charliemccraney

A presumably original injector stopped working so I bought a rebuilt throttle body. No problems before this. Nothing else done.

The throttle body has an injector pod spacer, milled air horns and thinned throttle shafts. The builder said this will not require a different tune. It's a newer unit with a different TPS and an adapter harness to work with the factory connector. IAC, TPS, fuel pressure regulator, injectors all new.
After first bolting it on, it will not idle below about 1025rpm with the IAC at 00 and TPS at .53v. I verified that the IAC is closed by observing the pintle and putting my finger in the hole to feel for airflow. I've checked for vacuum leaks but haven't found any. After performing the minimum idle air adjustment, I am able to get it down to about 800rpm before the throttle blades stick, IAC still at 00. I have not readjusted the TPS, yet. I checked timing just to confirm that nothing changed by some weird coincidence and it is the same. With the EST connector disconnected, it will drop to about 600rpm. The injectors have 5235279 on them, same as the old ones. I've driven it and it performs as expected otherwise. It just won't idle at the correct speed.

So, the question is, is there anything other than a vacuum leak that will cause it to idle higher than it should? The guy I bought it from doesn't seem to be too strong on the trouble shooting end and is suggesting that I put my old IAC in this new TB to see i that helps. Any merit to that, given that I can observe and feel that it is closed?

Uno122

Im not sure what computer you're running, but When you say you did the minimum idle air adjustment, did you do the IAC reset precedure because it was new first?

Just because the IAC is new doesn't mean it's good. I've had a few brand new bad sensors right at the get go. Trying the old IAC and seeing how it does might not be a bad idea.

And you said you've checked for vacuum leaks, I know the gasket under the throttle body had a tendency to leak.

Just my two cents worth, good luck

charliemccraney

I've replaced the base gasket just to see if I was somehow missing a leak there and it resulted in no change. So a total of two new base gaskets have been used with no change.

I don't know what the IAC reset procedure is. The throttle body came to me completely assembled. I followed the instructions exactly for the minimum idle air procedure. How do I reset it?

I understand that new sensors can be bad but we're talking about idle speed. When this is happening, I can observe and feel that the IAC pintle is completely closed, in addition to seeing the position at 00 in TunerPro. So if no air is getting by, how would a different one change that?

Schurkey

IF (big IF) the IAC is "closed" so that no air can get by; and
IF there's no leak at the throttle body gasket, and
IF there's no slop in the throttle-shaft bushing area, and
IF there's no missing or leaky vacuum plug on the throttle body, and
IF there's no "false air" coming in through the PCV, brake booster, EGR, evap canister, or other vacuum accessories, and
IF the intake manifold is properly sealed at the heads, and
IF the manifold isn't somehow cracked or missing a vacuum plug, allowing false-air entry, and
IF the base ignition timing is correct, and
IF the throttle linkage including the TV cable and Cruise Control are not interfering with throttle closure,
THEN the most-likely cause of high idle is throttle blades that don't shut properly.

This could be a result of throttle blades that aren't aligned properly on the throttle shaft before the screws were tightened, or it could be the result of a bent throttle shaft.

Given that the throttle shaft has been modified (thinned) and is therefore weaker than designed, and the throttle blades have to be removed to do this work, either is a possibility--but there's plenty of other items to verify.









Has anyone done dyno or flow-bench testing to see if pod spacers and smoothing the entry to the throttle bores ("milled air horn") actually has any significant effect on power production? Seems like a TOTAL WASTE OF TIME, MONEY. EFFORT, and ENTHUSIASM to me. (Especially the pod spacer) But that's just a guess.

Uno122

You could try this

Iac Reset

Good luck,

charliemccraney

2 new base gaskets have been tried. Manifold and throttle body surfaces in excellent condition
The throttle shaft holes have new bushings so that should be as good or better than new. By feel, it is definitely better than the 180k mile original throttle body, which idled fine.
No missing or leaking vacuum plugs.
PCV has been checked.
Forgot to check brake booster.
Didn't think to check EGR.
Evap cannister checked.
Other vacuum accessories checked.
Intake manifold not yet checked.
Base timing correct.
Linkage and cables are not holding the throttle open.
At the 800rpm idle speed it will achieve, the blades look as closed as they do with the original throttle body; alignment looks good.

So that's brake booster, EGR, and intake left to check. These would be coincidental given that it idled fine before but I can't deny the possibility.

