CIS Fast Idle Speed

[fred cook]

When I start the engine in my SC, it immediately goes up to about a 1600-1700 rpm idle speed. Once warmed up, the idle drops to a nice normal and steady 900 rpms. The engine was recently rebuilt with all rubber having been replaced and joints checked for vacuum leaks (non found). I haven't checked line pressures lately, but it is my understanding of the CIS system that a high cold idle speed is caused by excessive fuel pressure. Because it runs so well once warmed up, I don't want to change the fuel adjustment or timing. Is this going to be a WUR adjustment issue?

[tirwin]

Fred,

Just thinking about non-fuel pressure related sources... Do you have a decel valve? If the diaphragm in it leaks it can cause a high idle.

In the last few months I was troubleshooting and my basic mixture was lean. Once I hooked up the O2 sensor the idle on a cold start was high, in the same range as yours. After a couple of minutes it came down to normal. Once I richened the mixture I was able to dial down the idle to the correct range using the idle screw.

[boyt911sc]

Originally Posted by fred cook

When I start the engine in my SC, it immediately goes up to about a 1600-1700 rpm idle speed. Once warmed up, the idle drops to a nice normal and steady 900 rpms. The engine was recently rebuilt with all rubber having been replaced and joints checked for vacuum leaks (non found). I haven't checked line pressures lately, but it is my understanding of the CIS system that a high cold idle speed is caused by excessive fuel pressure. Because it runs so well once warmed up, I don't want to change the fuel adjustment or timing. Is this going to be a WUR adjustment issue?

Fred,

I'll be happy to get that 1600-1700 rpm when cold with my car/s. The big question is how long does it take for the rpm settle down to 1000 rpm, and finally to 900 rpm idle speed. The high cold idle speed is controlled by the AAR provided you don't have unmetered air. Check the opening and resistance of the AAR. The cold control fuel pressure has a direct effect on the idle speed too.

To get an idea how your system works, hook up a fuel pressure gauge during a cold start. The colder the weather the better. Record the time of the cold control pressure versus time till the CCP settles down to WCP. This is where the control pressure stabilizes at some point. If you get between 3 - 5 mins. for this test, your engine's cold idle speed should not be a concern of yours. Keep us posted.

Tony

[fred cook]

Originally Posted by boyt911sc

Fred,

I'll be happy to get that 1600-1700 rpm when cold with my car/s. The big question is how long does it take for the rpm settle down to 1000 rpm, and finally to 900 rpm idle speed. The high cold idle speed is controlled by the AAR provided you don't have unmetered air. Check the opening and resistance of the AAR. The cold control fuel pressure has a direct effect on the idle speed too.

To get an idea how your system works, hook up a fuel pressure gauge during a cold start. The colder the weather the better. Record the time of the cold control pressure versus time till the CCP settles down to WCP. This is where the control pressure stabilizes at some point. If you get between 3 - 5 mins. for this test, your engine's cold idle speed should not be a concern of yours. Keep us posted.

Tony

Yep, this is what I thought. To answer some of your questions, if I start the engine and immediately drive off, it will take a minute or so to get down close to the regular idle speed. Maybe a mile or two of driving. If I leave the at idle, it will take a three or four minutes for the idle speed to start dropping. I checked the AAR operation by putting it in the freezer for a few minutes to get full open position, checked to make certain it was getting +12v for the warm up circuit and that the AAR rotary valve worked correctly and smoothly. I just went out to the shop and started the engine. When first started, the A/F ratio was in the 12.5-12.9 range. After idling for just a few seconds, the A/F ratio started moving towards the 13.1-13.5 range. I think this tells me that the cold start valve is working properly. When fully warmed up, I have the A/F set at about 14.1-14.5. I guess the next step will be to break out the CIS fuel pressure gauge and see what the cold and warm pressures look like, Thanks for the feedback!

[70SC]

Fred , Check your thermo-time switch /wires, On the front timing chain box. They are susceptible to breakage & that will cause your issue . Getting pressure readings is allways a good thing . PK

[boyt911sc]

Originally Posted by fred cook

Yep, this is what I thought. To answer some of your questions, if I start the engine and immediately drive off, it will take a minute or so to get down close to the regular idle speed. Maybe a mile or two of driving. If I leave the at idle, it will take a three or four minutes for the idle speed to start dropping. I checked the AAR operation by putting it in the freezer for a few minutes to get full open position, checked to make certain it was getting +12v for the warm up circuit and that the AAR rotary valve worked correctly and smoothly. I just went out to the shop and started the engine. When first started, the A/F ratio was in the 12.5-12.9 range. After idling for just a few seconds, the A/F ratio started moving towards the 13.1-13.5 range. I think this tells me that the cold start valve is working properly. When fully warmed up, I have the A/F set at about 14.1-14.5. I guess the next step will be to break out the CIS fuel pressure gauge and see what the cold and warm pressures look like, Thanks for the feedback!

