5,7 TBI - low idle when engine is hot

[LuskenBF]

Hello. I have a problem with my truck. It's a 1988 C1500 with a 350 tbi and th700r4.

The truck works fine when cold, but after driving a while and engine is hot the problem starts.
It seems like rpm is too low at idle and when i step off the gas pedal when driving. And voltmeter drops down to 12-13v and lights dim a little bit.
It sounds like the engine is just about to stall, but that has happend just a few times.

Things i have replaced recently:
battery, alternator, egr valve, egr solenoid, intake manifoil + gaskets, gasket under throtle body, fuel filter, spark plugs, spark wires, distributor cap and rotor, knock sensor.
The problem is the same before and after all theese things were replaced.

 

[SkyHighColorado]

I would investigate your IAC and TPS you can check TPS with a volt meter. Also run your codes with a paperclip.

 

[Hood93z]

To me sounds kinda like a loose ground... Like sky high said run your codes they help a lot

 

[LuskenBF]

Tried the "paperclip diagnose trick" again, and it shows code 42.

I also tested the TPS with a voltmeter at it's at 0,63v at idle and 4,5v at wide open throttle, so i guess that TPS is ok.

 

[Hood93z]

Code 42 is "ignition bypass error" that's all I can tell you about that I'd check out the ignition system maybe you can find the problem there