930 start and run mixture difficulties

[T77911S]

could be the O2 sensor relay went bad. or more likely a bad solder joint.
check to see if the frequency valve is vibrating with the key on.
this would or should make it go lean if it is off.

if it runs bad once warm I would look at the speed relay. above about 3500rpm it puts the system in open loop mood. also look for bad solder joint.

check fuses

[worry9]

Thanks Brian,

I will try that tomorrow. Fingers crossed this is the culprit. I was fairly convinced the plunger in the FD was sticking form lack of use.

One step at a time...

Thank you again for your advice.

I will let you know how it pans out.

[worry9]

Thank you T77911S.

Do I understand that vibrating is normal and if not vibrating, a lean condition can occur?

Finally speed relay? Is this under the DS seat with the O2 relay? I see a lot written about this.

Will investigate and advise.

Cheers cam

[T77911S]

speed relay is under seat.
a lot of guys run without the O2 sensor including my brother BUT the FV still has to work.
I am not sure of what CP's he runs but I think they are close to stock. he has a nice rich mixture. his engine has been a great motor with around 140k on it and still going strong.
wish I could my AFR's like his.

[Rawknees'Turbo]

Originally Posted by worry9

Finally speed relay? Is this under the DS seat with the O2 relay? I see a lot written about this.

The O2 sensor relay is the rearmost relay on the front fuse/relay panel (aka - the rear fuel pump relay). That relay also supplies power for the WUR's heating element.


Non-turbo 911s with CIS/K-Jet have a separate relay for the O2 sensor that is not shared with a fuel pump, but the Turbos do not.

[RarlyL8]

Yes the frequency valve will buzz when working properly. Unfortunately that system cannot be defeated without fuel distributor mod or replacement.
Lambda system fail mode is lean mixture.

[worry9]

OK, FV appears to be inoperative. ie No "buzz" I can feel a slight vibration, but I would expect that to be just the fuel pump whirring

I have disconnected O2 at plug and reconnected. No change in either condition. I will try earthing and 1.5V as per Wayne's O2 advice. Then will work my way through O2 relays and speed relays and fuses etc.

Brian, if we set the AFR at idle at 13.0-13.5 ( I assume by using the mixture screw and idle speed screw), with the Lambda disconnected, how will the AFR look at high ie+3500 rpm when Lambda system is bypassed by speed relay? Or do we disconnect Speed relay too?

I do however, intend to sort out Lambda system anyway, but as a stop gap to use the vehicle, and my own knowledge,

Kevin930t ...happy New Year back at you...

[JFairman]

You have some cool cars
The lambda valve is an inline fuel injector so it should have some vibration. If you took some dowel rod and put one end against it and the other end against your ear you'll probably hear the buzzzz.

[worry9]

OK update. Thanks JFairman...like my cars!

Confirm no FV buzz.

It would appear I am getting no power to Lambda circuit at all. No power to either relays under the seat. ie Speed relay and O2 Control unit relay. I am also not getting any power to O2 heater. O2 sensor has 6 ohm resistance cold.

Both fuel pumps are running, swapped relays around just in case.



I am now wondering if we have a open circuit between fuel pump relay and lambda circuit?

[worry9]

OK SUCCESS!!

The Oxygen Sensor Control Positive Power Supply Relay I believe is in the DS engine bay and is the standard black relay nestled next to the Yellow Over boost relay.

There is a excel spreadsheet floating around on the forum with line 22 with question marks against its position. I hope I have solved this mystery.

I swapped this relay with a known item and suddenly the FV and all other items sprang to life. Happy Days.

Not wanting to peeve the neighbours any more, I will run the car up in the morning and reset the mixture to normal.

Will report/confirm final result then.

Thank you to all those who have contributed. Brilliant!

[worry9]

Folks, Can confirm all is well with the 930. Again, thank you to all the contributors. Learnt a heck of a lot. Much appreciated!! Off to buy a couple of relays form our host now!