85 Z51 ran and drove okay but the idle would fluctuate by a few hundred rpm. I figure a simple start at fixing this is removing the throttle body, cleaning it and replacing the IAC. Even found out the IAC that was already in there was hand tight, but figured I'd go ahead and clean/replace. Get the throttle body back on the car and suddenly the idle is WAY too high, like 1200-1500. I find out the shaft for the throttle body has a decent amount of play in it so that must be part of the problem. I order another used throttle body off an 87 that has new bushings installed already.

I clean that one and swap everything over to it with new seals and cap off the extra vacuum fitting. Start the car and the idle seems more reasonable, around 900 or so. It does surge to 1200 after running for a few minutes but it calmed back down when I plugged the vacuum port on the bottom of the TB (might be a fluke. It hasn't come back up since even if I pull this hose loose)

I go to set the minimum idle screw by maxing out the IAC and unplugging it, but when I do this it won't crank back up. It'll turn over, fire, then immediately shut back off. Plugging the IAC back in makes it run again.

I then notice if I give it gas it hesitates, makes a sucking noise sometimes, and then won't go back to idle. The throttle plates won't return home on their own while the engine is running. I have to turn them by hand to get it to chill out. Kicking the pedal doesn't seem to help.

I've been reading all sorts of threads from over the ages about this very thing. Practically none of them ever come to a solution, and if they did they didn't report back with it.

I guess my main question is what causes the throttle bodies to stick OPEN partially with the engine running? My only thought is since this was a rebuilt throttle body maybe someone didn't center the throttle blades properly when reinstalling them. Has anyone found a solution to this?

 

The fact that you can’t make it run with IAC forced closed suggests that your minimum idle screw is not opening up the TB blades enough to supply required amount of air. This may also be the reason it won’t return to idle as the velocity of air could be keeping it open.
Your hesitation may be due to you messing with the minimum idle screw.
Every time you alter minimum idle screw, you alter the throttle position sensor voltage.
That voltage has to be correct after adjustment of minimum idle screw.

 

So I did go back and adjust the TPS voltage which may have helped with the gasping upon acceleration, but the throttle not returning home still persists. I'll try adjusting out the idle screw, as I actually had to take it out of my old TB to install into this one, and I only tightened it just enough to touch the throttle arm. On the previous throttle body no one had ever taken the tamper plug out but the throttle arm was a few mm from even touching the adjuster screw at closed position 🤔

 

Update:

Started the idle minimum and tps adjustment procedure over. Turned the stop screw out enough to where it would actually idle with the IAC fully closed, then adjusted it to 550 rpm. Hooked the iac back up and set the TPS to .54 volts. Now the car idles like a champ but it still hangs open when I give it revs. Detached all the throttle cables to eliminate that and no change. Unplugged the vacuum T from the bottom of the throttle body and it also didn't change much 🤔

At this point could it be a problem with timing or a vacuum leak causing the plates to stick open? Or did whoever installed new bushings in this throttle body do a bad job?

Would a throttle plate being slightly off center also cause this? I read that when you rebush them you're supposed to install the throttle plates in such a way that you can see a tiny amount of light all the way around them.

 

I would pull it off and have a good close look at the blades and bores. Seems like it could be binding in there.

 

Update #2: I took the throttle body back off to recenter one of the throttle plates. It wasn't too far off but I loosened the screws and moved it over just a hair, reinstalled the screws with loctite. While it was off I realized the throttle body gasket I had installed looked different than the one that came off


I replaced it with a new one that matched the one that came off. Reinstalled everything and started the car. For the first while it seemed to be better! The idle was good and the throttle would return to idle when I let off. But just to be sure I let it get up to around 175 and sure enough the throttle started sticking again, worse and worse as it warmed up it seems.

I was able to confirm that this replacement throttle body does have a little slop in the throttle shaft. Better than the one that came off but it is there. I guess it's enough for it to bind up when it gets hot? Or am I missing something here?

 

You say it’s binding when hot,Both running and engine off ?

 

Only when running and only when the engine warms up it seems. When the engine is off it moves fine. When I first fire it up it returns to idle no problem, but as it approaches 170+ it starts hanging up.

 

Is it physically hanging up or has the idle speed gone up ? Is it resting on stop screw ?
When it’s hot and turned off, can you reproduce the hang up ?

 

I had the same problem on my 88. I tried all the solutions including the shaft bushings and nothing worked.

This is my fix. The spring is adjusted so there is just enough tension to close the throttle. It does increase the pedal pressure very slightly but nothing you cant get used to. I have had this setup for about 5 yrs and the problem has never returned.

 

It's like the throttle plates are either binding or being sucked open. It's not resting on the stop screw when it does this. When it hangs at ~1500 rpm I have to either physically rotate it back to the stop screw to get it to return to idle, or sometimes I can tap the gas pedal quick enough and it'll return. When the engine isn't running it'll return to the stop screw no problem, even if it's warmed up.

 

I've thought about doing this. Possibly as a last resort but it's good that it's worked for others.

 

I sent my factory throttle body off to SPR Performance to get rebuilt with needle bearings. Hopefully that resolves the issue.

 

Why not try your original TB and reset minimum idle.
Its possible that the springs may have not been installed properly when they put bushings in that one.

 

your issue is tb shaft bushings. an 8$ part and an hour of your time with a couple drill bit sizes, some tape to make depths on the bit, and a metal hacksaw, and a dremel with a stone to grind off the butterfly screw nibs, and the correct torx screwdriver size and red locktite.

yeah that seems like alot. so far ive done 4 tb’s. solves the problem immediately, returns the crisp throttle response. the screw will force the return yeah, but leaves the vac leak and sloppy response. a worthwhile winter project for anyone with this issue.

 

So far that seems to be the answer to this one. I've sent mine off to be professionally rebuilt and cleaned. I could probably do the process you mentioned myself but I really don't trust myself to pull it off, and the throttle body will be basically brand new when I get it back in a few weeks.

 

I couldn't actually do that because my original throttle body looked like this with the throttle blades all the way closed

This is with the factory plug still in the adjuster screw hole. The throttle shaft was so out of whack that it wouldn't even return all the way home. I had a good 2mm or so of play in that one. It'll be rebuilt soon. Can you believe I was driving around like that?

 

Got my throttle body back from SPR Performance and threw it back on the car. Fired right up and settled into an even idle on closed loop. Took it out and went flying down the highway no problem. Throttle response is perfect (it used to resist leaving idle when you pushed the pedal) and returns to idle on deceleration no problem.

Highly recommend SPR to anyone with this same problem. Well worth the price.

 

Sean does great work, he has some cool iac coolant bypass plates he sells too.

 

He does clean work.

While for the very budget minded its lossible to rebush, i have to say that workmanship is easily worth what you paid. Bearings should last forever. great result glad your car is good to go.