[EVRLET]

A PROM issue is likely. Did you check for codes? It doesn't matter if the CEL is on or off. If you get a 51 code, then PROM is suspect. Google your PROM code off the PCM label and see if you can find a replacement...good luck because it took me 4 months to find one.

But try wiggling the PCM connector and see if you have a loose connection. Read my story below about this.

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Here's my experience with OBS PROMS:

I worked on a 1995 C1500 and the issue with the truck dealt with idle rpm with and without a load, so very similar to yours. I worked on the truck prior to the rpm issues and I know timing and idle was dead on and all components were new or fairly new.

At idle with no load, rock solid at 750 rpm for about 10-20 seconds. Then it climbed up to about 2200 rpm all by itself. I put it in gear, didn't matter which one, the rpm went down to about 1200 and then stabilized at about 1500. I noticed the CEL was flickering between bright and dim. This was a late model 1995, so it had the 1996-2000 instrument cluster. I placed it back in park and let it idle with no load.

The flickering is what concerned me, so I went to the PCM behind the glove box. I was wiggling and pressing the red connector into the PCM and I noticed the rpm dropped to 750 and the CEL went off. Wiggled it ever so slightly and the rpm jumped up but no CEL. I was able to change the rpm by wiggling the red connector. The blue connector did nothing.

I jumped the OBD 1 connector and there was a 51 code set, which is a PROM error. I figured the loose connection and the voltage fluctuation caused the PROM to freak out and be intermittent. I mean, when I hold the red connector in the right spot, the truck runs great under no load and a load. However, computer chips are sensitive to voltage changes so loose connections are never good for electronics.

I replaced the PCM, which you already know doesn't come with a PROM. I searched high and low for a PROM because they are no longer made and you have to find the exact PROM that's listed on the PCM sticker. Mine was a BJYB and I was lucky to find one. Replaced the PROM and the problem was fixed.

PROM issues are not rare in the TBI trucks. Actually, it was a common error back in the day and I remember you could buy blank PROMS in 5-packs. The dealerships had the programmers to complete the fix. However, as these trucks became older, experiencing PROM issues is frustrating because replacement PROMs are so difficult to find.