Anyone know CIS on a 79SC?

IMR-Merlin · 07-11-2015, 03:16 PM

Just got the valves done on the SC as well as full tune up.

This car had been sitting for a while and it was not running. I am still chasing some electrical issues, but I have a weird idle issue that I am trying to figure out.

When you cold start it, the idle sits around 2k. It will stay there even after the engine is warm 180 oil temp.

I can turn the idle screw all the way in and the car will not go below 1900. If I let the car sit after warm up, I can then go start the car and it will idle fine or even real low with the idle screw in the same position. I actually have it stall a few times and have to back the idle screw out a little.

When it is idling correctly, I set timing, hoping that it was just too advanced, but this is not the issue.

My question is how does the warm up regulator affect the system? What about idle control? I am assuming this is the valve that is on the passenger side of the engine between the #4 and #5 intake runners.
(I have this disconnected and the issue doesn't change)

I have looked for vaccum leaks and with the exception of one injector oring that is worn, I haven't been able to find anything. Although i don't think vacuum is the issue because it changes after heat soaking.

T77911S · 07-11-2015, 05:05 PM

the warm up regulator (WUR) lowers the control pressure when the engine is cold to richen the mixture for cold starting/running. there is an electrical plug it that heats it up to raise the control pressure (CP) as it warms. there is also a vacuum control to the WUR that has 2 purposes. there is a thermo time valve (TTV) that blocks the vacumm to further lower the CP when cold. it is only active for about 20 sec. the other purpse is for WOT enrichment. this vac control drops the CP about .8 bar.

the aux air regulator (AAR) by todays standards is not a regualtor. it too is a very crude device that just opens when cold to let more air in to raise the idle. it has the same power that the WUR has to heat it up. then it closes and lowers the idle then serves no other purpose.

you also have an aux air valve (AAV). it only opens when starting. it is just like putting your foot on the gas to give it air. as soon as the engine starts vacuum closes it.

you have a decel valve (DV) this only opens when you let off the gas. it holds the RPM's up before letting them comne down.

make sure you dont have air leaks.
check your timing per the manual. also check the mechanical advance by itself, check the vacuum advance by itself. you can do that at idle by connecting it to a port that has idle vacuum. make sure the vac pot for the advance is connected to the correct port.

you need to check control pressures. cold, warm and system.
check the vacuum enrichment on the WUR. remove the vac line at idle and see if the CP goes down.

check the plumbing for all this crap. AAV,AAR and DV. it really helps to have a hand vacuum pump to check all this stuff. you also want to pull a vacuum on the brake booster line and booster to check for vac leak.

the DV could be hanging open. the AAV could be staying open,
if you are turning the idle screw all the way in it could be an air leak.
the idle screw is the large one

tabasco · 07-11-2015, 05:25 PM

Lots of troubleshooting info on this site : 911 CIS Primer - Index
but you probably know it already.

I would check the AAR, or at least do a quick test with the AAR plugged.
The intermittent idle you describe could match a stuck AAR.
It sounds like too much air, bypassing the throttle plate.
Check the vacuum drawing on the Jim's website and try to plug the devices / lines you want to remove from the equation.

The WUR changes the mixture, making it leaner as the engine warms up (by raising the control pressure).
If you have a CIS pressure gauge you can check if it's working. But it does not sound like it's your problem.

I suggest moving this thread to the 911 forum, you'd get more (and better !) answers.

IMR-Merlin · 07-11-2015, 05:28 PM

I am just wondering why the idle will return to "normal" after I let it warm up and sit not running for 15 minutes. I think I can rule out vacuum leaks for that. Although, I did check for leaks with carb clean and found one small leak on an injector o-ring.

I am thinking it is either the DV or AAR sticking open. It happens first start of the day and lasts until I shut the car off and let the engine heat soak for a few minutes.

IMR-Merlin · 07-11-2015, 05:35 PM

Looks like there is some good info in here for me to play with tomorrow.

Engine Fails to Start

Thanks!!

RarlyL8 · RarlyL8's Garage

It sounds like the thermol time switch or AAR electronics are sluggish.
To test put a clamp on the large vac line going into or coming out of the AAR while the car is running with the high idle. If the AAR is the problem the idle will drop when the vac is cut off.

T77911S · 07-11-2015, 07:41 PM

he said he had the AAR disconnected.

are you talking about the electrical connector

Alan L · Alan L's Garage

You can look into the AAR to see if it is working - with a mirror - just pull the hose off. The disc should be open partly when cold, and fully shut off when hot. Sometimes they get gunked up with the oil vapour etc circulating in the engine. I must say I had a problem like this with my SC - was OK cold. But on hot start would sit at high revs (2k) until driven. Weird. It finally stopped doing it all on it's own. It is possible heat soak into the AAR may finally close it for your hot starts. When you turn the key on from cold, you should be able to see it slowly close, solely from the bimetallic strip heating up. Maybe a broken internal wire and only working on heat soak?
Alan

RarlyL8 · RarlyL8's Garage

AAV just about has to be connected to get these symptoms, if not the idle would not return to normal after heat soak. None of the other devices will throw the idle RPM up that far.

Quote:

Maybe a broken internal wire and only working on heat soak?

Something like that is what I was alluding to. With age and heat the electronics become more and more sluggish, relying on engine heat soak to perform the function rather than electrical heat. Pinch it off an you'll know immediately.

IMR-Merlin · 07-12-2015, 04:15 AM

Quote:

Originally Posted by T77911S

he said he had the AAR disconnected.

are you talking about the electrical connector

Unplugged. Maybe it's physically stuck internally?

RarlyL8 · RarlyL8's Garage

Unplugged? If it is not getting electricity it will not close until the heat soak closes it.

T77911S · 07-12-2015, 07:36 AM

Quote:

Originally Posted by RarlyL8

Unplugged? If it is not getting electricity it will not close until the heat soak closes it.

that would take a long time for it to close.

did he have the same problem with the AAR connected?
if so, i would check power to the AAR and see if it is actually closing