Could the different TPS cause a problem even though it does seem do work correctly from idle to wot within TunerPro? The throttle body looks like a '93 unit based on the date code. It uses a smaller, round connector.

The IAC reset appears to be exactly the procedure for setting minimum idle air. So yes, I have done that.

charliemccraney

I swapped out the IAC and set minimum air again. Exact same result.

No vacuum leaks found in the intake or brake booster. Checked other areas again, still nothing found.
Haven't tried trouble shooting EGR, yet.

Schurkey

The EGR problem would be a failed gasket that allows air into the manifold vacuum passage.

charliemccraney

I noticed in past data logs that the TPS voltage at idle for the stock unit has been .65v since my first log. What effect would that difference in voltage have?

Sweatlock

iTrader: (1)

charliemccraney

I replaced the entire unit because it is worn and really does need a complete overhaul.

I also doubt all of the other stuff but I want to rule everything out before I blame the replacement throttle body. Coincidental things do happen.

A torque wrench was used.

Sweatlock

iTrader: (1)

The only wear point I'm aware of on the throttle body is the throttle shaft and/or bushings, which can wear and create a vacuum leak. But you said the car ran fine before the injector failure.

If the initial diagnosis was correct and the single injector had failed, I would have replaced only the failed injector - if the car ran fine after that, then the injector was the culprit. If the car didn't run fine, then either my initial diagnosis was incorrect or something else was amiss.

charliemccraney

I could observe that the injector was not working. I confirmed that the connector was getting the ecm signal with a noid light. I can now observe that both injectors work like they should. Diagnoses was correct.

jstadmud

I think what Sweatlock is trying to say is, you don't usually replace the engine just because you're having trouble with your distributor. Maybe your rebuilt TB has a problem that your original one did not. My suggestion would be, providing you left the original intact, to put the injectors from the "rebuilt" into the "original" and see if your problem goes away. It's not a hard swap, and maybe it fixes the dilemma

charliemccraney

I understand what he's trying to say. I'm also the one who has the original here to look at and it is time to take care of it all. I don't do half-measures.
I agree that there could be a problem with the replacement and that is what I'm trying to determine. I am planning to do the bare minimum to get the original back into working order to test if nothing else is found. Either way it goes, I want a like new or better throttle body once this is all resolved.

morgsie

iTrader: (1)

I assume this is a "rebuilt" TBI unit and has had throttle shaft "bushed". Very common problem with TBI units these days because of their age. Make sure idle screw tamper plug has been removed then turn idle screw in until IAC counts are 5-10 with engine fully warm. You may have to turn off car, wait 10 sconds then restart engine as turning the idle screw in too far may cause the ECU to think it's out of "Idle mode". I do this to verify my IAC counts stay the same after shutdown. I have done this method on many TBI vehicles after rebuilding and bushing with good success.

charliemccraney

Yes, new bushings and the screw is accessible.

charliemccraney

I installed new injectors in my original, reinstalled, and it idles normally again. So it was something to do with the rebuilt tb.

Uno122

Cool, thanks for posting your find, could help others in the future. Glad your trucks idling normal again.

morgsie

iTrader: (1)

maybe wrong IAC? you could try installing your old IAC in the "rebuilt" unit.

charliemccraney

I did that.

morgsie

iTrader: (1)

Have you contacted the "rebuilder"? I reread and you said the throttle blades "stick" if you set the minimum idle screw to low. this doesn't sound right to me. I rebuilt&rebushed a Qjet recently and had a similar issue, ended up playing around with the throttle plates until they were centred better. hold the throttle body up to a bright light, the exposed light should be equal around each throttle plate. Is the base flat? can check with a metal ruler and a flashlight. Only other thing I can think of is where the IAC pintle seats. GM did use different "cones" over the years. Are you using the IAC gasket. Are all the vac ports the same (full manifold, ported, etc) as your original?

charliemccraney

I have contacted the rebuilder. The unit will be on the way back to him shortly. It seems likely that the blades would stick if the screw is backed off so far, unless the blades are exactly the right diameter for the bore - maybe they are, in which case it could be bad alignment. I don't notice anything obvious. The port for the thermac uses a ported vacuum source on the rebuilt unit whereas the original is manifold vacuum. That should not have caused this issue, however. Otherwise, the ports seem the same.