Compression problem in cylinder 3

  • 3.0L
  • 3 CYL
  • 4WD
  • 93,000 MILES

Low compression in 3rd cylinder. What could be the cause and could it have happened when the timing belt or chain was replaced? Also, what should a rebuilt engine cost for this vehicle?

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Wednesday, April 5th, 2017 AT 5:19 PM

1 Reply


The 3.0L is listed as an "interference" engine, so timing belt problems can cause bent valves. If the timing belt broke while you were driving, the open valves would have been hit and bent by the pistons as they coasted to a stop. If the belt was simply being replaced for scheduled maintenance, it is unlikely a valve could be bent by turning the engine by hand while the timing was not yet adjusted correctly. Normal hand pressure is not enough to bend a valve unless the engine is forced really hard to rotate. If the timing was not set correctly, and the starter was used to crank the engine, you'd have bent valves in every cylinder, not just one.

A cylinder leakage test will identify the cause of low compression. You may be able to find the tester at an auto parts store that rents or borrows tools. It involves placing the cylinder at top dead center on the compression stroke, then you force in compressed air through the tester, into the spark plug hole. It will show the percentage of leakage, and you can listen for the results of the leakage. If an exhaust valve is not sealing, you'll hear air hissing at the tail pipe. If an intake valve is leaking, you'll hear the air at the throttle body. You can also identify leaking piston rings by the hissing at the oil cap or dip stick tube, and a leaking cylinder head gasket by tiny air bubbles in the radiator.

We don't get involved with costs here because there's way too many variables. Engines are not replaced for bent valves. That is way too extreme. Unfortunately too many engineers feel the need to build interference engines, and with many breaking timing belts, replacing bent valves has become a common repair.

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Wednesday, April 5th, 2017 AT 5:45 PM

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