1988 4.9 M5OD 2wd. Cold start (cold weather, overnight) starts very well and runs thru fast idle great. Then it starts dropping to 300-400 rpm and recovering to 1000-1200. Usually it will stall after a few times. Soon as it warms up a bit it runs fine. No codes ... Got those fixed with a new egr valve and TB cleaning (problem pre existed this). So far I've cleaned and/or tested the TPS, Idle air control, and the egr position sensor. Searched high and low and found no vacuum leaks. My next plan is to get a gauge to test the fuel regulator. Anything else I should look at?

 

Check ECT and ACT sensors. Ohm out the resistance for cold and hot. Colder will be higher ohms and hot will be lower.

If original to the truck, then this may be the cause as 26 years has toll on all sensors.

 

I'll check those as well. I just pulled the vacuum line from the fuel regulator and it is spitting gas out from the vacuum nipple. Gonna start with that i guess.

 

Replaced the FP regulator and fuel filter and the problem persists. Unplugged the ECT last night and cranked the truck this morning. It would idle much lower and the check engine light was on and sometimes die but the surge was still there. Did the same experiment with the EGR position sensor (the egr vacuum is unplugged due to the usual problem). Pulled the computer yesterday and it looks great, no leaking caps or carbon tracks. Ordered some replacement caps to put in later just as a preventive measure.
It runs very well otherwise except for the occasional puff puff straight sixes get so it can't be a major failure (yet). I'm stumped.

 

Did you ohm out the ECT and ACT sensors?

when is the last time the O2 sensor was checked or replaced. Its life expectancy is no more than 4 years and it only gets more sluggish to response as it gets older.

Does the MAP sensor have good vacuum?

Have you checked every old, brittle nylon line on the truck? On my '92, they would break if i touched them. Had to replace all because of a high idle, and it was located in the rear of the engine for the TAB/TAB, EVR solenoids.

Have you checked to see if the vacuum canisters haven't got rust pin holes in them? Both of mine did.