Simplifying vacuum lines


Most of us have 3 vac lines going to a triple checkvalve which then goes to our brake booster. This can be replaced to a single vac line and a single checkvalve. The advantages are that you get to reduce the # of places that you might get a vac leak, and you end up with a $9 checkvalve instead of a $50 checkvalve.

Vac leaks are the enemy.

There’s a number of different rubber intake boot designs for the E30 over the years. Some used an L shaped Idle Control Valve (ICV) and some use a T shaped. Most, but not all intake boots had a port to provide vac to the brake booster. Here’s a pic of a common setup. Note the T shaped ICV.

Here’s a pic of where you’re headed. Checkvalve is circled.

That’s a 2010 pic from Warren Turnipseed’s car. Note the single brake vac line and checkvalve. Unused ports in the intake boot and throttle body are plugged.

After I saw what Turnip’s shop had done, and this was in 2010, I scoured RealOEM to figure out if there was a way we could call this great idea “legal”. That’s when I realized how many different intake boot designs we had. Shawn has ok’d this as a “update/backdate” clarification.

If you’re highly motivated, you can find one of the E30 intake boots that have no vac port, and swap out your boot. But be careful that you don’t end up with a boot that isn’t compatible with your ICV. Remember that there’s both L shaped and T shaped ICV’s. Personally, I would just plug the vac port on your current intake boot and be done with it.

E30 checkvalve. Vaico - 34331156579 - Brake Booster Check Valve?

The hose is just generic vac line stuff. I don’t remember the dia.


Good Post Ranger.

Only thing I have to add - What did you plug the additional port on the throttle body with?


Nice! Do you have any details on which years had different intake boots? I’m looking at realoem and I’m seeing the same part for every e30, but I’m sure I’m missing something.


Gotta look harder than that. RealOEM isn’t great about subtleties. Hit the model years every 6 months for our car. Look at the diagram. Click on the part and look at it’s diagram. If the camera icon is next to the part #, click on that. Look for diagrams that are different or pics that are different. Write down the part #'s. Look for references to old part #'s. After an hour of this, you should have a collection of part #'s and studied lots of diagrams and pics. Then take those part #'s and start googling. Look for instances of part #'s hitting pics and diagrams of boots that are clearly different than what RealOEM said.

There’s 3 or 4 boot variations. T shaped ICV or L shaped ICV. Vac line port or not.

The problem is that BMW didn’t redo their diagrams every time they made a small design change. RealOEM is pretty good, but it’s not perfect.

Example. In preparing for a conversation with Shawn today, I used RealOEM and started with a '91 325i (or maybe IS) coupe. I noted that the “system” diagram of the boot was different then the “parts” diagram of the boot, which was different then the pic of the boot.


Gotcha, ok, thanks!


Each of my engines is different. One looks like the top pic, and one more or less like the bottom, except the intake boot doesn’t have anything like a port, as both of these pics do. The issue that both of mine seem to have is the fitting right at the throttle, the right angle piece with the vacuum hose attached, seems to almost want to come out of the throttle body. Any reason that is the case, and is there a solid engineering fix? One of my engines has what looks like black sealant on that fitting, but it’s still loose. I could epoxy the metal pieces together, but I don’t know what the long-term detriment from that might be, if any.

Update: One ICV is L-shaped, on my spare engine, and that boot is without the port. The ICV on my installed engine is T-shaped, and that boot has a port. Both engines have two over-under ports on the firewall side of the throttle body.


This boot uses the T-shaped ICV and has no “extra” port. (credit Warren Gould)

This Genuine BMW one uses the T-shaped ICV and does include the extra port.

Here’s one that uses the L-shaped ICV, and includes the extra port.
and the same thing, but Original BMW

The following non-return check valve is in-line, different from the 90 degree unit that Ranger linked.


Clean up the connections and then JBWeld the hose barb(s) into the tbody.


Thanks, @Ranger. Any word on a good way to plug the unused ports after the bypass surgery?


Does anyone have a TB disassembled that can measure the size of the port? If not, I will check tonight. Perhaps its close to a size of freeze plug that could be tapped in and RTV’d


For the tbody I just took a bolt about the right size such that I could thread it in. Not sure if metric or SAE. A steel bolt will happily cut it’s own threads in the AL tbody. Slather the bolt’s threads with JBWeld then tighten it into the hole.

For the boot, I think I did the same thing, except using silicone goobage instead of JBWeld. I might have used a small hose clamp too.

Remember that your sealing a vacuum, so these bolts you’re putting in aren’t going to blow out. Therefore as long as you slathered the bolts with stuff and threaded them in, they’re not going to leak.


Measured last evening. The small holes on the back of the TB are 10mm.

Something like this should work N0119103 Freeze Plug (10mm) | Corrado VR6

Throttle body vacuum leaks

hi, I would like to know too. what do you plug the ports on the throttle body with if you do a single vacuum line? Thanks


oh, I was just re-reading it. I think I understand it now. thanks.