Fred,

Based from the above information you posted, you have a good or would say a perfect running CIS engine!!!!! There is nothing wrong with your engine as far I could tell. What are your expectation?

Tony

[boyt911sc]

Originally Posted by 70SC

Fred , Check your thermo-time switch /wires, On the front timing chain box. They are susceptible to breakage & that will cause your issue . Getting pressure readings is allways a good thing . PK

What the has TTS has to do with high idle speed during a cold start? Would you care to help us understand the logic behind it? Thanks.

Tony

[Bob Kontak]

Originally Posted by 70SC

Fred , Check your thermo-time switch /wires, On the front timing chain box. They are susceptible to breakage & that will cause your issue . Getting pressure readings is allways a good thing . PK

Agree knowing pressures is a good thing. Pretty much mandatory for the DIY SC owner who want's to tend to the car with a degree of comfort.

However, the CSV/thermo time system ONLY works when the starter is engaged. If Fred has an issue, I am not picking up that the CSV system is the issue.

The car starts first/second turn when cold. Fred did not say, but I am thinking this is the case?

The number one issue is if the throttle bypass function of the AAR is taking too long to close off. boyt911sc states this "how long does it take for the rpm settle down to 1000 rpm, and finally to 900 rpm idle speed". This is the immediate concern.

Also, Fred is in the deep south and his car is probably getting colder weather as a result of this brutal winter. The car may be operating as designed and the cold weather causes "anomalous" operation from Fred's perspective.

On the up side, even if the AAR is tired, it settles down.

[Bob Kontak]

Originally Posted by boyt911sc

What the has TTS has to do with high idle speed during a cold start? Would you care to help us understand the logic behind it? Thanks.

Love me some Tony.

70SC, just roll with it and learn. I have been tuned up by Tony SO many times. He is a good man - and he is smarter than you and I regarding CIS.

[mysocal911]

Originally Posted by fred cook

I checked the AAR operation by putting it in the freezer for a few minutes to get full open position, checked to make certain it was getting +12v for the warm up circuit and that the AAR rotary valve worked correctly and smoothly. I just went out to the shop and started the engine. When first started, the A/F ratio was in the 12.5-12.9 range. After idling for just a few seconds, the A/F ratio started moving towards the 13.1-13.5 range. I think this tells me that the cold start valve is working properly. When fully warmed up, I have the A/F set at about 14.1-14.5. I guess the next step will be to break out the CIS fuel pressure gauge and see what the cold and warm pressures look like, Thanks for the feedback!

1. Both the engine case temp and the heater element in the AAR both affect the how
fast the AAR shutter closes. So when the engine is fully hot, the AAR is usually
closed and isn't affected by the heater element as would be case when fully cold.

2. "After idling for just a few seconds, the A/F ratio started moving towards the 13.1-13.5 range. I think this tells me that the cold start valve is working properly."

He probably meant to say warm-up regulator (WUP) and not "cold start valve",
which raised the confusion in a previous post.

[Bob Kontak]

Originally Posted by mysocal911

He probably meant to say warm-up regulator (WUP) and not "cold start valve",which raised the confusion in a previous post.

That sounds reasonable.

FWIW, Here is a question from ignorance. Why does the AAR dump air into the post throttle intake when a rich mixture is desired when starting cold?

OK, The WUR has low control pressure at cold, allowing the fuel distributor to piss plenty of gas into the cylinders. Why the extra air from the AAR? Is it because the mixture is so rich that the air bypassing the throttle plate is not enough to sustain combustion? This must be the reason, but I am just thinking vs knowing.

[mysocal911]

Originally Posted by Bob Kontak

That sounds reasonable.

FWIW, Here is a question from ignorance. Why does the AAR dump air into the post throttle intake when a rich mixture is desired when starting cold?

OK, The WUR has low control pressure at cold, allowing the fuel distributor to piss plenty of gas into the cylinders. Why the extra air from the AAR? Is it because the mixture is so rich that the air bypassing the throttle plate is not enough to sustain combustion? This must be the reason, but I am just thinking vs knowing.

