Can irun my 83 SC without the lambda box and other items under the pass seat?

[christopher@cam]

Removed my 83 sc motor from one car to another, do i need to use the lambda box. i am already running without the ox sensor.

Thanks in advance

[w21055]

I do not think so.

[Nickshu]

Yes and no. When you disconnect the O2 sensor on this type of CIS setup (which many people recommend doing), I think it pegs the lambda bypass (aka Frequency) valve on the fuel distributor at 60% open, a fixed value, so it still has a function, just not variable. I think if you remove the ECU under the seat you also will need to install a backdated (pre-1980) fuel distributor and lines which delete this lambda bypass valve.

[Vin-barrett]

you cant.
You need the lambda box to operate the frequency valve.

[psalt]

Yes, you can run the engine , but the fuel calibration will be lousy. The lambda CIS is calibrated with the Frequency Valve pulsing around 50% duty cycle. When you remove the lambda box, the FV is alway closed, resulting in a leaner mixture at all levels. You can attempt to compensate by richening the mixture but you will ruin the cold start, cold running, idle and cruise calibration. On a road car you are much better off with the lambda box and 02 sensor working. The system goes open loop after 35% throttle (default FV duty cycle), so you can tune the basic CIS settings for best performance, and still get better cruise and part throttle mpg.

[Formerly Steve Wilkinson]

Yes, but you need carburetors...

[don gilbert]

good answer steve. Chris, i sent you a long pm detailing the probs, you will have by leaving the k-jet contrl box out. If you could drive a properly set up 02 car and a non 02 car, you will pick the 02 car everytime as the one that has better driveabiliy, smoother power, and awsome fuel milage (25+ on a properly tuned car) Its a small harness and easy to swap over.

[Nickshu]

Originally Posted by psalt

Yes, you can run the engine , but the fuel calibration will be lousy. The lambda CIS is calibrated with the Frequency Valve pulsing around 50% duty cycle. When you remove the lambda box, the FV is alway closed, resulting in a leaner mixture at all levels. You can attempt to compensate by richening the mixture but you will ruin the cold start, cold running, idle and cruise calibration. On a road car you are much better off with the lambda box and 02 sensor working. The system goes open loop after 35% throttle (default FV duty cycle), so you can tune the basic CIS settings for best performance, and still get better cruise and part throttle mpg.

According to my Bosch FI manual the frequency (lambda control) valve is not closed with the O2 sensor unplugged, it is actually pegged at 55-60% open, a fixed value. Then mixture is controlled completely by the mixture control screw setting on the CIS air/fuel box, but the thermo-time switch still gives input, which would allow the valve to still function properly in the pre-heating phase of lambda control, when the O2 sensor input is usually ignored anyway. Therefore I think that if you just removed the ECU for the system it would peg the valve completely closed and a lean mix would result, or you would have to compensate for enriching the mixture screw significantly, which (as you pointed out) would create drivability problems under certain conditions....cold/hot start, etc.

[psalt]

Hello Nick,

I think you are confused, the thermo-time switch is a safety device to prevent flooding by the CSV, it can have no effect on the mixture once the engine starts and the key is not in the START position. Are you thinking of the 15C switch or the 35C switch ? As I said in my post, removing the lambda box results in a closed FV, which is the OP's question. When the sensor is unplugged, the FV is still pulsed by the ECU, but there are several default open loop settings. In practice, the system works best with the sensor plugged in, it can be made to run OK with an unplugged sensor, but it needs the ECU to get close to the proper fuel curve. Is your experience in tuning CIS lamda different ?

[Gunter]

Originally Posted by christopher@cam

Removed my 83 sc motor from one car to another, do i need to use the lambda box. i am already running without the ox sensor.

Thanks in advance

Best to run it with the Lambda brain and OX sensor connected for reasons mentioned.
Disconnect and plug the retard vacuum line for the ignition distributor.

If you don't want Lambda, change the whole CIS to the earlier '78-'79.

[911Freak]

Originally Posted by Gunter

Disconnect and plug the retard vacuum on the ignition distributor.

What is the advantage of this?
If I plug the retard port of the distributor do I plug that vacuum line as well?

I have basically the same set-up as the OP which is why I ask..

I get no where near 25mpg either...

Thanks

[Gunter]

Should be: Disconnect and plug the retard vacuum line for the ignition distributor.

Check for posts by psalt, he explains it beautifully.

