hey guys and girls i have a 1990 F250 and it is giving me heck. when i start it it runs fine when cold. but after it warms up the rpms surge like +- 600 rpm then dies if i dont keep my foot on the gas. ive replaced the IAC MAP TPS (they went bad before) but im thinking is there something else i could check? im getting all clear codes on self test on engine running im getting that the smog pump is bad (aka thermactor air) i know the pump is bad because it is locked up could this be my hole problem its currently bypassed with a smaller belt. any help would be great.

 

could be a whole range of things. Timing,plug gap, wires you name it. I'm kinda having the same problem with my 96 and am eliminating the potential problemms 1 by 1



Jim

 

could be the engine temp sensor. since it runs good cold and then starts stalling at idel it could be flooding. maybe?

 

never thought of that. Thanks



Jim

 

yea i think it might be a temp sensor maybe becouse my temp gage dont work

ive replaced the sender so i dont know what the heck is going on its starting to stall when i start it know.

 

First of all, the sending unit that drives the gauge, is NOT the one you gotta worry about for engine control. That one has a single wire on it, and is down on the manifold. The ECT (assuming you have a 5.0) is mounted on a tee right next to the thermostat cover, that also feeds the heater core. That's the one you REALLY gotta worry about. However, if it was bad, it would run bad cold or hot. Trust me on this one. You'd also immediately get a latched code and Check Engine Light if the ECT were out of range.

When it's cold, the engine runs off a standard fuel table, and is quite rich. Poor idle may not happen if the mix is rich enough. Lean it back to 14.7:1, though, and the idle will go all over the place if there's a vacuum leak.

Vacuum leaks are the most common cause of "surging" and oscillating idle and stalling. You said you replaced the IAC, but did you clean the throttle body too? Sometimes, carbon will build up on the back of the throttle plates, and this will play havoc with the EEC-IV. Check the EGR valve, and see if you have excessive leakage there. The best way to find a leak is with an unlit propane torch. You can point the gas at exactly where you want to check, and it doesn't make a dripping mess that's flammable like using carburetor or throttle body cleaner. One stray spark from a plug wire while using a cleaner, and you're gonna have a much larger problem than a surging idle. The propane blows away and gets diluted enough it won't catch the engine bay on fire. But if there's a vacuum leak, it'll pull in the propane, and the idle will smooth out and increase in RPM. Worst case, if you can get enough propane to linger to ignite it, it's just going to be a quick flash with a few scorched eyebrows. The liquid cleaners will burn much longer, and possibly cause a much bigger fire if there's any fuel leakage at all.

A better alternative is to isolate the intake from everything except the MAP sensor and FPR. Cap off all the nipples for the hoses, start the truck. Put a vacuum gauge one one of the nipples where hoses were removed. If you don't have at least 18 inches of vacuum at idle, you got a leak, and chanses are, if it's a 5.0L, it's the plenum/intake gasket. See my gallery for photos of idle problems on the '92 in my signature. If the vacuum is between 18 and 20 inches, then start connecting hoses one at a time until you find the culprit. Could be a bad diaphram in the brake booster, or any of a number of hoses. If it's EGR, then that will be the seal on the valve shaft, and probably not the vacuum line to it. According to everything I've read, the EGR should be closed under idle conditions.

A last possibility could be a wonky MAP sensor. They don't go bad often, though, and this would probably be my last suspect.

 

ive got the 351 w but i will try that one.

 

ok guys here it is i replaced the engine temp sensor and the air temp sensor and it starteds fine (even though i had to re time the distributor) thanks for the insight.. ohh yea the old air temp sensor was coverd with about 1/8 in of black crud.

 

ok started the truck today and the CEL came on (yes i reset the cpu before i started) drives fine but when i whent to check the codes nothing happend it didnt even give me code 11 CEL stayed off. hmm any input on what this could be?