911SC Stall at Idle and With Oil Filler Cap Off

[car_era]

Newbie here, 1981 911SC with 220,000+ miles. I've had it for a few months and just had a shop fix some known issues - triangle of death oil leaks, new clutch and flywheel, new breather hoses, new starter.

Before I took the car into the shop it leaked massive amounts of oil, though when I took the oil filler cap off the RPM's would drop a lot, but never stall. It also never stalled at idle and ran at steady RPM's at idle during warm up.

When I picked it up from the shop the oil leaks and clutch are taken care of, however the shop told me it's now stalling when the oil cap is taken off. They said this is a separate issue and it will need to come back to diagnose and fix. Also, they mentioned that before the fix one of the breather hoses was completely collapsed and blocked, and the brand-new breather hoses may be causing this problem to show itself.

The shop also said that the idle was set too high when it arrived to them at 1,100 rpms, but when they tried to decrease it to the correct 900 RPM's it stalled so they increase it back to 1100 and said I should get that fixed in the future too.

On my drive home it stalled twice when I depressed the clutch pedal (RPM's dived lower than normal each time the clutch was depressed, but it usually recovered back to idle right before stall - except twice it went dead.)

Now home three days later, when I start it up, it revs up and down when i first start it up. About one second up to higher RPM's and one second down to lower for the first few minutes it's warming up.

SO - what may be going on here? What would you look at to start going through this? And is the shop accountable to fix this up as a part of the work they did? Or are they right that this should be considered a separate issue from what they worked on? Thanks for helping me get my mind around it.

[rwest]

Hello,

With CIS, removing the oil cap should cause the idle to drop slightly. If it dies or stalls, it could be a sign of a too lean mixture.

With CIS, it is a good idea to get gauges and work through it methodically, otherwise you are just throwing expensive parts at it and hoping.

Here’s a very good thread http://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies.html

Likely that when the shop repaired other issues, the new ones popped up because someone had applied a “band aid” fix earlier and now it needs something else corrected.

[Aurel]

It is normal that the car will slow or stall when the oil cap is off. It is not supposed to run that way, and oil cap off messes up the Air/fuel ratio. But agree that if it stalls, mixture may be too lean.

[dlagerstrom]

Another thing to consider is you may have a leak in the intake. Removing the oil cap creates a path for unmetered air, but should not be enough to cause a stall if everything is running correctly. The number one problem for any CIS is air leaks. I have found if you have a small leak, and have 'tuned' around it, pulling the oil cap will kill the engine.

Do you have access to a smoke machine?

Good luck,
-Dave

[boyt911sc]

Originally Posted by dlagerstrom

Another thing to consider is you may have a leak in the intake. Removing the oil cap creates a path for unmetered air, but should not be enough to cause a stall if everything is running correctly. The number one problem for any CIS is air leaks. I have found if you have a small leak, and have 'tuned' around it, pulling the oil cap will kill the engine.

Do you have access to a smoke machine?

Good luck,
-Dave

Good advice. Not all good and honest mechanics fully understand CIS troubleshooting. If you find one, stay with him. They are hard to find now and a vanishing breed. Good luck.

Tony

[proporsche]

do you have this air restrictor in the breather hose in place?It has to be there .It is in the breather hose which goes on top of the engine from the filler neck .
That might be the problem
https://www.pelicanparts.com/More_Info/93010728900.htm?pn=930-107-289-00-M260

Ivan

[proporsche]

just so you have an idea where it goes

[stevej37]

I've owned my 78SC for over 25 years now. From the first year, it's almost stalled or stalled, when removing the oil cap.
With the cap on and tight...it runs perfect.

[pmax]

Originally Posted by stevej37

I've owned my 78SC for over 25 years now. From the first year, it's almost stalled or stalled, when removing the oil cap.
With the cap on and tight...it runs perfect.

But Steve, that means you can't check the oil levels half, pardon the hyperbole, the time ... surely not an operating point the Stuttgart geniuses designed to.

[Reiver]

Originally Posted by stevej37

I've owned my 78SC for over 25 years now. From the first year, it's almost stalled or stalled, when removing the oil cap.
With the cap on and tight...it runs perfect.

