I am working on my boss's 91 L98 auto vette and i checked the compression on 2 cylinders one cylinder said 13:1 about 187 and 12:1 about 175 the engine was up to temp, one cylinder seemed to be skipping on. like a half skip when its in gear black plug. and the other cylinder was fine brownish plug. now what is the problem here it seems like a stock 91 vette i can spin the 275/40's all through first when its skipping and throwing a code for the EGR and he seems to think its a vaccum leak on the intake gaskets. but anyways the motor was up to temp when i tested it. so anyone have any clues? car has 115k miles.

 

Compression readings seem reasonable. 'Vette L-98 had an actual compression ratio of about 9.5:1, by the way.

Fortunately you didn't find one cylinder at 180 and another at 90- that would be a bad sign. Differences like you're seeing can happen in an older motor with unknown quality of ring/valve seal and unknown quantity of carbon built up in various (non-running) cylinders. Being a roller cam motor it's unlikely you have a cam lobe going down on you.

I'd suspect something bad in the ignition (bad plug wire, bad plug, cap, rotor, etc.) or possibly one of the injectors is fouled up. That can definitely give you a "skip" on a single cylinder.

A vacuum leak can play havoc with proper running, but usually worse at idle and light throttle, getting better the closer you get to WOT. EGR code in the computer is often actually something wrong with the EGR system- valve, EGR solenoid, vacuum line routing.

 

why would it have such high compression? if it had that much i think it would sound different/different octane ect. im not sure on this one. but i already knew there is carbon build up in the 13:1 compression cylinder, i took the plug out and it was a cruddy black, the rest are brown(12:1). i swaped plugs with another cylinder, same half skip, then i switched the wires(Red MSD 8.5mm wires) and it has a hypertech "high voltage" ignition coil. im not sure how new the cap itself is. but i unplugged the vac. hose from the EGR and it didnt run any different what so ever. any chance the passage is plugged?

he wants me to change the intake gaskets and i highly doubt it has a vacuum leak he states someone saw it "leaking" and unless they used like carb cleaner and sprayed around the gaskets to see if it picks up RPM's i doubt they knew or not. but ill need the gaskets anyways if i change the EGR valve. and if i have to clean out any carbon in the intake base i can do that at the same time, im not sure exactly what i have to do, could be a cap or rotor i guess, it hasnt been firing on that cylinder for a little while now. and it does run rough at idle and better as you get tward WOT, when it shifts into OD and your going like 40 thats when you notice it, it wants to buck pretty good.

 

187 and 175 psi are not high compression readings. They sound stock to me. Over 220 is high. The cam also plays a part. Is the cam stock?

 

whole car is stock. besides the msd wires and hypertech coil. which amount to very little to nothing.