Truck tries to drive through the brakes

[Pinger]

Just took an hour drive on some 2-lane. Speed limit varied from 35-55. The truck almost acted like the cruise was set. If I was going 45 or faster I could take my foot off the accelerator and give only minimum input with the pedal to keep it at 45 or so. I had the ac on and i could feel the truck increase/decrease in speed as the compressor clutch kicked on and off. It took either a significant hill or brake pedal to slow the truck. Going downhill, it was a runaway; it would gain rpm and engine speed as long as I let it. Had to use a lot of brake to reign it in. The brakes will stop the truck, I don't want to sound like I'm being totally reckless.
truck.

Some of that reminds me of early days with my Suburban . The reason in my case was a higher than necessary level of fuelling in OFCO. Mine runs on LPG (propane) and the OFCO isn't necessarily set at zero. A setting higher than zero aids transition going back into the throttle. Too much and it feels like it's trying to run away.
Are you sure yours isn't somehow pulling fuel in on a closed throttle?

 

[PJones96]

Some of that reminds me of early days with my Suburban . The reason in my case was a higher than necessary level of fuelling in OFCO. Mine runs on LPG (propane) and the OFCO isn't necessarily set at zero. A setting higher than zero aids transition going back into the throttle. Too much and it feels like it's trying to run away.
Are you sure yours isn't somehow pulling fuel in on a closed throttle?

I've wondered if the fuel pressure regulator could be to blame. I'm not getting any DTCs about being lean or rich though.

 

[PJones96]

I'd suspect a faulty tach long before I blamed the ignition module.

If the module were sending out extra sparks, the truck would run bad.

What should the truck idle at when warm and the ac is off?

 

[Pinger]

I've wondered if the fuel pressure regulator could be to blame. I'm not getting any DTCs about being lean or rich though.

Leaking injector(s) (but I think you've eliminated that as a possibility) or maybe EVAP playing up. Can't think of any other source of unwanted fuelling. For sure, a touch of fuel where/when it wasn't needed caused mine to be always trying to get away from me.

 

[Caman96]

What should the truck idle at when warm and the ac is off?

Mine at temperature, IN GEAR and ac off is 650-655.

At temperature, IN PARK and ac off is 670-680.

 

[PlayingWithTBI]

If the module were sending out extra sparks, the truck would run bad.

Or maybe faster than the PCM is commanding, "680+/-". That's what's indicating something in the ignition system to me. Like I said, just a guess here.

 

[Schurkey]

Can't.

If the crank turns faster, the crank sensor provides that RPM signal to the computer. "Extra" sparks aren't properly timed, therefore either come too early or too late to do any good--but can do considerable harm if it causes over-advanced timing.

Besides--the RPM "should" be controlled via the IAC. I'm wondering why it isn't.

 

[PlayingWithTBI]

Besides--the RPM "should" be controlled via the IAC. I'm wondering why it isn't.

Yeah, I get it but, the PCM "thinks" it's at 680 RPM so, it's not gonna adjust the IAC because it "thinks" it's already at, or close to, target idle speed. The tach sees higher revs, AND the truck is pushing at idle, with the brake off, too fast so, there's a disconnect between the PCM data and actual speed. PCM/Sensors win 2:1. Maybe the IAC stepper motor, and/or the PCM output are/is bad?

 

[PJones96]

I had to move the truck yesterday out of the way. This is a cold start scenario. Truck started like normal, put it in drive and it stayed at a normal idle for a change. I drove about 50 feet and put it in park. Once in park the tach jumped to 1500 rpm. It would not "idle down". I plugged up the scanner and looked at the live data. The TP% kept moving between .4 to
.8 to 1.6%. I did not have my foot on the accelerator. The scan tool said the truck was idling at 1040 rpm.

The TPS was replaced within the last year. It was one of the first parts I threw at the problem. Can a parts store like Orielly or Advance test a PCM?

 

[Caman96]

I wonder, while it had that high idle, if you had gone out and manually closed throttle if it would’ve gone back to 0° throttle angle and stayed. Just to see if it was sticking mechanically.