Then I'd guess you have a minor air leak somewhere that can be compensated for by fuel enrichment/idle that becomes an over the top air leak when you pull the oil cap. I've had 2 SC engines... US and Euro and both became a bit rough when the cap was removed but ran without issue.

Dying or almost doing so is not the norm.

[pmax]

... or missing the #3 plug restrictor.

[car_era]

Originally Posted by proporsche

do you have this air restrictor in the breather hose in place?It has to be there .It is in the breather hose which goes on top of the engine from the filler neck .
That might be the problem
https://www.pelicanparts.com/More_Info/93010728900.htm?pn=930-107-289-00-M260

Ivan

This is amazing. When I saw your reply I ran out and checked the breather tube and sure enough the restrictor part is missing from the upper tube. I called the shop that did the work on Monday and they found the part accidentally tossed out in their trash. They are close to home so Im goin by in a few hours. Thank you!

[proporsche]

super, at least you do not have to buy one..glad to help...

Ivan

[boyt911sc]

Originally Posted by stevej37

I've owned my 78SC for over 25 years now. From the first year, it's almost stalled or stalled, when removing the oil cap.
With the cap on and tight...it runs perfect.

It is surprising to find these days how much people know about their car/s even after a quarter of a century of ownership. For new owners it would be understandable.

Tony

[Marc Bixen]

Originally Posted by car_era

This is amazing. When I saw your reply I ran out and checked the breather tube and sure enough the restrictor part is missing from the upper tube. I called the shop that did the work on Monday and they found the part accidentally tossed out in their trash. They are close to home so Im goin by in a few hours. Thank you!

Find another shop! If the guy replacing the breather hose on a 911 doesn't know to swap this part out, then he/they shouldn't be working on 911's.

[scotricker]

I've had my 911 since 2009, and Never Ever noticed a post about this !

I didn't know to check this, I never got the memo.

I have checked and checked , and looked for air leaks, and smoke tested, and replaced the airbox hoses and clamps,
and replaced the intake runner gaskets, and, and , and, until finally, just like stevej37 said, I gave up and thought to just live with it that way .
( I even secretly messed with the mixture, don't tell anyone )

I was so happy to see this post and finally discover the answer to my dilemma !


I got one of these from our host



and I've always wondered why there was a handy split with clamps on that hose?



So I removed all the clamps the other day, and removed the shorty piece, and looked into both hoses with a flashlight, oops, no, no restrictor!
It seems that a PO or retarded shop removed it way long ago, before I had it.

I took it apart again today, and jammed the restrictor into the shorty hose, see the bulge at the red arrow,
and ha ha, now I can check my oil correctly, while it's running, and Not stall, die !

I think this should be a sticky, required reading, how can we not know this ?
Now if someone searches, here you go, here's a big answer, thanks Ivan !

Best $25.00 upgrade ever.

[boyt911sc]

Originally Posted by scotricker

I've had my 911 since 2009, and Never Ever noticed a post about this !

I didn't know to check this, I never got the memo.

I have checked and checked , and looked for air leaks, and smoke tested, and replaced the airbox hoses and clamps,
and replaced the intake runner gaskets, and, and , and, until finally, just like stevej37 said, I gave up and thought to just live with it that way .
( I even secretly messed with the mixture, don't tell anyone )

I was so happy to see this post and finally discover the answer to my dilemma !


I got one of these from our host



and I've always wondered why there was a handy split with clamps on that hose?



So I removed all the clamps the other day, and removed the shorty piece, and looked into both hoses with a flashlight, oops, no, no restrictor!
It seems that a PO or retarded shop removed it way long ago, before I had it.

I took it apart again today, and jammed the restrictor into the shorty hose, see the bulge at the red arrow,
and ha ha, now I can check my oil correctly, while it's running, and Not stall, die !

I think this should be a sticky, required reading, how can we not know this ?
Now if someone searches, here you go, here's a big answer, thanks Ivan !

Best $25.00 upgrade ever.

Scott,

These are the two (2) different air restrictors you will find in SC CIS motors. The elbow type for the early models and the bell shaped for later SC’s. They are installed at different locations or spots as you would notice. The elbow type air restrictor/flame suppressor is installed by the CIS rubber boot (peanut) closer to cylinder 1-2-3. If you install it at the wrong spot (closer to cylinder 4-5-6), you would experience starting problem.





Tony