Another 75mm Professional Products TB problem thread

1FSTBRD

Registered User
I'd searched on here and obviously, there are quite a few of these threads, but here's another--I just couldn't get mine to run right, at all--hanging idle, idled high, etc. I'd adjusted the TPS with a voltmeter/ ohmmeter to read .95V. I'd adjusted the idle bypass screw in a few positions (closed, 1/4 turn, more than that)--no go. I'd adjusted the stop screw properly--still a problem. IAC is good--it was running fine on my stock TB, and was fine after I'd put the stock TB back on the car. After the 75mm wasn't running right, i'd also put some rtv around the IAC gasket, just in case--no dice. I'd sprayed around the throttle body for vacuum leaks--nothing.

:mad::mad:
 
I'd sprayed around the throttle body for vacuum leaks--nothing.

Sprayed what? Be sure that the computer isn't correcting & smoothing things out faster than you can try to affect it.

What does the idle do if you disconnect the IAC when the engine is warm?
 
I sprayed the throttle body area with carb cleaner. I didn't disconnect the IAC...if I put the 75mm back on, i'll check that. If the IAC was disconnected, would that be to rule out the IAC, itself?
 
It's just an idea to help gather data points towards figuring out what's really wrong - especially if you have more than one issue in play.

Be sure to pull codes as well.
 
The fix if required is pretty simple if it is the TB.

Pull the shaft, check the O Rings and replace as required.

RTV'd the shaft end blockers in place on re assembly.

Good luck
Paul
 
I've had two pp throttle bodies and never had a problem! In fact I like
them more than the mp throttle bodies because of the tps adjustability
 
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I'd searched on here and obviously, there are quite a few of these threads, but here's another--I just couldn't get mine to run right, at all--hanging idle, idled high, etc. I'd adjusted the TPS with a voltmeter/ ohmmeter to read .95V. I'd adjusted the idle bypass screw in a few positions (closed, 1/4 turn, more than that)--no go. I'd adjusted the stop screw properly--still a problem. IAC is good--it was running fine on my stock TB, and was fine after I'd put the stock TB back on the car. After the 75mm wasn't running right, i'd also put some rtv around the IAC gasket, just in case--no dice. I'd sprayed around the throttle body for vacuum leaks--nothing.

:mad::mad:

Did you check to make sure the idle bypass screw hole was drilled out into the bypass passage? That was one problem with mine.
 
Did you check to make sure the idle bypass screw hole was drilled out into the bypass passage? That was one problem with mine.

It looks like it's drilled into the bypass passage, since I can see it in there. Did they really forget to drill that passage on yours? :(
 
Okay, I've decided to take another shot at trying to get this thing to run right. I've done Ford's recommendation for setting the base idle, and the car idles fine. The problem now still is that when decelerating, on each downshift, it revs a little bit higher and then immediately drops back down to where it should be. Some guys (on Mustang forums) have said that with these throttle bodies, that they've let the car re-learn things in regular traffic driving. I've drove my SC for maybe 50 miles now, and it's still revving higher for a second or two and then dropping back down. The TPS is set to about .95-.96, which is in the range that it should be in.

Also, I should have mentioned that I'd ported/ polished my stock plenum to have a 75mm opening to fully accept the 75mm TB. When I go back to the stock TB even with the 75mm opening, the car runs fine. It's still the same IAC, and when going back to the stock TB, it runs fine.

Any advice?
 
I think I fixed this by re-adjusting the air bypass screw...it had backed out on me and was letting too much air bypass the throttle plate.
 
Okay, I've decided to take another shot at trying to get this thing to run right. I've done Ford's recommendation for setting the base idle, and the car idles fine. The problem now still is that when decelerating, on each downshift, it revs a little bit higher and then immediately drops back down to where it should be. Some guys (on Mustang forums) have said that with these throttle bodies, that they've let the car re-learn things in regular traffic driving. I've drove my SC for maybe 50 miles now, and it's still revving higher for a second or two and then dropping back down. The TPS is set to about .95-.96, which is in the range that it should be in.

Also, I should have mentioned that I'd ported/ polished my stock plenum to have a 75mm opening to fully accept the 75mm TB. When I go back to the stock TB even with the 75mm opening, the car runs fine. It's still the same IAC, and when going back to the stock TB, it runs fine.

Any advice?

It sounds like you may be getting a smidge too much airflow past the throttle when it is closed. That is a combination of the bypass screw and the leakage around the blade. (And leakage around the shaft, of course.) If you have the screw closed, then you would have to close the blade resting position a little to restrict airflow further.

