unplugging O2 on late SC's and fuel enrichment question
 

Please correct me if I am wrong...

81-83 SC's use the frequency valve exclusively for fuel enrichment.
These later SC's do not use the vacuum port on top of the WUR for enrichment as this is not a vacuum port at all like some older cars and is only a vent.

The blue sensor plate switch on the air flapper assembly along with the larger black switch on the throttle body determine the throttle is open. This, with the Lambda brain , Acceleration enrichment Control Unit , and oxygen sensor relay buzzes the frequency valve more resulting in a richer mixture.

Does the Lambda brain still allow all this to happen with the O2 sensor disconnected at a fixed duty cycle? Maybe not to protect the frequency valve from working too hard?

Thanks for any help

Vinny,

Here is the Lambda operation in a nutshell:

The Lambda unit does two things - It handles the enrichment that the WUR (with vacuum) used to do on the early CIS (up to '80). It also adjusts the mixture to Stoiciometric (14.7:1) during cruise conditions.

The blue plug on the airflow sensor housing is a safety switch for the fuel pump. It is not related to Lambda operation. The '80 CIS had one microswitch, the '81-'83 had two. There was an enrichment relay added in '81 controlled by the second switch.

During cold running (warmup) the system is running open loop (O2 sensor not controlling the ECU) and enriching the mixture to a preset value. It is also running open loop during acceleration and WOT. The O2 sensor is controlling the FV during warm cruise. The O2 sensor sends a varying duty cycle to the FV which can either richen or lean the mixture as it senses the Oxygen content.

Hope this helps.

With the O2 sensor disconnected, the ECU is sending a constant 50% duty cycle to the FV (neither rich or lean, but "just right"). The FV is still responding, off half the time and on half the time. So the system is now in open loop, and not being controlled by the O2 sensor.

If the O2 sensor corrects for lambda at cruise, then it corrects to a AFR of 14.7:1 for 100% gas. Throw E10 at it, won't it automatically try to get back to the set lambda, which requires extra fuel to make up for the reduced energy content?

In that sense, disconnecting the O2, and running E10, may result in too lean if the open loop was set perfectly, Right??


BTW, I saw in my owners manual, the car was designed to work with gasohol (~E10) from the 1980s. My CIS ran fine with E10, once I plugged the vacuum leaks at the injectors. ;)

Thanks for helping Jim

With a wideband O2 I have found my car going way lean off idle, as high as 17-18afr
Richening at idle helps somewhat but not that much (15-16) and makes it way too rich at idle anyway.

I feel I have no enrichment happening and am suspecting the enrichment box with the round plug next to the lambda unit.

the oxy relay works, dome lamp, FV buzzes...

I would like to know if the acceleration enrichment box is doing it's job
Does the engine have to be running to check the circled wires to the lambda? Maybe I could just jump the fuel pump. Any idea of what should I be seeing here if I move the throttle while putting a voltmeter on these?

http://forums.pelicanparts.com/uploa...1300757576.jpg

Thanks again
Vinny

The Lambda system in and of itself can only do so much. I consider it to consist of the components starting with the O2 sensor and ending with the Frequency Valve. Besides the O2 sensor, the ECU is fed by other input devices: the throttle switches and the 15˚C and 35˚C thermostats.

There is a single test point on the ECU where you can monitor the duty cycle being sent to the FV. This is the 3-pin connector, connected to pin 17 on the ECU, found on the left side of the engine bay. You will need a meter which reads duty cycle (a points-type dwell meter will do this job). If you connect the meter using some long jumper test leads so they reach the cockpit, you can monitor the overall operation during a normal driving cycle and determine if the Lambda system is performing as expected. Then depending on what you learn from this, you can perhaps narrow down your search for the source of the lean running issue.

There have been some lengthy threads here on Pelican that touch on the duty cycle readings for various conditions - i.e., cold starts, warmup, etc.

Here is a start: http://forums.pelicanparts.com/porsche-911-technical-forum/573516-measuring-dwell-help.html

FV location
 

Don't mean to change subject but I am in process of troubleshooting fuel system on my 82 SC and cannot find an illustration showing physical location of frequency valve.

I'm hoping it can be accessed without dropping engine!

Could someone please enlighten me?

