Well I feel like I am in the deep end now. My car was running great and I was headed home and there is a very long and steep hill I have to climb. I got about half way up the hill when I felt a sudden loss of power.

It proceded to run badly all the way home and has a very rough idle. Today I did a compression test and all of the cylinders were well in the 150-160 psi range except for cyl #5. It has no compression. The guage does not even try to move.

I suspect a burned/broken exhaust valve since it happened while doing lots of work and may have already been on it's way out from a previous lean running condition. I read on another car forum that I could have a hole in my piston as well.

I don't know if it is safe to drive it like this(?) but the car is my daily and I would like to fix it asap. It seems the best route is to purchase a rebuilt head if the pistons prove in good shape.

 

On both my xj6s it was #5 that was low on compression.On the 90 it was caused by a clogged catalitic converter(which caused the exhaust back into the motor through the exhaust valves and combined with the heat and pressure blew the head gasket).Dont know on the 93. Both were high milage cars. It might also be a stuck or bent valve (pull cover and check valve clearence on the valves.

 

Ok, sorry for taking so long to reply. Just got to work with it today.

Commenced a leak down test and got some wild results. I also measured valve lash clearance. All were in range except #5 had about a .38mm clearance on the intake valves. #3 had about a .38mm clearance on the intake valves and #6 had .38 on the exhaust.

I pressurized #5 up to 120 psi and found that it leaked out very slowly. I never timed it exactly but it was minutes before it lost more than 20 lbs. The leaking sounded like it was in the crank case somewhere but was too slow to detect it exiting anywhere. So I did a control test on a few other cyls and found the same exact leakage.

Obviously we can't have perfect seals in our engines so this is normal leakage from what I can tell. So how come it had 0 compression before and now it will hold 120 psi no sweat? I hooked it all back up and ran a compression test. The tester surged to over 300 psi on cyl #5.

I decided to drive it and come back for another test. Another test yielded a reading of 140 psi. but seemed a little slow to get up there.

I can't really fathom what is going on at this point. It seems a valve is sticking or not operating properly but not sure which or how.

On another note I managed to snap my distributor rotor in half. Yes, I superglued and JB-Welded it back together. Works like a charm, hopefully until Monday when I can get to the dealer.

 

I have found that these cars are like woman.
Its running good again?
I hope so because my issues seem to follow yours, sometimes months later.
Do you need a rotor? I have one.

 

Nope, not running good. It seemed a little better last night but same old same today. Could have been because the cyl was well lubed with transmission fluid. My Haynes said it could be a cause of excessive carbon build up. Considering the car was running fairly rich for a time I doubt it. I dumped aquart of Xylene in the fuel tank and some seafoam in the 20-50 oil and that cyl today. Just in case there is just some carbon fouling up the valve.

I guess the only way to find out and fix the problem is to pull the head. It could be any number of things. It is interesting to me though because I would think a stuck valve would probably cause my valve lash clearances to be off by a large margin. A burnt valve would probably not allow compression to reach 300 psi in 3 cranks. How is it even sucking enough air to do that? I suppose it could be a bent valve that has some movement allowing it to open and close intermitently and sporaticly.

As for the rotor, right now the superglue/JBKwik seems to be holding up very well. If your rotor is brand new I am interested otherwise I figure I might as well just get a new one.