06-2012, 05:44 PM   #6
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

Yikes! By "new engine" would this mean replacing the head?

Installing one of the J&C rebuilt heads off eBay is something I might tackle, but if it involved more than that, I think it might be time...

Which really stinks because I replaced the entire front suspension recently and it feels like a new car. It also stinks because it's getting 34+ mpg in winter and almost 40 in summer. <sigh>

I'll do the warm and wet compression tests and report back.
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Old 02-06-2012, 05:47 PM   #7
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

Quote:
Originally Posted by Sneaker Pimp View Post
Yikes! By "new engine" would this mean replacing the head?
Yes. or at least the valves/seals/gaskets on the current one, assuming the current head can still be re-used (IOW, it's honed/cleaned to specification, no warpage, cracks, etc)

don't forget you'll also need a new head gasket and bolts. While you're there I'd also check your intake and exhaust manifold gaskets as well as the manifolds themselves and replace as needed.
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Old 02-06-2012, 08:48 PM   #8
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

the situation is threefold.

assuming the compression problem is a burned valve, replacing the head should fix the compression issue. you also need to fix the underlying problem - a chronic lean condition - or it'll burn more valves. however, replacing the head most likely will not solve the very high oil consumption, which is usually caused by stuck oil control rings on the pistons. taking all of these things into consideration, if you want to keep the car the best option is to rebuild or replace the motor (and fix the chronic lean condition).

it's up to you to determine what is the most cost-effective option.

Last edited by magtec; 02-06-2012 at 08:57 PM.
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Old 02-06-2012, 09:06 PM   #9
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

I have a 97 sl2 and it used to burn 1/2qt every 100 miles misfires on 2 & 4 cylinders. I did everything that you did which helped alittle than it became to bad. So I finally had a engine rebuild pistons, rings, gaskets, and new timing chain & clean all carbon crap out. I got a pretty good deal and Im very happy. 500 miles so far and engine has not used any oil and runs like new. My car is in very good shape body wise and frame and suspension too. So I figured the $1800 it spent on rebuild is better than having a car payment of $200-400 per month.
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Old 02-07-2012, 10:57 AM   #10
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

Warm and wet compression results still bad:

#3 Warm: 80 PSI
#3 Warm and wet: 90 PSI

Why should I unplug the #3 injector? I assume it's to save the catalytic converter. What are the consequences of running it that way for a while?

Thanks for taking the time and sharing your experience.
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Old 02-07-2012, 12:15 PM   #11
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Default Re: Cylinder #3 very low compression. Please advise on options. New car time?

while unplugging the injector will help save the cat, what i'm most concerned about is unburned gasoline getting into the oil and diluting it. gasoline isn't a very good lubricant...

there shouldn't be any short term consequences. my sw2 had 0 compression on #3 when i bought it, unplugged the injector and drove it for a couple of months. no problems, dead reliable. only got 20-22mpg though.
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