Low Compression

[yater]

Hi,
I have a Vortec 330 HP in my '99 Maristar. The motor is making an intermittent popping sound at high rpm and surges a bit just above idle. My mechanic says that its running on 7 cylinders at the idle. Unfortunately I haven't been able to replicate the popping at high rpm for him.
A compression test showed 120 in one cylinder. All others were within range.

Mechanic indicates that a valve job is the way to go. An comments, suggestions? Thx

[MLA]

What is yours or his idea of within range? I would fallow up the compression test with a cylinder leak down test. This will actually pinpoint the source of any compression loss.

[yater]

The other cylinders were 185-195. What is involved with leak down test?

[MLA]

120 psi is definitely out of spec with the others.

A lead-down involves placing the piston at TDC and filling the cylinder with regulated air while watching a pressure gauge to see of the pressure drops. If it drops, there's a leak. if its past the rings/piston, you will hear it coming out of the valve cover. if its and intake valve, you will hear it in the throttle body. if its an exhaust valve, you will hear it exiting the exhaust. If its a head-gasket, you will hear it exit another cylinder or gurgle in the water passages.

[elliott]

120 is more than enough for the cylinder to hit on. so i dont think thats your prob. yea its lower than the rest, but it should hit. think you need to start looking else where.

[elliott]

the poping your hearing its probably running lean, causes backfiring through the intake. might have a weak valve spring, slightly bent pushrod,

[elliott]

plugged fuel filter or a weak pump

[MLA]

IMHO, a cylinder thats running 40% below the others is worth investigating. This can result in a decrease of crankshaft speed when that suspect cylinder is on its power stroke. Im gathering the tech has isolated a miss at idle to a particular cylinder? If so, system fuel pressure would not effect a single cylinder, but all of them. An issue with the #7 injector would effect #7 cylinder itself. Have the tech take it to the next step and rule out a mechanical problem with that cylinder with low compression.

[Kyle]

TxsRiverRat on here had the same problem.


I did the compression check and kept on diagnosing. We have the same year boats his is a 205 and mine is a 190 but both had the 351w. I changed the carbs out and found that his carb on my boat ran bad and mine ran bad on his boat but it ran better than his on his boat. Swapped them back and my boat ran great again. Ignition was great on both boats.

I pull the intake (cheap option) and inspected.


I found that 1 cylinder was not opening up all of the way. I pull valve covers off and loosen the rocker arms to remove push rods. They were straight. Pulled lifter and noticed wear.


Ended up being a bad cam lobe.


Replaced cam and lifters and she's been perfect ever since. He did replace carb as well but that was not his problem.

[Kyle]

Originally Posted by MLA

IMHO, a cylinder thats running 40% below the others is worth investigating. This can result in a decrease of crankshaft speed when that suspect cylinder is on its power stroke. Im gathering the tech has isolated a miss at idle to a particular cylinder? If so, system fuel pressure would not effect a single cylinder, but all of them. An issue with the #7 injector would effect #7 cylinder itself. Have the tech take it to the next step and rule out a mechanical problem with that cylinder with low compression.

I wonder if the compression check was run with the safety lanyard pulled. When the lanyard is pulled then the fuel pump is shut off so that the injectors are not triggered durring the test. If there was a bad cylinder and injectors are spraying durring test (just pulling coil wire) then the test is not accurate and the bad cylinder would read low if the injector was bad.


You want all cylinders to be within 10% of eachother. Sounds like 7 of 8 are within that parameter. The bad cylinder is way low.


Check that plug. Is it oily?


Pull valve covers and check the torque on the rocker arms. If it has backed off then the spring will not function properly.