1983 911SC Stutter

[Jim Schrager]

1983 911SC, USA car, been sitting a few years, we are bringing it back to life. Miles unknown, drained fuel and oil and it started right up and runs generally well, no funny noises, no black or blue smoke. Idles smooth and steady at 800 RPM, warm or cold. Put Techron in the first tank of gas. Runs well but stutters at light throttle or steady running up to about 4,000 RPM. Goes away if you give it throttle, pulls strong, but always comes back at steady throttle. One day last week, magically, it went away. I started the car, the idle set itself at 2,000 RPM (it was a cold day) and the car ran great all the time. Idle never dropped bwlow about 1200, but car ran great, fast, no stutter. Today, on a long drive, stutter came back at 50 mph. Now back to 800 RPM idle and stutter at light throttle. I am mostly an early 911 guy, wondered which of the sensors and adjusters to the CIS this might be.
Many thanks, Jim Schrager, South Bend, IN, jimschrager@compuserve.com

[Boxer_Airhead]

Bump...

I am interested in solving this mystery myself.

[Walt Fricke]

Jim

The usual drill for CIS related problems involves some familiar steps - checking other things just in case, like the distributor, plugs, fuel filters. Not likely to cause your symptoms, seems to me, but still good to know. Color of the plugs might point to something. Your distributor has two vacuum lines, red and blue (or black). These can get mixed up, as they serve different purposes, so that might be worth checking (I forget which is which, but that's easy to find out, and you could just do a search here on PP to find out if you have no manual to guide you).

Then one checks out the CIS system and control pressures using a gauge, which you can buy from JC Whitney for not too much money. This tells you if you are in spec there. You might be, despite the fact that, as you must have surmised, this looks to be engine temperature related in some way. Then a guy would consider checking the frequency valve, which is controlled by the oxygen sensor and the mini-computer under the passenger seat. Is it at least working? You can use a dwell meter or frequency meter on it, and there is a plug on the end of a couple of wires which stick out from the rear of your engine fuse box for use in this diagnostic procedure. Next might come a check of the auxilliary air slide over on the right side, mainly to see if its internal heater causes it to close as it should (if I don't remember that backward).

But perhaps you could cut to the chase by using a high quality oxygen sensor or other exhaust gas component measurer. Especially one of the recording kind you can use as you drive around, so you capture things as the stutter happens. Because my not too solid first guess is that there may be some air leaks if the distributor is working correctly. But this measuring would tell you if it was a rich stutter or a lean stutter.

Air leaks supposedly usually manifest themselves in a hunting idle and the like, but you say yours is rock steady, so that seems a bit less likely. You could still check for them in the time honored ways. The intake gaskets on the heads can dry out or compress over time, among other places that can admit unmeasured air.

Whatever it is, at part throttle the frequency valve ought to be able to compensate for an inherent over rich or over lean situation. Unless it is so out of whack that this device can't compensate, since it has a limited range of compensation. It works by reading the O2 sensor - if the sensor says lean, the FV drives things rich until the sensor says rich, at which point the FV drives it the other way.

You might want to look up and read the information Jim Williams has on his website on CIS. He includes some of the factory fault diagnosis trees, though I don't recall part throttle stutter being listed. This will get you up to speed some on how CIS works, and how the various add-ons compensate for this and that. Plus you will find a lot of CIS discussion stored here at PP.

Welcome to the world of the CIS. Maybe someone has experienced just this symptom and will share the experience gained.

Walt Fricke

[Jim Schrager]

Nothing like talking to a factory tech. I explained the symptoms to my ace wrench, Marty Hostetler of Bill's in Elkhart, IN, and without hesitation he said the frequency valve isn't working. This is because the stutter went away when I pushed on the gas pedal, and it idled rock steady at 800 RPM. He told me to check the relay under the passenger seat and if it was on the blink, that would explain all. I did, it was not working, and we gave it a few stout whacks, and the car drives great. Thanks to those who helped, I learned a bunch about the car going through this. As an early 911 guy, I've got plenty to learn about all the bells and whistles on the CIS. Best regards, Jim Schrager

[mack6820]

hmmm. I dont have a relay under my passenger seat. Could it be cause I have a 1980 engine in my 83sc?

[mack6820]

I found it. Its under my drivers seat. I have no idea why. Anyway, I read out the plug going into the box and didnt get any power. So I jumpered out the relay pins 30 and 87, then I got pwer to the box. So I replaced the relay and HOLY CRAP!! Ive had this car for a year and never knew exactly how much power I was losing. Its a completely new animal!! Ive been reborn again to my Porsche. Thanks for the help guys.

[Gunter]

Keep an extra relay and remember that the relay is powered by the same fuse as the dome light.

[rallyracer]

Originally posted by Gunter
Keep an extra relay and remember that the relay is powered by the same fuse as the dome light.

Too add a bit to this discussion, I had a short in a dome light that was blowing the O2 sensor circuit. I have a wire with a connector (not installed by me) at that fuse that appears to power the lights, but not the O2. Is this correct? I just unplugged it and taped off and no more blown fuse.

