Simplifying CIS

[pmax]

Originally Posted by OsoMoore

My SC was having leak issues. I eventually pulled the entire CIS (without engine drop) down to the intake headers. I found my hard line in the back, the metal one with the branching, and worn a hole by rubbing against the back of the engine bay.

I patched it with carefully applied JB Weld, and put plastic tube shielding over it to prevent future rubbing.

Not one of Stuttgart's finest moment, the overdesign of this contraption given the system works perfectly fine without the complicated addons.

[PeteKz]

Reliability engineering is a complicated field, but you don't have to study it very deeply to understand the obvious conclusion that simpler = more reliable.

[cmovic]

FWIW, I've been running my '78 911 SC with the same config as PeteKz and the Henry Schmidt engine referenced above. The only caveat is I had a ripped #5 boot too so my idle was wonky. It started and ran fine all summer and I'm happier not having all that plumbing in the way.

-Vic

[pmax]

No AAV, no decel valve.

[Hokie6]

A quick update on my quest to simplify my CIS system. I now have probably less than 100 miles on my engine after some engine work. The AAV and Decel valve have been removed, and the oil tank venting has been routed to the air box. The only issue I've noticed so far is the idle is too high when warm. Placing a restrictor in-line with AAR helps to manage the idle when warm, but kills it when cold. This I wasn't expecting, and the obvious answer would be a bad AAR or air leak elsewhere. I've gone through everything including replacing the rubber boots, injector sleeves, o-rings, etc. There really shouldn't be any air leaks. The AAR seems to be operating normally and mostly closed when warm. I'll go through everything again and maybe perform a smoke test if I feel really motivated.

I'm thinking about moving to an electronic air controller (Volvo unit). Might be a fun Arduino project. Maybe map it to the fuel control pressure or oil temperature. Need to do some more thinking on that -

[pmax]

Is the AAR closing properly?

Did you try turning down the idle ?

[cmovic]

Or a different tack on pmax's question: is your idle set cold or hot? I had mine set to about 1100 rpm hot and had to manage the idle at startup while the WUR ran rich and then leaned out the mix. This involved holding RPMs in the 1500-2000 range for 90 seconds or so. Once my engine warmed up it idled smoothly.

-Vic

[Hokie6]

The AAR is closing (small opening in the plate), and idle screw was bottomed out.

The idle was very fast when warm (hot). I tried installing a restrictor in-line with the AAR, and got the idle set to about 900 rpms when warm, but refused to idle when cold. I had to keep my foot on the gas to keep it running until it warmed up a bit. Next step is to spray carb cleaner around the usual suspects to see if there are any leaks.

[ahh911]

"Placing a restrictor in-line with AAR helps to manage the idle when warm, but kills it when cold."

When warm placing a restrictor in the AAR path should not make a difference, something is up with that path. Check your current through the AAR is as expected when plugged in. (Place an Amp meter in line with the wur in situ, or at least check 12V is present at connector end). Check that both ends of the air hose are tight and aren't leaking along that path. The window in the aar should look completely closed when hot.

[pmax]

Has been a while since I played with the AAR but yes I agree the window should be completely closed.

Maybe the heating element's busted, check the resistance.

[cmovic]

High idle with the screw all the way closed sounds like an air leak. This is how my car behaved with the torn #5 boot. Prior to that the idle screw was open a few turns to get a ~1k rpm warm idle.

Vic

[PeteKz]

Get motivated and do a smoke test.

[SlowSpeedMAN]

Side topic here, but I'm in the process of refreshing my CIS on 1982 911SC. My brake booster venturi plastic piece is in a poor state and the copper tube has pulled out of the throttle body. Can the venturi >- piece be removed and the throttle body hole plugged, then the brake booster vacuum tube connected directly to vac side of manifold? Why does the Brake booster need the venturi?

[Hokie6]

Side topic here, but I'm in the process of refreshing my CIS on 1982 911SC. My brake booster venturi plastic piece is in a poor state and the copper tube has pulled out of the throttle body. Can the venturi >- piece be removed and the throttle body hole plugged, then the brake booster vacuum tube connected directly to vac side of manifold? Why does the Brake booster need the venturi?

The below thread is a good read regarding the venturi, how it works, and consequences of removing it. I gotta admit, I really didn't understand what they had done there, but now it makes some sense -

https://forums.pelicanparts.com/porsche-911-technical-forum/727566-brake-booster-venturi-ok-delete.html

[Hokie6]

I've now had some time to work on the high idle problem that my car has been experiencing, and am no closer to solving this. I performed a smoke test, and there is no sign of any leaks in the system. Spraying carb cleaner around the usual suspects yields no change in the idle. The brake booster and lines are good - they all hold vacuum, so no problems with that system. The AAR was tested and seems to be working fine. I even swapped it with a known good unit with no change. Voltage to the AAR is solid - even put 12V directly to it.

Starting the car cold, the idle is under 1k rpms, but increases as it warms up - to around 1400 - 1500 rpms. AFRs are high 13s to mid 14s. Timing is good, though it seems to be set slightly more retarded than before (?). No retard line on the vacuum unit (retard is bad, advance is good - disconnected for test).

I'm kinda at a dead-end now. The only thing I haven't done is to check the throttle plate position, but that should not have changed. Throttle linkage is adjusted correctly and allowing the throttle to close completely.

This is all after replacing P&Cs (Euro), replacing intake boots, injector sleeves and O-rings, removing AAV and decel valve and backdating oil ventilation system (moving vent from above throttle body to air cleaner). The car runs strong with good power - just idles high. I hope to be able to solve this so I can take advantage of what nice weather we have left, but running out of time. This is really getting frustrating!

[Dave Kost]

The cold start should be ~1100 rpm. Then decrease to ~ 950 when warm.

Does the idle change when you open the oil cap?

How did you set the ignition timing ?

[pmax]

With the leaks e.g. intake boot out of the way, time to measure the CIS fuel pressures.

[Dave Kost]

You could monitor the control pressure from cold start to operating temperature.

[PeteKz]

Hokie: Let's review.
Have you removed any of the CIS warmup parts yet? Which ones?
Was the cold start and warm idle working correctly before you took the engine apart?

[Hokie6]

The engine vent system is now routed to the air cleaner, so no vacuum pull on it, and there should be no change in RPMs when oil cap is removed. Ignition timing was set with timing light - vac advance removed.

Cold and warm fuel pressure are within specs. AFRs are good.

The only parts that were removed are the AAV and decel valve. WUR and AAR remain. AAR is routed directly from above the TB to manifold vacuum below the TB as before. The only difference is the hose is 3/4", whereas the original pipes are 1/2" (?). I can't see where that would make a difference.

Everything was working fine before the changes. Should be a straight-forward modification. There are no vacuum leaks that I have been able to detect. I've run out of ideas as to what else could cause this. I keep wondering if the cam timing could be off, but not sure this would cause these symptoms. The car runs strong, just idles too high -