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Vacuum leak locations on E30 throttle body (M20)

March 1, 2022

In the vast majority of cases, vacuum leaks are to blame for poor or inconsistent idle issues on M20 engines found in E30’s.

Vacuum leaks basically mean air that is entering the engine where the air flow meter has not had a chance to meter it. Unmetered air, is unexpected air, and as such causes a lean fuel mixture.

So where is unmetered air able to enter on the throttle body?


1. ICV Elbow – These are press fit from factory and loosen over time – There is a limited run of 100 CNC units that we have made to solve for this leak:

CNC ICV Elbow – Fix for ICV elbow vacuum leak E30 M20 engines

2. Elbows to hoses going to brake booster – CNC version of this now available also!

CNC Throttle Body Booster Side Elbows – Fix for vacuum leak E30 M20 engines

Both of the below can be replaced with INA HK08122RS, a dual seal bearing.
3. Top ‘drawn cup’ bearing (OEM is 2 seal)
4. Bottom ‘drawn cup’ bearing (OEM is 1 seal) — This is why you get oil in your TPS, seal is dried up.

Note: Replacement is involved, but doable; with a 50/50 chance the plastic under the springs will break when removing and reinstalling. Generally you can forgo replacing these seals as long as everything else is sealed up well.

For removal: The key is to push down the needle bearings and install the inner bearing puller under the top lip of the cup bearing. Before hammering away its 100% critical that you apply a butane flame around the aluminum that surrounds the bearing. At about 250-275f the aluminum will have expanded more than the steel of the bearing and the bearing comes right out with a couple hits. For reinstall put the bearings in the freezer overnight.


Second note: Reinstalling the springs is what many people get caught up on; often leaving the springs 1 or 2 turns short. Before disassembly, mark the springs with a paint marker so when you reinstall you have a visual marker to show that the spring has been wound to its original state.

When you go to reassemble, you can use two picks to grab and pull the spring around and around. The picks make it easy work.

Put the TB in a vise so you have free use of both hands when using the picks.



5. Evap barb – the barb itself doesn’t leak, but it must either be capped, or there must be no leaks between it and the purge valve that connects to it.

6. Breather hose to valve cover – make sure there are no cracks in the rubber hose, and that the hose clamps are tight.


2 Responses to Vacuum leak locations on E30 throttle body (M20)

  1. Vince Caporale on January 24, 2022 at 9:39 pm

    How can order one of your CNC ICV elbows?

  2. admin on January 26, 2022 at 1:42 pm

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