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AndrewCologne 04-26-2018 05:16 AM

Backdate Fuel Distributor on a lambda based CIS to bypass the frequency valve

Im thinking about to remove the whole lambda stuff from the CIS of my 82' 911 SC (US version).
I read that the ECU unit and frequency valve are needed so the existing original Fuel distributor model provides the correct air fuel ratio.

Is it possible to just install a Fuel distributor like the one for 78-79 non lambda models (like the model 0438100031) so the frequency valve and ECU unit can be bypassed and therefore removed?

Would I be forced to change to a different WUR as well, which fits to the 0438100031 FD?
If yes, from where can I get the vaccum to be provided to the vacuum connection of the 78-79 WUR? Does there exist a vacuum connection at the throttle body of my '82 CIS beside the one provided for the ignition distributor.

Thanks a lot for clarification and best regards

30westrob 04-26-2018 06:09 AM

If your goal is to defeat the lambda just unplug the O2 sensor and the system goes into nil mode, which gives you set buypass. The lambda will no longer fine tune mixture.

I question your motives for your proposed change. I have found lambda system cars to give superior fuel management, where the system corrects for small issues, like fuel quality small air leaks, etc. Lambda also gives you enrichment at WOT. Rob

Sub8 04-26-2018 06:36 AM

Disconnecting the hego sets the duty cycle to 50% at the frequency valve.

You still get 65% duty when cold.

You still get 65% duty cycle when throttle microswitch opens, for 2sec.

So easiest way is just disconnect the hego, assuming everything else is working correctly.

Lambda does not give you continuous enrichment at WOT, only a 2s burst of 65% duty cycle.

Without hego fuel control pressure at the WUR can be modified to make WOT leaner or richer. Of course your part throttle cruise AFR will also be affected by this. This is an area that the hego system can help with as when closed loop it will still target a lambda of 1 helping fuel economy.

Same true the other way as well; if you want it slightly leaner at WOT. But without the Hego if you go in this direction you will have drive issues at part throttle as you are now lean of stoich fuelling (lean misfire).

PcarPhil 04-26-2018 07:56 AM

Edit: I just noticed this thread is in the 911 tech section, not the 930 section. I'll leave my post here as I believe this info still applies.

I did this with my '87 turbo (which has the aluminum lambda fuel distributor) to simplify the CIS system in preparation for adding the FrankenCIS system. Once I had the system dialed in the car was running so nicely that I abandoned my plans to install FrankenCIS! I did not have to replace my FD, or even extensively modify my stock FD, I just had to do some tuning.

To remove the lambda ECU and related wiring there is a jumper that needs to be added to the wiring on the engine side of the harness. If you do a search here on Pelican you can find the needed info.

Regarding the frequency valve and FD - I simply disconnected the frequency valve wire connector leaving the valve in the FD. Then I adjusted the WUR to get the pressures back to where they needed to be. Finally I adjusted the individual ports of the FD to get the ports as balanced as possible to each other (along with giving a little extra fuel to cylinders 2 and 5).

Having a WB02 sensor and gauge in place made the overall process fairly easy. Having a set of the CIS pressure gauges would be a good idea as well.

If you do all this and find that you're running lean in the upper RPMs (I am not) then you could add the FrankenCIS system to control your frequency valve to add fuel at the top end as needed.

T77911S 04-26-2018 10:30 AM

yes you can backdate the FD. you also need the WUR because the earlier WUR had a vacuum fuel enrichment for acceleration. if you live where it is cold I would suggest a thermo time valve for added enrichment for cold starts.

I prefer the non lambda systems just for reliability.
unless your system is in need of repairs I personally would not spend the $$ just to backdate.

you can go open loop mode as suggested. you may have to tweak the control pressures.

SCadaddle 04-26-2018 10:37 AM

When I obtained my 82SC, the entire Lamda circuit was inoperable due to the O2 sensor not being plugged into the wiring and the wiring from the Lamda box under the passengers seat to the Frequency valve had been shorted out sometime before as it was extra crispy. The car ran and drove fine---probably because the mixture was set very rich---but it wasn't how the system was intended to function.

I plugged the O2 sensor to it's connector, reaired the wiring from the Lamda box under the passengers seat to the Frequency Valve and set the mixture to the spec CO reading---which apparently meant a LOT leaner than it had been running. Everything was fine and dandy.

Then one day the actual Lamda box under the passengers seat decided to retire. Kaput! The car had absolutely NO power and actually idled so bad the rear end of the car became a paint shaker. Sourced another Lamda box and all was well again.

Long story short, if it aint broke don't fix it!

AndrewCologne 04-26-2018 12:12 PM

First thanks for all your support! Great community.

Im aware of the "Home mode" of the ECU and the resulting 50/50 duty cicle of the freq valve when the O2 sensor is disconnected. Thats exactly what I did and also raised the mixture to about 2,5% CO. And the Car runs smooth and everything is fine.
Some day the idle was bad and when just cruising the engine felt like a bit stuttering.
I went into the system and found out the the common guilty was the O2-relay below the seat, so I put a new one and everything was fine.

