Frequency Valve '82 SC, rough running.
 

Hi Guys,
I've done so much reading over the weekend that I am a bit confused so with your help I want to put together a quick synopsis on what my troubles might be and a path to the remedy.

1982 SC has slight stutter while driving at constant throttle at low to medium engine speed.
I have read that this is most likely the Frequency valve not working.
AAR valve works - checked for operation
WUR seems to be Ok because it warms up nicely.
Fuel pressures and AFR ok - just checked by a tech.
Timing, HT leads and plugs all just checked by a tech.

Trunk, interior lights and clock stopped working - Blown fuse. Feed it more fuses and it keeps blowing.

Traced the short to the trunk light wire, shorting on the steel holding clips and got it back working for a while but it blew again. Needs a deeper look.

So, I read that the frequency valve relay is powered off the same circuit as the courtesy lights and clock?

Please correct me if I'm wrong here but what I should do is;

1. Sort out the short circuit
2. Change out the relay. I read that it is in the K Jet control box under the seat. I can't find this with a look underneath but I haven't yet removed the left hand seat. Would it be anywhere else? The car is Ex UK, right hand drive if that makes any difference. Is there another name for this relay because searching Pelican Parts doesn't show a result for "frequency valve relay". Any idea of a part number?
3. Check Frequency valve for buzzing when 1 and 2 are sorted.
4. Get a new frequency valve. These are expensive! Is there an OEM Bosch or pattern part or a cheaper version of the same thing from another marque?

Would the most likely failure point for this problem be the fuse, the relay or the frequency valve? And is the stuttering in fact related to the frequency valve?

Thanks guys, much appreciated.

the frequency valve (FV) is not a hi failure item.

the fuse and the relay are. the fuse provides power to the relay. that HAS to be fixed.

post you numbers from having the pressures checked.
WUR can only be verified to work by checking pressures.

i am a bit confused. this is a UK car? with lambda?

I assume they were tested with a Ohm meter for resistance and continuity for the core which is something anyone can do with a volt meter. But was he able to test the integrity of the insulation for it's ability to contain a minimum 15kv? For that he would have needed a Megger, not a tool usually found in a tech tool box.

In other words, your problems could still be as simple as replacing your HT cables.


Cheers,

Joe

As was mentioned above, I'd be real surprised if you have a right hand drive car with Lambda control. Before you head off down the wrong path, post the type and serial numbers of your engine, so we can see what you have.

JR

Hi, thanks for your reply.
I don't have the pressure numbers but I could find out if they still have them.

I think i am the confused one. It is a UK car. Does this mean it won't have a Lambda, and maybe not a frequency valve? I just checked properly and there is no K Jet control box under the seats.

Thanks Joe.
I couldn't be sure of that, but I suspect not. I am a bit green on electrical things and I assume that only an Ohm meter was used.
A plug cap was replaced and that seemed to fix a cough under load. Other than the slight stutter at constant low throttle, it runs great.

Thanks JR,
The engine number is 63C4917, 930/10
New in the UK in 82.
Any help would be great.

If your ignition cables look like the originals, you might just replace them to eliminate any possibility that they might be at fault rather than trying to diagnose the entire FI system.

ignition has to the first thing looked at when it runs bad.
new plugs, cap rotor and good wires.
then i would look at the timing. make sure it is set properly and most important is the advance system(s) working.

then i would look at air leaks, control pressures and mixture setting.

make sure you have a mechanic that knows CIS.

A 930/10 is not fitted with an O2 sensor, so you don't have a frequency valve, either. I don't know what you mean by "stutter" but it sounds a little like a lean msifire to me. When you say the tech checked the AFR, do you mean he measured the CO? What machine did he use to check this? What value is it set at?

JR

If you have lambda, you should be able to disconnect the interior lights and clock at the fuse and leave the lambda circuit connected. There should be two wires coming out of that the fuse block -- one for lambda and one for the rest.

Hard to diagnose things you don't know you don't even have. So it might be wise to start with the basics, like new ignition cables and go from there.

Ok, will check over those things again, thanks for you advice

Thanks, yes, they do look pretty old.

Ok, thanks, I assumed they all would have a frequency valve but must be certain market models only. The stutter is an ever so slight constant miss at light constant throttle. Quite hard to detect but it is there. Goes away when you give it the gas. The AFR was checked with an EGA measuring the CO2. The value was set at approximately 13.6 at idle. Would that be an appropriate number?

No. Porsche specs reference the CO percentage, not the air-fuel ratio. There is a correlation between the two but with these cars, it is always best to do what the factory intended, using the correct tools. You need to take it to a shop that knows the CIS injected 911s and get them to set the CO using the correct meter and the correct procedure. There's more than one way to do it and several of those ways will not give you the results you want.

Make sure the engine is well warmed up when they set the mixture.

JR

The warm fuel pressure was 45psi. Cold wasn't checked as I couldn't leave the car there overnight to do that.

I must say that after the tech (very experienced guys) fettled everything it ran a whole lot nicer throughout the cold / warm cycle. Just this final thing to sort out and it is barely perceptible but if I can sort it out without spending big on chasing it then that would be a great result. I don't mind spending on new parts but it is the diagnosis that usually costs the dollars chasing ghosts if you let the pro's loose on it for days.

Thanks, good advice. I will ask around who has the correct equipment.

Just a look from a different angle.

The term constant miss can mean (or be construed as) a secondary electrical problem. Not saying it is the problem.

However, for fun, idle your car in the dark. Give your eyes 10 minutes to adjust and then look in the engine compartment for sparks. Rev it by hand to the "crusing RPM. Look around the coil, between wires on the dist cap, arcing to metal parts (e.g., AC brackets), blue light coming from spark plug hole (those valve cover thingies on the plugs always have a little gap you can see through)

If it is fuel related, maybe it's an injector that does not spritz too well causing a lean misfire, but, open the throttle and it pisses enough gas to make things kind of ok.

Try some Techron for a tank or two to see if a change occurs.

Thanks Bob,

The spark in the dark is a great idea. Not heard that before. I will try the injector cleaner as well.

The constant miss - stuttering - feels like soft continuous driveline shunt at low engine revs but very minor. I have to go looking for smooth roads to detect it from the somewhat choppy ride of the car.