'77 911S Fast Idle

[AFB24911S]

Pelican Parts Gurus,

Please help: I am trying to diagnose a fast-idle problem that only presents itself when the car is hot. So far, I have determined that:

AAR (Auxiliary Air Regulator) is good: when supplied with 12V, it takes 4 mins to close).

AAV (Auxiliary Air Valve) is good: holds vacuum.

Throttle linkage is not sticky; hand throttle and pedal return to normal correctly.

RPM decel valve is not a factor; not connected to system, a previous mechanic isolated it with a small hose connecting it to itself.

It takes 15-20 minutes of driving for the problem to present itself. After that, the car idles at an indicated 2,200 RPM. When I was working on it, I noticed that if the engine is idling high and I reduce the idle by opening the connection to the air plenum (and then closing it) it the idle drops and returns to the correct 1,100 RPM.



When the engine is hot, if I remove the oil fill cap the RPM does not change. Previous posts lead me to believe that means I have a vacuum leak. So, I connected an air mattress filler to the main air supply (downstream of the big air throttle plate), plugged the exhaust, and used soapy water to check for leaks. Found none, checked the air box, the injectors, and anything else that looked like it carried air.

Checked whether the vacuum advance is working correctly: I used my vacuum tester to pull 6" Hg and observed that the plate moves.

What are your thoughts? What should I check next? Could it be ignition timing? Thanks for reading.

(Car info: 1977 911 S Targa, 206k mi, engine rebuilt at 130k. CIS car, CA car. I've owned it for 8 months, it's given me no issues except this and a busted distributor rotor. Temp gauge doesn't work, but engine is never *HOT* to the touch even after bumper-to-bumper stop and go. Getting normal fuel mileage. So far, I have replaced all the ignition system wear-and-tear components and a few old hoses, adjusted the valves, changed oil, and changed transmission fluid. THANK YOU all for being such a cool community of tinkerers, love reading the posts and learning!)

[stlrj]

"... I noticed that if the engine is idling high and I reduce the idle by opening the connection to the air plenum (and then closing it) it the idle drops and returns to the correct 1,100 RPM."

My guess based on this evidence would be a weak #2 (Aux air valve). But on second thought, it might just be as simple as your base idle speed being set too high. 1100 rpms seems high to me and bringing it closer to 900 might just solve your problem. Always doing the simple things first is probably the best approach before going totally nuclear after finding yourself totally frustrated.

Cheers,
Joe
87 Carrera

[T77911S]

1100 is too hi.
I was able to idle mine at a nice 900.
if it is idling too hi it could be a vac advance issue. if the throttle plate is past the port (or you have it connected to the wrong port) then you could be pulling vacuum to the advance.
I believe you also have vacuum retard.
make sure the vac hoses are connected to the correct ports.

get the idle down below 1k.
remove both vac hoses from dist.
pull a vacuum on each port on the dist. one should raise the idle, one should lower it.
check you vac hoses for vac.
the one that has vac at idle goes to the retard port.
raise the idle and make sure the vac goes away as you raise the idle.

would like more info on the DV.

is it still connected to the throttle body at all
there are 3 hoses going to it. one small one and 2 big ones.
if the small one is the only one plugged then it could still be the DV.
I would remove the DV and plug the 2 big hoses also, just to be sure.

[AFB24911S]

Thank you both so much for the suggestions, will try this weekend once I have a new cam oil line in.

I misidentified the DV in my first post, apologies. Re-read the literature and found that it’s in place and connected correctly.

My distributor only has one vacuum connection. When I took the vacuum hose off, idle rpms rose. Will re-verify when running and pull vacuum with my handheld tool to see if that brings idle back down.

I even had the fast idle condition with the AAV out of the car and both connections taped over. The function of that valve is to close once there’s enough vacuum, correct?

Also, what is my mystery valve that I misidentified before? I’m ordering a Bosch CIS book, but it hasn’t shown up in any of my prior research on the CIS system. It’s on the right hand side of the motor, shaped similar to the AAV but mounted flat (like you’d throw a frisbee) to a black manifold. Two vacuum hose connections, small, one topside and one below. Hard silver line that runs from the top side of the engine compartment to the bottom, and runs forward from there.

Marking on top reads PE 20 332, Pierburg KG.

[stlrj]

Egr?

[boyt911sc]

Originally Posted by AFB24911S

..........

You are using the incorrect diagram for your vacuum system. If your car is a ‘77, you need to find the correct one. Please take note how the WUR’s vacuum connection is located.

Tony

[Walter_Middie]

You said you checked the vacuum advance. What about the centrifugal advance springs? They came from the factory with small plastic spacers/sleeves that hold the springs just slightly away from the posts. As they age, the plastic disintegrates and the springs sit loosely on the posts and cannot completely pull the advance weights back in at idle and you end up with a slightly elevated idle speed.

[T77911S]

tony, I don't think the correct one is out there.
I only ever found the one for the 78, just have to note that for the 77 the vac hose connects on top of the wur, not the side.
I doubt the TB connections are correct either.

OP
if you DV is still connected that can hold the idle up hi.

I would still get the idle down.

yes, only vac retard.
I was not sure about vac advance.
sounds like it is working.

[allaircooled]

I have been out of the game for awhile but I believe this is the one I have for my 77 engine.

[boyt911sc]

Originally Posted by allaircooled

I have been out of the game for awhile but I believe this is the one I have for my 77 engine.

Chris,

The schematic diagram you just posted has incorrect vacuum hoses routing for #4, #5, and #6. It is a mixture of ‘76-‘77 and ‘78-‘79 for #4, #5, and #6. In summary, your diagram has more errors than the one I am trying to correct.

