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Rob the plumber · Joined: 25 Aug 2008 Posts: 122 Location: Utica, Michigan

Why would a 931 run rough when it is first started? On a cold start my car started running worse. It always had a bit of a stumble if I accelerated too quickly after a cold start. Yesterday it seems to have gotten worse. It idles around 500 rpm and barely wants to run. Once it warms up it is fine.

I also had the CIS removed yesterday in an attempt to install an intercooler. I ended up re-installing all the stock stuff, because of some fitment issues. Everything went back together fine and I don't believe that I left any vacuum lines disconnected.

I am still quite a noob to the mechanics of the Bosch CIS system, so I don't even know where to check. My first guess would be whatever I touched last, and next would be an issue with the WUR?

Thanks!
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1981 931

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Low idle on startup could be AAV, vacuum leak, WUR. How does the exhasut smell when it first fires up?

And what intercooler setup were you playing with?
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Rob the plumber · Joined: 25 Aug 2008 Posts: 122 Location: Utica, Michigan

What is the AAV? I will go out this morning and see if the exhaust smells unusual. I was playing with a sidemount set-up that I bought from a fellow board member. I think it won't fit with my stock 931 fenders though. Simply not enough room. I think I am going to relocate the battery to the hatch, and mount the intercooler in the factory battery location. The negative pressure at the base of the windshield should be better than behind the headlight anyway.
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1981 931

Rob the plumber · Joined: 25 Aug 2008 Posts: 122 Location: Utica, Michigan

I rechecked my work and found I left a clamp loose on the bellows that connects the CIS to the inlet of the turbo.
False air/vacuum leak making it run like crap. I imagine that the rubber must have sealed better when it warmed up or something.

Runs like a top.
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1981 931

Smoothie · Posted: Mon Sep 29, 2008 2:29 am Post subject:

AAV is auxiliary air valve - it's on top of the intake manifold, behind the charge tube.

Careful there...the cowl area at the base of the windshield is actually a high pressure area. That's what makes vent systems on cars work even with the fan shut off. -And cowl induction systems. In the following diagram, the base of the windshield (20 to 21) is +0.3 to +0.4 (below the 0 line is +pressure, above 0 is -pressure (or as I sometimes say, "vacuum")).

Rob the plumber · Joined: 25 Aug 2008 Posts: 122 Location: Utica, Michigan

So, at the base of the windshield, the air is leavin the engine bay, and escaping up the cowl area?
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1981 931

Smoothie · Posted: Mon Sep 29, 2008 3:07 am Post subject:

No, you should have a rubber seal along the top of the firewall that seals between firewall and hood.
The +pressure at the cowl area happens because air is rushing over the hood, then hits the obstruction of the more-upward sloped windshield. It needs to go somewhere, but before it does, it's under pressure from the base of the windshield until more than halfway up the windshield. -So the cowl area air is under pressure - obviously most of the air goes up over the windshield (and spikes to a -pressure at point 27 just past the top of the windshield where the surface contour changes direction back down again), but some air will flow down and through the cowl area if given a passage such as the hvac intake.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Rob the plumber · Joined: 25 Aug 2008 Posts: 122 Location: Utica, Michigan

I would assume that an intercooler mounted in the battery tray would get decent airflow. Like cowl induction? Or is the problem that the air has nowhere to vent to when it pressurizes the cowl area?
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1981 931

tuurbo · Posted: Mon Sep 29, 2008 4:32 am Post subject:

What's the hvac intake?
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1980 924 turbo, MSD, Meth. Inj, otherwise stock.

Paul · Posted: Mon Sep 29, 2008 4:56 am Post subject:

Heating, Ventilation, and A/C air intake.

http://en.wikipedia.org/wiki/HVAC
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

HERE is an example of making that location work for an air/air unit.

HERE is an air/water unit in that location.
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

tuurbo · Posted: Mon Sep 29, 2008 5:52 am Post subject:

Thank you!

Hey that diagram is great for effects on the surface of the car.

Our four nose vents are at position 9 then (.45)? How much air is actually making it past the next four vents between the engine bay and the nose, where the hood latch is located? Would it be very much? Or is it just to vent the engine bay and not much?
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1980 924 turbo, MSD, Meth. Inj, otherwise stock.

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

If you do a search over at Autospeed.com on the term "magnahelic", it'll bring back a bunch of articles that discuss how to use an inexpensive magnahelic gauge to perform just such an assessment. Try it:
http://autospeed.com/cms/search/index.html?keywords=magnehelic&x=0&y=0
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erstwhile owner of just about every 924 variant ever made