Hey guys, I'm pretty new here and was recommended to try this forum out. I have been reading some past posts and might have a better idea of what the problem is. I have an '88 F-150 with a 4.9 in it. My problem is that when I first start my truck in the morning, the engine fires up, but then shuts down. On the second start, it fires right up and stays up...however the idle varies from about 500rpm to about 2000rpm. Once it's warmed up the idle evens out. I have replaced: Fuel pumps, fuel filter, and the idle air bypass valve. Not exactly sure where to look.
If anyone could give me a clue, I would greatly appreciate it! Thanks guys.

Chris T.

 

Perhpas try cleaning the IAC (Idle Air Control) on the throttle body.

 

by idle air bypass valve do you mean the idle air control valve like sprocket said?

 

yes. it is a new one.

 

the valve and the motor, or just the valve. have you checked the EGR valve?

 

The valve and the motor. I figured since it has 311K miles on it it wouldn't hurt to replace both
I haven't thought about the EGR valve though. I wonder, do you know of any way to see if it is bad?
Also thanks for you help and patience. It is greatly appreciated.

 

im not real sure on this but i think the computer will open the EGR on cold starts to help with the rich condition. so i guess take it off and check to see if its working properly. should be able to push on the little plunger in the end and it will move up and down.

 

I don't believe that your egr opens on start up, but anyway to test it, you need to wait till the engine is running then hook a vaccume pump (hand operated) to it and apply vaccume, your engine should stumble.

If it were mine, I would start by cleaning the injectors, I think that should help your problem. Once the engine is warm it adjusts off of the oxygen sensor, correcting your problem. The 4.9 is legendary for bad injectors.

 

I agree that you should clean your injectors, but i don't think that will effect your problem. Did you reset your ECM after replacing your IAC (IAB)?

 

no I didn't think to reset it.

 

By the way, thanks guys for the help. THis problem has been an ongoing issue for me for a while. now it's getting a little out of hand and I want to keep the engine going for a little bit longer! I am going to stay tuning in and hopefully help someone out with my own experiences. THanks for the replies!

 

Try that and see what happens. Anytime you replace a sensor that feeds information to the ECM, or a device that the ECM controls, you need to reset the ECM so it can relearn the values of the new part.

Best way to reset the ECM is to disconnect the neg battery cable, then turn on the headlight switch for a few minutes to drain the capacitors. Turn the switch off, connect the battery, and drive it like you stole it for 20 minutes.

 

Your problem sounds similar to mine. . .
Only mine is a 1990 302 automatic. On starting, it would normally rev to about 1200, hold there about 45 to 60 seconds then drop to a normal idle.
Now, on start, it revs up to about 1200 - 1500 then drops dead.
If I try it a couple of times and manage to get it in gear before it dies, it will hold a normal idle.
I have to keep pumping the gas to keep it from dying.
Another connection / symptom was while driving at a moderate 35 mph, it would tend to buck. . .as if it was dropping to idle, then kicking back up to cruise speed.
Do you have any idea what is causing this.
90,000 original miles.

 

Throttle Position Sensor maybe...
Mine would rev up & down then die... and if cruising and I'd come to a stop sign or go down a hill it would die if I wasn't on the gas

 

i had simular problems with my 90' 5.0 , i ended up replacing the EGR, IAC , TPS , EVP , TAD and MAP, throughout it all i still had issues until i replaced the MAP, then it ran great . the other stuff had thrown codes except the map . i just happened to have another 90 ford truck i took the map off of to test it and it solved the problem . i would run the codes even though and see what the ECM says.