Truck tries to drive through the brakes

[tayto]

Bought a used TB from a used parts seller online. The truck it came off of had 130k miles on it. That's about half of what mine is at right now so that's pretty much new in my book. It had a IACV and TPS on it out of the box. I took them off and cleaned the whole thing, all the nooks and crannies. I put the IACV off my truck's TB on the "new" TB but put the "new" one's TPS back on it after cleaning it because why not.

It was dark by the time I got the new TB on the truck. I cranked the truck and it acted exactly the same as before. The idle blips intermittently at random when cold. I didn't let it run that long because I was frustrated and went inside and poured a glass of bourbon.

This morning I hooked the scan tool to the truck and could see that powered on without the engine running TPS was reading 0 and seemed to have no dead spots or issues as I played with the pedal. When I started the engine the TPS would go from 0.0 to 0.4 to 0.8 sometimes, almost constantly changing. I drove a 5 mile loop around my house with a couple traffic lights.

It tried to drive through the brakes at the first stop light with the TPS reading 0.4. I held it with the brakes for a second or two and then it idled down to normal. The TPS read the same the whole time.

At the next traffic light I stopped at the TPS read 0.4 but the truck slowed down and idled down like you would expect. No issues at this traffic light even though the TPS read the same as when it wouldn't idle down. I stopped at two other intersections without having an issue.

I pulled into the driveway, put the truck in park and it revved up to about 1500 for a second and came back down to normal pretty quickly.

I do think that the new TB is better than what I had. I'm trying to not psyche myself out just because I put a new part on the truck, but throttle response seemed better and it idles smoother. I think I have a multi-faceted problem here and the throttle body was just one part of the issue.

how much play is in the throttle shaft? i haven't found a TBI yet that i havent had to put throttle shaft bushings into....

 

[PJones96]

With the new TB the truck drives really well if I don't use AC. When I cut the AC on is when it wants to increase speed on it's own and tries to drive through the brakes when I come to a stop. I've gotten so used to it now that when I know I'm going to park or something like that I just start cutting the AC off ahead of time so the engine idle will come back to earth.

I know that idle speed increases slightly when the compressor turns on. Is that a vacuum related issue or is just the PCM opening the IAC valve?

 

[PJones96]

how much play is in the throttle shaft? i haven't found a TBI yet that i havent had to put throttle shaft bushings into....

Where do I get throttle shaft bushings for this TB? I still have the truck's original TB so I may try to repair if if it's something I can do.

 

[Schurkey]

I got mine and the "special" drill bit from Cliff Ruggles, the Q-Jet guru.

Haven't installed 'em, though. And I don't remember which size I bought.

3/8″ Throttle Shaft Bushing Kit – Cliffs High Performance Quadrajets

5/16″ Throttle Shaft Bushing Kit – Cliffs High Performance Quadrajets

My machinist buddy drills 'em on real live machinist fixtures, then cuts-down bronze thinwall valve guide inserts to fit. Enormously less expensive...if you've got the fixturing, and if you've got valve guide inserts in-stock in a bunch of different sizes already.

The difficult part is getting the screws out of the throttle plates. They HAVE to be ground-down on the staked side, or you WILL break the screws. When you're done, they get put back in with threadlocker, and re-staked if at all possible. The re-staking has to be done without bending the throttle shaft, or you've done all that work for nothing.

 

[PJones96]

I've made no progress on this. Same issues as when I started this thread. I'm going to send it to a mechanic I trust and I'll report back when it's resolved.

 

[PJones96]

Turns out the problem was the Idle Air Control tables needed to be cleared. I had replaced the IAC valve and the truck was trying to run on values that it learned from the original IAC valve. I didn't know that I needed to clear anything after replacing the IACV. The mechanic I took it to cleared the tables and we drove the truck a few miles with no problem.

 

[PJones96]

Final Answer...The used throttle body was the fix. The IACV that I put on the truck was defective. I could grab the plug end of the IACV and spin the entire unit inside it's housing, so I think it by itself was a vacuum leak. When I purchased the used TB it came with a TPS and IACV still attached. I took both of them off to clean everything but I put the new stuff that I bought back on it. When I took it to my mechanic and he cleared the IAC tables he told the IACV shouldn't be able to spin in it's housing, if the symptoms return try putting the old IACV back on the TB. The issues started again two days later. I put the IACV that came with the used TB back on the truck and it's been driving like a dream every since. So if your truck is trying to drive away from you, check the throttle blades in the TB. Don't automagically assume it's the IACV.