Ford Mustang Forum banner

Help with hanging rpms after TB and plenum install

2729 Views 9 Replies 5 Participants Last post by  tripleblack
Ok so I bought a accufab plenum about a month ago and installed it about 3 wks ago and everything was great.

About two wks ago i bought a 70mm polished accufab throttle body and a motivator throttle body spacer and just installed them like two day agos.

My rpms were hanging up so i made and installed one of those metal restictors that goes between the plenum and the iac. the rpms werent hanging as much as before but now i have a hissing noise every time i touch the gas pedal

Also here is the major problem- just bit tapping the gas pedal a tiny bit the rpms raise up to about 1.5k and sit there for about 5 seconds and drop down to regular idle. Also if i keep the rpms up in a gear like 3rd for example to liek 4k and pull into neutral the rpms will fall to 2k for like 5 seconds and then to 1.5k rpms for another 5 seconds and then to normal idle

Also once the car idles normal and i turn the ac one, the idle goes to 1.2k everytime.

Not open for further replies.
1 - 10 of 10 Posts
Sounds to me like you most deffinantly have an air leak, to isolate the leak withe the car idleing spray carb clener or wd40 arond the throttle body and plenum until you hear a change in rpm, then just repair your leak
ok thanks ill do that, Also i wanted to tell you guys that the throttle body is used so is there any adjustment i have to make or have to do anything with the tps with that voltage thing?

I have a very similar system on my 02, accufab 75mm and c&L plenum. I wonder if you still need that spacer with the new plenum... I also suspect the spacer gaskets may be where you're leaking.

The other suspect would be the used tb. If someone messed with the thing, it may be leaking from the pivot shafts or even from a hole drilled for nitrous or some such nonsense.

You may also need to adjust your tps, but I'd locate and fix any leaks first.

Let us know what you find.


GTRmustang95 said:
ok thanks ill do that, Also i wanted to tell you guys that the throttle body is used so is there any adjustment i have to make or have to do anything with the tps with that voltage thing?
There's no need for the spacer with an aftermarket plenum. They already hold a larger volume of air which is what the spacer does for the stock plenum.
What should I look for to see if someone messed with the throttle body? There are no tapped or drilled holes, i mean this throttle body is spotless.............

How what is the voltage i want in the tps to be i think .98 to .99 right ? what setting on the multimeter and where do i put each of the two leads to ?

Thanks for all your help guys i can wait to find out what this is and let you know.
A big problem for me when i installed my throttle body was that i forgot to plug my MAS back in which caused high idling and reving. Make sure that your MAS is plugged in and that all your hoses are properly attached to your intake. Lastly did you disconnect the battery before the install? You need to disconnect the battery when installing new parts in order for the computer to reset and work with the new parts. If you dont the computer may react badly to the new goodies you just installed.
Ok I have made some huge progress, today in the morning I took the throttle body back off and closed off the throttle body until the blade got stuck and then opened it up a 1/4 turn as suggested by someone earlier. However on in their throttle body install how to they said to unplug both the tps and the iac bypass. then once installed to only connect the tps adjust idle and plug the iac back when finished

I did this, with the iac unplugged the car runs great, once it is plugged in, it idles normal but when I tap the gas ever so gently the rpms shoot up to 1.5 to 2 k rpm range and give me the symptoms as before. So now i am driving around for now with a unplugged iac because with it plugged in- if i pull up to a light and put the car in nuetral the rpms stay high for 5 or so seconds and then drop.

So my iac broken or what? Also remember I installed the plenum first with the stock throttle body and everything was great then I installed the accufab throttle body and a spacer and the iac starts acting up.

the car has 14k on it so i doubt the iac would go bad!!!!!!!!!!!!!!!!! :sosad:
See less See more
As mentioned above I also took out that gasket I made with the smaller hole- which was the cause of the whistling sound

This thread's gotten a little tough to follow, but if I read you right you've probably got 2 problems:

1. Spacer. Not needed, and may be source of problem. As the Darleks would say, "Exterminate", ie, get rid of it.

2. IAC. Infamous source of Mustang tuning problems. Several reputable mechanics I know (well, actually they're mostly "dis"reputable) suggest replacing the IAC any time you do the plenum/tb swap - the darn things are just delicate as hell. So, yes, even though it has few miles on it, it may well be damaged.

1 - 10 of 10 Posts
Not open for further replies.