Idle oscillates at deceleration - 1988 3.2

[guyster]

When letting off the accelerator, the car drops to idle speed (925 rpm) but then surges up 300 rpm, then back down 300 rpm, then back up 300 rpm and so on. The problem is intermittent. Sometimes, it will oscillate continuously, other times it will oscillate several times then "calm down" to idle. At a stop light, I can ease off the clutch so it engages, then press in the clutch and the car will go right to proper idle.

Thoughts?

I'm a newbie 911 owner with moderate mechanical skill. (Building a 1971 Datsun 510, currently). Thanks for looking.

[proporsche]

hi there do use the button on right top :search: I would start with taking off your idle control valve and clean it .
here is a little video
https://www.youtube.com/watch?v=pPv40JF3-Rk

Also check your linkage micro switch ...if it is pushed in when you let of the throttle..
this little black in the middle has to be off when letting go of the gas for idle.


this is just to start with....

Ivan

[john walker's workshop]

Idle speed 800 up to 86 and 880 87 up. If it's too much off the factory spec the idle valve will try to control it, repeatedly. The idle micro-switch function is very important too.

[stlrj]

Originally Posted by john walker's workshop

Idle speed 800 up to 86 and 880 87 up. If it's too much off the factory spec the idle valve will try to control it, repeatedly. The idle micro-switch function is very important too.

Which means you need to turn the idle air bypass in to prevent this annoying hunting to stop.

Cheers,

Joe
87 Carrera that no longer hunts at idle.

[Jonny042]

I had the same thing with my 87 when I first got it. I think I felt the idle was a bit low and fiddled with it (the bypass screw), with the same result.

[guyster]

Thanks all. I now realize that there is plentiful information on this topic and am a bit embarrassed by my newbie post. Anyway! I'll start by cleaning the ICV and go from there.

[proporsche]

do not be embarrassed ..this is the reason the forums are here..keep us posted...

;-)
Ivan

[guyster]

Originally Posted by proporsche

do not be embarrassed ..this is the reason the forums are here..keep us posted...

;-)
Ivan

Thanks for the reminder that it's all about community! No longer embarassed.

Cleaning the ICV didn't work, so now it's on to diagnoses. I reviewed the Bentley manual and will start checking off the boxes.

[guyster]

Originally Posted by stlrj

Which means you need to turn the idle air bypass in to prevent this annoying hunting to stop.

Cheers,

Joe
87 Carrera that no longer hunts at idle.

Hi Joe! It's all quite interesting: Upon engaging the clutch, the idle will hunt, then drop to 1,000, then under 900, and sometimes come to rest at 750, but sometimes it will rest at a perfect at 880. Of course, this isn't repeatable every time. Thoughts?

[steely]

Welcome!
I am just going to submit a guess, but you might want to check the idle switch to make sure it is closing, or the Throttle body linkage - to make sure it is making contact with said switch. (it might not be closing all of the way)

If you look at the picture in post 2 of the hidden side of the intake, the idle switch is the black micro switch with the red button in the middle of the photo. In idle, the linkage should depress this puppy.

You can check 2 things - with the engine off, reach in there and gently move the linkage, you should hear the switch click. You can also do a continuity check of this switch as well, refer to the pic, see/follow the small black cable to its connector located just above the round Wide-Open-Throttle switch. Disconnect the connector there and check for continuity at the bracket by moving the linkage.

If you want to go further, with engine running, you could disconnect the harness from the switch at that bracket, goose the throttle, let the linkage go to idle position, and see if this replicates your hunting. (your throttle will be in idle position, but the DME/ICV won't know b/c you disconnected the switch). That switch tells the DME (ECU) when it is in idle, so the DME then can adjust idle speed using the Idle Control Valve. If the switch doesn't close, the ICV won't do it's job.

Other potential related causes for this theory: old dirty switch, linkage needs adjustment (we have info on that), throttle bellcrank on engine deck springs (2) are missing /weak, pedal cluster and/or bellcrank nylon bushings are disintegrating, stuff like that.

good luck and let us know?

[stlrj]

Originally Posted by guyster

Hi Joe! It's all quite interesting: Upon engaging the clutch, the idle will hunt, then drop to 1,000, then under 900, and sometimes come to rest at 750, but sometimes it will rest at a perfect at 880. Of course, this isn't repeatable every time. Thoughts?

Idle mixture might be too rich.


Cheers,

Joe

[guyster]

UPDATE: This was very educational, so THANK YOU to everyone who chimed in. I used the Bentley manual (page 240-20) to troubleshoot this problem, and learned how to use my multimeter by watching a YouTube video.

As of today, there is no 300+ RPM hunting. I don't know that I clearly identified the cause. At this point, I'm thinking the combo of cleaning the ICV (which didn't appear to fix things, at first), and disconnecting the plugs and harnesses (thereby resetting the connections) may have done the trick. In other words, perhaps there was some corrosion affecting one of the switches, plugs, or harnesses.

There is a slight variation upon deceleration, and the idle will settle between 880 and 750 when warm, so, still some work to do. Any thoughts about that are welcome. My plan is to buy a tachometer meter so I can adjust the idle.

Happy Wrenching, y'all!

[steely]

Wow congratulations Guy. Intermittent problems are a pain, but it sounds like you were on the right path (and still are)! Glad the immediate issue backed-off. Thanks for the update!