Compression Problem...

[rmj134]

hello everyone, this is my first thread, but I feel the topic is beyond the 'newbie' status. I appologize if this thread was incorrectly placed.

I will give as much information as I can to help you get an idea of previous attempts to diagnose my problem.

let me first start off by saying that a year ago I had new valves installed due to a failed timing belt tensioner. with the head off I installed a Cometic head gasket and ARP studs.

after a year of good vacuum, I now have a low reading. 11hg at sitting idle, 14hg at cruising idle. after warm-up, and driving, if I press down to about 40% (or more) throttle, and release quickly, the vacuum will go to 21hg and sit there for a few seconds, then 'snap' to 14. occasionally it will rapidly fluctuate between 14 and 21, then stay at 14 (the new valves made me not so convinced it was a sticky valve). I did a compression test on all four cylinders and they read :

cylinder 1 : 160
cylinder 2 : 30
cylinder 3 : 160
cylinder 4 : 110

this is what I tried as fixes to further give you an idea of where I am at...

I tried a wet compression test and all four cylinders did not increase enough to make me think the lack of compression would be piston/ring related. I ruled out a blown head gasket as the adjacent cylinders had good compression and I had replaced the gasket with a Cometic a year ago when I had the valves replaced. I then tried a leak down test by only listening (I didnt have a gauge to tell me how much of a percentage I was losing) and could not conclude where the leak was coming from. I also have made sure I do not have any obvious vacuum leaks. the timing is fine. also, this past weekend I did a visual inspection of the valve train and replaced all the lifters (incase the leak was from a collapsed lifter). I saw no visible indications of broken springs, worn lobes, ect.. I also have not taken the exhaust manifold off to look for a burnt valve. I keep a pretty good mixture at all times between the AFC, A/F gauge and EGT, so I figured a burnt valve in about a year would be unlikely...

well, thats about it. sorry for the breathy thread, but I am at a loss and hoped someone out there could shead some light on my situation before I rip the head off of my daily driver. thanks in advance.

 

[destrux]

A collapsed lifter will give you low vac readings, but only because the valve wont open far enough... but it will stay closed and give you good compression readings anyway, even with a collapsed lifter. I would say you either have something stuck in the valve holding it open slightly, or a cracked valve seat/cracked head, or a burned valve. You dont need to have a bad AF ratio to burn a valve, it can happen if you have a blown exhaust manifold gasket and cold air gets into the port when the engine is very hot. I've burned valves on motorcycles this way a few times before (smashed exhaust header pipes let LOTS of cold air in when you shut the bike off, ). The only way to really know is tear it down, it definetly doesnt sound like something that can be fixed with the head on the motor, unfortunately.

 

[Tom N]

You are going to have to take the head off anyways. So you might aswell start there and look at the head and head gasket.

 

[Big Woo]

Well with a 30 psi reading on #2 and no signifcant change when adding a little oil. It could be a burned valve, or a bigger ring/piston problem then a little oil can compensate for.

Too much boost, or lean air fuel ratios can burn the newest of valves in short order, additionally a poorly seated valve or one with the incorrect amount of seat width can aslo limit the amount of valve cooling due to poor heat transfer, reguardless of boost levels and air/fuel ratios.

 

[rmj134]

well, the idea of the blown exhaust manifold gasket causing a burnt valve sounds like the winner. with the exhaust manifold stud threads starting to wear, the gasket blew a few times before I helicoiled the studs. I didnt think about that. looks like new valves are in order.

thanks for the quick replies everyone.