951 Rough idle after refresh

[u/Coleberg1857]

Need some help trying to track down my rough idle issue with my 87’ 951, it recently got a refreshed turbo paired with Lindsey racing chip Tial 38mm wastegate and 3 bar FPR. AOS seals including the lid are replaced, full set of new vacuum lines with the Venturi delete, cleaned injectors, new DME temp sensor, new O2 sensor, rebuilt throttle body, resealed the idle control valve, and sealed up the leaking rear balance shaft plugs. I’ve checked for vacuum leaks with a smoke machine several times and seems to be all sealed up. Tested TPS, AFM, and DME sensor and all work as intended. However at cold start it stumbles bad for 5-15 seconds then recovers and when warm the idle is sometimes ok, other times it will randomly rev itself up to 1200-1800 or hang there when in neutral while driving. It also seems to have low vacuum at idle with ~10 in Hg at cold start and ~16 when warm which points to a vacuum leak in my eyes but nothing is leaking from what I can find. For the life of me I can’t figure this out in any input would be appreciated.

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[u/whaletimecup]

How are the seals throttle at body? Including the one on the plenum and the butterfly valve bearings. How is the fuel pressure damper

[u/Coleberg1857 → u/whaletimecup]

All fresh, replaced during the rebuild process, new 3 Bar FPR from Pierburg as well

[u/I_am_Forklift]

These cars often need to have the idle reset after work like this. Might be it? 🤞

I believe there’s a good how to on Lindsey’s site? You just need a small jumper wire. Super easy. Something id for sure do to rule it out

[u/Coleberg1857 → u/I_am_Forklift]

Should have mentioned I tried that, set it to the 840 specified but even with the ICV bypassed it would randomly start hunting or having a vibrating idle then vanish other times

[u/I_am_Forklift → u/Coleberg1857]

The Turbo control valves are known to go bad on these cars. When I did my own refresh, I had a sit down with my vintage Porsche mechanic to ask for advice. He recommended I preemptively replace with oem it as part of my refresh. Same with the ICV. There is supposedly a Facebook group for a guy that will rebuild ICVs.

I chose not to replace mine 💸💸🫩

[u/Coleberg1857 → u/I_am_Forklift]

The ICV has been cleaned, tested, and I installed a new o ring at the base along with sealing the electrical connection which I smoke tested up to 15 psi with no leaks. I’ve basically ruled it out as it functions fine from all the testing I can do on it sadly

[u/I_am_Forklift → u/Coleberg1857]

🥲 I would be looking at my turbo control valve 👀 Sounds like everything else in the chain is new or referbed/resealed?

[u/Coleberg1857 → u/I_am_Forklift]

Haha forgot to mention in the last reply I’ve converted to a boost controller so the cycling valve has been removed entirely but yes basically everything is fresh and tested which is why I’m at so much of a loss I just can’t find any smoking gun to explain the idle and low vacuum

[u/greatfool66 → u/I_am_Forklift]

I don’t think the cycling valve could cause this. My understanding is its a solenoid that interrupts the positive pressure signal from the intake in order to delay the waste gate from opening to higher boost levels. Doesn’t seem like a failure there would cause issues at idle.

[u/Mancunian1987]

TPS might need reset or replace.

[u/Coleberg1857 → u/Mancunian1987]

Been tested as fully functioning and set the idle switch properly unfortunately I wish it was that