Hi All -

I picked up a high mile ( 280K ) but very good condition 2WD Crew Cab Dually Lariat, the truck runs very good just rolling around town in stop and go traffic and most of the time at highway speed. The truck idles great, no misses.

But some of the time the truck chugs and bucks, chugging along like a train. I have been trying to figure out why and am coming up empty handed. It generally does this when going up a slight grade, but not always sometimes it will happen on flat ground, it will chug and buck with either manual throttle control or cruse control. If it starts doing it if I stop on it it will accelerate out of it.

I have noticed that I will get a micro-second flash of the Service Engine Soon light that does not stay on and with both a snap on scanner and AE there are no codes stored in the truck. See # 4 below.

So where it gets odd is if I unplug the IPC sensor the truck smooths out and stops bucking but seems to lack a bit of power and throttle response. Of course this does set a full time Service Engine Soon light. See # 3 below. IPC replacement made no difference.

I have tried different fuel stations, additives and even ran some farm fuel in the truck to see if it could be a fuel supplier issue, none made any difference.

Here is what I have done so far:

1. Changed oil and filter
2. Changed Fuel Filter
3. Replaced IPC and Pig Tail
4. Scanned truck with two pro level scanners, no codes
5. Truck passed buzz test
6. No contribution errors, even #8 did not throw an error.
7. Tested fuel pressure, 70 psi at idle, 66 psi @ 65 MPH, 62 psi @ WOT acceleration.
8. When truck chugs I do not see a drop in fuel pressure.

At this point I don't just want to throw parts at is as that is expensive and dumb can anyone suggest where I should start looking next ?

The truck is now locked up in a barn for winter, so it will be a bit of time before I get back to it but I am looking to figure this out.

Thanks

 

You said the magic word: "Bucking" - and "when going up a slight grade". This is a classic symptom of a loose injector (or more). The UVCHs are under the valve covers, and they frequently need attention as well (click the UVCH link in my signature). If you go under the covers to check the injector bolt torque, first set the torque wrench to 50 INCH/pounds. If the bolt tightens, you need to pull the injector and replace the O-rings and copper washer on that stick.

Before popping the hood, a CPS can cause similar symptoms - so getting a Motorcraft CPS and installing it from under the truck is a safe (and inexpensive) thing to try. Even if this doesn't help, it's always a good idea to carry a spare known-good CPS and a 10 mm wrench on board - because the CPS has a jaded history.