OK here goes; my 16 yr old daughter and I have a project 89 Iroc that came with the l98 that was blown. We changed it with a 89 l98 corvette engine and things were great for about 200 miles. Then it started to run like it was running out of fuel, we changed the fuel filter, ignition module, plugs, wires, cap rotor, coil, and four injectors that were running around 12 ohms. All this did just about nothing in fact it got so bad we could barely start the engine and keep it running.

Fuel pressure is a steady 41 psi
We set the timing (without ecm) at 6'
Cleaned the MAF sensor - no change
Replaced all injectors with a new Bosch 3 set (stock flow rate)

Then we discovered that the car will start and idle great with the computer advance wire disconnected.

We then swapped the ecm with a spare we had and it does the same thing.

So the big question is what sensor(s) are part of the ECM ignition timing curve?

Oh one last thing, during all this we lost a head gasket (rotted out) and had to pull the motor again and the mechanical end looked great including the timing, valves, and cylinders. So we truly believe it is a sensor issue.

Any help would be great, thanks.

 

Any abnormal noises? The knock sensor could pick these up & retard the igntion.

 

No knocking at all but the sensor did feel a little "loose" almost like it was coming apart. Looks like I now have a direction, Thanks Max

 

I would consider the coolant temp sensor.

 

We changed the knock sensor with no change. Then we wired the sensor directly to ground to eliminate any engine noise - still no change.

Which coolant sensor are you refering to?

 

If you have the L98 TPI engine you say you do, there would be a coolant temperature sensor in the front of the engine behind the water pump. That sensor feeds information to your ECM.
Plus remember when you are checking / adjusting the timing to disconnect the computer first. ( brown wire connector by the heater core area ) after timimg is set, shut engine off, reconnect the wire, then disconnect your positive cable to your battery for at least 30 seconds or so to reset the code set by the wire. Reconnect the positive cable, and start your engine to see what results you have.

 

Does the coolant sensor actually help control the ignition timing control? The reason I ask is because when the computer (brown wire) is disconnected the car runs good at a true 6 degrees, but once the car is restarted with the computer adjusting timing the car will not run for long and when it is running the timing goes crazy.

 

The ECU is wrecking your timing. Check the diagnostic to see what codes have set.

 

I have tried a spare ECU and it has the same exact problem. Also the only codes are that the ECU is good (12)and ONLY when the timing (brown wire) is disconnected we get a (42 or 43 i forget now).

 

The coolant temp sensor gives the computer the correct info on the temp of the car, for warm-up and during normal operating temps. If it is defective, the computer will over adjust the timing, as well as the fuel mixture, and you may have issues as I have recently been through.
The only other thing I would check is the control module under your distributor cap. Make sure it has enough white lithium grease under it, and the two connectors to the distributor are firmly attached.

 

I'll give the coolant sensor a try since we have a spare. As far as the ignition module it has already been replaced and we used all of the heatsink grease. But we will recheck the connections. Thanks

 

Well we finally found and fixed the problem. It was when we changed the bad module the pick up wires got reversed. The car still ran but way out of time so we assumed that the module was not the fix.

Since then we have:

* corrected the module pick up wires
* re timed the 89 l98 Corvette engine to initial 6' degrees
* put everything back together "normal"

Now the car runs great again and I've restored my daughters faith in my troubleshooting skills.

Thanks for all the help