Hladun1

In the fall of 2000 I purchased a 1978 911SC Coupe (number 76 of the SC’s). Last winter, when cold and idling, my car began surging to over 2,000 rpm. I live in Calgary, Alberta in Canada where winter temperatures can reach -25C in the winter. I use my car as a daily driver so it’s a problem I had to cure. The surging is hard on the engine and very embarrassing to be sitting next to someone at a stoplight with your engine revving up and down. In checking the literature I discovered that this is not an uncommon problem for CIS equipped cars. I have successfully solved this problem and, as an added bonus, my car never backfires at startup (which is another common CIS fault).

The problem is that the 911SC engine has an auxiliary air regulator (warm up regulator) plumbed in parallel with an auxiliary air valve. The auxiliary air valve is a round device at the back of the right hand side of the engine. Its function is to supply additional air during starting and manifold vacuum should close it once the engine starts. Presumably, it is to help hot starting since when the engine is cold the auxiliary air regulator is also open and it provides an air bypass. Now when the engine is older and cold (auxiliary air regulator is wide open), it cannot develop enough manifold vacuum at idle to hold the auxiliary air valve closed so this valve opens due to its internal spring. This is the same as stepping on the accelerator. The engine speed surges, it develops more manifold vacuum and the valve closes. When the valve closes the engine idle speed falls and the valve once again opens. The result is a resonance (instability) that develops and that is the problem.

The solution that will stop this problem forever is to remove/block the auxiliary air valve (do not remove the regulator). Like the human appendix, the auxiliary air valve serves no useful purpose. It is supposed to close immediately after the engine fires so it does nothing when the engine is running. You can mimic its function by stepping on the accelerator when you are starting the engine, but it’s not necessary or recommended to do that. However, what the auxiliary air valve also does is cause a very lean mixture during startup and that is the cause of the infamous backfiring that has sold all those pop-off valves and air boxes. My car never backfires.

To deal with the extreme cold in Calgary, I have made two additional changes to my engine. The first change deals with the crankcase ventilation hose that runs to the oil tank. This is the lower hose and there is also an upper hose that runs from the tank over to the rubber bellows above the air sensor. The theory is that blow-by gases are vented to the tank and are then drawn into the intake manifold and burnt in the engine. Now blow-by gases are essentially exhaust fumes and they are laden with water; the product of combustion. Since the oil tank is normally colder than the engine, particularly when the engine is cold, the water condenses and begins to fill your oil tank with water. This is not a good thing.

But there is something even worse that happens, in my car I found that the return vent hose had a low spot and in it about a cup of oily water had accumulated. Now imagine if this water is drawn into the intake manifold; it will destroy your engine. Even if the water stays in the hose another bad thing can happen if you drive in cold weather. The water can freeze and the result is that your engine has no crankcase ventilation so it builds up pressure in the crankcase. The result is a mass of oil leaks as the oil forces itself past the gaskets.

The solution is to take the crankcase vent hose, the lower one, and connect it with a "T" to the upper vent hose as near the rubber bellows as you can. Cap the connection for the crankcase vent hose at the oil tank, but leave the upper tank vent hose in place to allow tank venting. Vent gases are thus redirected into the intake manifold before they can condense.

The second modification I made was to the auxiliary air regulator and I would recommend this change only to people who intend to drive their cars in -5C or lower temperatures. The problem here is the cold air flowing through the auxiliary air valve cools it so effectively that the minuscule (11 watt in my car) heater in the regulator cannot close it. The result is that the engine stays on fast idle and races at 1,800 to 2,000 rpm in cold weather. The auxiliary air regulator is probably made for all those liquid cooled engines that get hot even in very low ambient temperatures. The location where the regulator is placed on a Porsche engine never gets very warm in cold ambient conditions. The solution here is to make a "U" out of 5/8" copper tubing, wire tie it to the oil return pipes from the head under the right engine bank and run hoses from the rubber bellows along the front of the engine to the "U" and then back up to the regulator. What you effectively build is a pre-heater. With this in place my engine warms up in about 5 to 10 minutes and settles to a smooth 900 rpm idle even in the coldest weather.

With these changes and Michelin Alpin tires, my SC is a blast to drive in the winter. Its traction on ice and snow is phenomenal. If nothing else you get a great deal of practice with the legendary 911 oversteer at speeds that do not cause any damage. I also fixed the heater, but that’s another story.

One last item. While reading the Rennlist archives there was a question last summer about how the deceleration valve works. For those that are interested…read on. The two larger hoses are plumbed as a bypass to the throttle. The smaller hose on top is plumbed to a point below the throttle. When you take your foot off the accelerator the throttle closes and the engine produces a very high manifold vacuum. This vacuum is applied through the small hose to a diaphragm that opens the bypass. This allows air to flow around the closed throttle and this keeps the engine rpm up until the engine slows to a point where the manifold vacuum is low enough to cause the deceleration valve to close and the engine reverts to idle. The simple way to check if this system is working is to rev the engine. The rpm should drop to 1,500 rpm or so, hold for a second or two and then smoothly settle to the idle speed. When the deceleration valve is not working and you rev the engine, the rpm will drop under the idle speed and then come back up, and sometimes the engine will just stall and stop.

VWpete

Nice write up!
I have the CIS blues sometimes and will keep this in my archives...
Thank You,
Pete

Matt Chamblin

Nice write up!

HarryD

Interesting set of observations. My '73 T does the same thing sometimes. I'll need to take a look and see if this is the culprit.

Thanks for a great article.

HarryD

Hi,

My 73 T Surges and gets stuck at high idle sometimes.

I looked in my engine compartlment for the device on the right hand side of the injector control module and found nothing. The only thing I see is the one on the left hand side. Can someone post pics of the two valves described in this article so I know what I am looking for?

Thanks in advance.

Hladun1

In reply to the question from HarryD, I believe he's right and the 73 CIS engine doesn't have the dreaded auxiliary air valve. I looked at my Haynes manual and all that the 73 engine has is what is called a deceleration valve on the SC. The early cars also didn’t have an auxiliary air regulator because they had a manual throttle.

Just to make things confusing the deceleration valve of the SC’s is referred to as an auxiliary air device on the earlier engines. (I can now see where the confusion arises.) The function of the auxiliary air device is the same as the deceleration valve I described in my first posting.

The SC deceleration valve (73 auxiliary air device) can fail in a number of ways. If the diaphragm ruptures then you will not be able to adjust the idle speed down. The valve can also open at too low a manifold vacuum (it's adjustable) and the engine will rev up. The engine may then develop enough vacuum to close the valve and surging at idle could occur. Or the valve might stay open and engine rpm would stay high. This sounds like the problem HarryD has. Adjust it or replace it. You do need it because without it the engine may die when you take you foot off the throttle.

The suggestion of pictures is good and I'll scan and post some pictures on this posting next week.

Hladun1

http://boards.rennlist.com/upload/AirValve.tif

I said I'd post some pictures and thanks to Pelican Parts here they are. (Hopefully the link works, it's the first time I've done it.)

Hladun1

http://boards.rennlist.com/upload/airvalves.jpg

I posted this file previously as a TIF image. QuickTime screws up Internet Explorer on my machine when I try to open the TIF. I've posted it as a JPG.