'86 930 Idle Surging

[dos531]

Once it gets warm the idle starts to surge between 1500 rpm and dropping down to around 500 rpm. I hear a noticeable click right before the idle drops and right as it catches at the bottom. Are there any solenoids etc that control idle on this car? The AAR, AAV, and Decel valve are all removed and plugged.

Mods are gt35r, tial wastegate, Leask WUR with rpm enrichment, Mahle 3.4 pistons & cylinders, etc.

It seems a previous owner was having the same issue and apparently never resolved it before selling the car to the owner I purchased it from. Here is the thread from before. He didnt mention anything about the click, which seems to me like something is opening and closing in relation to the surge.
Inconsistent idle surge

I have attached a video of what it is doing. Ideas?
Porsche 930 idle surge - YouTube

[Robbbyg]

That is strange,
Ive had the clicking sound before but it went away after i pulled the top off a few times.

I took decel and cold start off my engine, i had a clicking sound but it went away and it seems fine now i think my car got used to them not being there anymore somehow.

I have no idea what could cause the surging though,

I could assume its not electrical

That leaves Injection and mechanical i think.

Mechanically it sounds -good (to me) idle is smooth if you take out the surging

So maybe something with injection ...maybe AIR and fuel head area? wur? (whatever Wur means thought id mention it because it seems its always a problem with some cars)

Fuel delivery?
Vacuum leak then vacuum sealing? is that possible?

Hopefully one of the Jedis here can help?

[dos531]

I am thinking it is a fuel issue. I smoke tested the motor and found no vacuum leaks so it's not that. Once I get my wideband back I will start tweaking the fueling.

[Robbbyg]

Have you seen this??

Porsche surging idle repair-How to--HD - YouTube

[Ronnie's.930]

The clicking is the diverter valve pistons hitting the end caps because the wax-like washers/bumpers that go on the inside of the caps have degraded and fallen apart. That is also likely an air/vacuum leak that is contributing to the surging idle - info in the linked thread.

Changing 930 dump-gaskets, pictures galore

Is the lambda system hooked up on your car (O2 sensor hooked up, too)? I ask because if it is, the surging idle is likely due to an overly rich idle air/fuel ratio and the lambda computer is trying to correct it back to the pre-set high 14s afr. The above mentioned air leak can be a cause of the rich idle mixture - would need to fix that, and any other air leaks, before adjusting the fuel head idle mixture screw.

And no, there are no solenoids, or any other idle control devices on the 930 fuel system.

[dos531]

Yeah I saw that but the fuel distributor has already been inspected.

[dos531]

No o2 sensor on the car. Actually after inspecting it during surging I had a suspicion the sound was from the diverter assembly, but I thought it might be normal function. I have a long neck intercooler coming so that will be gone soon. Hopefully that resolves it!

[dos531]

Update: Yesterday I installed a long neck intercooler, thereby deleting the factory diverter valve and the problem is GONE. As soon as I started it up after installing the new intercooler and bov setup it would barely idle. I had to turn the idle screw out 3 turns or so before it would idle and now it is rock steady at 950rpm and runs better than ever. Diverter valve was definitely malfunctioning.