1996 LT1

it has a leaking intake manifold gasket

and the compression is all screwed up

optispark was just replaced

has a performance cam that i believe is WAY too much for a stock PCM/ stock motor

what would make this engine misfire?

also what should the compression gauge read as correct compression?

 

how do you know the compression is all screwed up and not know what the gauge should read? i'd look for a 10% deviation and fix the intake leak and your compression problems.

 

because its my friends motor
he told me that the compression is way off

i asked him how bad

he said 120, 160...........

right there thats a terrible difference

so i told him that 160 is way too much

and he said thats because he has a high compression engine

which i didnt believe (would cause the high compression readings)

which is why i am asking this question



i looked up the camshaft he put into the engine and its not street legal, it says a custom PROM is required, and the lift is VERY high

his engine runs like complete **** and i am certain my V6 camaro could take his car

 

The first thing I would have guessed as I started reading was the Opti-Spark, but that's apparently all good.

Later LT1s have 10.5:1 compression or better. If you call that "high", then so be it, but 160 PSI on a compression test is not out of the ordinary if the test is done correctly. If all the plugs are removed, and the battery is fully charged, and the throttle is blocked/tied open, that pressure is not out of the question. The big problem in the two readings you gave is the difference - over 30%.

Also, you didn't give the cam specs, but an LT1 PCM can "live" with a .510/.510 - 220°/230° - 112° LSA cam profile with no real problems (Comp 07-306-8). That cam isn't "street legal" either, by it runs on my street. The MAF on the LT1 system compensates for a lot of the vacuum change, and makes the installation survivable. Of course, a reprogrammed PCM is going to be better since the cam isn't going to provide a lot of vacuum, but it at least runs without misfire and will definitely take out most V-6 cars. It likes to munch on Mitsubishi 3000's as a between meal snack, if that tells you anything.

An intake gasket leak, on the other hand, is really going to screw up the works, and should be the first thing to replace. In addition to lost performance, you risk damaging the engine from a lean condition, coolant leaks, and possible oil consumption and/or contamination from the coolant (even Dex Cool). It really shouldn't skew the compression readings like that, but before you get out the wrecking ball, you might want to do a leakage test instead of a straight compression test for better data.

 

ummmmm ok, i didnt make the V6 statement meaning that i wanted to race him

just for an example of how bad his car is running

he floored it and it barely went anywhere...............

would the leaking intake manifold cause a misfire though.......?

what other things usually cause misfiring?

i guess his cam isnt too extreme then

 

I didn't think you were planning to race him, but merely pointing out that a respectable V-8 with that type of cam should be pretty healthy, not nearly as anemic as you are describing. I'm sure your V-6 is more than capable of holding it's own.

As for the misfire, the typical reasons are an ignition miss, fuel delivery, or mechanical problems. We'll presume the ignition is O.K. since the Optispark is new, and all plugs and wires should be reliable if the job was done correctly.

Another possibility is mechanical damage, which can be a burned or bent valve, holed piston, cracked head or cylinder, failed head gasket, worn rings, worn cam lobe, bent push rod or rocker, etc. The disparity in the compression test results indicates some sort of a mechanical problem.

The other possibility is the fuel system. LT1s use sequential injection and can miss if one or more of the injectors is not firing correctly. Firing timing is derived from the Optispark unit, so we'll again presume that the system is functional since it's new. Since you already know that the intake is leaking, the fuel mixture on at least one cylinder is not correct (most likely lean), so that should be repaired ASAP. This lean condition will also skew the oxygen sensor readings at the PCM, so the system will tend to enrich that bank to satisfy the sensor, possibly contributing to more misfire.

In the process of repairing the leaking intake gasket(s), you may also discover the problem causing the skewed compression readings. Look closely at the valve train for starters, since that was evidently altered from stock, and is therefore a more likely candidate for failure.

 

i guess i forgot to mention its an ex-state trooper car and has 120k miles on it.....................................

he paid $5000 for it and its in horrible condition

he thinks that can sell it for $5000

i was like, bud, you got another thing comming

 

So it has iron heads instead of aluminum. That makes it a little more durable. Unless it has been severely overheated, the chance of having a warped or damaged head is greatly reduced. That also means the change of a blown head gasket is reduced, but either cause is still a possibility. It would be good to have all the compression test results to see if there is low compression on adjacent cylinders.

Another possible cause is still the OptiSpark, since both injector and ignition timing are derived from the high resolution optical iterruptor in the unit. If the unit was not kept very clean when it was reassembled, it may be providing erroneous position information to the PCM. This could cause late timing and injection pulses on one or more cylinders. You'll need a fully capable data scanner to determine that.