We could go over and discuss the discrepancies if anyone is interested to know the correct routing.

Tony

[allaircooled]

Well, like I said, I have been out of the game for awhile and don’t remember what diagram I used when I hooked mine up. Anyways disregard my post then.

[T77911S]

if you look at the TB and were the ports actually enter the throttle body you can figure out the hoses.
what you are looking for is where the ports are in relation to the throttle plate.

you will see one that enters "just" below the throttle plate. that one goes to the vac retard.
you want the retard to come out as soon as you give it gas. (a lot of people don't like the retard. I look at it as a form of advance. I also like the way it idles with less advance at idle).

the 2 large hoses on the DV. one goes below the plate (manifold vac) and the other is above.
its purpose is to bypass the throttle plate.

4 above is not even close to correct.
there should be a small port below the plate that has manifold vac. that one goes to the top of the DV then is "Teed" and goes to #4, then to the top of #5.

the bottom of #5 goes hi above the plate. its only purpose is for fuel that would leak if the diaphragm ruptured. you do NOT want vacuum on this one.

[93nav]

I would be interested in the correct info for a 1976 North American 911S. Is there a valid, accurate OEM diagram for it?

Thanks

Originally Posted by boyt911sc

Chris,

The schematic diagram you just posted has incorrect vacuum hoses routing for #4, #5, and #6. It is a mixture of ‘76-‘77 and ‘78-‘79 for #4, #5, and #6. In summary, your diagram has more errors than the one I am trying to correct.

We could go over and discuss the discrepancies if anyone is interested to know the correct routing.

Tony

[boyt911sc]

Originally Posted by 93nav

I would be interested in the correct info for a 1976 North American 911S. Is there a valid, accurate OEM diagram for it?

Thanks

The ‘76-‘77 CIS have a slightly different vacuum routing than the later SC’s except those with non-vacuum assisted WUR’s (engines with lambda).

Below is the correct vacuum routing the following WUR’s:
WUR-045
WUR-069
WUR-089
etc.
‘78~’79 SC (all model year)
‘80~’83 SC ROW



Since ‘76 & ‘77 CIS used a completely different vacuum configuration for WUR-033, where the vacuum is applied at the top unlike the later WUR’s listed above (vacuum on the side), you need to switch or change the vacuum hose connections.

Using the same diagram above and WUR-033 (‘76-‘77):
a). Connect the vacuum hose (BLUE) to the top port of WUR-033.
b). Connect the vent hose (YELLOW) to the side port of WUR-033.

In summary, BLUE hose goes to vacuum port and the YELLOW hose goes to vent port. You could use WUR-045/-069/-089 for your ‘76 & ‘77 CIS with -033 in case your -033 is having problem.

Tony

[93nav]

Thanks Tony!

[AFB24911S]

Thank you all for such great input! Good news, I finally got my oil hose back on and started to play with the deceleration valve (DV). When I pulled the top hose off, the single small hose, idle dropped immediately from 2,200 RPM to 1,600 RPM. Then, I disconnected and plugged the two larger hoses and when I restarted the motor I couldn't make it idle faster than 1,200 RPM.

This weekend, I'll check my vacuum hose routing against the new diagram (thanks Tony!) and see if I can't find and stop my other vacuum leaks.

Here's my thought on the DV valve, based on what I observed: since it idled at 1,600 once I pulled the single hose and then it idled at 1,200 after I disconnected and plugged the other two, there's a vacuum leak AND the DV valve is leaking air as well.

Granted, I am plugging hoses with blue painter's tape and screws; I will go to Home Depot this weekend and find more suitable stoppers to re-confirm my results.

But! Progress! Appreciate the input from the community, thank you for being the generous sources of information and advice you are.

[T77911S]

go to advance and get rubber plugs

you can remove the DV if you want to.

set the idle with the engine warm and check to see what it does when cold.

you should be able to set it to around 900

[AFB24911S]

OK, here's the update, in order of my operations. First, I isolated the deceleration valve with rubber plugs and went for a test drive: it took longer for the fast idle condition to present, but it did and it idled at 1,600 rpm (lower than the previous 2,200). So, I drove home to continue adjusting.

Turns out, the idle adjustment screw was three and a half full rotations out of whack. It took that many turns in to notice any change in the idle speed. I was able to get the idle down to 900 rpm, still ran well at that speed. Re-incorporated deceleration valve, idle rose a little but drivability improved (less need to rev-match on the upshift).

Next day, second test: cold start was a little rough, idle very lumpy at around 500 rpm. Tried opening throttle with gas pedal to stabilize, idled well, let off, nearly died. Shut off, turned idle screw out a half turn, restarted motor, clean idle at 600 rpm.

Next steps: test WUR, ensure it is providing correct lower control pressure to raise fuel distributor piston. Then, I'll check the cold start valve, make sure it's dispensing fuel. But first I have to figure out how I'm going to do that.

Any tips on where to get an in-line tester like the Bentley manual shows? I have a vacuum/fuel gauge, a handheld vacuum pump, a bunch of hoses, and a bunch of ribbed hose connectors. Will ribbed hose connectors work with fuel, or do I need ones that fasten with threads?

[Ginger77]

Did you get the CIS dialed in? What did you do to get there?
How did you route the vacuum hoses in the end?

I'm in the same boat, so I'd love to hear the end of it.

[kach22i]

Maybe your gas pedal is sticking?

Fixed mine.

https://forums.pelicanparts.com/porsche-911-technical-forum/728449-what-did-you-do-your-911-today-449.html