I've been working on this for months now… My freshly rebuilt 93 starts easily and runs and idles great when cold and for about 15 or 20 minutes after reaching normal temps. Then the idle gradually deteriorates becoming rough and lumpy and slows down to around 600rpm as indicated by the tach.

I've replaced the water temp sensor, air intake temp sensor, TPS, and idle air control valve. TPS voltages are adjusted nicely within spec.

I'm puzzled and have been unable to find any reference to this problem on the forum.

Any thoughts about where I should be looking?

I'm basically running in stock trim except for a down pipe.

Thanks,
Tom

 

Stock ECU? Porting?

 

Put a timing light on the plug wires when the engine is cold and hot. Change the plugs as they foul easy after rebuilds if there's to much assembly lube. If that doesn't work check the coils and igniter.

 

Stock ECU, mild Pineapple Racing street port.

 

Can be a number of thing causing this problem , but starting with
Pimeapple street port , if your running stock Ecu it will not run smooth as it will needs more fuel on idle and most probably on boost too .

The reason on colds start its ok , cause it runs more fuel than usual in the mode.

Another thing , is the engine run in , how many miles done ?

And maybe vacuum hose not on connectly , more sure the fuel regulator has vacuum to it.
But I think you will need a after market Ecu , like Apexi pfc and get a good tune.