Sticky/high idle when warm

lightbulb8817 · 08:28 PM

I'm having a problem with my idle. The vehicle is a 1975 911 s 2.7L CIS.

When the car is cold, it doesn't have this problem. As the car gets hotter and warms up, the RPM will hang around 1800-1900 then eventually drop to idle around 1000-1100 RPM. If the car is really warm, sometimes it doesn't even drop. With such a high idle RPM, sometimes shifting from neutral to first will grind

I've searched the forum and have done the following:

1) Cleaned the air sensor plate. It was somewhat dirty and I thought maybe it was getting hung up. No changes.

2) Plugged the hose to the top of the decel valve and reconnected the vacuum line (with plug in). This didn't seem to make a difference, which surprised me.

Any suggestions? I've been reading about the aux air reg (AAR) and aux air valve (AAV), but I'm not sure if my car has these units...??? I do see a hand throttle near the e-brake that doesn't appear to do anything when I pull on it. I'm starting to think that might be the culprit and will look that direction unless you guys have other ideas.

Thanks for the help.

javadog · 12-31-2015, 03:07 AM

That can be caused by a fuel mixture setting that is too lean. Check for air leaks, set the mixture using a gas analyzer, then set the warm idle speed.

The hand throttle is not your problem. The plastic part that engages the throttle rod is probably broken, which is why it does nothing.

Plugging the decel valve also won't fix this.

You won't have an AAR or AAV on a '75. You do have some components on a '75 that are different from other years, so it would be a good idea to verify you still have the correct ones, like the WUR that was used that year. Lots of CIS systems get "fixed" by installing replacement components that are wrong for a particular year and don't work well when installed.

JR

ossiblue · 12-31-2015, 07:16 AM

Javadog has given you the best guess on what may be the problem as well as clarifying which components your car has and does not have.

I've owned a 75, and the most likely cause is an air leak. You have lots of hoses that could develop cracks that are difficult to see with the naked eye, as well as some that are not visible from the rear of the car. A thorough pressure test of the intake system is called for, IMO. Not too difficult and many threads on how to do one yourself.

One other thing just to check. The plastic cam for the hand throttle is probably broken as javadog mentioned, but it may be possible that a broken piece is preventing the cable from fully returning the throttle to idle. Take a look at where the cable attaches to the throttle body and check to be sure the throttle is completely resting on the stop and the micro-switch is closed when the hand lever is fully down.

lightbulb8817 · 07:49 AM

Thanks for the help. Your responses will definitely save me hours of research.

Here is a video I took showing my problem in action.

https://www.youtube.com/watch?v=1Ia_s3jK2gM&feature=youtu.be

Attached are photos of my decel valve that I plugged, as well as my WUR. Through reading other threads, I ruled out the WUR because I thought it only affects the car when it is at a completely cold state, but I must be mis-understanding. There appears to be one wire cut and spliced to my WUR. Service records also show that one of the previous owner had it looked and that idle was a problem when it was cold. Idle does jump around right now if I park the car outside in ~40-50F weather. No obvious problem when car is parked in my 70F garage overnight. Perhaps they were compensating for the idle problems by adjusting the AF ratio, but once the car is warm, the AF ratio is now off.

Are you able to tell if it is the correct WUR and if the cut wire is supposed to be that way based on these pictures?

I'll bring the car to a Porsche shop soon to get the CO measured. I'll also check the plastic cam.

Plugged decel valve (at top with hose reconnected)

WUR picture 1

WUR picture 2

javadog · 12-31-2015, 08:05 AM

The '75 WUR had a vacuum attachment that was designed to enrich the fuel mixture when the engine was under load. So, not all WUR are the same. You might want to study Jim Williams' CIS primer to start off, just to get familiar with what you have.

I really can't see much from your photos of the WUR. I see a cut wire, but not much else.

JR

Steve@Rennsport · 12-31-2015, 08:08 AM

Check the distributor to ensure the advance weights are not sticking as that will cause the same kind of problem.

lightbulb8817 · 08:29 AM

I'll work on getting better pictures of the WUR.

Would you be able to share some insight on how I can check the advance weights in the distributor? I'm afraid of taking apart pieces that require special tools to align or calibration to put back together...

I'm thinking it's something sticking, more so than an incorrect mixture. The reason I say that is because the idle has a much higher tendency to return to normal speed specifically during hard deceleration... it feels like the hard deceleration knocks the "stuck" piece loose, allowing the engine to drop back down to the correct idle speed. Maybe its in my head... once the car gets really warm, this affect seems to go away and the idle will just stick regardless.

lightbulb8817 · 12-31-2015, 12:07 PM

Shops are closed today and weren't able to check my CO levels.

Here are pictures I just took. You can see the WUR much better. As you can see, the wiring appears to be manipulated. Is there anything else that stands out? I also removed my non-working heater and associated piping. Where can I get plugs/caps for the exposed heater openings now? I looked on pelican parts store, but wasn't able to find it.

distributor opened up. can i spray lubricate directly into it? i'd like to try something less extensive before opening up to check the advance weights

lightbulb8817 · 12-31-2015, 12:10 PM

more pics.

i'm learning as i go, thanks for all the help and support

boyt911sc · boyt911sc's Garage

Lightbulb,

You got good pictures and that helps a lot in inspection. One thing that caught my attention is the decel valve vacuum hose connections shown in your picture/s. The T-connection is located at the bottom of the decel valve instead of being at the top. Where does this hose from the T-connection go? To WUR? What is the resistance (Ohms) of the WUR? Keep us posted.

Tony

lightbulb8817 · 01-23-2016, 02:51 PM

The problem seems have went away on its own. Weird! Maybe it just needed to be driven....

jason2guy · 01-23-2016, 09:09 PM

i was thinking distributor as well. at a minimum you should pull the dist and give it a good looking over. what i mean by that is take it apart and clean/lube it. think about how old that part is- and how important its job is. when was the last time its had any maintenance at all? just a thought about the logic- air leak causes high idle. thats what i used to think, before i had a real air leak. by air leak i mean- false air. air that has not been measured by the air flow sensor. it does not cause a high idle, it causes a lean condition and will misfire.
think about it, extra air is getting in without getting extra fuel. the high idle issue with SC's is either timing that creeps in advance as the car heats up or the mixture enriching as a result of heat

lightbulb8817 · 01-24-2016, 08:23 AM

that makes sense.

i was installing backdate heater parts and had to remove the distributor cap / wires to get those parts in. maybe agitating it indirectly fixed my high-idle problem.

i'll try to find some resources on taking the distributor apart... don't want to bite off more than i can chew.

Thanks for all the help. I appreciate it very much.