That's correct. It's like the carb system where there's a choke linkage (more fuel)
and a fast idle linkage (more air) that work together when then engine is cold.
It's the same on the 3.2 DME ECM where the mixture is rich at cold and the idle
valve provides more air for the fast idle. The additional air provided by the AAR
does not bypass the sensor plate and as such causes additional fuel, so the AAR air
does not offset the mixture change from the WUR for the cold running mode.
This is the same condition as in the DME system where the idle valve does not
bypass the AFM during cold running.

[47silver]

It should idle high when cold and then drop. Tony is correct nothing is wtong

[porwolf]

Originally Posted by 47silver

It should idle high when cold and then drop. Tony is correct nothing is wrong

That is the way my 1979 SC does it since I bought it brand new.

[Nostril Cheese]

Consider yourself lucky it idles so high when cold. As the others said here, it's pretty much normal. My SC would go to 1300 cold and down to 900 warmed up.

[Dr J]

Perfectly good behavior. No need to mess with anything.

[fred cook]

Thanks to everyone for the input. I had gotten into my head that the initial idle speed was a bit too high, but after reading the above posts, the initial idle speed is something that I can live with. Also, I expect things will settle down a bit once the local temps go back to normal here. Considering the engine that this CIS system is sitting on (3.3SS) and the mods that I made to it, it does work well! The air box and runners are from an early SC (large port) and the remaining hardware is from the airbox off my 1980 SC 3.0 engine. The system has been stripped back to 1973 1/2 levels with most if not all the emission control stuff removed. It starts easily even after sitting for a few days and runs well with good power up thru the gears. Once I finish the break in period (500 miles), adjust the valves and change the oil/filter, I am going to take it in for a dyno run. The engine consists of 100mm Mahle 10:1 pistons, LN Engineering cylinders, 964 cams, Electromotive XDi twin plug ignition, Carrera cylinder heads, SSI exhausts and the modifed CIS system. Compresson on all cylinders is a nice ever 160 lbs. Even with the moded CIS, I'm hoping to see about 250 - 260 HP at the rear wheels. Here's a picture.

[82 SC]

Nice thread. What would be the cause if the idle keeps staying high for a longer period of time? I can go about 10 mins or more sometimes with a high idle on my 82Sc when really cold. When i first got my car a few months ago (first Porsche) it did well and then i noticed that the heated element connector on the drive side was broken. Fixed that and a few other things but I did not get to start it for a few weeks while I was waiting on parts (tune up items like wires and such). Idle would go to almost 2000 sometimes when cold and I could drive 3 miles to my kids school pick them up and still have a high Idle. It will go down eventually. Some times i even get the idle bouncing up and down really low to 1000 rpms. Still new to this but I do all my own work. I came from old vw bus world of carburetors. I did adjust the idle up a bit and it helped with the idle issue for the most part. Still sometimes it will do a crazy bouncing Idle out of no where.

[boyt911sc]

Originally Posted by 82 SC

Nice thread. What would be the cause if the idle keeps staying high for a longer period of time? I can go about 10 mins or more sometimes with a high idle on my 82Sc when really cold. When i first got my car a few months ago (first Porsche) it did well and then i noticed that the heated element connector on the drive side was broken. Fixed that and a few other things but I did not get to start it for a few weeks while I was waiting on parts (tune up items like wires and such). Idle would go to almost 2000 sometimes when cold and I could drive 3 miles to my kids school pick them up and still have a high Idle. It will go down eventually. Some times i even get the idle bouncing up and down really low to 1000 rpms. Still new to this but I do all my own work. I came from old vw bus world of carburetors. I did adjust the idle up a bit and it helped with the idle issue for the most part. Still sometimes it will do a crazy bouncing Idle out of no where.

82 SC,

If I were in your shoes, I would check the following:

1). AAR (auxiliary air regulator). Measure the resistance (Ohms) of the heater, perform oven/freezer tests, and finally do the 12-volt test.

2). FP (fuel pump) and fuel pressures. Without knowing the control, system, and residual fuel pressures you are facing too many unknown variables.

3). Unmetered air or air leak/s. Test and confirm.

4). And others. The above 3 items would keep you busy for the moment.

Keep us posted.

Tony

[82 SC]

Thanks Tony, I will check that list out when the weather gets better. I also don't have the ox sensor and am running the SSI heater boxes. Not sure if that makes a difference or now. I keep finding mods on my motor as there is no smog and other little things on it. Heck the rear wiper motor switch is a kill switch and I don't have a rear wiper. The more I read on this forum and the more i get to play and learn on my 911 the more I find out how it was modified. I think my suspension has even been lowered! lol!