Fast Idle Question

Is there a temperature at which the engine will not engage in a fast idle at cold start, or should it always have a fast, cold start idle?

I can't find any guidelines that explain when the Auxiliary Air Regulation system should operate (such as temperature ranges). I would assume if the ambient temperature is high, then the AAR would be closed and not allow a fast idle, but I don't know if this is a good assumption or not.

Also, I realize the Thermo Time Valve contributes by blocking the vacuum at the distributor, so is there also an operating temperature range for this as well?

Thanks!!

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Michael
'78 911SC Euro

Michael,

I've never been able to find any temp specs on any CIS parts other than the WUR. Good luck, and let me know if you locate something.

BTW, the vacuum to the distributor should not go through the thermovalve, it should be a direct connection, as shown in this diagram. Again, let me know if you find something that shows otherwise.

http://albums.photopoint.com/j/View?u=1349791&a=10050384&p=44329680

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Bill Krause
'79 911SC Euro

I've consulted several diagrams, and I simply do not have all of the separate, throttle body vacuum connections shown in any of them.

I have one small vacuum connection on the back of the throttle body with the following hose routing:

1. From the throttle body, to the top of the Deceleration Valve, to the center nipple on the Thermo Time Valve.

2. From the offset nipple on the Thermo Time Valve, it "T's" off and routes to the bottom connection on the WUR and the distributor.

The vacuum connection on the front of the throttle body has only one line going directly to the top connection on the WUR.

I've read so much about this stuff recently, that I don't remember the source, however the Thermo Time Valve evidently provides 2 functions during the cold start process, at which time it should be closed:

1. It prevents a vacuum from getting to the distributor. When present, a vacuum to the distributor retards the timing, so preventing this vacuum during cold starts advances the ignition, thus increasing engine RPM's. As the Thermo Time Valve opens, vacuum is presented to the distributor, which retards the timing, thus slowing the idle. This was evidently implemented in '78 to prevent poor cold starts and possible backfires through the induction system.
2. By blocking a vacuum to the WUR, it allows the WUR to increase control pressure during the warm-up process, leaning the mixture as the engine comes up to operating temperature.

What I'd really like to know is:

1. At what ambient temperature does (or should) the Auxiliary Air Regulator close.
2. At what ambient temperature does (or should) the Thermo Time Valve open?

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Michael
'78 911SC Euro

By the way, as I understand it, the warm-up process goes as follows:

When started, the Thermo Time Valve is closed, and prevents a vacuum to the distributor, thus advancing the ignition and increasing engine RPM's. This is the first stage of the warm-up. Next, the Auxiliary Air Regulator takes over the second stage of the warm-up process by allowing additional air into the induction system.

I'm sure there's overlap in the functions of these devices, but they each seem to contribute to the cold start process.

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Michael
'78 911SC Euro

I guess this is the first time that I've heard the distributor included in a description of the cold start process. It's still not clear to me that the distributor on a 78-79 Euro SC has a vacuum retard anyway. I'm not saying you're wrong, I just can't find any published information that supports it, but I'm still looking.


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Bill Krause
'79 911SC Euro

I'm sure most everyone has been to this site, but I figured I post a link anyway.

http://www.users.bigpond.com/INTERJECT/KJETRON.HTM

They have some pretty interesting information on K-jetronic, yet as you have found, I cannot find any specifications on the vacuum system, Thermo Time Valve or Auxiliary Air Regulator.

I sent these folks an e-mail, and I'll post their response, if any, to my questions.

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Michael
'78 911SC Euro

Here's the answers to my questions:

> 1. At what ambient temperature should the Auxiliary Air Regulator Close?
75 DEGREES CELSIUS
> 2. At what ambient temperature should the Thermo Time Valve open? 0-20
DEGREES CELSIUS

Based on this, it seems that at any temperature above 20 degrees celsius, the AAR alone handles the fast idle function. Below 20 degrees celsius, the thermo time valve contributes by blocking the distributor vacuum.


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Michael
'78 911SC Euro

"Below 20 degrees celsius, the thermo time valve contributes by blocking the distributor vacuum".

I'd still like to see something published that shows this or describes this. Not even the factory emissions sticker shows the distributor connected to vacuum through the thermovalve. Perhaps it is a worthwhile modification to make.

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Bill Krause
'79 911SC Euro

I have a similar question related to the cold start. When I first start a cold (overnight) engine it sputters and backfires for ~5 seconds before it kicks up to 2000 rpm. It idles high for ~ 1 minute then drops down to a normal idle at 950-1000 rpm. What should the exact sequence be in time? I will try to get an accurate reading next cold start.

GB