I have an 89 C4 that shows no codes through the check engine light. The check engine light comes on like it's supposed to on start up, but doesn't light up during driving. When shorted to read codes, it will not flash any codes. The initial problem(s) started after driving the car. I parked it for 2 hours. When I went to restart it, it was difficult to start and would only run at 500 RPMs. It would not throttle up and it sounded as if the timing was way off. I had it towed home. The next day, it started right up! I ran it up the road and put it back in the garage. I attempted to restart it, and it did the same as the day prior.
I hooked up an code scan tool and pulled up 9 codes. Codes 41, 43, 46, 51, 52, 54, 61, 62, and 63. Being new to the C4(I have only had it for 1 year), I figured the ECM must be bad!(Rookie mistake!?) I ordered a reman ECM and a Hypertech 128982 Prom. I thought I'd chip it up a bit! New ECM and Prom arrived. I pulled the old ECM out and discovered that the Prom in it was a Hypertech 128982! Change both ECM and Prom and fired her up. Started and ran OK, once! Same issues and codes. I obviously don't have the original Prom. Any suggestions as to what might be causing multiple errors? I have checked conections and pins. I just find it difficult to have 9 Codes be legitimate.
The only changes I made to the vehicle was a fresh set of plugs, wires and cap /rotor and that was over the winter. Car had been running since with no issues. Help!

 

Most likely your scan tool has compatibility problems with your 89 Corvette ECM.
Post the make and model and maybe someone can confirm if they have used that model with no problems.

Normal operation.
Turn the ignition On.
Service Engine Soon indicator should be On solid.
Start the engine and the indicator should be Off.

Diagnostic mode.
Connect Pin A to Pin B.
Turn ignition On. (Don't start the engine).
The radiator fan should turn On.
The Service Engine Soon indicator should flash 12 three times then any DTC's three times and the cycle repeats.
You may hear a clicking sound which is the Idle Air Control motor being retracted in its bore which is normal.



If your fuel injectors are the original Multec gray body colored injectors they can cause a poor running engine to one that won't start.
You unplug each fuel injector connector and use an ohm meter to verify the coil resistance is in the 16-18 ohm range. Do the test when the injectors are cold and when hot if you can.

You never mentioned if you verified the fuel pressure which should be in the 40 psi range.
Pull the vacuum hose from the fuel pressure regulator and verify there is no gas in the vacuum line. Verify the pressure holds after you turn the ignition off.

 

I would get a hold of a stock prom and give it a try. 51 is prom and 55 is ecm. I really hope it is the prom because you are going to be doing a lot of back probing if it isn't. Do you have the FSM with electrical diagrams?

 

Thanks! That was kind of where I was leaning, but good to hear it from someone with some background. I do have the FSM and it has become a "Bathroom Reader"!!

 

Thanks for the reply. I have Innova 3120 Scan Tool. Prior to using the Scan tool, I connected pin A to pin B, turned the ignition switch on, and absolutely nothing happened. No flashing on engine light, no fan bump. When using the scan tool, the fan kicked on and all else proceded as advertised. I will be back under the hood this weekend and will verify injector resistance and fuel pressure. Thanks for the tips!!

 

I see no listing for a code 61, 62, and 63 so I wouldn't trust that scan tool on your 89.

Take a jumper wire from the negative battery terminal and connect it to Pin B on the diagnostic connector.

Turn the ignition On.
Does the Service Engine Soon indicator flash 12 and does the fan come on?

If that doesn't work, contact Hypertech tech support and verify if they disable the pulling of codes by connecting Pin A to Pin B in their Eprom.

 

Never thought to contact Hypertech! Thanks for the info!

 

Put the factory EPROM back and sell the Hyperjunk to the next guy who is dumb enough to by it. Call it lesson learned.