3.4 NO compression in cylinder #5

Been working on this truck that I have no history of. According to OBD2 scan, misfires on cylinder 1, 5 and 6. Changed plugs and wires yesterday, no change. Ohm tested coils and got acceptable readings on all 3. Decided to do a compression test on the cylinders. Took all the spark plugs out, and noticed that several of them were already quite black on the ends. The motor only ran for about 3 minutes on the new plugs before I shut it off yesterday.



So I removed EFI fuse and began dry testing. This testing was all done on a cold motor.

#1 - 180 psi
#2 - 172 psi
#3 - 170 psi
#4 - 171 psi
#5 - 0 psi
#6 - 170 psi

At first when I got to #5, the starter would turn but not engage the flywheel. Could not get the starter to engage unless I had a plug in another cylinder. Not sure if it has anything to do with the low reading, but I thought I might as well mention it. #5 was also the last one I tested so I went back to #3 and re-tested to make sure the gauge wasn't have issues. I then conducted a wet test on #5, no change. The needle on the gauge doesn't even move a little bit. It kind of surprised me that all cylinders could be so high and one with absolutely no compression at all. I also noticed when removing the air intake tube, there was some oil pooled in the "J" shaped resonator tube on the bottom of the intake tube. Not sure when anything was done on this vehicle so it could very well be the PCV is bad too but I haven't had a chance to mess with it yet. From what I've read, if it was worn rings the compression should at least read something and change on a wet test, unless they're completely gone. Getting no bubbles in radiator and coolant is clean. Oil isn't a milkshake and there doesn't seem to be any condensation on the cap. From what I can tell, I'm most likely looking at a valve issue. It also no longer has stored misfire codes for some reason. I don't know much about these engines so maybe I'm missing something?

 

What do you mean the starter wouldn't engage the flywheel while testing #5? If the engine wouldn't turn over you can't get compression...

 

https://www.youtube.com/watch?v=ynolAwrRTmo

 

The starter wouldn't engage the flywheel when testing #5 unless another cylinder had a plug screwed in. I tried switching the tester to another plug and everything worked just fine but as soon as I put it back on #5 the starter would no longer engage the flywheel. I ended up putting a plug in #1 and then I was able to get it to turn over with the tester in #5. Not sure why

 

Ended up dropping a boroscope down the spark plug hole and looked around. Cylinder wall looks immaculate, very clean and cross hatching looked brand new. No buildup around piston crown to speak of. Turned on side view to take a look at the valves and wouldn't you know it, 3 of them were open and one closed. Cranked the motor briefly and checked again, now only 2 were open, but one more than the other. Cranked again, cylinder now was at TDC and still one valve was open. Based on rotation of the camera it appears to be that the rear exhaust valve is stuck in the open position.

 

Good investigative work. I wonder what caused the carbon fouling on the plugs from 1, 4 and 6.

 

I'm leaning towards bad/old gas. I don't know much history of it but I do know that the PO bought it from another person where it had been sitting for approximately 4 years. He put more gas in it but that was 6 months ago now. I'm betting the injectors are sticking/leaking causing a rich condition. When I smell the tip of the fouled plugs they smell like stale gas as well. I'm wondering if the sitting for years has something to do with the stuck valve as well.

 

The starter drive spun but did not extend into the ring gear

I have experienced strange things But I think that is now number 1

You have some very strange issues .This is more of a concern then the valve issues.

 

Pretty weird to me too. I've since cranked the engine 10+ times and had no further issue. Could be a coincidence but at the time it felt related haha