911SC High Idle

[DDD]

1983 SC, no modifications other than popoff valve.

I've been struggling with a high idle issue for the past year. I've searched and searched and it seems like this is a common problem, but I have NEVER seen a resolution on the technical board here. The threads all just kind of trail off.

A few data points to consider --
Car always starts perfectly. Fires right off, struggles for a moment around 600 RPM, then immediately raises up to about 2000 RPM. Stays there for about a minute, and then starts dropping off, and after two minutes idles right around 900-1000. All textbook.

General pattern, though I can't guarantee it always does it exactly this way, is that after about 3-5 minutes of driving, and any amount beyond that, idle refuses to drop down below 1400. I can bog the engine down to get it to 1000, but as soon as I depress the clutch and the engine is free, it goes right back to 1400.

If I stop the car, and immediately restart it, it will idle perfectly at 1000 for about 15 seconds, and then it immediately rises back to 1400.

If I let the car rest 10 minutes or so, and restart it, even if it is warm (near first mark on temp gauge), it idles perfectly for a quite a few minutes. But, if I drive it, it will be back at 1400 within a minute or two.

Decel vacuum line is plugged with a golf tee. Seemed like I was getting a much faster idle last fall (2000 maybe) before I did this, so I originally thought I fixed it when the idle dropped down so much, but then I put it in storage for winter and forgot about it. Does the decel unit itself need to be capped off too, and not just the vac line leading to it?

I checked popoff valve. Good seal, and when I wiggle it idle doesn't budge.

When I remove oil tank cap, idle drops quite a bit, so vacuum seems good.

AAR is working as it should (open when cold, closed when warm), and I closed it off by plugging both the line coming out and the unit itself just to be sure. Idle still remained at 1400.

I disconnected the O2 sensor. This solves it. Car idles perfectly at 950 or so. So, I bought and installed a new O2 sensor (it had been 50K miles at least since I had changed it) and reconnected the lambda circuit. Right back at 1400 again!

I tried adjusting the idle while warm. Idle screw was about one turn away from closed. I closed it, and that was only able to drop the idle to about 1200. So, clearly air is getting in somewhere, right? The fact that it will idle perfectly without lambda tells me I have an air leak, and when connected the O2 sensor is asking for a richer mixture, which is increasing the idle? Once it gets the mixture it wants, which results in 1400, it is satisfied?

I tried spraying starter fluid around the intake runners, back behind the fuel distrib,near the injectors, and pretty much everywhere -- nothing. No change in idle. I shot a bit into the air cleaner, and idle increased immediately, just to be sure I knew what to listen for.

Why would it idle so nicely even when warmed up after sitting for 10 mins? If I have an air leak, it should be happening then too, right? And, why does it idle so nicely even upon immedidate restart , but for just 15 seconds? Is lambda not active yet?

I am stumped. I guess air leaks can be impossible to track down, so I shouldn't be surprised. Could I have an air leak given oil tank cap behavior described above? Or, do I definitely have an air leak given the O2 info I've described? I've heard AAV maybe (which is buried back behind the fuel distrib, so I haven't dug that deep)? What about distributor advance? Sticking weights? Seems unlikely given that this condition seems entirely contingent on elapsed time since startup and engine temp. Something amiss in the lambda circuit logic? Again, the fact it idles perfectly when warm after "resting" 10 minutes is very strange.

I've heard WUR can go bad, but more likely to create a cold start issue, and not a warm idle issue. I don't have CIS gauges.

One other data point -- when this started happening last fall, it seemed like it was strojngly correlated with outside temp. If cool outside, this happened. If warm, it didn't. But, it is warm today (first time this year), so that theory is shot.

Any ideas out there? Thanks. -DDD

[fred cook]

There is an adjustment screw that the throttle lever stops against. You can get to it by removing the heater blower motor. You will find a locking nut against the bottom of the throttle body flange that will have to be loosened before you can turn the adjustment screw. The adjustment screw is slotted on the bottom end and will take a very small flat tipped screwdriver blade. You might also need to adjust the link that connects the throttle lever with the throttle console mounted on the engine. I don't know if this will solve your idle issue, but it is a place to try. Good luck!

[fred cook]

One other thing. Remove the dist. cap and try twisting the rotor. It should turn a few degrees and then spring back into place. If not, then the advance weights are frozen and will affect the way the engine runs.

[kodioneill]

What fred said, also check your vacuum advance. SC's have a unit that pushes and pulls and they tend to wear out. I've seen many failures. If you find this is bad, have it rebuilt they are $180.00 new.

[psalt]

DDD,

My advice would be to hook up a dwell meter and read the duty cycle of the FV if you have a working lambda system. If you don't know what this is, searching and reading the archives will help. Set up the hot idle mixture as described in the archives and see if the problem changes. Vacuum leaks can be hard to find and some do increase with heat, an unlit propane torch works better than spraying liquids. You need to check the throttle shaft cold and hot and isolate all the vacuum hoses, including the brake booster. I would not attempt to adjust the idle with the throttle stop screw, but I would check the setting to see if a PO screwed up the factory setting.

[DDD]

Thanks everyone for your responses on a saturday night. I have no idea how to evaluate the centrifugal distributor weights, but I took the cap off, and they do turn freely about 5 or 10(?) degrees (CCW I think), and then snap back, but they don't exactly snap all the way back, nor would I describe the return back to be "sharp". More of a sloppy half-hearted return back in the direction from whence they came, but not all the way back. Does this explanation/description help at all? And, if this is the problem, would it be consistent with a perfect idle when O2 is disconnected?

