CIS and Engine Idleing Problems
 

I have had a persistent problem with my 1979 911SC idling all season around 2,000. This was the first year I had the car back on the road after a top end re-build (with 98mm Max Moritz P&C, 964 cam grind, SSIs, lightened flywheel, and M&K 2 in 2 out exhaust, and supertrapps to quiet it down on the road) I tried playing with the mixture at the fuel distributor and the idle mixture screw. It would work and then the next time I started it would go up again.

I then went back to the basics and looked at the vacuum line schematic. I found that I had two problems. The vacuum line to my WUR from the thermo valve was wrong, it should be the outside line on the thermo valve to the WUR and I believe more important was that the line on the Vacuum control that T's with the thermo valve. This was connected to a low vacuum port on the throttle body. It is my understanding that a low port vacuum is constant and a high port vacuum depends on the throttle. Well as soon as I changed this to a high vacuum port my problems were solved.

I hope this information helps someone else.

dog

This was connected to a low vacuum port on the throttle body. It is my understanding that a low port vacuum is constant and a high port vacuum depends on the throttle. Well as soon as I changed this to a high vacuum port my problems were solved.


the SC uses vacuum advance where the earlier cars used vacuum retard. The vacuum advance comes from above the butterfly and the retard comes from below the butterfly.
Above the butterfly as the engine revs it will suck the advance mechanism to advance spark.
The earlier cars, 2.2, 2.4 and 2.7 use retard of the distributor to retard at idle when the butterfly is closed to meet emissions.
By 1980 the distributor used both advance and retard of the distributor to make acceleration and meet emissions along with the lambda. 1980 to 1983 USA cars have 2 lines to the distributor and its important to have them right.
Bruce

My problem had nothing to do with the distributor and the vacuum advance.

Everyone knows that Flat6pac has a photographic memory. However, he forgets the film on occasion. Roll with it. He is righteous.

when you moved the vacuum line from the WUR, you lowered the control pressure making it run richer.

first, make sure you dont have any air leaks. either have a smoke test done or just replace anything that can cause a vacuum leak.

check your aux air regulator (AAR). make sure it is not stuck and has 12v going to one pin and ground on the other

next check your decel valve. that should be T'eed with the WUR and going to a port BELOW the throttle plate(manifold vacuum). (it should have vacuum at idle, well, all the time until you go WOT). make sure it is closed at idle. it should only open when you let off the gas and vacuum is much higher than normal.
i checked mine with a vacuum hand pump. pull a vacuum and note that the idle increases.

check the thermo time valve (TTV). it should close (no vac) for about 20 seconds when cold and should also have 12v going to it.

now check your control pressures, cold and warm. make sure you have 12v and ground going to the WUR. you should also see the control pressure change when vacuum is removed and applied to the WUR.

check your dist. make sure the mechanical advance is working and timing is set.
vacuum retard goes to a port just barely below the throttle plate. that way retard is removed as soon as you give it gas.

last, have the mixture set.

thats great!!

that goes along with,

i am looking for something, problem is, a cant remember what, so i dont know what to look for.

I'm not sure if we are talking about the same systems.

The vacuum line on the base of my WUR goes straight to the outside port on the Thermo valve as shown in diagram in the Porsche parts catalog. The top vacuum port on the WUR goes to a high port on the throttle body, again as shown in the diagram. Neither of these vacuum lines are T's with the Vacuum Control valve (AKA decel valve). As per the diagram the inside port on my thermo valve is T'd with the top port on the decel valve, the bottom part of the decel valve is a larger vacuum line that does go to a low port on the throttle body.

My distributor only has a vacuum advance that goes to a high port on the throttle body.

one port on the WUR is a vent. it should or can go to something above the throttle plate. it has no effect on performance.

the other port on the WUR is a vacuum enrichment, or, lack of vacuum enrichment. it goes to the TTV. the other port on the TTV should go to the top of the decel valve (DV) where it is T'd and then go to manifold vacuum. its a port in the rear below the throttle plate.

the 2 larger lines on the DV bypass air around the throttle plate to keep the RPM's up when you let off the gas. i would guess the failure mode for the DV is it would not work.
they do hang sometimes though, keeping the RPM's up. mine was doing that, due to too much oil blow-by i think. when i removed my throttle body, a lot of oil came out of the DV.

the auxilary air regulator (AAR) rasies the RPM's when the engine is cold. it can get stuck and not close. check for power and ground on that along with the WUR and TTV.


i understood that you moved the vac line from the TTV that T's with the DV to a port above the throttle plate from one that was below the throttle plate and it ran better. that is not how it is routed.

What actually did I do. Did I reduce vacuum to the WUR by going with a higher port? Does this cause the mixture to be rich when driving?
How did that affect my idle?

I have two diagrams of the Vacuum lines. The one from the Porsche 83 parts catalog (which seems to show my 79 year and not an 83 since it only has one vacuum line from the distributer while 80 and above had two) and it shows the T being attached to the front of the Throttle body. Another from the CIS thread here shows a 1978 vacuum line diagram (it has a "Diverter valve" which I do not have) and the T goes to the back lower port as you say.

Now when I start the car it idles around 1,800 RPM for a few minutes until the car warms up and then drops to 950 or so. When I had the engine apart I checked the Auxiliary air regulator and it closed when electricity was applied. The Auxiliary Air valve I wasn't too sure if it was working properly. I have checked for vacuum leaks and when I take off my oil cap the revs drop so I believe that there are no air leaks.

just when I thought it was running fine! thanks for taking the time. dog

here is one for the 78 (79).

http://forums.pelicanparts.com/uploa...1324579654.gif

here is a pic of the back side with the T from the TTV to the DV and then the throttle body:

http://forums.pelicanparts.com/uploa...1324579747.jpg

changing the vacuum on the WUR changes the control pressure and changes the mixture.

Holy Smokes! That and the others are great answers. Thanks T7911S, I can't wait to review my entire system and your your great explanation :)

But how did a mixture adjustment solve my Idle problem.

This is a picture of the vacuum control (decel valve) the vacuum lines comming from the bottom are on the left is from the T with the thermo valve and the one on the right is the vent from the WUR.http://forums.pelicanparts.com/uploa...1324582130.jpg

sorry the vacuum line on the right is from the T with the thermo and the left is the WUR vent

just look at the pic i posted.
B and C are both below the throttle plate, A is above.
just run the lines like the pic i posted has it. C to the decel valve with a T, then to the TTV (either port) then to the WUR.
check your timing/ advance, check your control pressures then set your mixture.

make sure that the control pressure goes down when you remove vacuum from the WUR. the vacuum circuit is there to make it run richer at WOT, which you need with your mods or you will run too lean.