QuestionTruck has 200k miles, head gasket replace in 8 months ago. This seemed to start more than 2 years ago as an intermittent problem. Has deteriorated to the point where it's running rough and bucking 100% of the time - too violent to drive. Compression test: #1cyl 175psi, #2 160, #3 176, #4 152, #5 181, #6 171. Have found that pulling number #5 plug wire makes no difference in rpm's or the the way it runs. Have replaced plugs, cap, rotor, MAP sensor, #3 and 5 fuel injectors, pulsation damper, fuel pump, fuel filter, ignition wires, coil, TPS. Spark looks strong on all cylinders, fuel pressure good, verified signal to all injectors.
Am pretty sure at this point that it is not fuel related or lack of spark at the plugs. Is it possible to have a these compression values and still have a basic engine problem? Suggestions?
AnswerYes, there is still aproblem with the #5 cylinder, since we know that the compression is good and also all those other parts are good the cylinder is still not getting one of all the conditions it needs for combustion, you can rule out compression, spark(it was confirmed to all cylinders)if it was a timing problem it would affect all cylinders, there are only a couple of things I can think of, there may be a broken valve spring, this will still indicate good compression but will cause a misfire when the engine is running,or it's still not getting fuel, when the injector pulse was checked was a small light used connected directly to the connector, just checking to see if there are 12v at the injector terminal to one side doesn't mean too much, there has to be pulse voltage between the two terminals, if that checks ok I would seriously consider checking for a broken valve spring, I had this very same problem a few years ago where everything seemed to check ok but we finally found it was a broken spring.
