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L28ET ignition jumps at idle


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I have a stock 1983 L28ET swapped into a 77. My issue is that my timing shifts from 20 BTDC to around 30 BTDC, after about 30 seconds. It is not mechanical, because I can shut off the engine wait a few minutes and when I start it back up timing is at 20 BTDC. I have checked AFM, TPS and CHTS per the 83 FMS.

 

I think this points to an ECU problem, because the TPS is supposed to lock the timing at 20 BTDC at idle.

 

Am I missing something basic or is the next step to swap the ECU. I hate to just swap parts, but I am out of things to troubleshoot.

 

Has any one seen timing shifts at idle before?

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