suddenly my truck is running like garbage. at low rpms it bucks, vibrates, and surges and just has no *****. at higher rpms it seems to clear up and run fine. i am frustrated because almost everything on this truck is new or relatively new. cap, rotor, plugs, wires, air filter, all that tune up stuff is always kept on top of. plus tons of other things like the fpr, coolant temp sensor, every vaccuum line, its too much to list. i have two suspicions:
1. my exhaust manifold leak is getting worse. is it possible that the gasket blew out so bad that its affecting the intake manifold?
2. the iac valve has been acting up despite the fact that its a motorcraft and i replaced it about 6 months ago. the truck screams when its cold, and i mean screams (about 3500 rpms).

 

Here's something to check that may explain everything.

Click this link:A9x ECM's (and same years ECM's) Failures Due to Age

Several of us out here are in agreement that the computer/EEC/ECM/PCM or whatever term you'd like to use, WILL EVENTUALLY fail on these older trucks.

You can replace everything under the hood, but, if the computer is toast you'll be pizzin' your money away on the wrong thing(s).

Also, grounds are VERY important on the 4.9's
I't be wise to find all your ground points and clean till shiny and closely examine the negative battery cable.
A resistive ground(s) will throw the truck into complete chaos as far as driveability.

Keep us posted.

Bob

 

Answer to #1: Yes it is possible that could have happened. Rare, but possible. To top it off any exhaust leak upstream of the O2 sensor will make the PCM think the truck is running lean. In turn it starts dumping in more fuel.

Answer to #2: I doubt the IAC is at fault here. You could have a vacuum leak (if the theory in #1 holds true, this is very plausible), you could have an unrelated vacuum leak or there is a sensor sending wrong info to the PCM. You stated you replaced a bunch of sensors but did you ever test the old ones?

Have you checked for codes? Even without a Check Engine Light on there can be a failure and/or stored codes.

 

Ive never looked into a bad pcm, but it definitely sounds like a possibility because my truck does some unexplainable things. Il have to take mine out and investigate. Its just so cold out! As for the bad grounds, i had a no start problem a few months back and cleaned every ground there is. (Turned out to be a bad coil tho)

 

I dont think i have a vaccume leak other than the manifold leak. I the manis redone twice and both times it leaked again within weeks. I think i have a warped head so il prolly never fix that correctly unless the motor blows. I replace sensors when the truck throws a code for it. I havent read the codes lately but i will as soon as i get home in time with some daylight.

 

update: i decided to take a look at the spark plugs. i found a plug with a blown off electrode! put in 6 fresh plugs and now shes purring again. thanks for the replies, i still want to check out the pcm and see if i have any bad capacitors.

 

Let's hope it keeps running well.

 

I suspect you'll find that you do. I've yet to find a single F150 PCM of this era that does not have at least one failed cap!

The good/bad news is that they will still function as they fail; complete failure seems to take a while. Try not to wait that long.