My car is a '92 LT1 with the ZF6 manual transmission and FX3 suspension.

I have, what I think, is an electrical problem that only seems to happen when driving -- never at idle. And only when the engine is nice and hot, after about 10-15 minutes of driving.

The tach needle will nosedive faster than the engine can possibly decelerate, and the car speed will feel like I hit the brakes. Then it goes back to normal.

When it first started happening, I could have mistaken it for running over a pothole or bump. Then it became more noticable. When I saw the tach during one of these dropouts, the tach dropped by about 500rpm and then went back to normal in less than a second.

It is becoming more frequent and severe when it happens. The last time I drove it to work in the morning, the tach dropped to 0 for a couple of seconds and the SES line came on, and the car was stumbling, then the tach went back to normal and the SES light went off.

On the way home that night, it had two severe dropouts. The first time, the tach went to 0, stayed there and I lost all power. I pulled over the side of the road, shut off the engine, restarted it and continued on my way. Then 5 miles later it happened again. This time I put the car in neutral, removing the load from the engine. The tach and engine-power went back to normal. I put the car back in gear and continued driving. Both times, the SES light came on while the problem was ocurring, and went off after the problem cleared. There were also several minor dropouts during the commute.

I have the FSM. I pulled the codes and got H54 and H74 for Module 1 (CCM):

Code 54 is FEDS - Fuel Enable Failure. From reading the description for this error in the FSM, the SES light is behaving exactly as described for this error: If there is a voltage problem while the engine is running, the light will only be illuminated as long as the voltage problem is there. Once the voltage goes back to normal, the light goes off.

Code 74 is LED Display Dimming Output circuit open or shorted to ground

When these incidents happen, there is no problem with other electrical items. The radio continues playing. I tried to make a point of checking the battery gauge during a dropout and I don't think I saw anything -- but I was also a little distracted trying to maneuver the car to the side of the road in heavy traffic.

Any suggestions?

 

Sounds like a grounding problem, or a loose connection somewhere. I would start checking all the grounds and computer/ignition/fuel harnesses and connectors. It might be a pain to trace out, but it should not cost anything but time to find. Good luck.

 

I would pull the battery clean the connections then clean the connections to the block behind the battery and the fuse block. then go for the other end of the cables and grounds and check the cables themselves

 

..... Based on code 54 , I would be looking at the fuel pump circuit ... the relay and the relay connections ... the harness connector at the rear of the car for the fuel pump ... and the oil pressure switch that enables the fuel pump on the back of the engine ... look for melted plastic connectors or corrosion ... also try a new or known good ignition control module ... it tells the ECM that the engine is running and its OK to allow the fuel system to operate .......

 

Thanks everyone. Looks like I have some work to do!

 

Have you been into the ECM for any reason (PROM change or..) recently OR have you maybe been cleaning in the vicinity of the ECM and maybe disturbed the connections to the ECM?

I don't know that the FP or related circuits directly affect the communication ECM/CCM .. I believe NOT

Is this the factory PROM, OE ECM or a service ECM and maybe a "tuner PROM"?

 

No, the ECM is stock and hasn't been touched. My assumption going in was the car is almost 24 years old, and the connectors, wiring and insulation are showing their age.