911 SC (1982) feels a bit 'jerky'?

[Buzetti]

Hi, I have just collected my new 911 SC - it seems to run great, but has a slightly ‘odd’ feeling at fairly low revs – say up to around 2.5k revs and when not under load (say doing about 30mph is second gear on the flat) & off the throttle – sounds almost as if the engine is ‘hunting’ and progress feels kind of slightly jerky – does that sound odd? Or is that what they are supposed to feel like?

Clutch feels fine and the car goes like a rocket - engine is very smooth.

Car seems to be idling when hot at about 700-750 RPM - is that right?

Cheers

Buzz

[emac]

RPM should be around 850-900, probably going too slow to be in second gear which would cause the jerking. Be advised they love to be rode hard
Ernie 81sc

[ManniB]

Buzz,

with my '81 SC US engine, I can feel a slight surge when coasting around town. It comes and goes with a frequency in the several seconds range and is only noticeable when I just barely touch the throttle. I found out that it is in sync with the CIS system regulating the mixture. Comments from fellow Pelicans: Thats the joy of CIS ;-)

Regards, Manfred

[Buzetti]

Thanks.

Gave it another try this morning - is worse from cold - feels almost as though it is 'missing' slightly under low revs and over-run - but idles smooth and revs up smoothly as far as I can tell....

[Nickshu]

Had this problem on a VW rabbit with K-Jet CIS years ago. Turned out to be a bad fuel distributor. The problem with CIS is that it's great when it works, but when there is a problem there are about a zillion things that could cause it. You can do some reading here on the forum (TONS of info) on troubleshooting CIS. Looking for vacuum leaks and checking fuel pressures seems to be the place to start. I seem to recall someone with this issue finding out it was the fuel pump in their case, but again there are several possible causes.

Many issues with CIS arise from people trying to adjust things and make up for vacuum leaks. A good place to start.

[Richard C2S]

Just what might be a somewhat silly suggestion - try cleaning the fuel injectors. If you can't pull them out to do it with an injector cleaner, use some Chevron Techron. May not cure the "surge" but may help the "jerky"!

[Buzetti]

New information - when I bough the car on Saturday, the dealer had fillled the gas tank right up - he has e mailed me to say he thinks he might possibly have used UK standard unleaded (95 RON) rather than UK Super unleaded (minimum 97 RON) - I wonder, could something so simple possibly cause the problems I am experiencing?

Any views please?

I am planning to fill up the tank (about half full currently) with Shell V Power which is 99 RON to see if that helps.

[tevake]

If the car has been sitting for sale for a while, It may just need some driving to work the bugs out. As you get good fuel in the tank try to take it out for a few hours of brisk driving, It a tough job, but someone has to do it. Heard it called an Italian tune up, but it works for German machinery also.

Have you any pics of the new ride? We like pics!

Cheers Richard

[stlrj]

Have the base mixture checked. It may be running too rich for the 02 sensor in closed loop.

[CorsePerVita]

Don't let anyone tell you that CIS has "behaviors" that cause it to run like crap. When properly in tune, it will run fantastically. When well maintained, it'll be butter smooth like a new car.

There are things that CIS will do when certain things do not run properly. As nickshu mentioned, there's the possibility of a few things. Others mentioned the O2 sensor. Either way, it is NOT normal for your car to do that. I have an '82 SC as well and the only time it has behaved erratically is when something is out of whack.

First off, since it is CIS and you're seeing behavior where it is hunting, check for vacuum leaks. A vacuum leak in itself can sometimes cause issues. The other thing to consider with CIS is that if you "fix" an issue with CIS when it has vacuum leaks, that when you finally fix the vacuum leaks you can create another issue of it running too lean or too rich again.

So start off with the basics of checking your vacuum leaks:

- Vacuum hoses on the ignition dizzy
- Vacuum hose to the CIS boot from the oil tank
- Check the injectors to see if they "wiggle" - if they feel loose and can rattle around, this is a sign that the seals/o-rings have shrunken, are likely hard, and causing vacuum leaks. They're a $.50 part if I recall.
- Breather hose to the engine
- Check the CIS boot for cracks
- Check and make sure the straps at the CIS boot are nice and tight

When you can be sure that all the above has no vacuum leaks, continue on to checking mixture. It would be best done with a sniffer (if you don't have a cat). However, while it is doing the hunting idle, you can do the "poke test" which is to put a mixture wrench or allen wrench on the mixture screw at the fuel distributor and lighly just push down, just a hair. This emulates a richening effect, so if the car is lean, this would briefly add fuel and if the hunting stops then we're potentially dealing with a car that is too lean. The set screw adjusts counter-clockwise for lean, clockwise for rich.

However, you cannot just turn it and potentially fix a mixture.

Consider this:

- If we adjust the mixture to be more rich, but have a vacuum leak, then fix that vacuum leak, now less air enters the system, and we end up going TOO rich. We're on the opposite side of the fence.
- If we don't adjust the mixture to be more rich, but have a vacuum leak, then fix that vacuum leak, now less air enters the system and we end up being at a "normal balance" until we fix the mixture.

There are a lot of CIS cars out there where owners dabble with the mixture and have a ton of vacuum leaks. In the end, they are compensating a mixture on the fuel dizzy for an ultimate issue that is really something else. So as the leaks get worse, the car runs worse. The car is always being tuned and dabbled with, and seems "problematic" but people are attempting to bandage an issue that is really a totally different underlying problem.

So I stress to check all of your vacuum areas for potential leaks. There is everything from the old fashioned "starter fluid squirt" test around areas you think are leaking (as that causes the idle to raise) to feeling by hand, to plugging or kinking hoses to see if it affects anything.

You should also test your O2 sensor if you can, or inspect the wiring as sometimes those have a tendency to be frayed and covered in road grime and make a terrible connection.

