Bad compression in all cylinders. Need help!
 

Recently I bought an LM4 5.3 Engine from an Envoy XL at a local junk yard. Supposedly the mileage was 150,000 km and 170 compression in all cylinders...

Today I tried doing a leak down test and got no compression at all. The best cylinder had 40% leakage, and the rest were 80% at best. Most of the air is escaping through the exhaust valves, but some intake valves are also leaking. I can't tell if anything is getting past the rings at this point. When I took the manifolds off, there was quite a bit of dirt in the ports and around the valves. Could this be preventing the valves from seating properly. I wasn't planning on taking the heads off, but I guess I have no choice. Would soda blasting be good way to clean things up?

Is there anything else that I can check? I find it really weird that all cylinders are leaking past the valves so much.


This is my first time doing a leakdown test, but I think I did it properly. I put a balloon on the line to check for a compression stroke, then used a screwdriver to gauge for top dead centre. The inlet pressure was set to 100 PSI. The engine is drained of all fluids, which is probably not idea for checking leakage past the rings, but I can't see this affecting the valve seals...

I find it easier to take/loosen all of the rocker so they are all closed and remove all of the spark plugs so that I can still rotate the lower end and bring the piston to TDC. Basically with the rocker off, you can pretty much do a leak down test where ever the piston it is at.

If you found dirt, carbon and ??.. it might be that you do have something in between the valves and the seats.

Try blowing compressed air into it to see if the dirt clears out of the valves. Then try a leak down test.

with 150k.. might be a good idea to do the top end rebuild anyway... just remove them.

Good luck..
BC

i have had some junkyard motors do this just because they were sitting so long. put a starter on it on the engine stand, fog the cylinders and turn it over with the starter and test it again. I bet its OK.

I'd try the above mentioned first. But usually low compression in all cylinders is a timing chain issue.

Unless every damn valve is bent I bet it's just sat for a while. Do what posted above.

Thanks for your advice guys. I ended up taking off the heads. I figured it wouldn't hurt to clean things up and put on a new gasket while the engine is on a stand. Gonna try lapping the valves to fix the problem.