Chris liszewski

Hi, I'm Chris. I've been working on my dads corvette. I have been chasing this problem but I have exhausted my expertise or perhaps become too frustrated to think clearly about it. A little background, the corvette blew a head gasket and the engine needed to be rebuilt. The car sat for over two years until I completed the project for my dad. since them it has had an ongoing problem I have not been able to resolve.

The problem:
The engine cuts out at about 3800 rpm, the problem seems to happen very consistently almost every time, there are no engine codes. The engine will rev to 5000 under no load. I reproduce the problem by driving the car until warmed up and then accelerating from rest until it gets to 3800 rpm in 2nd gear at wide open throttle. the car runs well for normal driving conditions, idle is very smooth and feels peppy but once it gets to that certain rpm it just refuses to go any faster.

This is a 1990 C4, I believe this is an L98, single roller cam, aluminum heads, iron block.

Here is what I have done so far.
Replaced EGR vlave, the old one was bad.
Replaced O2 sensor
Replaced Fuel filter
Replaced Fuel Injectors with Bosch 3 from FIC
Swapped ignition coils and replaced distributor cap and rotor
Double checked ignition timing and firing order
Tested fuel pressure ~47 when problem occurs, seems okay.
Did a cylinder drop test by disconnecting fuel injector connectors and while subtle I think there was a change with each cylinder.
I checked the plug wires for arching.
The spark plugs are new and are standard NGK copper plugs, I have always used these with no problem.
I checked for an exhaust restriction ~1.5psi at idle, ~3 at 2000rpm ~5psi at 4000rpm, seems okay.

I would like to try another ECM but I don't have a spare however I did remove the JET prom and put the factory prom back in but no change. I would also like to try another MAP sensor. Could I be missing something with the distributor setup, its been out a few times and put back in with no change. Perhaps cam timing is off, is there an easy way to check this?

Does anything come to mind that I should double check, or what should I do now?

joe paco

check a FSM, sec 6e3-c4-4. some references there about what may seem normal, but can be a problem -firing plugs- at load.

93Rubie

Without reading into a whole lot and thinking too hard, my gut says old worn out valve springs. Lets face it they are 21 years old and you did not say how many miles on it.

Bad springs can definitely cause what you are describing, runs fine pulls good then falls flat on its face around a certain RPM. Sounds like you covered the basics, so that would be my next step.

Try checking engine vacuum at idle and such and report back. A cylinder leak down test may also be a good idea just to make SURE the internals are ok and sealing properly. Do you have a scan tool that can view sensor data, that would be very useful in this situation.

As I recall that is a MAF car, try cleaning it and see what happens. Use the appropriate spray available at any auto parts store. Again scan tool would be useful.

joe paco

93 Rubie, valve float. I hear so much about injectors and wiring I forgot about that.
I think the 89 thru 93 are MAP. 91 is for sure.

joe

Chris liszewski

It has a MAP sensor. I will get back on the vac, the distributor is currently out while I have parts ordered to replace the pickup coil and Ignition module. On page 6E3-C4-4 says "if there is spark while EST connector is disconnected than pickup coil output is too low". This is under the no-spark condition, but failed this test anyway. Pickup coil resistance is within spec(500-1500 ohms), 1100 ohms at normal temperature, 800 ohms cold.

joe paco

I am sure you read it all, but the facing page to c4-4 has inputs on polarity, tach grnd, etc. Several place in the fsm it refers to "normal" at low rm but failing at high rpm, load.