2.8L S-Truck, won't idle initially when warm

[grassulo]

i have a 1987 GMC s15 2.8l, which won't idle when warm. The truck sputters and will eventually die unless you give it gas, then it idles fine. This condition only occurrs when the vehicle has been left alone for about an hour, and will idle perfectly fine once the accellerator has been pushed down a few times. From a cold start the truck starts up and idles fine. The fuel pump and engine coolant temperature sensor have been replaced, I'm beginning to think its either my MAP sensor or possibly my idle air control valve. The truck dosen't have an intake air temp sensor. I'd heard of this problem awhile back with other 2.8L cars and trucks and hoped you here could provide me with some insight into maybe whats causing this odd condition.

-Garret

p.s. I dont think its my IAC valve, because when the truck is cold the idle will rise and gradually even out, like the valve is moving unless its not moving enough, how do i test it without a scan tool?

[geoffinbc]

Temperature dependant idle problem may be due to a bad O2 sensor or a bad CTS sensors (yes i know you preplaced it but test it) the ECT sensor needs to read 3400 ohms at 21*C(70*F) resistance gets higher as it gets colder.

[grassulo]

it very well may be my o2 sensor then, because about a week ago it threw a code 13 (o2 sensor circuit) at me while it was idiling in the driveway, i cleared it and wrote it off as an intermittent problem since it never happened before, but this may well be it. I'll test the cts with an ohm meter when i get home from work and I'll backprobe the o2 sensor with my oscilloscope, what sort of variations in oscillation of voltage would a healthy o2 sensor put out, or should I just replace it since their only about $20?

-Garret

[geoffinbc]

Repace it. Don't use BOSCH!! Buy a Delco

[grassulo]

I replaced the o2 sensor with one made by Wells, the truck runs a lot better now but is still exhibiting the same warm rough idle condition. I'm beginning to think its my fuel pressure regulator, or the fuel pump relay not turning on immediately when the truck is warm. It feels like its not immediately getting fuel delivered, but once somthing "clicks" or kicks in, fuel gets delivered and it idles fine, anyone ever hear of this happening?

-Garret