Low compression misfire number two cylinder

STUARTG

MEMBER
2007 CHEVROLET MALIBU
4 CYL
2WD
AUTOMATIC
116,000 MILES

We failed the California smog test because of the number two cylinder misfire. The test does not actually measure the exhaust after it senses the misfire. We have had this intermittent problem (misfire) for over two years and passed two smog tests. We do not think that this car is worth a new engine approximately $7,000. It runs smoothly, there is no smoke coming out of exhaust, it does not overheat, and no water leakage. We would like to get some more miles out of it. CA requires $650.00 in repairs for a one year wavier but two different shops have quoted a minimum of over $2,000.00 to start (remove head). Anything (inexpensive) we can try?

7 Replies

STEVE W.

MECHANIC
13,443 POSTS

What has already been tried and what tests have already been run? For instance you have "low compression" in the title, what are the actual compression numbers for all four cylinders after both a dry and wet compression test?
Has a leak down test been run to determine where the compression is being lost?

Without knowing where the issue is it would be pointless to suggest anything. Have the plug, coil, injector already been tested and ruled out?

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Monday, October 23rd, 2017 AT 11:24 AM

STUARTG

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Thanks for the reply Steve.
The first shop: connected "snap on motus ultra" scan tool to access computer found po300 - checked all cylinders - cylinder two at 80psi and cylinder three at 60psi. Do not recommend valve job as bottom end of engine may be compromised.
Cleared codes, fuel adaption memory and did fuel trim and ecm sensor drive recycle relearn.
Reccommended new engine $7,100.00
The second shop paper work says: performed system tests and found cylinder two with misfire performed compression and found cylinder two at 75 psi below spec.
Recommended removed cylinder head for further inspection and repair $2,000 plus additional labor and parts based on repair.
Nothing else about where loss of compression, but they both ruled out electrical.
Thanks for the help.
Stuart

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Monday, October 23rd, 2017 AT 12:56 PM

STEVE W.

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Okay, if you feel like doing a bit of work and own an air compressor (even a small one like an air nailer style will work) you can do a leak down test and discover that your issue is probably going to be valve related or a bad head gasket between cylinders and not in the bottom end (unless something real strange happened). A leak down test is simple. You remove the spark plugs, then using an adapter and the tool you add air to the cylinder with it locked at top dead center. Then you read the gauge on the tool and listen at the throttle body, other cylinders and the exhaust for the air leaking past.

I say it is not in the bottom end primarily because you say it runs smooth and because of the compression numbers. In most engines you can take the piston rings off and still get 80-90 psi in the cylinder, but if a valve has carbon build up or valve seat damage they will lose compression much worse.

Now one thing with carbon build up is that in many cases it can be removed without opening the engine using what is called a top cylinder cleaning. Basically you spray or draw in the cleaner into a warm engine. Shut it off and let it set for an hour or so, then restart it and blow out a lot of crud.

I would start with the leak down test. It will point a finger at where the engine is "leaking" then you at least have a real idea of what the problem is and could plan accordingly.

Couple videos showing what the test involves.
https://www.youtube.com/watch?v=44ZIhDHonwA
https://www.youtube.com/watch?v=oFCPeqPKGNU

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Monday, October 23rd, 2017 AT 4:04 PM

STUARTG

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4 POSTS

Thanks so much Steve!
I will get the adapter and tool and see if we can get out of this without losing the use of this car.
Best,
Stuart

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Monday, October 23rd, 2017 AT 5:01 PM

STEVE W.

MECHANIC
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You should be able to "borrow" the tester from a parts store. They "loan" you the tool by selling it to you, then when you bring it back in the same condition they sent it out, they refund your money.