91 RS all stock 305 TBI auto, 140k.

I did a dry compression test and got 100-125 psi on 1,3,5,7
and 195-200 psi on 2,4,6,8
Which side is the problem? I know 100 is low, but isn't 200 really high?

And I have a post in the tbi section I started (spark knock), but when I posted this question no one's answering it so I thought it might be better here.

 

Mismatched heads maybe - has your engine ever been rebuilt? 200 Does seem a little high, but it depends on the compression ratio. If you happen to have the valve covers off, you could check the casting numbers on both heads.

 

Even mismatched heads shouldnt make a difference that big. Something went wrong during the test. Did you hold the throttle all the way open? Was the battery still good by the last cylinder? Were all the plugs out of the other 7 cylinders? I'd say redo the test.

 

Yeah I'm gonna redo the test. I didn't have the throttle open, or all the plugs out. But it sure is a pain with those air tubes in the way. I'm doing it underneth but still. I've only had the car a month, and I'm pretty sure the engine has never been opened.

 

Ok - when you do the test, you have to have the throttle all the way open, and you have to have all the spark plugs out, to do it correctly.

Would be wise to have a battery charger (preferably with 'engine start') hooked up as well if its possible, just to keep from running the battery down.

 

I just redone the test with plugs out, throttle open, and engine hot. I only did cylinder 1 and 3 because of the results. Cylinder 1 had 100 psi, with 3 squirts of oil it had 120 psi. Cylinder 2 had 80 psi this time (lower), and 90 psi with oil. So it seems like bad rings but how come it doesn't smoke and how could only one bank (the drivers side head) be low and the other so high?

Could it still be a head gasket? I'm pretty sure someone got it hot because the connector on the temp sensor for the fan was broke off when I got it. But there is no sign on coolant leaking in the engine or oil in the water.

And just a reminder it runs really good other than a rough idle. That's what has me so puzzled.

 

How many times are you spinning the engine over? Should let it spin over at least 3-4 times for each cylinder you test. Have you adjusted the rockers recently? If they are a bit too tight, the valves won't completely seal and the readings will be low.

 

That sounds like the most plausible explanation for the R-L split on the readings. Maybe the driver side was misadjusted at some point and the passenger side was not touched. Just a WAG. Cant hurt and not all that hard to check.
Shoulda checked just one on the other side to make sure your reading was still a lot higher... but I'm thinking your original results were probably fine.

 

A leak down can give you a better idea where it is, valves or rings.

They just showed how to build one the other day for cheap on the HP block with a hollowed out used spark plug n a air hose fitting.

 

I thought about the valves being tight too right after the first test and checked them. They where actually loose. I put them at zero lash and half a turn, even though alldata says 1 1/4 turn. That just seems too tight. As for the leak down test, I don't have a tester but I was thinking about putting some compressed air in the cylinders and listening around for it. I really want to go ahead and pull the head off and look at the gasket. Probably going to be a dead end though. I have a neibor with a lot of engine assembly experience, and he told me if I get the machining done he would assemble it for $200.00. I'm seriously thinking about just jerking it out and getting it over with.

 

5 plus revolutions

 

Thats what a leak down test is, blowing compressed air into it to see where it leaks. If ya search [im not doing it for ya] the HP TV website you should find the vid on how to build it.

 

An update just in case anyone followed this thread. I have the engine tore down and found quite a few problems. First off, the side with high compression had a rebuilt head. I'm thinking this is why it had more compression. May be it was warped and it has been shaved to make it true again. I'm not sure. And the valves wiggle quite a bit when pulled a little ways out indicating pretty bad guides. Next thing was the piston ring gaps were lined up. I'm sure that doesn't help compression. And the rings are wore pretty bad. .032 gaps

 

Finally found the problem! All four intake valves on that head are shot. I flipped the head upside down and put gas in the valve chambers and they leaked like crazy. The valve seats in the head still looked good, so I took one of the good intake valves out of the other head and put it in and did the gas test again. It didn't hardly leak at all, little seepping. Does anyone know what would have caused these intake valves to do this? And if the seats are still good could I get away with just lapping four new intake valves in?

The valve on the left is the bad one, the right is a good one.