Low compression cylinder 4

Asked by May 05, 2016 at 02:03 PM about the 2007 Chevrolet Cobalt 2 Dr LS Coupe

Question type: Maintenance & Repair

Leak down found cylinders 1-3 are 174 lbs  and 4th is 74.......dealer wants
$130/hrs to tear it apart.....I don't have that kind of money.....this seems to be a
widespread problem ...this car is a 2007 with 78000 miles....no
enginecwork...never overheated....never low oil....what gives

9 Answers

610

The 1-3 cylinders at 174lbs is good number 4 is losing compression. maybe caused by a blown head gasket. Bad ring or maybe even a bad piston.

Best Answer
44,485

Head gasket, piston, piston ring or bent / burned valve is likely the issue. Did they do a wet compression test by putting a small amount of oil in the cylinder. This will tell them if it is the piston, piston ring or if the problem has something to do with the cylinder head (gasket, valve, etc.).

1 out of 1 people think this is helpful.

Thanks so far, those are all the things my mechanic said it could be...he only did the leak down.....the dealer wants $130/hour to get to the head to diagnose one of those issues....my question is.....why would this happen to a car that is only 8 yrs old, has less than 100,000 miles, never overheated, oil and filters changed every 3000 miles....plugs and coils changed too...many vehicles seem to have this issue

44,485

It's a Chebby, what did you expect? Sometimes it is difficult to determine why a problem occurred. Could be just worn out, could be abuse, could be design issue, could be part issue, could be a maintenance issue, could be.... it just goes on and on. It's broke get over it and get it repaired. The very best of luck to you.

44,485

My best guess is it is the head gasket. Let us know what you find out.

17,955

Hmm, 78,000 miles?? Not a ringing endorsement for your car. Your in a pickle here, it's not worth much not being fixed, and I don't know how much or if you feel it's worth fixing.... My only advice is, fix the car, drive it another 25,000 miles and sell it while it's running good.. You might as well get something for your money spent on getting this repaired.

3,825

It's a machine, and machines break and wear out and they all have weaknesses somewhere. Maintenance is all part of the fun of owning a car, no matter what brand it is. Now that we have that out of the way, on to the fix. Much as you may love your dealer, he is going to charge you and arm and a leg for work that a reputable non-dealership shop will do for only a leg. The sort of work you need done should never be done at a dealership unless its warranty. Find a good private shop, (not your uncles' friend's neighbor) get it fixed properly and motor on. They're good cars and you should be able to get a couple of hundred thousand miles out of it.

Not sure how this ended up for you but... having worked at a dealer, and performance shop, this is how I am tackling the same problem on a friend's 2010 cobalt. Misfire at idle on #3 only. Moved coil, misfire didn't follow Replaced plugs, should have swapped but oh well they're cheap, no fix Compression test showed about 150psi for #1, #2, #4... #3 was 100psi. Dialed compressor down to 150psi, connected to #3 through compression tester. Put #3 on tdc Looked for bubbles in coolant reservoir, none. Listened for air leak through brake booster hose, it would seal as I rotated Listened for air leak through exhaust pipe, no matter where I rotated engine, would not seal. Tells me #3 exhaust valve isn't sealing. Now why??? Cracked/burnt/bent valve? Head would have to be removed cuz I don't have scope. Broken valve spring? Only valve cover would have to come off. Currently running air induction cleaners, lots, at varying rpms. Decided to not waste money on chemicals so I started running water/methanol 50/50 (like the injection kit on my xterra, which has really clean valves) If cleaning doesn't work, I will remove v/c and check spring, if it's good, THEN the head has to come off.

Oh, I dropped some oil in the cyl before I hooked up the air compressor to rule out rings/piston. No change in compression. Also no blow by through oil fill cap.

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