911sc stalling problem

[et cetera]

I hate that this how my air cooled ownership is starting, but I’m having a problem with a 1978 911sc that I just purchased.

The car was great on the test drive. When cold started it idled a bit high but would settle into a nice 1k idle after a few mins.

On the hour drive home it was good on ther freeway, with just a few bucks at 2500 while cruising.

When I pulled off at my exit problems started. I pushed in the clutch to coast to the light and the car just died. It started back up, but would die every time I came to a stop.

I was able to make it home. I left it in the driveway for 10 mins or so then went to pull it into the garage. It fired right up and idled with no problems.

I took it for a drive a few days later. It was great again until it got warm, then it was the same problem.

I’ve been reading up on the CIS for the last 3 days. I did pull the oil cap and the idle dropped a bit but I don’t have the tools to properly diagnose the problem so I’ll likely take it to a shop, I just want to make sure I know what I’m talking about when I got in there.

I get the feeling that one of the valves in the CIS is sticking because the car wasn’t driven that much. I just wanted to see if I’m on the right track of if there is anything minor I should try before taking it in.

Also, I’m not afraid to pull anything apart and rebuild it, I just don’t want to be throwing money at it when I’m not sure what problem is.

[T77911S]

probably too rich.

hi idle when cold is good.

drive it with the oil cap off.
you can remove the air filter and pull down on the sensor plate at idle to lean it out. if it idles better it is too rich.

if at idle, it starts to surge it is too rich.


invest in a set of pressure gauges.

one other thing. you may have 2 vacuum hoses going to the warm up regulator(WUR). one should have vacuum on it at idle. if not that will make it run rich too.

[timmy2]

Could be an ignition problem. You are describing a common CDI failure mode. It shuts down when car gets warm.
Is the coil black or silver?

[pete3799]

Coil was my thought also.

[T77911S]

it starts right back up from what I gather.

the coil needs to cool before it starts back up or it will act like it is trying to fire up but wont.

IE, it dies at idle

[et cetera]

Thanks for the quick responses.

I'm 90% sure coil is black, I'll confirm when I get home.

After it dies it will fire back up, warm. If I keep some throttle on it I am still able to run and drive. Its only dying when no throttle is applied and it attempts to idle, while warm.

Once it cools down it idles with no problems.

I'm also reading this (911 CIS Primer - Components) while "working" and trying to come up with a short list of things I can troubleshoot w/out doing a fuel pressure test.

Replacing some of the "cheaper" items isn't a problem. I just want to avoid having to buy a rebuilt WUR or FD if I'm not sure they will fix the problem.

[et cetera]

Confirmed, black coil. Blue Perma-tune CD.

[Brtuffli]

Hi, I have similar issues with my 78. I have an appt with a garage to check it out (mixture) as I have guages and all pressures are ok. I will post back in a few weeks after the appt.
By the way, I can get me car to not stall if I brake to a stop light and leave it in gear to slowly bring the revs down to 1100 ish. Then when I put in the clutch, it idles fine

[TibetanT]

First and foremost, welcome to the Pelican Porsche community and P-car ownership!

Second, and probably most important…when was the last maintenance done? Check the records which came with your car since they will provide you with very useful information about the care taken prior to your ownership.

You may find that the car just needs a little bit of adjustments, such as:

1) a tune-up
2) new components (spark plugs, air filter, fuel filter)
3) valve adjustment
4) Timing set
5) Oil change

By what I gathered from your information posted here, your idle should settle around 950 rpm when the car is warm. One thing to check is the distributor itself…does it move when you try to rotate it to the left or the right? If it does, this could be a problem and there is a nut that can be tightened up to secure the distributor from moving while driving. Just a thought.


Do you have Wayne's book 101 projects, or the Bentley Manual for the 911SC? These are good sources for information, as well as the Porsche Factory Manuals.

As mentioned above, the CDI system could be the culprit but I hope not since that is expensive to diagnose and replace. When you turn the key on, there should be a high-pitched whine coming from the engine compartment which indicates the CDI is working.

I do hope it is something basic and not something major which provides the fix.

Good luck with your investigations and have fun with your car.

[et cetera]

Thanks for the warm welcome, I appreciate everyone patients while I figure out how the new car works.

It was maintained regularly, but could absolutely use a good tune up, valve adjustment, etc.

Right now I’m just trying to get it running long enough to smog it and transfer the title. Once that’s done I’ll give it a whole once over.

I’ve also noticed that there is some play in the fan belt. It’s got a wobble while running and I can deflect it about an inch. I’m guessing that’s out of spec, not sure if it can cause this problem.

[et cetera]

Oh and this is actually my second p-car, but my first air cooled.

I call it an obsession, my wife calls it an expensive problem.

[et cetera]

Pulled the fan pully off to see if I could adjust the tension on the belt. Only 4 shims all on the inside.

I get the feeling I’m going to find all sorts of fun things while I get this thing fixed up.

[emcon5]

With it idling, take the oil cap off, does the idle change? If not, you probably have a decent sized air leak letting unmetered air past the sensor plate.

[et cetera]

Originally Posted by emcon5

With it idling, take the oil cap off, does the idle change? If not, you probably have a decent sized air leak letting unmetered air past the sensor plate.

I gave that a shot when idling around 1000-1100 it would drop 200 rpms.

Should it be dropping more than that?

[timmy2]

I agree with the possibility of it being overly rich now that more information has been posted.

Not sure about the Permatune CDI’s failure mode characteristics.

You are lugging the motor at 2500 rpm... they don’t even start to breath until 3200 rpm..

[Targalid]

If you post your location in your signature block it may prompt a local Pelican to come over and help. Someone familiar with these cars can do a quick diagnosis that might take you much longer to discover. The solution could be something as simple as turning out the idle screw.

[et cetera]

Originally Posted by Targalid

If you post your location in your signature block it may prompt a local Pelican to come over and help. Someone familiar with these cars can do a quick diagnosis that might take you much longer to discover. The solution could be something as simple as turning out the idle screw.

Good idea, I should probably register with my local PCA as well.

Do these two screws come out of spec that often?

I spoke with the PO and he had been going on 100+ mile trips every few months without issue.

[T77911S]

check for power at the WUR connector

check power at the thermo time valve(TTV). check vacuum at the valve at to the WUR

[et cetera]

Originally Posted by T77911S

check for power at the WUR connector

check power at the thermo time valve(TTV). check vacuum at the valve at to the WUR

I think we’re on to something here. I went to check the Thermo Valve and before testing it I noticed the vacuum line to the wur was extremely loose. I zipped tied it on and took it for a drive. It did die once but really tried to catch the idle. It started right back and held the idle w/out staling again.

I’m guessing I still have a few vacuum leaks I need to hunt down.

[et cetera]

Welp, good news bad news.

Took it to the smog shop and it passed, but it was only because the guys were cool about it stalling a few times.

I can at least get it registered and insured.

It's back to square one. From what I can gather the troubleshooting process is this:

Vacuum
Ignition
Fuel
CIS components

I'm curious, are there any numbers from the smog test that would help me better understand if the car is running rich/lean?