5.9L No Compression In Cylinder #8
 

Hi all,


I recently had my Check Engine light come on and noticed the engine was missing a bit, as if one cylinder wasn't firing. It was overdue for a tune-up anyway so I brought it into my local mechanic and had him replace plugs, wires, cap, and rotor.......all to no avail. It still had the Check Engine light and his diagnostic tool indicated a misfire on Cylinder #8. I was there at the time and we checked the wire routing to make sure it wasn't in contact with another wire, and then pulled the plug only to find the plug was wet, with fuel, not with oil. He then did a compression check and found that all cylinders were within 130 to 140 with the exception of Cylinder #8 which had ZERO compression.


My question is this: Is it more likely to be a valve\head issue or a ring, and what would be the next step in determining the problem? Bring it to an engine shop and have them pull the head? I don't want to spend the kind of money it would take if its a ring issue as this truck is old and not worth it, but a valve job is still doable to me. It's a 1996 5.9L 1500 4x4 with 160,000 miles so you can understand my having to draw the line soon.


Any help would be appreciated.

there's 2 more tests you can do, 1) go back and do the compression test (dry)on cyl. 8 then do it again after you squirt a little oil in the cylinder, this will temporarily seal the rings and raise cylinder pressure, if so the rings are shot, if not, its head related..there's also a cylinder leak down test you can do with compressed air to locate the problem also..

Zero seems too low, unless it's a stuck valve. Yup, try the test again.

Not necessarily too low. I had a motor (the one I was running before my 408) that would test zero, but would start working around 2500~3000 rpm range. Of course it had a gawd awful noise too. The piston was slapping so bad it sounded like a rod knocking.

To me it sounds like a stuck valve or broken valvespring. Typically worn rings or a head gasket would result in low compression. As mentioned previously, dump a teaspoon of oil down the cylinder and rerun the test. If still zero then do a cylinder leakdown by bringing the engine to tdc compression stroke on cylinder 8. Add air down the spark plug hole and seal off around it and listen to where the air comes out.

You could pull the valve cover to have a look at the valve springs. Crank it over and watch for proper motion.

I meant for it to be a piston ring issue.

I follow you, and generally you are correct.

I should have added that with mine, the rings were full of carbon from a bad plenum (burning lots of oil). The reason they would start working in the higher rpms was because of the cylinder pressure forcing them out. Having stuck rings is what caused the cylinder wear leading to the piston slap.

Thanks to everyone for the responses.


I decided not to trust the mechanic that did the compression test and went to Sears and bought my own tester. It turns out the compression on cylinder #8 wasn't zero, but rather it was 50 PSI. Probably not that much of a difference in the grand scheme of things as it should be somewhere near 130 but worth noting none the less. I then put a couple of shots of oil in the cylinder and tested it again: Still at 50 PSI. So I guess it's safe to say it's a head problem. Probably a stuck valve.


Since I'm in Minnesota and it's getting colder out and I don't have room in my garage to do any work, I'm going to have to bring it in at this point to have it serviced. Hopefully the worst case scenario is that they have to replace a broken valve spring.


I do have one last question though: A friend told me that I could try draining half the oil and add a bottle of Seafoam to it and run it for 15 minutes, then do an oil change. If it's just stuck, that might clean the $hit out of there and get it unstuck. Does this sound like a good idea? Can it cause any harm or is it just a waste of time?

Well i dont think half of the oil needs to be drained out a quart will do if your worried about overfilling. Marvel mystery oil may be another option. My 2 cents, one of those may work, it just depends if you have time and the supplies. Definitely cannot hurt (to my knowledge) to try, if something if broken than these methods wont change anything except your parts will be cleaner