I recently replaced the ECM in my 1991 Cam RS. Before the ECM was replaced, my cluster instruments read okay. After replacing the ECM, my tach read 350 rpm too high at idle (instead of 750) and 1100 RPM too high at 60 mph (instead of 1900) & my temp gauge neared red line (instead of 190 degrees). After checking with laser, the temp really WAS that high. My mpg was down to 11.5 (from 14, city driving). The air dam is okay. This suggests something wrong with the ECM; I don't think it's a sticky thermostat because of the tach and mileage problems. I don't want to try replacing the ECM again; it took three tries last time--two were DOA from sitting on a shelf for decades, and, anyway, threads here say ECMs from 1984 to 1991 were identical, regardless of Chevy model. So . . .

could the ECM be reading a V6 (the original engine) instead of a V8 (355 cubes)? Can the ECM be reprogrammed to fix the problem? Would an after market fix be required? Would a performance chip be a fix? Something else?

Thanks,

 

The tach would be off if it was a v6 cluster. Did you go from a 3.1l to tpi and are we talking about the 1227730 ecm for tpi or tbi 1228746 ?

 

Don't know. I'll try to find out what ECM is in there now. I think the original engine was 3.1 tpi, but I didn't do that swap.

The cluster was replaced earlier; it wasn't reading correctly. When I installed a new resister in the cluster's tach board, readings were okay. When odometer jammed, I replaced it with the 140 mph cluster, which worked okay.

 

Get the letters off the memcal it should be anjf or aujp if it’s for a 350 automatic and the 730 ecm.

 

Will do. Thanks.

 

The ECM is correct. The temp gauge is reading correctly, so the engine is reading hot. When tapped, the tach reads correctly. Should I install a temp gauge on the dash, just to make sure? Also, do 350+ engines come OEM with dual fans for radiator? Mine is one fan after engine swap to 5.5 tpi. Would upgrade to dual fan and/or larger radiator make a difference?

 

You would need to rewire to get dual fans to work at the same time.
Stock the primary fan is ecm controlled. The secondary fan is controlled with ac on or cylinder temp switch in the passengers side cylinder head.
If you feel like your over heating a dash mount gauge would be a good way to tell for sure. A larger radiator will help with cooling or you might have the wrong water pump or stuck thermostat.

 

Thank you.