f350 5.4l stalling problem

chrisdvorak · Mar 22, 2015

sorry for the late response, as weather has been bad and haven't been able to work on the truck. anyway today I replaced the wiring to the OSS sensor (figured I would rule it out) but as luck would have it I still have the same problem. I was driving it today and it still shifts great into all gears. if I clear the code I can drive it no problem until I get stopped at a long light and it will stall with the same code po720, it has also started stalling in park? I am out of things to try and this is driving me crazy!

HHR · Mar 22, 2015

chrisdvorak said:
sorry for the late response, as weather has been bad and haven't been able to work on the truck. anyway today I replaced the wiring to the OSS sensor (figured I would rule it out) but as luck would have it I still have the same problem. I was driving it today and it still shifts great into all gears. if I clear the code I can drive it no problem until I get stopped at a long light and it will stall with the same code po720, it has also started stalling in park? I am out of things to try and this is driving me crazy!

I have a gut feeling that you are getting some kind of RF interference with the signals going to the PCM from the sensor. I would try wrapping the harness from the sensor all the way to the PCM with reflective aluminum foil to rule this out. Heck, cover the engine with the stuff. Maybe the new coils are somehow causing this.

chrisdvorak · Apr 3, 2015

here is an update, I finally got a new code to come up for the throttle position sensor I replaced the sensor and it has not stalled yet! however I have noticed that with the new sensor the idle is about 100RPM less (just over 500rpm), like I said truck hasn't stalled yet but it has been on my mind.

sld · Apr 4, 2015

Did you disconnect the battery to reset the computer yet?

chrisdvorak · Apr 5, 2015

yes I did, and it did not raise the idle. I thought that the TPS sensor would have some sort of adjustment but I cant seem to find anything?

sld · Apr 5, 2015

The only thing I can think of that would cause a low idle is the TPS sensor or the IAC bypass valve. The IAC is cleanable. I would take it off and blast it with carb cleaner and compressed air. Keep the cleaner out of the electrical connector. After cleaning (or replacing) reset computer.

The only other thing would be if someone messed with the set screw for the throttle plate. Despite the warnings it is adjustable. As a last resort I would do this:

1. run the screw in a couple of turns
2. disconnect the IAC connector
3. start the truck and adjust screw to correct idle (make sure it's at operating temp and might be idling as high as 2k depending on how far you turned the screw)
4. shut truck off and disconnect battery
5. plug IAC connector back in
6. after 10 minutes or so, reconnect battery and restart truck

I have never done this to the 5.4 but the same process works on many Fords that I have done. The IAC's sole purpose is to handle idle speed deviations (put in drive, compressor on, alternator load, etc..). The throttle blade always let some air in.

3930dave · Apr 5, 2015

Had a quick read, didn't see this mentioned, so wanted to ask....

When was the last transmission and rear-diff service done ?

If it's a 4R100, the service intervals aren't super-long, and old diff fluid can cause funky OSS readings if the contamination level is high enough.

Rgds, D.

chrisdvorak · Apr 5, 2015

well today had the truck stall on me again while backing up a slight hill, I have removed and cleaned the IAC. trans and diff was serviced 30k ago and fluid is clean. I guess the TPS was not the problem just masked it a bit. this seems to happen now only when the truck is cold or more often anyway. and for that reason its making me think fuel injectors?

3930dave · Apr 5, 2015

Miles on truck ?

Is the fuel pump original ? Fuel filter changed recently ?

Can't remember if you've ruled out ECT sensor. (Coolant temp).

Rgds, D.

chrisdvorak · Apr 5, 2015

there is 116k on the truck fuel filter was changed about 10k ago the fuel pump is original as far as I know, but I don't think its a fuel supply issue as you can put the pedal to the floor and it will take off at 3k rpm with no problem. this is the first i've heard of the coolant temp sensor could you elaborate?