New Throttlebody Doesnt Close All The Way

[93@VIR]

Hey all, I just finished building my top end on my 93 5.0 and when i started the car, it wants to idle around 2.5k rpm. It seems to be an air leak, so we checked all the vacuum lines and looked for a gasket leak, but no luck. We pulled the intake tube off to look at the new Summit Racing throttle body and noticed that it wouldnt close all the way. If i recall its supposed to have some gap to prevent vacuum lock, but it seems like too much to me (pictures below). It wouldnt close enough for the throttle stop to hit it, even when we adjusted it out considerably. We figured this is probably the air leak, but before i contacted summit to return/replace the throttle body i figured i would see if anyone here could confirm that the gap is too large, Thanks.

 

[Mustang5L5]

I'd say yes. Too much gap. At idle, it should be closed with the idle air controller regulating how much air the motor sees at idle

 

[General karthief]

Loosen the bolts and see if it closes.
Make sure throttle cable is not holding it open.

 

[jrichker]

There is no such thing as vacuum lock Your choices are limited to vapor lock or liquid lock, neither one would apply in this case.

Put you hand or cover over the TB air inlet while the engine is running. If it keeps running, you definitely have a vacuum leak.

With the engine running, disconnect the IAC connector and the throttle cable. Then slowly loosen the throttle stop screw. If the idle speed does not drop, and you have done the above test, then the throttle body is defective. Return it to Summit.

If the idle speed decreases, here is the recommend procedure to set the engine base idle speed.

Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM

Remember that changing the mechanical idle speed adjustment changes the TPS setting too.

This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.

Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.

Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it. Anything between.6 and 1.0 volt is good. There is no advantage to setting it to .99; that is a BOZO Internet myth, complete with red nose and big floppy shoes.

When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.

 

[MFE92]

There is a stop screw under the TB that controls how far shut it can go. However, yhrottle tip-in can become very stiff and hard to modulate when the plate is allowed to close all the way.

 

[NIKwoaC]

I had a similar problem with my Accufab TB, but not when it was new. Over time, the plate wore a burr into the bore and then the plate wouldn't close all the way and the car would idle very high.

Throttle bodies are simple devices, I would take it off the car and open/ close it by hand. If it closes all the way when off the car, the problem is likely with your cable or linkage. If not, look to see if it's a machining issue in the bore, or some other feature is blocking it.