Firstfives.org

Hello,

I have a 1975 E12 that has issues once the engine is warm. The car will start up right away and runs okay until it gets warm and then it runs rough almost to the point of stalling. If I drive the car, it runs normally until I take my foot off the gas and then it wants to stall (once warm). Looking at forums online I was directed to the aux air valve. I took the part off and tested it with boiling water pictured below. I’m not sure if this aux air valve is causing the problem or if it’s something else? (the "warm" picture is the extent that it closes which is not totally sealed, is that normal or should it close with no gap?) I’m pretty novice so any direction would be appreciated.

https://drive.google.com/drive/folders/ ... sp=sharing

Thanks,
Alex

Welcome to the group!

Unlikely to be the AAV, it just serves to let in additional air when it's cold. It's usually a problem when cold because it's stuck and not letting enough air in for a "fast idle" sort of thing.

I would look for a vacuum leak or check the ignition timing, usually if it's not running right it will want to die when it's cold, not be a problem when it's warmed up. Especially check the hoses going down to and back from that AAV, unless replaced they are pretty old and hard now and prone to cracking.

Mike W. wrote: Mon Oct 13, 2025 12:37 pm Welcome to the group!

Unlikely to be the AAV, it just serves to let in additional air when it's cold. It's usually a problem when cold because it's stuck and not letting enough air in for a "fast idle" sort of thing.

I would look for a vacuum leak or check the ignition timing, usually if it's not running right it will want to die when it's cold, not be a problem when it's warmed up. Especially check the hoses going down to and back from that AAV, unless replaced they are pretty old and hard now and prone to cracking.

Along the lines of what Mike said, early 530i cars had intake plenum leaks, which were sorted out at the factory by welding a joint instead of it being epoxied. However, maybe not all cars had this retrofix

My car had vac leaks at the dipstick and the oilpan gasket that caused roughness like you describe, among other symptoms. I found them after I had verified no leaks in any of the hoses. I used a large-diameter length of shrink tubing around the top of the dipstick. The oilpan gasket leak was solved by tightening to spec - and not inadvertently omitting any of the more hidden bolts like I had previously done.

Yeah, I’m kind of late in replying, but there is another, rather simple reason for engine stall.

Troubleshooting for a friend decades ago, he had a tuneup which included replacing the distributor. The car ran great until it sat in traffic with the engine warm. It would just stall. Had to wait 10 minutes or so before it would start and run.

In THIS CASE, the shop that replaced the distributor forgot to install the air baffle that blocks the hot air from below from HEATING THE DISTRIBUTOR. A quick fix was to double line the bottom of the distributor with crumpled aluminum foil (heat shield).