You can go lower on the TPS starting voltage, down to about 0.80V. The computer takes a baseline reading when you first start the car, so it shouldn't be confused by this. The only thing to worry about is that you don't want the blade to stick in the bore, so you can't back the screw out entirely.
 
I had actually adjusted the throttle blade stop screw so that it wasn't a factor.....adjusted it so that the blade would seat itself. I drove it for a bit and the blade didn't stick against the TB bore. I still had the same issue, even after adjusting the TPS to compensate. The TB shaft has o-rings that appear to be snug, but there must be a fair bit of air getting past it, still.
 
I wasn't sure how much leakage I had past the o rings though they looked perfect..

What pushed me to rtv was one of the bushing falling out during disassembly.

I popped out the other side as well and rtv'd the pair.

They appear to be press fit and could see them going out of tolerance with hot and cold cycling.

Paul
 
The problem with air bypassing the throttle plate is that the IAC can mask the problem by simply closing a little further and bringing the idle down to normal range. When you take your foot off the throttle, the computer positions the IAC to a preset place for a moment - which is what you're noticing.

Just to verify - you have the throttle plate and bypass screw set such that the car will barely run with the IAC unplugged (fully closed), right? As soon as you unplug the IAC, it should start running very rough and/or die completely.

I don't think saying that the car needs to "relearn" anything after the throttle body swap is accurate (except maybe idle mode strategies).
 
Just to verify - you have the throttle plate and bypass screw set such that the car will barely run with the IAC unplugged (fully closed), right? As soon as you unplug the IAC, it should start running very rough and/or die completely.

It should be able to idle without the IAC - even if just barely. Otherwise, you will have problems with rough idle and stalling when pulling into parking places.

The idle without the IAC should be like 600-650 rpm.

If you have a big low-vacuum cam, of course, you will need to go a little higher. But you will also need to make the target RPM higher in the IAC, or things will not work that well.
 
It should be able to idle without the IAC - even if just barely. Otherwise, you will have problems with rough idle and stalling when pulling into parking places.

The idle without the IAC should be like 600-650 rpm.

If you have a big low-vacuum cam, of course, you will need to go a little higher. But you will also need to make the target RPM higher in the IAC, or things will not work that well.

Correct - After the throttle plate stop and bypass screw are set properly.

Except, the IAC doesn't close when pulling into a parking spot, etc. It will be in varying positions during driving, but is rarely (if ever) closed completely.
 
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There is never a need for an IAC bypass screw on an aftermarket throttle body because a bigger TB is naturally going to bypass more air than stock, so artificially increasing it is going to be counterproductive no matter what, therefore they should always be closed completely shut. Any adjustments needed for airflow should be made using the idle stop screw.

One of the problems with the PP TB's is that the throttle blade is too thin and it flexes at times. This causes it to stick in the bore when closed abruptly from high rpm. To get the blade not to stick you have to set it a bit more open and this contributes to the problem already present from a larger than stock throttle bore naturally allowing more air to bypass the blade. Adjusting the bypass only makes it worse, never better.

Another problem that can exist with these TB's is bore to throttle blade tolerance. Sometimes the blade might be a tad small, or it might not be centered in the bore. These are manufacturing problems that may be present on one TB and not the next so it's always something to look at.

If you can't get it to idle down properly by adjusting the throttle stop then something is wrong with the TB. TPS can be set anywhere from .5v to 1.2v and still work completely fine, so don't worry about that too much. The EEC will learn the resting position and work everything relative to that.
 
The problem with air bypassing the throttle plate is that the IAC can mask the problem by simply closing a little further and bringing the idle down to normal range. When you take your foot off the throttle, the computer positions the IAC to a preset place for a moment - which is what you're noticing.

Just to verify - you have the throttle plate and bypass screw set such that the car will barely run with the IAC unplugged (fully closed), right? As soon as you unplug the IAC, it should start running very rough and/or die completely.

I don't think saying that the car needs to "relearn" anything after the throttle body swap is accurate (except maybe idle mode strategies).

Yeah, I unplugged the IAC and it ran a little rough, and then plugged it in and it still ran rough, but then I adjusted the idle bypass screw so that it idled a bit higher and didn't run rough anymore, but it was still somewhat higher than it needed to be in order to idle correctly. Also, the hanging idle when coming out of cruising speeds when slowing down and the revving higher for a second on the downshifts just wouldn't go away. I agree that the car shouldn't have to re-learn anything, because on my Accufab for my Mustang, i'd never had any problems.

Back to stock for now. I'd ported and polished the stocker and it works well.
 
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