Aux Air Valve FAQ

Peter Florance, Robert Bondi, and Sean (Kaveman) Steinman

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The valve is opened when cold to bypass extra air around the throttle plate to keep the idle up to spec while the oil is thicker. It's action is very similar to fast idle solenoid on a carb. Note: the aux air valve does not effect the fuel injection air/fuel ratio. It is a throttle type device only. Any additional air that enters the engine through the aux air valve is measured by the air flow meter. The fuel injection adds the extra fuel to keep the mixture correct for the increased air volume.

As the engine warms and loosens up, the valves are heated (both types) and slowly close. At full warm operation both types of valves (79 and earlier and 80-81 types) should be fully closed.


The aux air valve is located on the valve cover on 80-81 models next to AC compensation valve. Labeled below. The valve closes on rising temperature which it senses from the valve cover. It also has a small heater inside to help it warm up. There is a small nut on the side that can be loosened and then moved to adjust the valve. You might try that before replacing it. If the hoses are brittle (mine were like plastic) you may consider replacing them.

Adjustment: Before starting the car, loosen the 7mm nut on the side and pry or slide the nut towards the outside of the valve to increase the cold idle. Tighten the nut and check the idle real quick. Not high enough, keep moving. Too high; move back half way and try again. Don't keep the ignition on long as the valve is heating up and trying to close.

If you can't get a change in the cold idle speed, it's probably stuck ,WD40 may free it up; but be ready to replace it.

Here's the location of the aux air valve and AC compensation valve on 80-81 528i's. Note the two black hoses on the aux air valve that make up the throttle bypass path through the valve.

80-81 Valve cover area showing aux air valve and AC compensation valve.
Photo by Craig Peck of Karl Koenig's 528i w/ e34 3.5L

79's and earlier aux air valves are coolant temperature actuated (close on rising coolant temperature) and are mounted on the coolant pipe below intake runners. Much less reliable and not adjustable. It can stick and sometime be freed up with 'Carb and Intake Cleaner' sprayed in the air side. This is the device recently referred to as the "Jules Verne Device". Not sure why. Anyway, there it is below.

530i and 528i (79 and earlier) aux air valve shown on coolant pipe.
Photo by Sean Steinman
530i and 528i (79 and earlier) aux air valve out of the car 
Photo by Sean Steinman

Make Your Own Custom 3-Port Vacuum Hose for 1980/1981 Systems [Robert Bondi]

It is a subject of some debate, but BMW does not seem to manufacture vacuum hose 11-61-1-271-604 any more.  This procedure is not easier or cheaper than buying the hose new from BMW, but a viable alternative if you need a new hose.  In my opinion, a rubber hose is one of the last things you want to buy used.  You might find one new on somebody's shelf, but for those less fortunate, you can make your own with some fittings, 3 segments of hose or tubing, and a few extra hose clamps.  I will describe this procedure and list the parts you will need.

Some tools and supplies you will find useful include a bench grind stone, a vise, sharp knife, electric drill, a 1/8" NPT tap, and some Permatex Ultra Black silicone (sensor safe).  Most of the fittings can be found at a good hardware store, but the 45 degree barb might be tougher.  It came from .

I personally despise American units, but pipe thread fittings are probably the easiest to work with and find.  The good news for metric lovers is that once you construct the fitting assembly, you won't be taking it apart often and you can probably escape with adjustable wrenches.  All fittings used are brass.  Brass provides more than enough strength and temperature range for this application, but is still a reasonably soft material for easy grinding, drilling, and tapping.

I gave this a lot of thought before I started.  Initially, it seemed like just a tee and some hose barbs, but it got a bit more complicated.  I decided a standard tee wasn't going to cut it for the confined volume we have to work in and some grinding was also needed to shrink some of the barbs.  I also gave some thought to the hose selection.  It is much easier to find hose or tubing that will stand up to positive pressure, in contrast to vacuum.  Failure under positive pressure requires wall rupture or blowing off a connection, while vacuum failure simply requires collapse.  I consider the AC compensation valve hose connection the weakest in my design.  It's a longer segment and will need to operate under warm to hot conditions, which will soften the hose.  Constructed in January, I have not been able to test it under real heat.  Tables 1 and 2 list the major parts you will need for my design.

