Can you feel a duty cycle valve running?

[A/SP911]

I've got a 1980 SC that has developed a bad stumble throughout the RPM range. Unfortunately I'm a 12hr drive from the nearest Porsche dealer so I am on my own on this. Basically it feels like it's running on 5 cylinders under load warm or cold.

The engine has been breathed on slightly with S cams, a Jacobs ignition, some headwork that included a 60/45/30 valve job, a B&B muffler, and the usual upgrades (Thank goodness for the popoff valve as it backfires now too)

I've pulled the plugs and they are all the same with top of the tips kinda white. The compression shows 170 on all cylinders and there is no shorts between the plug wires that I can find. I also check the fuses and relays for bad connections per a post I found here on Pelican Parts, but the connections look good.

I did notice that there seems to be two vacuum hose nipples on the passenger side of the engine towards the back that are just venting to air, is this right? Also I had a VW mechanic note that when he puts his hand on what he calls the duty cycle valve located in the top middle of the engine between the air box and the intake plenum that he cannot feel or hear any cycling of the excess fuel in it. Could this be a sign of a probelm?

Any help would be greatly appreciated!

Thanks
S.Chapman

[Doug Zielke]

Duty Cycle Valve??
Also called, Frequency Valve.
Lambda-equipped CIS 3.0's have one; approx. where you describe it.
It does vibrate when operating. If not, it can be NFG, or the Lambda computer, or it's relay....
Without the FV operating correctly, your motor will run poorly.

[A/SP911]

Thanks from the right coast of Canada! I hear your getting some rain in BC...

Anybody know of a way to test this Frequency valve?

I assume the Lambda is the computer under the passenger seat. Anyway to test it?

And assuming that the relay is the red one up front is there anyway to test it? Can you switch in a black one?

Thanks
S.Chapman

[emcon5]

What Doug said. Put your hand on it, You can feel it vibrate with the engine running. It is located on the front of the engine, with an electrical connection coming out of it, looks like a EFI fuel injector.

I posted a picture of it once, I can't find it right now, but if I do I will post it again.

The red relay is the fuel pump.

The relay is under the passenger seat, and gets power from the dome light fuse (do the dome lights work?).

S cams? Like 2.4 S cams? Did you change the pistons? Has it ever run well at low RPMs? High lift-high duration cams are not supposed to work well with CIS.

Tom

[Doug Zielke]

The Lambda relay is *not* located with the other relays. It is a small (1" x 1") Bosch relay plugged in to the exterior of the Lambda computer, which is under the passenger seat.

If you un-plug the relay, and the car runs worse, then you know the Lambda system is porbably ok. If there is no change, try a new relay; they are cheap.

[emcon5]

Found it:

It is hard to see, but it is part of a thick fuel hose behind the throttle body, with a electrical plug on top.

This is a pic of the photo from the Benteley SC manual. To give you an idea of where this is, the little half circle cut off at the top of the pic is the idle adjustment screw. According to the book, the test is to put your finger on it to see if it is vibrating. If it isn't, pull off the electrical connection, and check the resistance of the winding, it is supposed to be 2 to 3 ohms.

[Doug Zielke]

And a close-up of the FV on the 'Bomber's 3.0....



It's tucked in behind the air box pretty well, but you can feel it buzzing when your fingers find it.
And....**Don't get yourself caught in the fan!!**

[A/SP911]

Thanks guys! I'll check the dome light thing and voltages in the morning.. ah wait look it is morning here. Unfortunately the relays may be cheap but unless you know of a VW/Audi part that may work I'll have to fly one in.

As for the cam type everything the previous owner did was per Bruce Anderson's book (every time I had a question the seller would say something like "I'm not sure what the torsion bar size is I had the size Bruce recommended put in").

On page 145 it says something about "S" cams and the fact that they needed additional valve clearance... maybe he did put in pistons. I don't know what kinda of cams they are but on the track I can leave stock 3.2's behind on the backstraight but not quite catch the older Turbo's. The idle was a bit lumpy but very drivable on the street and made a beautiful sound with the B&B dual exhaust... blub,blub,blub,blub

S.Chapman

[emcon5]

Originally posted by A/SP911
Thanks guys! I'll check the dome light thing and voltages in the morning.. ah wait look it is morning here. Unfortunately the relays may be cheap but unless you know of a VW/Audi part that may work I'll have to fly one in.

The one in mine is an Audi part. I'll try and get the number off it tomorrow.

Tom

[A/SP911]

I replaced the relay with a fog lamp rely of the same specs but it didn't help.

So I dug out my wiring diagram and put a voltmeter on the connectors going into the Lambda computer and found that there is no activity on terminals 15 or 17 going to the frequency valve which makes me believe that I need a new computer... Any thoughts?

S.Chapman

[emcon5]

You check the dome light fuse?

Tom

[A/SP911]

I didn't check the fuse but the dome lights are working fine. Any chance the computer is getting a wrong signal from somewhere and therefore isn't actuating the frequency valve, ie oyxgen sensor? (Note that the problem exists whether it's hot or cold though)

S.Chapman

[Doug Zielke]

The FV operates with or without the O2 sensor connected. But make sure you are getting 12 volts to the computer power input.

[emcon5]

Here is the VW/Audi version of the frequency valve relay:







Make sure the relay is correct, just because it fits doesn't mean it is the same. I know there is a 944 window relay that will fit in that socket that will not work, the guts are different.

The wiring diagram for the Lamda system is here:
http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part6-2.jpg

Not sure what you mean by "terminals 15 or 17", the relay should have terminals 30, 85, 86, 87, and 87b. Terminal 30 is powered from the dome light fuse (and is hot all the time). Terminal 86 is powered from the fuel pump relay, and should be hot only with the engine running.

Did you try and feel if it was buzzing with the engine at idle?

Doug, I will see your tiny picture, and raise you a wider shot, with a spiffy red circle :




Tom

[Doug Zielke]

Thanks for the circle, Tom!
What a wonderful maze of pipes and parts!!
CIS: In your heart you know it's right!

[emcon5]

So did you figre it out?

Enquiring minds want to know.

Tom

[A/SP911]

Sorry about not getting back to you sooner, I've been out of town buying another tow vehicle for the P-car as my old half ton wasn't quite up to the job

Actually I was headed to a Solo I event at Atlantic Motorsport Park when the car broke. I usually tow to the track but a friend of mine with a WRX STi offered to take some of my tires so I thought I'd drive down. If I had been towing it to the track I would have probably gotten at least a morning session in... Arragh

Anyway getting back on topic... as I was getting power to the computer I knew that the relay was fine so I got a VW mechanic friend to put a voltmeter on the back side of the connector going into the Lambda while I tried to keep it running. He found that although there was power going to the Frequency Valve it never changed readings on terminals 15 and 17. He figured that the reading needed to go up and down so that the valve could do it's job. So I located a computer out of a 1981 SC plugged it in and she fired right up, problem solved. Too bad there is no more events this year...

I want to thank you guys for your help, it got me checking the right things and was a big help.

S. Chapman

[thabaer]

Most frequency valves are switched too fast (ergo "frequency") for voltmeters to show it, you want a meter that can read the duty cycle. I usually just listen for FVs, seems like I can always hear them running without needing to feel for the vibration.