My question is if my car runs fine, unlike when the fuse blew, then is the circuit ok for the relay?
Thanks,
David

[mack6820]

just read out pin 30. If your getting power to 30, then you should be ok.

[Gunter]

David:
The relay is not for the OX sensor, it's for the FV.
Normally OEM, the fuse for the dome light also gives power to the relay on the Lambda brain under the seat. That in turn operates the Frequency Valve (FV) metering the fuel. The FV is behind the airbox and you can feel it vibrating when it operates.
With engine running in idle, carefully pull off the relay from the brain.
Any changes? Engine idling crappy now?
Put some dielectric grease on the pins and re-install.
To find the FV, or any other components, get the Bentley Repair SC Manual, you'll love it!

[SCWDP911]

sub'd for future reference!

[74threeliter]

Does anybody have a pic or an idea where in the engine compartment the frequency valve is receiving its power. i am trying to trace an electrical issue. i believe there should be a fuse within the line from the engine compartment, i could be wrong tho. i had a shop fix the stutter i had and they told me its the connections but i dont know where they are, the FV works, the O2relay is new, the ECU is working... but there is some electrical fuse or connection that is being interrupted and driving today i must've hit a pothole or something because it was running fine in the morning then all of a sudden my idle dropped to 400 rpms and only ran good if i had the rpms over 3000. i am getting the Bentley, but id like to know if someone has an idea(explanation), or a pic displaying where on the FV receives its power from. this is aggravating me as i have sent the car to the shop "3" times for the exact same thing. But i would like to conquer this one with some forum help if possible. When all is good i can hear and feel the buzzing FV. thanks in advance

[74threeliter]

Bump - does anyone know if the wire from the ecu to the frequency valve goes straight to the FV or reaches the engine compartment fuse box first?

Also if my relay under the seat is HOT! is that bad? or expected? Could i be having a power surge going to this relay? I mean this thing can really warm up.

I also read somewhere there's a wire that goes to the FV with switched Ground? or is it switched Power?

Where would the common locations be of electrical gremlins associated with powering the FV?

-82SC engine with some electrical gremlins, behaviors include moving the turn signal switch to on for the right turn will TURN ON with engine off, no high beams for H1 lights. Could any of these items factor for what i am seeing with the FV?

[porsche930dude]

how do i test the FV? My car idles at 1500 rpm occationaly changing to 1100- 2000 rpm. This is all after it has been sitting for winter. I changed the enrichment control box under the seat because that was obviously bad and the idle fluctuation seems to have been fixed. When i unplug the jetronic box the idle goes down from 1500- about 1100rpm but fluctuates slightly. When i push in the clutch the idle goes up 100 rpm and goes down 100 rpm when i turn the lights on. Its very strange. Im thinking a bottle of techron tomorrow and a ride to see if things clear up. Any clues before i do so?
Thanks

[Walt Fricke]

Dude

Test 1 for the FV is the hand's on test: feel it. It should feel like it is vibrating.

Test 2 is to hook your dwell meter (saved from long ago when you worked on cars with points) to the test port, which is a small wiring loom sticking out the back of your rear fusebox. There is a way of converting dwell readings to FV percentages.

What you call the Jetronic box controls the FV through the FV relay. Until the engine is at least warm, the box uses a warm-up value for the frequency. Then it switches to looking to what the O2 sensor is saying (which also needs to have things warmed up, as it is a one wire dealie), and adjusts the frequency based on that. And at WOT it switches again to a WOT frequency.

And it has a program, so to speak, for when it isn't getting anything from the O2 sensor due to a broken wire or a failed sensor.

As I recall from installing an '82 in my '77, all of the wiring for this runs through the separate big loom from the box back to the engine.

A wiring diagram ought to answer your question about whether the relay grounds or supplies + power to the FV.

My engine didn't start easily or idle well when I tried to shortcut things and leave off the box and its stuff. Much betta after I put it all in.

Walt

[psalt]

There is a test connector behind the plastic cover on the left side of the engine compartment. If you hook up an analog dwell meter to the pin with the green and white wire, you can see the FV duty cycle. The system is open loop when cold and the meter should show a steady 58 (65% duty). Once the engine is above 15C, the needle should start fluctuating between 40-58 in a steady rhythm. If the needle doesn't fluctuate you need to test the 02 sensor. If the needle fluctuates between 20-30, the mixture setting is too rich, between 60-70, too lean. When you set the mixture on this system, you are really only altering the open loop mixture (cold and WOT), the system corrects the mixture back within the range. I usually set it around 30-40, because I have found the WOT mixture too lean on many of these engines.

[porsche930dude]

Wow loads of info! My problem is kinda solved but now i have another. We tried tesing the FV as the hayes manual states 2-3 ohms of resistance at the FV. That registered zero. We tried removing it to better test it but the fittings were reluctant. As we were doing this my brother suggested we try a different oxs relay. so we stuck everything back together and tried it. Pow it worked great! So we went for a ride and i noticed it doesnt have the power it usually does above 3k rpm. its almost like the advance isnt kicking in. other than that its fine. Im going to check it over again tomorrow and reveiw all your info when im more awake.
Thanks guys