81 CIS Low Cold Idle
02-01-2002, 09:40 PM
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81 CIS Low Cold Idle
I received the attached e-mail from Bill Barr as a follow-up to some postings I’ve made on CIS surging. I’ll post my response here because it may be interest to others and I can’t answer Bill’s question completely. Maybe someone else can.
Taking out the auxiliary air valve will help with cold surging and backfiring, but I don’t thing it will help with too low a cold idle. Malfunction of any of the air side systems, except the auxiliary air valve, will cause an uneven or high idle. The deceleration valve and auxiliary air valve can only introduce extra air flow. So I don’t think this is an air-side problem.
That leaves fuel and fire. It could be an ignition problem, but those don’t tend to be temperature dependent. Bill’s changed out a lot of pieces. Let’s assume that its not the WUR because it’s new. A fuel system pressure check should isolate that problem. But before that, 81’s have the oxygen sensor. My car doesn’t. In looking at an 82 wiring diagram there are temperature sensors associated with the oxygen sensor. I suspect that the oxygen sensor, which controls richness, is not working correctly and Bill’s car is too lean when cold. Can the system be unplugged to check? I’m not sure.
As far as books go, Bosch puts out a good book on their fuel injection systems. The problem is that the CIS systems they discuss don’t have the "extras" that are hung on Porsche CIS systems, i.e., auxiliary air valve, deceleration valve, emissions, etc. Where I learned most about these systems is fixing my 62 XKE and 56 XK140 Jaguars. The CIS system is just a big SU carburetor. That is, fuel flow is controlled by manifold vacuum. Apologies to all the purists.
Bill’s e-mail;
"I would post this to the list but I have forgotten my login. Your explanation was great. I have been studying the CIS system for several years and there is always something to learn. Any books out there that I have not heard about?. My 81 SC has a cold start problem. Once warm it runs great. I have dropped the engine and replaced the airbox, new aux air valve, WUR reg, new aux air valve. on start it starts goes to 400-500 rpm and stays there until it warms up them will go to about 900 rpm. Aux air reg is open and closes when he engine warms. It is supposed to start go to 1200 rpm then kick back down when warm. It drives me nuts. Do you think taking the aux air valve out of the loop might help?
Bill Barr
Charleston, SC
81 911SC"
Taking out the auxiliary air valve will help with cold surging and backfiring, but I don’t thing it will help with too low a cold idle. Malfunction of any of the air side systems, except the auxiliary air valve, will cause an uneven or high idle. The deceleration valve and auxiliary air valve can only introduce extra air flow. So I don’t think this is an air-side problem.
That leaves fuel and fire. It could be an ignition problem, but those don’t tend to be temperature dependent. Bill’s changed out a lot of pieces. Let’s assume that its not the WUR because it’s new. A fuel system pressure check should isolate that problem. But before that, 81’s have the oxygen sensor. My car doesn’t. In looking at an 82 wiring diagram there are temperature sensors associated with the oxygen sensor. I suspect that the oxygen sensor, which controls richness, is not working correctly and Bill’s car is too lean when cold. Can the system be unplugged to check? I’m not sure.
As far as books go, Bosch puts out a good book on their fuel injection systems. The problem is that the CIS systems they discuss don’t have the "extras" that are hung on Porsche CIS systems, i.e., auxiliary air valve, deceleration valve, emissions, etc. Where I learned most about these systems is fixing my 62 XKE and 56 XK140 Jaguars. The CIS system is just a big SU carburetor. That is, fuel flow is controlled by manifold vacuum. Apologies to all the purists.
Bill’s e-mail;
"I would post this to the list but I have forgotten my login. Your explanation was great. I have been studying the CIS system for several years and there is always something to learn. Any books out there that I have not heard about?. My 81 SC has a cold start problem. Once warm it runs great. I have dropped the engine and replaced the airbox, new aux air valve, WUR reg, new aux air valve. on start it starts goes to 400-500 rpm and stays there until it warms up them will go to about 900 rpm. Aux air reg is open and closes when he engine warms. It is supposed to start go to 1200 rpm then kick back down when warm. It drives me nuts. Do you think taking the aux air valve out of the loop might help?
Bill Barr
Charleston, SC
81 911SC"
02-03-2002, 11:32 AM
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I finally figured out my password and am back in business. My original problem started with a cracked air box. I dropped the engine, replaced the box and while I was at it replaced anything that was hard to get to while I was at it. After it was all back together and started I discovered that the control unit ( under the passanger seat had been drowned. Not quite sure how but it was completly corroded. The previous owner had adjusted the system so it would operate without the CU. I replaced the CU and have tried to restore the engine to all of the default settings. I have a fuel pressure tester and know that all the pressures are set at the factory default called for in the manual. I replaced the WUR because I watched a mechanic "adjust" it with a hammer and a punch. The AUX air reg opens and closes like it is supposed to but I get no increase in rpm on start. I know if I adjust the WUR to be on the rich side it will probably work but I don't want to band aid the situation and adjust something that does not need to be adjusted and not solve the problem. Anybody with any thoughts?
Bill Barr
Charleston, SC
81 911SC
Bill Barr
Charleston, SC
81 911SC
02-03-2002, 03:15 PM
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Bill, I think you've just confirmed my diagnosis. You replaced the CU, but what about the plug to it? You probably have corroded contacts. There are also two temperature switches on the 82 diagram and it could be one of those.
A better way to adjust the auxiliary air regulator is to use some small sockets and a vice. You use the bigger one as a support and push the plug with a smaller socket. (The way you do universal joints.) Hammering is much loved by mechanics, but is not good.
Remember too that if you never want to replace your air box again the take out the auxiliary air valve.
A better way to adjust the auxiliary air regulator is to use some small sockets and a vice. You use the bigger one as a support and push the plug with a smaller socket. (The way you do universal joints.) Hammering is much loved by mechanics, but is not good.
Remember too that if you never want to replace your air box again the take out the auxiliary air valve.
04-26-2010, 08:49 PM
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To bring a solution to this problem for any search, I would like to state as the newest owner of this car that the problem with the low idle was cause by an incorrect Cold start valve flange that had the nipple for the air pipe blocked off. I found out that the Airboxes come with 2 different CSV flanges, one blocked and one open. I had the open flange in my possession from Bill and determined that this must have caused the problem. The car now starts and works it way to 1,200 rpm after a few seconds stays there for a short while depending on the temperature. I am still working on a problem that may have caused the original back fire that blew the box.
https://rennlist.com/forums/911-foru...-computer.html
https://rennlist.com/forums/911-foru...-computer.html