Also, clean and lube your distributor first, then set the timing:

Distributor service (Clean and lube) real easy without removing the pinion gear! - Pelican Parts Technical BBS

[911Freak]

Originally Posted by Gunter

Should be: Disconnect and plug the retard vacuum line for the ignition distributor.

Check for posts by psalt, he explains it beautifully.

Also, clean and lube your distributor first, then set the timing:

Distributor service (Clean and lube) real easy without removing the pinion gear! - Pelican Parts Technical BBS

Excellent info...

I'm preparing to service the dizzy as per the thread link. Can't wait to see the condition, how dirty etc.

I have several weird conditions with my dizzy and setting timing, i suspect it is b/c it's dirty and the vacuum advance pot is probably gummed up and not working properly.. I'll report back with pics on that thread...

Thanks again!!

[Gunter]

It's not the vac pod that gets gummed up, it's the center bushing for the Magnetic Pulse Generator (805). When frozen, it prevents the PG from moving when advance vac is applied.

You'll see when you dismantle.

Pix appreciated.

[Nickshu]

Originally Posted by psalt

Hello Nick,

I think you are confused, the thermo-time switch is a safety device to prevent flooding by the CSV, it can have no effect on the mixture once the engine starts and the key is not in the START position. Are you thinking of the 15C switch or the 35C switch ? As I said in my post, removing the lambda box results in a closed FV, which is the OP's question. When the sensor is unplugged, the FV is still pulsed by the ECU, but there are several default open loop settings. In practice, the system works best with the sensor plugged in, it can be made to run OK with an unplugged sensor, but it needs the ECU to get close to the proper fuel curve. Is your experience in tuning CIS lamda different ?

Yes my experience is different. This topic has been discussed here before and the concensus is leave the O2 sensor disconnected on 80-83MY 911SC's for several reasons.
The K-Jetronic system with the O2 sensor connected yields an air-fuel ratio of 14.7:1, so you maximize efficiency (MPG) and lower emisions, but this is at the cost of increased heat generation and reduced engine performance. This was driven by federal emissions mandates which took effect in 1980.

If you unplug the O2 sensor on K-Jetronic CIS (1980-83) and dial the idle mixture to 3-3.5% CO (measured at the tailpipe) you get an air/fuel ratio of 12.5:1, which produces higher torque and allows the engine to run at cooler temperatures. This is especially true if you have removed your CAT and replaced with SSI's.

The other advantage of O2 sensor disconnect is that it stops the chronic problem of idle speed "hunting" which this system causes when it gets worn/old because it stops the frequency valve from constantly playing with the fuel/air ratio at idle (based on fuel pressure).

If you disconnect your sensor, then using a CO meter adjust your mixture to 3-3.5% you will find the car runs smoother, cooler, and has quicker throttle response with no negative effects on engine operation across all conditions. I don't have a CO meter, so after disconnect I had a local Porsche shop adjust the mixture using their CO meter. Without the O2 sensor, this is the only way to adjust mixture on these cars.....disconnecting the O2 obviously precludes the O2 inline sensor meter you can use alternatively to adjust.

It's all in Charles Probst's book about Bosch Fuel Injection Systems....a great reference and repair book for CIS-laden 911 owners.

As for the Thermo-time switch, according to Probst's book the TT switch gives input to the K-Jetronic ECU so it doesn't allow the Frequency/Lambda valve to restrict the mixture too much at cold startup...this is to avoid stalling when cold. Also at WOT the WOT switch on the throttle opens the frequency valve all the way, so at WOT this system has no influence disconnected or not.

Also I don't think on K-Jetronic CIS systems there is any relationship between the vacuum advance on the distributor and the Lambda control/Frequency valve system. When we have discussed this topic (O2 sensor disconnect) previously on this board many Pelicans have gotten WAY confused because people don't separate the effects of this on CIS K-Jetronic vs. the later DME/Motronic pulsed injection cars (1984 and later except 930). It's a totally different story with the later FI systems.

[Paulporsche]

I have been running w/o a lambda system on my engine for about 17 years, in temps from about -5C to about 30C. I have a 78/79 WUR to provide vacuum induced enrichment @ WOT. Once I cleared up a poorly functioning WUR and a vac leak this spring, the engine starts and runs beautifully.