In the case that it is hunting this wildly, it's possible to be running too rich possibly. My '82 SC had this issue in the winter time when it was VERY cold outside and tended to run very very rich when it was cold. The best I could describe it would be a violent hunting idle for 2-3 minutes till it evened out. It would be "jerky" as if it was loading up really hard until the car was hot or warmed up. I ended up fixing the vacuum leaks which made it even worse (rich) and eventually leaned it out just a hair and it was fine. However, it took some time of working on the vacuum leaks and dabbling with the mixture after that to get it right. A new O2 sensor also helped a ton.

[euro911sc]

Your SC, when in proper tune should be butter all the time... Something needs to be fixed and its not the gas.

[Buzetti]

Hi, thanks for your thoughts on this.

I am told the car had pretty much a ful tank of 'standard' UK unleaded fuel just before I collected it - 80 liters I think - UK 95 RON octane.

There is a red lettered label in the filler flap which says 'minimum 98 RON'

I have managed to add half a tank (40l) of fuel with a 99 RON rating (the highest available at a regulat gas station in UK) - and I think the car runs better (but could be placebo effect....)

My question is, given that K Jetronic is relatively unsophisticated compared with modern injection systems (which can take account of widely different fuel Octane and adjust for it?), what are the LIKELY driving effects of running my SC on a tank of 95 when the car is meant to use 98? (there is no 'pinking' evident so far as I can tell - just a slight 'jerkiness' at low revs in low gears and the over-run, plus a low idle of aroud 700 RPM) - it seems to get 'better' when properly warmed through - worse from cold.

[Buzetti]

Just to add, before I bought the car, I had a full pre-purchase inspection carried our by one of the UK's experts on older 911s - this was about 30 miles before I collected the car and before it got a full tank of 95 - he noted no problems on his 10 mile test drive, but I (not an expert!) thought the car felt 'odd' (jerky/ uneven from cold) almost straight away after collection - a handful of miles after the tank was filled....

[CorsePerVita]

Technically speaking, CIS cares nothing about what octane of fuel you run. Nor does the octane have anything to do with the sophistication of modern day fuel injection or not. That has everything to do with what the engine is made for. Octane is the resistance to burning vs compression. Or basically how resistant it is to being burned when you don't want it to be, if you think about it. Some cars of today can adjust/retard the timing a bit if it senses pinging or knocking. But that has more to do with variable cam timing, ignition timing, knock sensor, etc etc and a whole load of other things that modern day ECUs can do. CIS is just a fuel system. Think of it like a really fancy vacuum run carburetor.

But we're going off tangent here...

If you have an engine that is made for say 95 RON and you put 85RON in it, you may get pinging. This is because upon the compression cycle, the fuel may burn and you get detonation. This means the fuel goes up in flames before the spark plug can make it do that. That's bad for your engine and can hurt it. This is why it makes no sense to me when someone says, "I put the good stuff in it" because fuel is still fuel, and the extra octane rating is how hard it is to burn, not "how good it is" (although if you go to a gas station and the gas is $1 cheaper, well, you may want to wonder why if it's for the same octane).

Anyway...

If the car was pinging and firing early (pinging) then yeah, it'll run like crap and it is NOT good for the engine. But that usually comes with an audible pop/bang/shake or just overall sound of the engine running like crud when that is happening.

If the car isn't missing or pinging, it doesn't sound like it's having any disagreements with the gasoline you're running.

It still sounds like something in the CIS system is off. Again, if it is hunting on an idle, or if the idle behaviors change when warmed up and it only runs well when warm then it's something during the warmup cycle. If it still sometimes runs like crap when it's warm, or still hunts when warm, something is still amiss.

[SC-targa]

This is a 204 DIN HP Euro car. No cat or O2 sensor, right?

Have you checked the mechanical spark advance? Make sure it's lubed and moves freely.

[Starless]

My 1980 Sc was doing the exact same thing. My problem turned out to be that the pulse valve ( not sure if this is the correct term ) was not working due to a bad relay.

[Buzetti]

Thanks all for your comments.

The SC is now undergoing a full set-up with my local specialist JMG Porsche in Poole - the technician who took a quick look thought this was probably just a tuning/ set-up issue...will let you know!!

[TX911SC]

Just a suggestion....I had a low speed bucking problem on an '83 911SC that was caused by a bad frequency valve relay ($6 USD). Replaced it and car runs strong from 0 to 5k RPMs in a very gear!

[83_Silberpfeil]

Originally Posted by TX911SC

Just a suggestion....I had a low speed bucking problem on an '83 911SC that was caused by a bad frequency valve relay ($6 USD). Replaced it and car runs strong from 0 to 5k RPMs in a very gear!

Very interesting to know TX! Thanks for sharing.

My 83 SC does also have a very slight buckling at low RPMs. Would you mind sharing the info on part # or a pix of what the part looks like and where it goes?

For $6, I'd like to give that a try!

Thanks!

[TX911SC]

Originally Posted by 83_Silberpfeil

Very interesting to know TX! Thanks for sharing.

My 83 SC does also have a very slight buckling at low RPMs. Would you mind sharing the info on part # or a pix of what the part looks like and where it goes?

For $6, I'd like to give that a try!

Thanks!

83_Silberpfeil -- We probably shouldn't steal Buzetti's thread, but here goes. My work order shows "321951253 Oxy Relay $6.50". The technician found the problem while replacing my alternator. While my receipt shows "Oxy Relay" he called it a "Frequency Valve Relay". He showed me the relay and it looked like.......a relay; about 1" by 1/2 inch. He said it was beside the computer under the passenger seat. I tried plugging the number into Pelican parts page and it didn't return anything. Hope this helps.