hi,
i'm just new here in the board,but i have been working on a similar (or maybe the same) problem for six weeks on a 3.0 l engine.
@ohecht: there are two different types of deceleration valves: a small one right around the throttlle valve (seems to be used more often) and another type, looking like the auxiliary air valve with an aditional vacuum connector, mounted in the anterior vacuum section with the AAV and the AAD (quite seldom); both shown in the pelican part diagrams k-jetronic 911SC 930 (page 1 and 2).
there was a remark about opening the oil cap: is there a remarkable overpressure in the oil tank? if so, that may be suspicious for a leaking or broken piston ring blowing oil in the intake tract and enriching the mixture when engine is hot.
in my car , 3.0 l running with spare part catalyst, oxygen sensor and a volkswagen control unit, there is a similar (but very well tolerable) behavior:
idle of about 1200 after starting cold, dropping at 900 when getting warm, after restarting warm engine idle of about 600, getting up to 900 after about a minute.
i think, that the mixture control system is not perfectly adjusted yet; finally there are no vacuum leaks after dis- and reassembling the intake system several times (as described in an earlier posting).
checking electric system could be another hint.

with regards
pomaro

Any luck in solving your problem? My 82 SC started the same thing last week.

I adjusting the CO to spec, rebuilt the WUR and adjusted the fuel pressure to spec, adjusted the valves and ignition timing, replaced the plugs, rotor and distributor cap.

Now, when I start the car up from cold, it starts perfectly, warms up and seems fine, then after a few minutes, it seems that something warms up and the revs race, the idle runs up around 2,000 RPM.

I have sprayed ether around the engine looking for a vacuum leak, and found just the slightest sensativity on one vacuum fitting. I have verified that it is not the auxiliary air valve, the one that lets more air past the throttle to boost the cold idle speed. and the little thing next to it that keeps the revs from dropping too fast. I checked both of those by opening the hoses and plugging them with rubber stoppers. No change.

I also removed the copper tube that runs from the throttle body to the bottom part of the airbox, under the heater blower motor, (It may be some sort of vacuum limiter) and plugged it with rubber stoppers too. The car starts fine and then races after it warms up.

Pomaro, I hope your theory about the piston rings is wrong! I did a compression test a few months ago and they were all fine, but that does seem possible. Is there another way to check if oil is getting into the intake path? The car does not really use any amount of oil, maybe half a quart every 3 tankfuls of gas. There has been oil residue every time I've cleaned the throttle body, but nothing unusual for the amount of time between cleanings (I think).

I have the later-style decel valve that looks like an AAV. I pulled it and checked it, it opens at 20" of vacuum, which is around what the engine is pulling at idle. I have since disconnected it and plugged the connection at the other end, and the problem is still there, but definitely tolerable. It starts cold at around 1600 rpms, then drops to 1000 rpms in a minute or so. It will idle there if I just let it sit, but it will drop to about 7-800 if I drive it, and then rise back to 1000 at operating temperature. If it goes above about 200 degrees, the idle will rise to about 11 or 1200. I want to use a gas tester when it is hot and idling high to see what the mixture is at the tailpipe then.

SC-Targa, I hope you find your problem. I have found a few small leaks, and broken AAR wire, and other issues that I thought would solve it, but I have been disappointed every time so far.

I really hope it's not the rings! Could it be if it is using no additional oil, pulls strong, and blows no smoke?

Olivier

I've nearly resolved my problem. It was operator error. When I set the timing, I pulled the red vacuum line, but I didn't see the blue one. (It was dark and late) My timing was way advanced. Now that I have the timing correct, it's idling fine.

I still had to close off the idle screw, but I think I've isolated the problem. Does anyone know what the copper U shaped tube is that runs from the throttle body, under the heater blower and back into the base of the airbox? Is there a restriction in this to keep it from pulling too much air around the throttle? When I pulg this, I can adjust the idle with the idle screw, when I connect it, I have to completely close my idle screw.

Regards,

Jerry

I am not sure what that tube is for. I will have to re-check my timing as well.

Olivier

I rechecked the timing, and advanced it a few degrees (to about 10 BTDC at idle with the vacuum lines disconnected and plugged) to see if that has any effect.

I did notice that the timing seemed to "vibrate" above about 3000 rpm, making it hard to read the exact amount of centifugal advance. Is that normal (or typical), or could a spring or something in the distributor be weak or broken?

The timing did not seem to affect anything on the test drive, but I will monitor it over a few days.

hi,

hope i didn't you shock too much with the piston rings....a little amount of oil (a little bit of moisture) in the air cleaner housing is normal, if a piston ring is leaking, there is a heavy overpressure in the oil tank and in the decompression line to the housing at the engine running..... i think it to be o.k. in your engine (when pulling well, no excessive oil consumption and no smoke, as you wrote).
plugging the decel valve is not a very well solution, because the engine refuses (with otherwise correct settings) to take gas (or even dies) after idling down a hill or decelerating from high rpm's , and you have to 'reset' it with turning off the ignition for a moment at open throttle to achieve a combustible mixture again.
if there is no 'hanging', you're running the engine far too lean. i'd really check the emissions. unfortunately i can't tell you the specs of the aav and the decel valve, even at 'porsche zentrum' and bosch service (usually those both are made by bosch) there was no information available. what i know is that they are quite expensive as spare parts. maybe a check of the electrics (plugs, cables, co sensor, control unit) could be useful, too. i've heard from problems like this caused by bad soldering spots in the control unit - that was a 928, but anyway.....but first clean up the vaccum system.
when setting up my engine the first time (after engine crash by a burned-out piston), there was exact the same behavior with too high idle; after dis- and reassembling the intake tract several times (for some other reason) and an adjusting of the emissions it went finally o.k. (not completely perfect yet, but well driveable); i think i have to afford a professional adjustment :-)

good luck
pomaro


p.s.: read your posting about timing an distributor:
'jumping' checkpoints could be caused by a worn out driving gear of the distributor, a loose shaft (worn out bearings) or maybe simply a lack of lubrication... check the vaccum device too, sometimes they leak, and then there is no correct timing towards 'early'. the specs for adjustment for 3.0 SC USA are 5 degrees after 0 with both vaccum lines on at 950 rpm idle. Should alternate to i think 35 degrees before 0 at 3000 (without 'jumping'!).