Truck tries to drive through the brakes

[PJones96]

Try checking vacuum level, air fuel mixture, throttle linkage, timing, make sure the ignition is functioning ok. And then after all that, go ahead and check the brakes.

Roger that, I'll check the blinker fluid while I'm at it.

 

[Schurkey]

Actually, verifying the brakes is a good idea.

I've owned vehicles that "drove through" the brakes. Both times, the rear brake hose was plugged. No rear brake action at all. All stopping was done with the front brakes, on a RWD vehicle. Replacing the rear hose made a huge difference in how the vehicle behaved, especially on ice/snow covered roads.

But in this case, I'm thinking IAC action, because the idle speed varies without the throttle position changing. The question is "why".

 

[PJones96]

My TP is 0° at idle and while driving with foot off pedal it also returns to 0°.
Watch it while driving and what do you see.

“

1. Failing throttle body​

A car accelerating on its own without the driver’s input is commonly caused by a malfunctioning throttle body. If the throttle plate of your car sticks, it cannot return to the correct position, so the car will accelerate as a result.

I pulled the TB off to clean it. It didn't have much build up at all. I attached 2 pics because there are two holes on the throttle plate that have rivets in them. Isn't this a mod that's done on these TBs sometimes? Is this the reason it lunges?

If I drill the rivets out will this go away?

 

[Caman96]

Looks just like my 96.

You must be registered for see images attach

 

[PJones96]

Sorry...I just answered my own question. Those rivets hold the restrictor onto the throttle blade. I will go put this back on the truck and let you know if it improved anything.

 

[PJones96]

Looks just like my 96.

You must be registered for see images attach

I put the cleaned TB back on and the scanner reads 0.0-0.4 for the TPS now. The idle still blips intermittently and it revved higher when I put it in drive.
One thing to note that I think may be another symptom is when I first start the engine it revs straight to about 3500 for 1 to 2 seconds and then slows down to a normal idle speed. That's every time I start the truck.

 

[Caman96]

Well I figured wrongly it was the TB, but still an easy thing to check and never a bad idea to clean.

But in this case, I'm thinking IAC action

But now I’m in this camp. ^^^
(He’s usually right anyway).

 

[454cid]

When you had the TB off did you pull the IAC and TPS (so it won't get damamged) and clean the passage out?

 

[Caman96]

I did when I pulled mine, makes cleaning that passage easier.

 

[thinger2]

I've got a 96 C1500 with a 5.7vortec with 250k on the odometer. It seems like I have a ghost in the machine here. I need some help diagnosing an idle issue. The idle flutters intermittently. I wouldn't call it a surge because it's a very quick blip of 50-100rpm then it goes back to normal. That's a cold start without the ac running.
The real fun happens when I put it in drive. I have to hold the truck back with the brakes. When going down the road it feels like the truck wants to drive itself, meaning that it will gain speed without me using the accelerator. The pucker factor has gotten up there a couple times, not gonna lie. I can stop the truck and once the truck is stopped it eventually will idle down to a normal rpm. I took a short drive from a cold start. Put the truck in park and it revved to 3k. I had to put it in reverse to bring the rpm down low enough to shut it off. There's no CEL or pending codes.
I've replaced the MAP sensor, IAC and EGR valve. I've cleaned the mass air sensor. I realize I've described two different scenarios but I figured I would give as much info as possible. I'm trying to get this truck ready for my 16yo to drive but I can't give it to him like this....

I think you have two seperate issues.
I think the idle stumble might be an unrelated red herring.
When you inspect the throttle plates for binding you also need to look at end play and wear. Does the throttle shaft move up and down or side to side or in and out.
That throttle plate can seem free and clear on the bench but still bind when it is pulling air.
And that can change as it heats up.
The next thing is the throttle linkage and any cable or bushing all the way back to the gas pedal.
The last thing, though ive never seen it on a tbi so im not entirely sure that this could happen.
Look at the motor mounts.
Back in the late 1960s the public could buy some pretty hi torque V8 engines that used pretty much the same "open" motor mount design as the straight 6.
When the rubber broke, the engine would torque over and pull the trottle wide open. Killed a few people
This led to what at the time was the biggest vehicle recall in history.
That was about 1971 or 1972 or so.
But, by 1973 the gas crisis hit and those 60s cars never got repaired.
Mom and Dad stuck that V8 in the back yard and bought a Pinto or god help them a Vega.
When I was in high school back in the early 80s all of those cars were backyard 200 bucks cheap.
Nobody ever told us about motor mounts.
I am pretty well convinced that most of the awesome blown up engine burnouts I witnessed back then were broken motor mounts.
It will idle just like normal, but the slightest touch to the pedal cranks it wide open and away you go for quite the ride.
I really do not recommend it.
The response to that recall was the "clamshell" type of mount.
That type of mount kind of "interlocks"
It keeps the engine from torqueing over far enough to have wide open death throttlle.
But it will certainly allow enough slop to pull 3k rpm.
To me, I think the telltale sign of a binding linkage or broken mount no matter what the cause is that you can stop that over rev by putting it in reverse.
Does it thump?
At 250k, It needs motor mounts and trans mounts anyway.
I think its worth a look.