Hey guys, I am having an interesting issue and was wondering if anyone could help me out. I have a 91 F-250 351w and its idling high and surging. It will only idle high after the gas pedal has been depressed and then when it returns to idle, it will idle at around 1200, then it will surge from 1000 to 1200 once about every second. I was thinking TPS and IAC, but both have been replaced and verified to be working correctly. The motor was freshly rebuilt so I don't think its a vacuum leak. It seems like a computer issue. I did recently replace the throttle body with one from a junkyard. The one the truck had on it had holes on the butterfly valves and was idling high could never get it to come down. The "new" one idles at 900 before the gas is pressed so I don't think its an issue with the TB itself. Any help would be appreciated.

 

That's a pretty obvious sign of a bad Tps sensor. Borrow or buy a cheap oscilloscope and test it yourself. The o-scope line should look like this. If there is any dips or bumps in the line while you smoothly move the throttle, your TPS is bad and it's confusing your ECU to idle high.

 

Awesome, I will test it later today. Thank you.

 

Alright got around to testing it, I don't have an oscilloscope but I used my DMM, it measured .91 volts at closed and 4.45 at WOT, there were no dead spots.
I also ran it with the IAC unplugged, the idle speed didn't decrease, it stayed at 1100, still surged. The ECT checks out. I unplugged the MAP sensor and it stopped surging, I plugged it back in and it stopped surging but still idled high. I got my vacuum gauge on it and it was reading 18 inches of mercury, it fluctuated with engine speed as it surged. I am going to look into get the MAP tested or replaced. My only other guess is the ecm is failing, or failed. I replaced some leaking capacitors around November of last year.

 

Have you tried starting it up with the iac unplugged? Another possibility is that the IAC is stuck in the open position. I try to leave "replace the ECU" as the last resort.

 

Yes, it doesn't help with the high idle, I just bench tested the IAC it is in working order electronically. But in the closed position it is allowing air to get through it, I just blew into the holes, its not a lot of air but it does make me wonder if they are supposed to be open a little bit. I am going to get my dmm and see if the computer is commanding the IAC to open causing it to idle higher, but unplugging it doesn't seem to help, I will double check anyways. But it wasn't necessarily stuck open more like its just not closing all the way, but I have no idea if this is normal.

Sorry for the late reply, I was checking my ignition timing to see if the computer was advancing it allowing the engine to idle higher, when I moved the harness my engine shut off and wouldn't start back up. I eventually pulled the whole harness and bench tested the entire thing last night so I couldn't chase this issue. I did install the harness back in today and the whole engine shutting off and not starting thing seems to have resolved itself, but time will tell.

 

Well a lot has happened in the last 24 hours.

I have been dealing with some intermittent stalling, I recently replaced the ICM and ignition coil, it was running really good Friday, then Friday night it kept stalling and wouldn't start up, I have ruled out all the wiring regarding the ignition, but when it did stall the coil would not fire, also the ignition ground of the ICM (pin 16) is not connected to the negative post on the battery, which is why I have been lead to believe that this stalling is ignition related. Also unplugging and plugging in the connector to the ICM helped it crank back up and run for about 10 minutes and then it would die again. I pulled the ECM harness to test the wiring harness, plugged the harness back in and the fuel pumps stayed on for a couple of minutes and flooded the engine. Took out the spark plugs and disconnected the ignition coil cranked the engine to get the gasoline out, boom, raging fire, luckily it was put out in a matter of seconds, no damage other than the electrical tape melting.

Went to start it up this morning, she did not want to start up, took a minute or so of cranking till she fired up, it acted like the MAP was unplugged, was idling really low like 600 rpm, and surging between 400-900 and it sounded terrible. Got code 126, I replaced the MAP, no avail.

The map is getting 7 volts, but I need my other DMM that can read the frequency.

I think there is a bad ground somewhere in the system because the stalling issue was intermittent. I am also trying to figure out why its throwing code 126, as the map was fine yesterday, because I tested it. But I'm stumped it was running "fine" and now it just died.

Its probably something simple as it always is, but for some reason I can't find it. Sorry for my essay I am just trying to provide as much information as possible.

 

A fuel pump that runs constantly is a sure sign of a failing ECU. Have you removed your ECU to look inside for leaking capacitors and corrosion?

 

I replaced some leaking caps back in November. It only cycled it for that long period of time once.

I got rid of the 126, it ws a loose connector.

Still chasing this stalling, it maybe worth to swap in another ICM. Will keep updated.

 

Alright I replaced the ICM, brand new motorcraft was bad out of the box, just got a cheapo from the local parts store until I can get another motorcraft one shipped to the house.

It stumbled for a second, but I can't get it to do it again so maybe just a fluke, I don't have high hopes for this cheap ICM.

It is still idling high, but it will idle high after it idles normally for a second, then it will slowly creep up to 1k and then surge ever so slightly maybe 50rpm. I gotta find a way to test the ECM, I will do some reading into it tonight. Thanks for the help in trying to narrow down this annoying issue, sorry I kinda hijacked it for a second.

 

What about smoke testing the intake just to make sure there aren't any air leaks?

I have found leaks in other cars in places I didn't even know I had...

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One more suggestion, visually inspect your ECM relay and the relay base. If there's an intermittent connection there, the effect would be high idle, like the normal high idle when you first start the truck.

 

My ecm relay actually just died this morning, I replaced it with a spare one I had laying around, I will get a new one just to rule out the possibility entirely.

 

I don't have a smoke machine but I do have some steam machines, I will try it with those.

 

Well it ran good for a solid 5 minutes. It is now stalling again, 2 new ICM modules and a new ignition coil. Its more like stumbling then actually stalling, the tach will drop down to zero and then shoot back up and the engine stutters for a second and then it repeats 3-4 times and then is fine for a couple of minutes. Whenever the stumble does happen there is a voltage surge at the coil by about 2 volts then it drops back down, seems like a short somewhere in the system or a break in the wiring, but I also believe it is the ecm I will double check both, I am at my ropes end with it.

During the time it does run it is still idling high and surging, it acts like a vacuum leak but I am throwing that on the back burner right now until I get this spark issue figured out.