another idle thread

[Drisump]

My car (an 85 Euro), has an idle issue that I hope someone can help me with. The car starts great and pulls like a freight train but.... upon start up, it idles at 1000 rpm (smoothly) and after warming up, it idles at 1200, and with a miss. I changed out the plugs, cap, rotor and wires this spring but it hasn't effected this issue. When I changed the plugs the ones that I replaced looked text book beautiful (a nice tan colour with no crusty build up). I have to say that the rpm issue has been the whole time that I've owned the car (two years), the miss, I think, has gotten more obvious over that time. Other threads with idle issues have mentioned the O2 sensor and a temperature sensor but none, that I have found, have had these specific symptoms. On a completely different topic, does anyone know where someone can get Kendall gear oil (for the transaxle) in SW BC? I've been running Amsoil syn. but I've read the opinions of the Pelican gurus who all say only dino for 915's. Thanks

[techman1]

Start by checking your idle switch, sometimes called closed throttle switch, some other names. It should make a clicking sound when your throttle comes off open to closed. But don't stop at the click, unplug the wire, hook up a meter to the switch, and make sure it is working. Next, check your ICV or idle control valve. there is a few threads on cleaning, checking and replacing, if necessary.

[Drisump]

Thanks for the response techman, would a problem in those systems cause the miss? Or do they control exclusively the rpm? Thanks

[Vincent Hill]

My "feeling" is something different! I think it may be mechanical as in too tight of a Valve Clearance and most likely an Exhaust valve. (They heat up first and receive the most abuse and shrink into the head) a slightly open exhaust valve would make it not run right on just 1 cylinder but under power still make enough that you would think everything is OK.

WHen is the last time the Valve clearance was checked?

[techman1]

Not a miss. Here is a quick breakdown for the ICV, idle switch and DME:

At idle closed, the idle switch sends a signal to the DME. DME then takes over adjusting the idle by sending a signal to the ICV, the ICV opens or closes, regulating extra air into the engine.
If the idle gets too high, the DME reads this, and cuts the signal to the injectors or the spark, I am not sure which. It then restores the signal when the rpm has dropped.

What usually happen - the ICV gets dirty, and the valve is stuck in a position, letting in a certain amount of air. This may be good for a cold idle of 900 rpm, but as the engine warms it is enough to bring it up to 1100, maybe higher. The problem is the DME will cut that signal at around 1200, so if the ICV is open too much, you get a pulsing effect of about 1 second frequency. Not really a miss.

Want to see how this works? Hold the idle switch closed, in the engine bay, and manually blip the throttle. As you get too high, the DME should cut the signal. this is a way to test the throttle closed switch circuit. Mind you, if the DME does not cut the circuit, this does not say the switch is bad. It could be the wiring to the DME or the DME itself.

I have just a basic understanding of how the whole thing works, Ingo and others here are the masters.

Hope this helps!

[Vincent Hill]

Makes sense to me if that is what it is doing AND fairly easy to check. I hope it is as simple (Easy for me to say) as this.

[Drisump]

Originally Posted by Vincent Hill

My "feeling" is something different! I think it may be mechanical as in too tight of a Valve Clearance and most likely an Exhaust valve. (They heat up first and receive the most abuse and shrink into the head) a slightly open exhaust valve would make it not run right on just 1 cylinder but under power still make enough that you would think everything is OK.

WHen is the last time the Valve clearance was checked?

I did the valve clearances 4000 miles ago (2 months ago, before going on a tour). Since this was the first time at doing valve clearances on a 911, I certainly don't dismiss the theory..... a bloody awkward job..... a few of those adjustments are very difficult to see and execute properly (crossed fingers). The only thing is, the miss was there before the most recent valve adjustment, but perhaps the miss wasn't quite as obvious (I think?). I'll definitely look into the idle system to correct the rpm but I'll probably wait until the next oil change before examining the valve clearances. Thanks....any other places I should be investigating? Cheers

[techman1]

Need to find if it is one particular cylinder.Easiest way is pull a plug wire, one at a time. with only 5 cylinders, hopefully you will notice if another cylinder occasionally drops out. Keep doing this for each cylinder, until you do not notice an additional cylinder drop out.

Note of caution- I had to do this (found bad plug wire) and after a while it gets hot, and you are starting and stopping the engine. Definitely good to have a friend help to avoid a bonehead move. Almost got the small towel I was using sucked into the fan! I was using it to shield my hand from the hot engine when I laid it in the wrong place and went to start the engine!

[Drisump]

Good idea Techman.... now where did I put my asbestos gloves?