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I did set my mix again on Saturday - it wasn't off actually. Using the sensor plate lift method. There is no way that it is running that lean! With the sensor lift, I'd read here that mixture should be very close when you get to the point where lifting the plate a 1/16" causes the RPM to decrease and lowering the plate also causes RPM to lower. That is exactly how mine was set. However, re-reading Jim's 911 CIS Primer again last night I found this: 911 CIS Primer - Testing: Mixtures

Quote:

With the engine warm and running, slightly lift the air flow sensor plate.
If the RPM immediately falls off, the mixture is probably too rich.
If you pull down, and the idle falls off, you could either be too lean, or about right. Do the step immediately below to find out.
If the RPM slightly rises, then falls off as you continue to raise the sensor plate, it may be close to right.
You are trying for about 14.7:1.
The required movement of the sensor plate is slight, on the order of maybe a 1/16" or less

According to this, I'm too rich! However, in re-reading some forum posts again, I came across this thread I had bookmarked: http://forums.pelicanparts.com/porsche-911-technical-forum/212120-adjust-mixture-now-i-have-tool.html#post1819071 See post #5

Quote:

This can be set by ear and its very acurate for best performance. If tuning CIS w/ lambda, disconnect the O2 sensor first.

The trick to find your engines happy CO is to get the engine at least running and with the airbox cover and filter off, SLIGHTLY lift the air sensor plate via the exposed elbow with a finger, knuckle or whatever. All is needed is very slight pressure to notice any change in RPM. If the engine RPM increases it wants to be richened. This is also true if you were to pull down on the sensor plate. If RPM's increase, then it wants to be leaned out. Make very small adjustments with the 3mm allen as necessary. The trick is to get the engine at a max RPM so when you either push or pull on the sensor plate, the RPM's will drop (from either being too lean or rich)."

This one says that the O2 sensor should be disconnected for this sensor plate lift method but I've never done that. What is correct?

So I'm not traveling for work this week so I am going to do some diagnostics. I think the best approach would be to work through Probst's Checking Lambda Control section 6 page 21 for 2 reasons: 1) to verify that I understand how my instruments (dwell gauge and oscilloscope) work and 2) to verify the CIS components - which I have not done in totality for 18 months.

I like Probst's approach as it starts with a simple test on the "thermoswitch" disconnected and connected to measure the open-loop, cold and closed-loop, warm-up duty cycles. Since these are documented targets (open-loop: 60% duty cycle stable, closed-loop: 50% duty cycle stable) I can verify that I know how to use my dwell meter and oscilloscope to test.

I do have a question about this though, the Porsche documents and Jim's 911 CIS Primer have two "temperature switches" and nothing called a "thermoswitch". In fact, even though the Primer talks about temperature switch 1 and temperature switch 2, the CIS Component Description Page 911 CIS Primer - Components doesn't list either! but it does have a thermotime switch.

What I make of this and lots of other reading is that the 81-83 SCs have TWO temperature switches.

Temperature Switch 1 is the switch Probst is calling the thermoswitch in section 6 page 21. It is closed below 15°C and opens above 15°C. This switch is used in what Jim calls Mode 1 (911 CIS Primer - CIS Lambda)

Temperature Switch 2 is not discussed in Probst. It is open below 35°C and then closes above 35°C. Jim calls this an enrichment relay and is associated with Mode 2

Does all this make sense?

A quick update...

tirwin and I met for dinner on Tuesday while he was in town. Great to talk about 911s and other things. It was a great end to a bad day (I got axed at work that morning - good thing I didn't drive to Rochester to find that out). Anyway, the temps here have been too cool so I have not been able to get the temp up enough to observe the idle surging. It is supposed to get to 80 this weekend so that might be enough. In the meantime, I prepped the O2 sensor with a disconnect so I can check it disconnected and then be able to reattach.

My FabSpeed cat bypass also came in yesterday. I need to get the existing cat off so I can find the best place to weld in the bung for the wideband O2. It will go on the rear tube of the bypass as I posted in another thread today. http://forums.pelicanparts.com/porsche-911-technical-forum/998803-fabspeed-2nd-bung-location-sc.html

When I was looking under the car today I discovered that there is a steel "flange" that is part of the engine compartment/body that extends to the cat/muffler so really need to dry fit the wideband to make sure it won't interfere with that flange - nothing is ever simple is it!

I had started another thread about the FabSpeed bypass but I wanted to update here for closure since this has been a long term problem that is now resolved!

So I installed the cat bypass with a new Bosch O2 sensor since I could not get the old one out. I should say that I know the CIS Lambda was working as I saw the correct dithering behavior, etc on the test port.

After installing the new bypass/sensor it was immediately obvious that something changed for the better. Firstly, I noticed that the gas odor was completely gone. I drove on a few pretty warm but not blistering hot days and the idle surging did not appear. I also noticed an almost 2MPG improvement in fuel consumption along with feeling a bit more power.

Things have been great for three weeks - no idle surging after a "hot" ride at all. Today was the ultimate test - it was hot hot hot and humid here in Boston today. I went for a 60 mile drive at - ahem - highway + 30% - speed. The engine temp barely got above the first white tick and idle was dead steady when I got off the highway. I had a return trip 30 minutes later. This surge issue would absolutely have presented itself under these conditions today. So I claim victory! I suspect my cat must have been on its way out and was partially blocked. I can't get the old O2 sensor out to eliminate it as the problem but the cat makes more sense to me.