Table 1.  Fitting Assembly Part List        [Robert Bondi]
Part Description
 tee 3/8"x3/8"x3/8" FPT; could use a 90 deg elbow and drop the plug.  I recommend one with square outer cross-section; this will simplify drilling, tapping, and holding in a vise.
plug 3/8" MPT
barb 3/8" MPT & 1/2" barb (to throttle body)
barb 3/8" MPT & 5/8" barb (to aux air valve)
45 degree barb 1/8" MPT & 3/8" barb (to AC valve)


Table 2.  Hose Connections from Fitting Assembly       [Robert Bondi]
Connection Description
AC compensation valve 3/8" ID fuel hose
throttle body port 1/2" ID silicone tubing - This stuff is very flexible and subject to collapse 
under vacuum; however, we will escape by butting the barb right up to the throttle body port.  This is the toughest and shortest connection; you will appreciate the flexibility.
I purchased this tubing from .
aux air valve As part of a lambda conversion, I found that one of the 530i hoses has a nice elbow I could use.  The hose is 11-61-1-261-816 and appears available on the parts CD.  I'm sure other hoses of the same ID can be used.  After coarse cutting of the hose with a knife, I used the grind stone to get just the right shape. 

Figure 1.  The hose we wish to emulate (left) and the fittings I used (right).  [Robert Bondi]

Procedure:  First, you will drill and tap the tee (or elbow) with a 1/8" NPT tap as shown in Figure 4.  We wish to make a fitting with ports on 3 independent axes.  Next, do a bunch of barb grinding on the 3/8" MPT / 1/2" barb.  I reduced the barb length at least 50% on my part, but the length of the barbs may vary somewhat.  I also ground down the tee branch
facing the throttle body a little, but this doesn't accomplish much in the end.  Instead of compacting the final structure, you just leave more pipe threads exposed on the barbs, so don't bother.  I believe this is a consequence of the tapered threads.  I didn't grind the 45 degree barb at all and I only ground a little on the end of the 3/8" MPT / 5/8" barb to round the end more from the original blunt design.  This was just to facilitate insertion into the vacuum hose elbow.  Now, prepare for assembly by washing your parts to remove all metal filings.  Figures 2 through 4 should make assembly self-explanatory.  The heavily-shortened 1/2" barb goes to the throttle body, while the 5/8" barb goes to the aux air valve.  I used a little Ultra Black silicone on all threads to help insure a good seal.

Now, use just need to cut the three hose segments as listed in Table 2.  For the silicone tubing and fuel hose segments, I just held the fitting assembly in place and estimated my cuts.   Neither segment is more than a couple inches.  Size the silicone tubing segment first by butting the barb right up to the throttle body to estimate your cut.  For the 11-61-1-261-816 vacuum hose, make a coarse cut with extra length.  Then, grind the ends to the precision length and angles you want.  It surprised me, but the material grinds decently; my fuel hose did not, probably because of the embedded braiding.   My silicone tubing segment was translucent, but I coated it with Ultra Black for a more stock appearance.

Finally, collect 6 hose clamps (12-20 mm range is a good size for all of them), your 3 hose segments,  the fitting assembly, and put it together.  Having already removed this hose assembly a couple of times, I find it easiest to leave the silicone tubing segment on the throttle body, so stick it on the throttle body port first.  I leave the other two hose segments attached to the fit assembly for removal.

Figure 2. View 1 of Completed Hose Assembly  [Robert Bondi]

Figure 3.  View 2 of Completed Hose Assembly  [Robert Bondi]

Figure 4.  View 3 of Completed Hose Assembly [Robert Bondi]

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