Cylinder 4 - low compression after valve job

VinnyCarbuncle · #1 08-29-2020, 08:08 PM

I’m getting low compression in cylinder 4. Had a valve job done on the head but I’m still getting around 60psi in #4. Does anyone know if it’s related to the tumor on the exhaust side and what might’ve caused the growth? I’m hoping I can just replace the piston/rings to fix the problem but thought I’d check here first. Thanks!

tomboy · #2 08-30-2020, 09:14 AM

Is it a hole that goes through the piston head or just something that is wedged there, if idts a hole, it could be from timing too far advanced, post a better close up pic, and more info on what happened.

tomw0 · #3 09-01-2020, 09:29 AM

Are you talking about the bump on the top of the piston? Doubt it would do anything to compression.
Is it possible one of the valve lash adjusters is stuck? If so, it could be holding the I or E valve off its seat, leading to poor compression.
You should be able to lean on the support end of the cam follower, pushing downward hard for a while. I like to use a hammer handle, putting the wooden end on the top of the follower. The follower should collapse a bit, perhaps .050".
If you cannot compress it, you can work the follower out from under the cam, off the valve stem and the follower, and pull the follower. It may be sticking due to deposits, but can be disassembled if you want to take the time. They should be replaceable without any special consideration. No 'size', they are all the same. Used would be cheaper.
If you don't want to do that, get a Tbsp or two of oil, put down the spark hole, and check compression. If it comes up a lot, i.e., from 60 to 130-150,l your rings are likely leaking. Not good. If it stays the same, general thought is the valves are not seating. That is, if the head gasket is functional.
tom

VinnyCarbuncle · #4 09-06-2020, 06:05 PM

Thanks for the good ideas. I ended up pulling the engine and taking out the piston. As you can see from the photos, something happened with the rings/grooves. Not sure what went down. Hopefully the cylinder just needs to be honed.

tomw0 · #5 09-11-2020, 10:53 AM

The pics are too small for me to see, but if the cylinder wall is not scored, you could replace just the rings and put back together. Oh, and replace the piston. I assume the 'lands' have been damaged/broken, along with some damage above the top ring. That erosion can happen due to pinging. Did you experience any ping noise?
tom