I just installed an EFI Hardware pro-race tapered throttle body on a racing beat manifold. The car starts every time and got it idling at 900 rpm. The issue is that when I start revving it, the throttle plates won’t close all the way. They slowly close and finally keep the car idling around 2500 rpm after. Seems to be worse when the engine warms up. I have to push the throttle lever on the throttle body by hand to get it back to 900rpm.

A bit about the engine, it’s an S5 turbo block that I built about 8 years ago. Built it with a mild street port and NA rotors, since it was staying NA. Originally had it tuned with the stock S5 turbo intake manifold and had been running it like this since.

For the throttle body, I did buy the heavy duty throttle return spring. Throttle plates snap shut right away when the car is shut off, so not sure if it’s a vacuum issue with the car running. To keep things simple, I did turn one of the fuel rails into a vacuum manifold. The brake booster and the Haltech internal MAP sensor are hooked up to it. Not sure if this could be causing my issue, maybe nipples lower down on the intake manifold would be better?

Anyone experience this before with ITBs?

 

Here’s a pic of my custom fuel rail vacuum manifold. You can see the line for the brake booster at the rear end and the line for the MAP sensor in the middle.

 

Two things I have learned over the years:
Aftermarket throttle bodies are usually garbage as far as sealing to the bore at closed throttle is concerned. They don't have the resources to make tooling to make 100,000 identical units so they tend to bind a little bit. Some finessing of the fit for the throttle plates helps a lot. This is just the nature of low volume production parts, you're going to have to do some finish work to make things really good.

The fact that it seems okay with the engine off also makes me think that the throttle body is built wrong. Throttle plates should not be round, they should be slightly oval so they are at an angle when closed. The shaft should also be a little off center so that the blade is slightly longer on the side that rotates up, so vacuum tends to close the blades. Yours may have been built backwards, so vacuum opens them.

Second, and most importantly, the throttle return spring must pull against the cable. Meaning it needs to be inline with the cable, on the other side of the throttle body, so it pulls the cable "directly". If the return spring pulls the other side of the lever, on the same side as the cable, this will cause the throttle shaft to bind. Over time it will also wear out the bore that the shaft sits in. No it doesn't look as "clean install" to do it this way, but function must take precedence over form here.

I had a hell of a time with throttle hangup near idle with a Holley, and stronger springs seemed to make it worse, until someone older and wiser told me that. I made a new return spring bracket that overhung the throttle body the other way and all of my idle hang issues were SOLVED.

 

Thanks peejay! The throttle return spring on the EFI Hardware throttle body is on the throttle shaft. I will try adding a second spring to pull against the throttle cable as you suggested. I will let you know how it works out.

 

Thanks again peejay, worked like a charm! Here’s what it looks like. Not overly pretty, but it works! I could probably make a nicer spring bracket later on.

 

yeah these EFI hardware units have terrible return springs, the HD one is too light. I have had to add one to mine and its weird because of the circular pull you need to balance with

I experienced the same thing you described and fixed it with a similar solution recently.