[kodioneill]

Bad vacuum unit.

[psalt]

would it be consistent with a perfect idle when O2 is disconnected?

No. If you suspect a problem with the mechanical advance springs, check the timing with a light at idle and high rpm.

[911freek]

Airbox crack? At the lower body of it (front and left) was my case.

[DDD]

At this point, I guess I don't see how it could be distributor-related given the perfect idle without O2 sensor connected. Am I missing something? Doesn't the O2 behavior prove I have an air leak?

[psalt]

DDD,

No, you could have a bad ECU, bad wiring, defective FV, etc. These problems usually can't be solved with guesses and assumptions. You need to set up the timing, mixture and idle to the spec to get a baseline, then perform specific tests if it continues. This can be done with an analog dwell meter and a timing light in a few minutes.

[boyt911sc]

DDD,

Before you start messing and changing your settings (air mixture, idle speed, etc.) check for air/vacuum leaks. Not being able to locate one (vacuum leak) does not mean you don't have one!!!!! You don't have to do any guess work if you want to know if you have a vacuum leak some where in your system (engine). Simply pressurize (10 psi or less) the system and it would show up like a sore thumb. Once you have confirmed that you have no significant air leak that could effect your mixture, then you could concentrate on the OXS and ECU troubleshooting.

Lastly, fuel pressure gauge kit is mandatory for fuel injection system troubleshooting. Regardless you have an EFI, Motronic, or CIS. Either get one or have the pressure checked by some else. Keep us posted.

Tony

[DDD]

Tony -- No, I would never touch the mixture without having CIS test equip. I messed with idle only as a test, and then put it back to where it was because I could see it.

I'm pretty sure that I DO have a leak. I did double check this am and RPMs dropped about 200 or so when I unscrewed the oil cap, so vac is good, but I guess that isn't necessarily definitive.

Do you concur that good idle with O2 sensor points to air/fuel mix or ECU/lambda circuit issues, and not timing/vac advance/centrif advance issues? And, how does one "pressurize" the system? You've lost me there. Apply 10 psi to a vac line? Any vac line?

Thanks in advance for everyone's help. --DDD

[DDD]

A quick update with one more data point...
It has gotten pretty warm here in MN. 90 yesterday, mid-80s today. VERY muggy. And, my car has idled around 1000, maybe 1100 tops. I generally have been under the impression that when it is cool outside, this problem is worse, but I wasn't sure since it has been so cool this spring. Today seems to be consistent with that.

I've never understood how a car can behave differently when warm depending on how warm it is outside. Once the engine is warmed up, what possible difference can it make whether it is 60 degrees or 90 degrees outside?

Does this shed any more light on anything? Maybe a small air leak is not so bad in hot humid weather? Stuff swells up? Maybe WUR issues?

I didn't have a propane torch, and really should anyway, so I went and bought one tonight. My next step is to shoot propane all over the bottom of the airbox (did some research today on where they are prone to crack/split) and back by the AAV. It is supposed to be only 55 here Saturday. I'll bet I'm back at 1400 RPMs by then.

[psalt]

I've never understood how a car can behave differently when warm depending on how warm it is outside. Once the engine is warmed up, what possible difference can it make whether it is 60 degrees or 90 degrees outside?

DDD,

The difference between 60 and 90F ambient makes a big difference in an IC engine. Engine block and head temp, intake air temp and density all change. A worn out throttle shaft may not effect idle when cold, but when everything heats up and expands, it can boost idle several hundred rpm. Modern engines have closed loop active idle speed control that can compensate. CIS and carbed engines do not and you have to find the right compromise of fixed settings by adjustment at the same operating temperature, after driving several miles under load. Have you read the lambda duty cycle when hot ? It will give you a good indication what is actually happening without any guessing.

[DDD]

Dwell meter is next up. I don't own one of those either, though I think there are two back home in my dad's garage in Denver. Never figured I'd need one again in the days of electronic igntions. Propane torch last night yielded nothing (though clearly something isn't right there, as I shoved it into the air cleaner/intake, and got no response either). Stay tuned.

[psalt]

Old analog dwell meters work best for this task, tell him to send one to you. The lambda ECU duty cycle will tell you a lot about how the car is running. An 83SC can be tuned to fire immediately when cold without touching the throttle, settle to a 1500 rpm cold idle, dropping to a rythmic 900-950 rpm in hot closed loop. Are your injector sleeves, O rings and seals original ? If you can wiggle the injectors try looking for leaks there.

[Vincent Hill]

Originally Posted by 911freek

Airbox crack? At the lower body of it (front and left) was my case.

Same Here! I used WD40 or even carb cleaner and sprayed all around my Airbox and all of a sudden the engine would really paick up speed. I tried everything and finally replaced the AirBox and everything is Perfect now!

I even put a load on the engine to pull it down Just like you did and tried everything else but HAD to replace the AirBox.

[DDD]

Vincent -- Did your idle go higher with colder outside temps? I have a popoff valve, and have for many years, so I would be surprised if my airbox has a problem, but I don't know that the popoff valves are a 100% guarantee against a blown box.

My car starts just as Paul describes (fires right off with no touch of the throttle, goes up to about 1800 for a minutes, and then slowly declines back down to around 950 after about two more minutes). It is after 10-20 minutes of driving that the fun starts.

[911Freak]

You mentioned disconnecting your O2 sensor and the issue going away, have you checked your o2 sensor relay for proper functionality?

Mine is located under passenger seat next to CIS brain, a faulty relay will effect symptoms.

I believe the Bentley manual has troubleshooting for this in the electrical section?

Just a thought...