Now since some days "sometimes" the idle again shows these symptoms and also when cruising. And then after a few seconds everything runs fine again. To me it seems that theres an unsteady electrical open circuit issue present at some of these CIS "ECU/freqValve" parts.

I hope its again the relay, if not then I have to check the ECU Unit and ... the freq valve itself. But .... replacing one of these devices is expensive.

So THATS why I got the idea of backdating to the 78-79 Fuel Distributor and the matching WUR. So getting rid of the lambda parts of the engine which kills the risk of getting something kaputt which isn't present anymore ;-)

don gilbert 04-26-2018 04:34 PM

Fix your system! It works great when sorted correctly, gets great fuel mileage, and overall just better drivability.

Sub8 04-27-2018 02:36 AM

Yeah I get the occasional slightly low load / lower RPM rough running as well on my CIS-lambda. I have been using the Hego sensor boss for my own wide-band sensor.

This slight roughness is when the system just runs a little lean, like 15 - 15.5:1. This is the trade off of not having the Hego plugged in. Give the idle mixture screw a 1/8 turn rich.....or plug the hego back in!

If you have a suitable multimeter you can also log the dwell or duty cycle (depending on meter) at the connector under the passenger seat.

T77911S 04-27-2018 04:37 AM

yea the relay IS the weakest point,
also check fuse 18(?). spin it to clean it off.

take the old relay and resolder all the joints. its probably just a bad solder joint and this will make it better than new.

AndrewCologne 05-01-2018 10:52 PM

Thanks for all your input.
Yesterday the car was running a lot and I figured out that the stutter especially at high RPMs starts when the engine is hot/warm. The engine suddenly doesn't pull that strong as it was.
I stopped the car and the idle was normal. In my experience, if the Relay is the guilty then the idle should be rough or not normal as well. So I did pull the OSX-relay and yes, the idle drops and gets rough as it should be. So ... Im a bit confused.
Then at the evening also the idle sometimes gets rough as well ... in the same way like known from a bad OSX-relay. But suddenly .. as written above, the idle gets stabilized again.


Originally Posted by Sub8 (Post 10016724)
If you have a suitable multimeter you can also log the dwell or duty cycle (depending on meter) at the connector under the passenger seat.

I thought that can be only done via the testing port in the engine compartment?
Where is that plug/contact/connetor under the passenger seat? I assume its somewhere at the ECU, right?

However, ... that diagnostic above tells me that there is somewhere an electrical circuit problem in the ECU/FV system. So now I've to figure out if the ECU is bad, the Frequency valve, the wiring loom or if the relay again is flawed.

As first step of the test I will do is to pull the relay and use my special y-wiring to bypass the relay via that kind of jumper so both 87 terminals of the OSX-relay socket will get terminal 30 current directly.

I hope it won't be a bad Frequency valve, as its quite expensive. Not mention the price of an ECU.

Best regards from Cologne!

pmax 05-01-2018 11:49 PM

Just to narrow down the possibilities, what is the number on your WUR ? It should be a 090.

If you have a CIS fuel pressure tester, check the control and system pressures.

T77911S 05-02-2018 03:47 AM

your mixture is probably not set correctly due to air leak, wrong fuel pressure, mixture has been adjusted, engine wear.
could also be injector issue, ignition issue

AndrewCologne 05-11-2018 04:40 AM

Did a few more tests.

I pulled the OXS Relay ... and used a jumper cable problem still remains.

Then I swapped the ECU but ... problem also here still remains. As said, especially when engine is hot after a highway drive. When leaving the highway I again recognized that also sometimes the idle drops for a few seconds and stabilizes again.
And at THIS moment when the idle dropped for a few secs I did the test and pulled the jumper cable at the plug of the OXS Relay and ... nothing changes. This means to me that the system of ECU and Freq valve is the guilty one and not the WUR or control/system pressure related.
So ... as last test I'll try to swap the Freq valve but its not easy to get one here.
But ... normally engine temperature should not be an issue in case of the freq valve?! As the gasoline should keep it cool and it doesn't touch hot engine parts.


your mixture is probably not set correctly due to air leak, wrong fuel pressure, mixture has been adjusted, engine wear.
could also be injector issue, ignition issue
The engine was overhauled in 2013 and if an air leak would be the guilty one, I think it the idle wouldn't drop that fast and stabilizes that fast again. I have the feeling that some electrical circuit ist the problem and pulling the relay where no change occurs maybe proofs that.

T77911S 05-11-2018 05:27 AM

the O2 rly only provides power.

if you removed the jumper, which would be like removing the O2 rly, and nothing changed then you either did not do what you think you did or the mixture is way off or something is not wired like it should.
when you remove the O2 rly the mixture should go extremely lean.

if the fV vibrates, it should be fine.

checking the duty cycle of the FV is a better test. you can do it with a dwell meter.

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