So today i traded a 4dr eg for a eg coupe with a blow auto tranny and zc swap. The car doesnt drive because of the bad tranny but we started it and everything looked good. Got it home and did a compression test, results were 210, 210, 150, 60. Any idea why cylinder 3 is low and cylinder 4 is almost dead?

 

Valves, valve seats, head gasket, rings, cylinder walls, mating surfaces. Take your pick.

 

Thanks for the reply but this isn't my first rodeo. I know what could cause a engine to lose compression but my question is why are the first 2 healthy and the other to so far apart. doesn't sound like rings. Hoping its just the valve lash and not burnt valves.

 

Just checked the valve lash on cylinder 4 and they were way to tight. Adjusted the valve lash for that cylinder and did a compression test, still the same.

 

if this isnt your first rodeo, then why are you asking the same question again....

 

I'll take your response as you're very familiar with internal engine work. Open it up and see what you find.

 

Thanks for the reply. Yes im no stranger to the internal engine. I was hoping not to have to split the motor but it seems like i will have to. I should have the head of by monday and will post updates on what i find.

 

Yeah I hate to say it but the only scenario for a correctable low compression condition that I can think of is over-tightened valves and you eliminated that. Aside from a burnt valve look to see if maybe one of them is slightly bent. You can also test all valves to see if they all seal while the head is removed. Do you know the home method for checking if the valve seats are good?

 

I've used water to check which valves leak. Idk if thats the method your talking about. Thanks for the reply, bouncing ideas always helps diagnostic

 

I use brake cleaner but same concept. If valves are dry on the combustion side, all good. Damp is usable but shows signs of imperfect seal. Wet/drip shows bigger imperfect seal. Waterfall well...zero seal (just including explanation for anyone reading that might ever need to try this trick).

I've actually had a head with similar results before, when I opened it back up I found one valve that was ever so slightly bent. Could barely see it when it was in the closed position but that trick made it clear it wasn't sealing at all.

Another idea when it's open, look at the pistons at the half way point of its stroke and see if they are still centered. A long time ago I bought a cheap block that had ovalized cylinders. At TDC all pistons were perfectly centered but halfway down you could literally take your finger and swing the piston side to side about a mm or so (I was able to see the pistons sitting at an angle relative to the cylinder walls).