My '93 C4 base manual with 160k miles just developed an issue that started off as a minor stumble. No codes from that. It finally got to the point where the tach would start bouncing around and completely shut off. I was able to limp home with it occasionally shutting off completely. Sometimes it would come right back, other times I'd have to restart. It always started up completely fine. When cool it will do the same thing and run for a while then stumble. I started going through the steps of testing the ECM, ICM and Opti. I am pretty sure it may have failed the ECM test. Voltage is supposed to read between 1 and 4 AC and I would get a 1volt for a split second and then it would bounce around between 0.1 and 0.4 AC. I ran out of time to continue testing but wanted to see if this looks like a definite ECM or if I should keep going through. Opti was replaced with aftermarket not too many miles before I bought it and I haven't put that much mileage on it since. I know aftermarket optis can sometimes be pretty much DOA. I'm hoping to get some time this week to go through more diagnosis with the FSM but want to track down an ecm or ship mine off to get fixed as soon as I can if necessary.
Thanks
Brad

 

Went to pull codes again this morning after driving around last night and got the service engine light to flicker on during one of the stumbles. H16 so I'm guessing that is most likely pointing at the Opti. I'm guessing a weak signal from the opti to the computer is possibly causing the low voltage from the computer to the ICM?

 

If I were you I would look very closely to see if there is a leak from the water pump dripping onto the opti. I have experienced the same symptoms before on my '96 that I bought in '99. I have changed out 4 opti's. The original, two bad ones from the get-go, and now the last one is operating very well. It is a Petris. The only brand I will ever consider from here on out. You might consider getting a new water pump while you have it off. Be sure to get the two gaskets that have sort of squared off edges in the bolt pass throughs so they will stay in place while positioning the pump to tighten down by grabbing onto the bolt threads. The gaskets that have round holes keep falling off the bolts while trying to line up the pump with the engine block threads. No gasket sealant needed for a good seal and that is the only way to keep the round hole gaskets in place. Actually, it is a very messy job to install another pump down the road if needed when the need to scrape off the old gasket sealant. For that reason I don't coat the gaskets. Never needed to.