'90 GMC Sierra ECM Timing

[Ken Richardson]

Hello all.

I've had some spare time on my hands and decided to do a little tuning on
the 5.7L engine in my Sierra. I tried changing the Main Spark Bias from 9.8
to 7.7 using the CATS Engine Tuner to give me 2 degrees more advance. I
burnt an EPROM of the CATS modified ASDU image and plugged it in my ECM.
When I start the truck I think it runs in "limp in" mode. It starts fine,
idle is smooth then very rough, then the engine surges and stalls. No "check
engine" light. I've modified the BPW constant successfully before (not using
the same source image) so I'm not sure what's going on here. I plugged the
EPROM in my burner and read back the program, it is correct, no errors.

These are my theories:

1. Can't change the Main Spark Bias in the ASDU image for whatever reason.

2. My ASDU source image is bad. I can try to download the stock chip again.

3. The ECM can't read a Toshiba 2732D EPROM (what I'm using).

Has anyone run into this before? What is the likely cause? Would I be better
off changing the Main Spark Table instead of the Bias?

Thanks
Ken Richardson



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