Dean Tanner

I've been trying to track down an idle problem. My idle hunts from about 760rpm to about 825rpm. I have checked for vacuum leaks and I've replaced o2 sensor, cylinder temp sensor, cap and rotor. I just pulled the ISV and cleaned it and checked resistance according to my Bentley manual. Their chart shows resistance between outer terminals to be 40ohms and center to outer terminals to be 20ohms. I get 30 and 10 respectivley. Could this be causing my problem?


Sorry folks I had zeroed my ohm meter in the "high" setting then switched to low All readings are fine, any suggestions on what to check next?

scottb

Dean: I had a hunting idle problem on my 84, though the range was larger than yours. I replaced the ISV, O-2 sensor, and head temp sensor, but nothing worked. Then I had my mechanic adjust the mixture, and its been rock solid ever since.

RockyMcD

If Scott's suggestion to adjust the mixture dois not cure your problem. Carefully check the timing and the advance. This is sometimes the problem.

Steve Weiner-Rennsport Systems

Dean:

Hunting idle problems are mixture related. A rich or lean condition will cause these problems.

You really need to hook a 4-gas (or 5-gas) exhaust analyser to the car and see what your CO is, both before & after the cat. That will pinpoint the trouble and allow you to proceed in the proper direction.


RockyMcD:

A Carrera's ignition timing and advance is all done by the ECU (chip); you cannot really adjust timing like you can on the SC's, Turbo's and older cars.

Dean Tanner

Steve,
So after taking CO readings and I find I am lean (or Rich) what direction do I go?
I looked at my Bentley manual and it looks like mixture is adjusted at the front corner of the AFM. I looked in the opening and it doesn't look like any screw I've ever seen, is that the adjustment?

Lorenfb

Possible causes of idle hunting:

1. intermittent idle switch
2. bad ISV (although you've cleaned it, it may be bad)
3. bad performance chip
4. fuel mixture problem
a. fuel pressure
b. misadjusted AFM (screw and/or spring)
c. temp sensor connections
d. O2 sensor connections
e. AFM connections

Note: All good running early 911s (O2 system car & pre 996) will always hunt a little at idle in the closed-loop mode because of the way the O2 system works. A 50 to 100 RPM variation is normal and depends on the O2 sensor (OEM vs aftermarket) and its' condition.

So, disconnect the O2 sensor and determine if the hunting continues. If so, check the CO level (1.0 to 1.5%) before the CAT.

Bottomline: Your 65 RPM variation is probably normal.

Peter Zimmermann

CO first - this can be done wrong - be sure that your technician knows how to do it! The spec for USA cars is 0.6 - 1.0% @ 880 rpm +/- 40 (changed from the original requirement of 800 rpm...). The car must be at operating temp (front oil cooler hot to the touch), O2 sensor disconnected, and measurement made before the catalytic converter. I know of many '84/85 cars that had to have new air flow meters because they would not lean out below 2 - 2.5%. If all is well, replace the Idle Stabilizer. Remember, your '84 is the first of its kind. '85s were smoother, '86s were smoother than '85s, etc...
Pete

jimcabc

65 rpm normal idle variation? Could this be true? Or should Carrera's idle be rock steady?
( Know mine isn't!) - Jim
'87 911 Cabrio

Steve Weiner-Rennsport Systems

Great input from Loren and Pete!!

Take that as gospel,......

Dean Tanner

I have been reading other posts about idle speed and mixture, so I decided to check a few things on mine. As per these other posts, I disconnected the 02 sensor, hunting still occurred, just at a higher rpm. I jumpered the isv and my idle jumped up to about 950-975 and smoothed out nicely (+/- 40rpm). Is this telling me anything? I will hopefully be able to get a 02 reading sometime soon.

theasphaltgambler

The one thing to check for is throttle blade varnish/gum build up. The base idle RPM is set by a specified air-flow rate past the throttle blade opening. Take the boot off of the t-body and look down inside, you may need a dentist style mirror to fully see in there. By hand open the throttle blade and see if there is any build up.

If so use a good upper intake chemical spray (can) and wipe with a clean towel while you hold the blade at WOT. After it is clean start it and see if it still 'hunts' This would be the absolute second thing I would do after confirming that there are no vacuum leaks.

scottb

Dean: Unless I've missed something, it appears you haven't had the idle checked yet. That's the simplest, least expensive alternative. If you haven't had it checked, you really should before tearing into other stuff.

Steve W

In addition to the advice above, generally you'd first check induction air leaks by disconnecting the O2 sensor and ISV, and spraying engine starting fluid around any suspected leak points from the throttle body, to the intake manifold, to the gaskets between the manifold to the heads - those are notorious for shrinkage and being sucked in. If your idle shoots up, you have an air leak that needs to be fixed.

Next check and adjust your idle mixture via a gas analyzer, or you could also measure the O2 sensor voltage disconnected from the car and adjust so it fluctuates evenly between 0.2 and 0.7 volts at normal running temperature. Below 0.2 is too lean, over 0.7 is too rich. You are trying to adjust the mixture to where the O2 sensor wants to see it, so it doesn't have to work as hard to force it back towards its expected operating window.

A standard 84 Carrera is programmed to idle at 800 rpm, and when jumpering B and C of the test socket for the ISV, you want to set the base idle speed also on the throttle body to 800 rpm to assist the ISV to operate around that window. 975 is a bit high and can cause idle hunt. A subtle 50 rpm variation at idle is normal and is caused by feedback from the O2 sensor constantly modulating the mixture back and forth around the stoichiometric ratio of 14.7:1 . The rise and fall in speed corresponds with the mixture swing from about 14.5:1 to 14.9:1.

Dean Tanner

Thanks guys. I am planning on getting the idle speed and mixture checked. I just wanted to do a little investigating first. I appreciate all of your input. Steve, I had been checking for leaks by spraying carb cleaner around possible leak points. I had not heard that I should have the ISV and 02 sensor disconnected, could it be I've got leaks I missed because of this?
Also, just for my knowledge, where is the mixture adjustment? It seems I saw something that said it is in the hole on the bottom left corner of the AFM. I looked up in there with a mirror and it looked like a little silver piece with a hole in it. The hole didn't look like an allen head, it just looked like a hole about 1-2mm in diameter. Am I looking in the right place?

Steve W

You could have - disconnect them and try again. You want the influence of the O2 sensor and ISV out of the loop when testing. The mixture screw may be plugged with an aluminum anti-tamper plug. If so, drill two small holes in them and use a pair of needle nose pliers to pull it out. It is located under the red plug pictured, and adjusted with a long 3 mm hex wrench.