Thanks,
Warren

http://forums.pelicanparts.com/uploa...1301146303.jpg

With a wideband O2 I have found my car going way lean off idle, as high as 17-18afr
Richening at idle helps somewhat but not that much (15-16) and makes it way too rich at idle anyway.I feel I have no enrichment happening and am suspecting the enrichment box with the round plug next to the lambda unit.
the oxy relay works, dome lamp, FV buzzes...
I would like to know if the acceleration enrichment box is doing it's job
Does the engine have to be running to check the circled wires to the lambda? Maybe I could just jump the fuel pump. Any idea of what should I be seeing here if I move the throttle while putting a voltmeter on these?

Vinny,

The extra cold acceleration enrichment only works when cold, open loop and only provides enrichment for 2 seconds. It is irrelevent to a WOT test under load when hot. When you say the AFR goes lean, are you measuring it under load when hot ? Measurements at rest with no load are not relevent to what happens when driving the car.

The lambda system is primarily an emission driven device to extend the life of the catalytic converter. The cat needs 14.7:1 for a long life. No engine management system can produce a steady 14.7:1, they all need some way to dither the mixture lean then rich, so the average mixture is as close to 14.7:1 as possible. CIS lambda is designed so that the "K Basic" (mechanical) AFR calibrations are all too lean. The FV is a variable pressure regulator and it must be pulsing in the mid range for the fuel curve to be anywhere near the requirements. If the FV stops pulsing, lower chamber pressure goes up and the whole fuel curve is leaned out. Unfortunately, the BOSCH engineers chose "too lean" rather than "too rich" as the failure mode of CIS lambda. The ECU pulses the FV in the mid range, the O2 sensor reads the results, the ECU alters the pulsewidth to "trim" the mixture in the right direction toward stoich, then the O2 sensor reads the results and dithers the mixture back the other way. When you are reading the duty cycle when running closed loop, you can infer the mixture from what the correction factor is, i.e if the needle is dithering between 20-30 dwell, the "K Basic" settings are rich and the system is correcting back to stoich. Most cars will run best around this setting because it provides a richer mixture under WOT, and stoich for idle and cruise.

If you want to test the WOT switch, disconnect the main temp switch and test for continuity to ground when you open the throttle with the engine off. A better test is to view the duty cycle on a meter while driving under load with the engine hot.

If the lambda system is working properly and your mixture adjusted to 20-30 dwell, and it goes lean under load when hot, your problem may be outside the lambda system, fuel delivery, weak pump, clogged filter or injectors.

Don't mean to change subject but I am in process of troubleshooting fuel system on my 82 SC and cannot find an illustration showing physical location of frequency valve.

Warren,

The FV is behind the air box. It looks like a pulsed EFI injector in a return fuel line. The best way to test it, is to connect an analog dwell meter to the green white stripe wire in the test connector under the left side engine compartment cover. Then read the duty cycle, cold in open loop and hot in closed loop.

Roger that Paul.

Quick question. Why is there a vacuum line on my WUR? Usually when you have a device with a diaphragm such as this one, you also have a vent to atmosphere or the diaphragm won't displace. Is this designed so that engine vacuum assists the regulator? High vacuum (idle) would help open the regulator to bleed fuel back to tank and low vacuum (acceleration) helps shut off the return line for more control pressure in fuel distributor.
Does vacuum provide a calculated bias to the action of WUR?

Thanks,
Warren

Warren,

No, its a vent.

Reply
 

The car has always had a vacuum (?) line attached. At least I assume it's a vacuum line. Not sure of anything at this point. I'll go out later to check it out.

Also the line has a plastic piece - either a filter or a check valve - I would hope a filter to keep outside air from being suck in when diaphram moves in one direction.

I haven't traced line to other end. Maybe it goes to charcoal cannister perhaps (to capture possible gas fumes)?

Thanks again for input.

Warren


http://forums.pelicanparts.com/uploa...1301233365.jpg

As Paul stated, it's a vent line. It should go to an attachment point on the throttle body above the throttle plate where it sees only atmospheric pressure. Your WUR should have a series of numbers ending in 090. This unit does not have a vacuum diaphragm.

The plastic piece is just an adapter between the WUR and the vent hose.

Thanks to all
 

Yeah I figured it was routed somewhere that saw a small amount of filtered vacuum.

I think next step is to get a dwell meter. Back in the seventies everyone had a dwell meter. No telling where mine went to. Next stop - Ebay!

I shall return.

Thanks again guys,
Warren

http://forums.pelicanparts.com/uploa...1301246305.jpg