compression testing variables

had a 2004 grand cherokee 4.7 liter with a dead misfire. found it to be the exhaust rocker arm that came off. so the exhaust valve was stuck shut. had all cylinders with compression of 140 psi. i went with compression just by the way it cranked. the vacuum gauge looked pretty solid no bouncing or anything. is there a way you guys have to check for stuck exhaust valves? the valve cover was easy to get off but if it wasnt it would be nice to know so not much time is wasted. i was thinking maybe timing as well

Are you saying the cylinder with the dropped rocker had same compression as rest? All the mopars I've seen drop the rockers were caused by the valve seats falling out of the head and holding the valve open..

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Are you saying the cylinder with the dropped rocker had same compression as rest? All the mopars I've seen drop the rockers were caused by the valve seats falling out of the head and holding the valve open..

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Short of an in-cylinder pressure test watching the gauge on old style compression test with the Schrader valve removed should show a difference in the way the needle moves it should bounce twice as fast because there is an extra compression peak on the exhaust stroke.

A First-Look pressure pulse test on the exhaust or inlet should show something but interpreting First-Look tests to diagnose a particular fault isn;t easy.

yes ben. same result as the rest

Relative compression with a scope would show it. If the exhaust valve is closed you would basically have 2 compression strokes on that cylinder. So on the relative compression you would have one peak much higher than the rest because you would have 2 cylinders at peak compression at the same time. I will post a picture of a scope capture later today or tomorrow

i could tell there was an issue during cranking. at the time i dint know if it was out of time or if it was a compression issue at the time. vacuum gauge was super steady at idle no fluttering or anything i was taking vacuum at the brake booster

Added a pic from a class I took in Pennsylvania. It shows relative compression from a 2002 Dodge 4.7 with a #2 misfire. See how how the #3 compression is? That's because the exhaust rocker on #2 is off causing the misfire. When #3 is at TDC of compression #2 is at TDC of the exhaust stroke. But with the exhaust rocker off that puts compression on #2 and #3 at the same time. Which is why#3 is so much higher than the others on the relative compression test

Nice capture

cheryl hartkorn wrote: i could tell there was an issue during cranking. at the time i dint know if it was out of time or if it was a compression issue at the time. vacuum gauge was super steady at idle no fluttering or anything i was taking vacuum at the brake booster

I never really learned the skill of using a vacuum gauge as it was not useful on the engines I learned on which were mainly 4 cylinder twin carb.

I've seen the same thing as Bruce on a relative compression waveform. This is my go-to test when I think a rocker arm fell off a 3.7/4.7 engine. I used to go in-cylinder to prove it, now I just sell valve cover labor. :lol: Nice job going with your gut and the cranking sound.

I'm really surprised your vacuum gauge was steady, Cheryl. :huh: It's gotta be dumping 100+ PSI into the intake when the intake valve opens... Why doesn't the vacuum gauge notice?

Tyler wrote: I've seen the same thing as Bruce on a relative compression waveform. This is my go-to test when I think a rocker arm fell off a 3.7/4.7 engine. I used to go in-cylinder to prove it, now I just sell valve cover labor. :lol: Nice job going with your gut and the cranking sound.

I'm really surprised your vacuum gauge was steady, Cheryl. :huh: It's gotta be dumping 100+ PSI into the intake when the intake valve opens... Why doesn't the vacuum gauge notice?

I wonder if the brake booster vacuum take off had the one-way check valve built in atthe manifold end ? butsurely even then I would have expected some pulsing on the gauge.