cajun_4x4

As it states the compression is low and as soon as the motor stops spinning the compression drops to 0. But it gets up to 90psi and the truck runs fine thinks nothing is wrong. I was thinking worn compression rings or washed out rings. But after a lot of reading I'm thinking a stuck valve sounds more likely. What are yalls opinions? Also the truck has 135k on the odometer.

Chris_1

iTrader: (1)

Squirt a few drops of oil in to seal the rings and do another compression test (wet test).
If compression jumps up it's rings. No change - valve.

cajun_4x4

Ok. I'll give that a shot this weekend. Now if it's rings I should be able to just change them and be good. Right? If it's a valve issue I should be able to suck some seafoam into the motor through the vacuum hose and knock it loose. Does all this sound plausible?

Crownman

If you had really low compression on one of the cylibers you would feel it when the engine is under load. If it's running fine as you say then what prompted you to check the compression?

cajun_4x4

I checked it because I found some maintenance papers in the glove box when I got home with the truck that said check the fluid that's splattered on the hood and at the bottom it showed that they did a compression test and it was 0. So I decided to check it. I got 90psi while it was spinning over and when I let off the key it drops to 0psi. They also changed the plug wires, cap, and rotor at the same time.

Silent Echo

If you replace the rings then you will need to hone out the cylinder so that new rings will seal to the cylinder wall. While you are in there I would replace all the rings so you will be set compression wise.

If it is the valve, I doubt simple seafoam would do anything. I'm a boat mechanic up in Alaska for the summers, and I had a 150HP Yamaha outboard that was driving me insane. I couldn't figure out why It was running so rough until I opened up the exhaust plate to find that both intake valves on cylinder 1 were stuck open. I have no idea how they got stuck, the engine only had 260 hours on it and has its oil changed every 2 weeks. (The boats are used a lot for the company.)

I had to use a 5 pound sledge to drive them out of the head, then have new valve stems put in, along with new valves and had the seats cut to match.