M20 (Jetronic) What is this vacuum port meant to be connected to?

[detroit 4]

So I've had my e30 for almost a year now, It's finally time I try to address the high idle (1000-1200rpm, when warm in neutral).

I have replaced the obvious stuff (Intake boot and a visibly cracked hose). But I noticed when doing my timing belt and water pump that on the distributor there was a vacuum port with no line connected to it. I'm guessing to advance the distributor timing. Idk what took me so long, but I'm finally going to plumb that back up, I looked on realOEM and found the section I'm looking at but cannot figure out what the vacuum line goes to. I haven't taken the cold side off to have a proper look but it's not immediately obvious.

Here's the diagram. If you look at where #15 connects to the distributor, I believe that's what is in my picture. It goes to #12 but #12 isn't listed on realOEM, so I have no idea what it is. So, What is #12 and where in the engine bay would I find it?

 

Here's my image:

 

My vacuum lines aren't quite routed the same, and it doesn't appear that I have the second vacuum port on the distrubutor (shown in the realOEM diagram), but I haven't looked closely either.

Also, for the jetronic experts out there. Is it likely that this leak could cause this big of a jump in idle revs? I could always just bite the bullet and pay for it to be smoke tested.

As always, thanks for the help! You guys have saved my ass once before.

[B.M.W Ltd 950]

From memory the diagram is showing two systems !2 & 16 are both dampers. It can have either/or. 16 is a plastic damper and 12 is a heavier mounted one. I could be wrong. Long time since I worked on these

[E30 325i Rag-Top 2959]

Great memory Glenn, I would be positive that is the case, that is why some of the identification numbers in the parts drawing are "greyed out", there were few iterations of L-Jet, the big metal damper was on the early version, then switched to the later plastic one.

If you have the metal one, it is normally bolted to a bracket on that side of the engine bay, possibly off the oil dip-stick bracket?

It is more likely to give an issue at high revs I would think as I believe this vacuum is used to advance the spark timing? More revs = more vacuum = more advance on the ignition IIRC.

In terms of the idle problem, have you got the wax-stat cold start thing-ummy bobby under the throttle body? These are very temperamental at the best of times and often cause issues with the idle.

[detroit 4]

3 minutes ago, E30 325i Rag-Top said:

In terms of the idle problem, have you got the wax-stat cold start thing-ummy bobby under the throttle body? These are very temperamental at the best of times and often cause issues with the idle.

You talking about the Air Slide Valve? If Yes, I cleaned it and bare bones tested it when I did the timing belt. I tested it by placing the coolant side in hot water and there was movement in the air or vacuum side. That being said, I do have a used spare in somewhat unknown condition. I bought it just in case but haven't put that in yet. 

If you believe that to be more likely of the culprit, I could switch it out.

Thanks for the help though, I'll have a better look for the damper.

[B.M.W Ltd 950]

The water Inlet & outlet fittings on the hose ends often get blocked too

[detroit 4]

12 minutes ago, B.M.W Ltd said:

The water Inlet & outlet fittings on the hose ends often get blocked too

I'll be sure to check this too then.

Thank you.

Edited June 14, 2020 by detroit

[detroit 4]

Ok I'm going to update this, just in case someone else has the same issue at some point :).

After posting here I did a bit more digging and came across a thread on another forum. The poster of that thread had a different issue, but a high idle still, they had the exact same symptom I had (there was no line connected to the same port of the distributor) they also had the same exact cap over the same exact same port on the throttle body that I had and they too, couldn't see that valve that is mentioned earlier in the thread. It was strange, it was like looking at my car but from the UK. Anyways, this is what they did and I subsequently did too.

Basically they just plumbed the capped port on the throttle body to the port on the distributor, therefore bypassing the "valve" that is supposedly there. I took the car for a test drive and the idle is significantly better, if not fixed. It seems to idle at around 800 when warm, but (I think) less when cold, around 600-700, which is sort of the opposite of what it says should happen? Even if that's not typical I can't see how a lower cold idle would cause issues? I will be double checking this tomorrow and seeing if I notice any other changes, especially in the higher revs now that the distributor timing should be able to advance.

But if any of you jetronic experts would like to chime in and let me know if not having that valve is dangerous please do, if not, I'll continue to run this!

 

Thanks for all the suggestions and help!

Edited June 16, 2020 by detroit

[B.M.W Ltd 950]

The valve is there to stop sudden, violent vacuum advance / retard operation in the distributor causing timing and points bounce issues

Edited June 17, 2020 by B.M.W Ltd

[detroit 4]

4 hours ago, B.M.W Ltd said:

The valve is there to stop sudden, violent vacuum advance / retard operation in the distributor causing timing and points bounce issues

Hmmm, ok. That sounds a bit more vital that I initially thought. I'll look into getting a second hand one then. Do you think it's dangerous to run without one in the mean time? or at all?

Thanks for the help!

I took it for more of a drive today, and noticed the car seemed to be pretty much normal, but may be running a bit less rich. Before it was quite rich and smelly, now it isn't, like at all. It also seems to have a tiny bit more pep, and maybe even a slightly more dramatic deceleration (in terms of power, very subtle changes, that could be my imagination). As for the idle. It's so nice to have a regular idle. Despite my previous post saying otherwise it does idle at around 600-700 for maybe 30 seconds right on cold start, then kicks up to 1k, then down to 800 when warm.

[E30 325i Rag-Top 2959]

Yes, plumbing in that vacuum should give you a bit better performance as it will be advancing the ignition timing as the revs increase as is intended on these older "mechanical" type distributors.

It won't be heaps of improvement, but possibly enough to notice when driving and not your imagination.