[psalt]

Hello Nick,

You are still confusing the switches, I suggest you reread Probst book, which does not state anywhere that the thermotime switch has anything to do with the lambda system. It is strickly for the CSV. Probst book is OK, but it does not cover many of the adaptions Porsche made to CIS lambda for the 911. There are several temperature switches for the lambda system, but they are not "thermotime" switches. I also suggest you hook up a dwell meter to the test port to actually see what the system is doing. You will see the 35% throttle switch does not open the FV "all the way", it is a pulsed injector and no BOSCH ECU runs pulsed injectors at 100% duty cycle.

In my opinion, you are deluding yourself to think that disconnecting the 02 sensor produces higher torque and cooler running. Torque is produced under load with the throttle open. As soon as the CIS lambda on a 911 senses >35% throttle, the system goes open loop and the peak torque AFR is identical, whether a sensor is present or not. It may make you happy that the idle doesn't fluctuate, but the incorrect fueling at part throttle is not worth the imaginary increase in torque.

[Nickshu]

Originally Posted by psalt

Hello Nick,

You are still confusing the switches, I suggest you reread Probst book, which does not state anywhere that the thermotime switch has anything to do with the lambda system. It is strickly for the CSV. Probst book is OK, but it does not cover many of the adaptions Porsche made to CIS lambda for the 911. There are several temperature switches for the lambda system, but they are not "thermotime" switches. I also suggest you hook up a dwell meter to the test port to actually see what the system is doing. You will see the 35% throttle switch does not open the FV "all the way", it is a pulsed injector and no BOSCH ECU runs pulsed injectors at 100% duty cycle.

In my opinion, you are deluding yourself to think that disconnecting the 02 sensor produces higher torque and cooler running. Torque is produced under load with the throttle open. As soon as the CIS lambda on a 911 senses >35% throttle, the system goes open loop and the peak torque AFR is identical, whether a sensor is present or not. It may make you happy that the idle doesn't fluctuate, but the incorrect fueling at part throttle is not worth the imaginary increase in torque.

Paul,

Thanks for the detailed reply. My knowledge of CIS has come from reading Probst and this BBS, but apparently flawed. Some experienced 911 mechanics who post here have advocated disconnecting the O2 sensor in the past, including John Walker's Workshop. While I believe you that on a 911 some of the sensors/switches are unique, Probst does actually say in his book (I just looked again) that the TT switch gives ECU input to bypass the FV under cold/startup conditions, but as you pointed out the 911 is likely unique/different here.

So how does the system know 35% throttle is open? There's not a potentiometer on the throttle like on pulsed injection. Does it use the fuel distributor plunger??

Where can I read more about specific modifications of K-Lambda for the 911??? (for my own knowledge).


On a separate note, is the CSV controlled by the same ECU as Lambda...if he removes it will the CSV not work correctly, or is it direct link from TT switch to CSV?

Thanks!

Nick.

[psalt]

Hello Nick,

The CIS lambda on a 911 is unique and even the factory manual does not describe all the functions correctly. The Bentley manual has a good descripiton for a novice, but there are several factual errors in it concerning the sensors. BOSCH publications are generally the most accurate and "Gasoline Engine Management" is very good, but not Porsche specific. Books are nice, but there is no substitute for hands on knowledge of what the system does and when, by actually viewing the FV duty cycle on a running engine under load.

You still do not understand your confusion about the switches. A " thermotime" switch is a unique device that is a temperature switch with a built in heater. Anyone who has experience with BOSCH injection knows this device is not connected to the ECU, it is in a completely different circuit for the sole purpose of shutting down the CSV to prevent flooding. If you have any doubt, look in the wiring diagrams. Probst does understand this, and his book does not say what you keep repeating. Re read it, it does not say "thermotime" in connection with any lambda devices.

Your version of CIS lambda has two seperate discrete throttle position switches. The 35% switch is in a black box between the throttle body and the air flow meter. The 15% switch is an exposed microswitch next to the thottle linkage. This is for extra cold enrichment under acceleration. If you trace the wiring diagram to all of the switches and sensors, you will find that the system is quite simple and works quite well on an engine with decent compression and no vacuum leaks. The recommendation to unplug the sensor is a bodge in my opinion to cover up other issues which should be address separately.

[Nickshu]

Ok, so I just reread the Probst book. He calls this switch a "Lambda thermoswitch" and he specifically mentions 911's where he says it is located on the cylinder head. He states it's function is to peg the FV at 60% duty cycle to allow slightly rich mixture at cold start. I assume this is distinct from the TT switch?