More 3.2 Carrera Idle Hunting Problems!

[PcarRacr]

Hello Again,

Well its been a week now and I put a stock chip as well as a new IAC motor and instead of the slow rise in idle then the drop the car now does a rapid rise then drop...kinda like it has a cam in it.

Ok here's where it gets interesting...if I unplug the IAC nothing changes...but then I pinch off the upper hose to the IAC an instantly the car settles downs and Idles smooth as glass...unclamp the hose and bam the idle madness begins again. The car will do this whether the IAC is hooked up or not. So as long as I pinch off the hose the car idles great.

I don't think that there is a vacuum leak because the way the car changes when I pinch off the hose.

Needless to say I'm all out of ideas...Anyone have any to share with me?


Thanks again for your insights!

Chad Armstrong

[Lorenfb]

You have localized the problem to the ISV operation.

The ISV valve may have damaged the output idle stage of the DME unit.
You can check this with a test light connected to +12 volts. Touch the
pointed end of the light to each of the ISV pins (outer pins). If the light
is on brightly, then that pin is shorted in the DME. At a normal idle the
light should be the same brightness (1/2 brightness) on each pin.

[PcarRacr]

Hi Loren,

Thanks again for you help with this. I have a stupid question for you...I noticed that the clip for the ISV only has metal connections in it for two of the 3 connections. Is that normal. It the center one and one of the outside ones.

I will test to see if it is shorted out later today and let you know what I find out.

Also if it is shorted at the dme then What? Do I have to buy a new computer?

Do you think if I put a freeze plug with a small hole drilled into it in the tube to the IAC which should make it idle smooth...that it would be worth a shot to get smogged?

I spoke with my mechanic this morning and he insists that there has to be a vacuum leak somewhere. He even thought that it could be the line to the brake booster and that there is a little plastic oneway valve up there that goes out and causes a vacuum leak. Any opinions on that?

Thanks again,
Chad
85 911 V4 POC #192

[stlrj]

What is so difficult about lowering the idle speed?

My 86 was cured of the same symptoms when I turned down the idle after my mechanic gave up after trying everything including replacing the ISV.

[911boost]

by idle jumping, do you mean it jumps from about 900-1100 RPMs when you are sitting still? Mine does there every once in a blue moon. Its wierd. If I let the clutch out just enough to feel it starting to engage, its stops.

Bill

[PcarRacr]

ttt

[Lorenfb]

The ISV must have all three connections. The center is +12, the others
are; idle up & idle down - basically, as the DME varies the duty cycle
of each. Try and find the third wire.

[Por_sha911]

IAC motor? Vas is IAC motor?

[stlrj]

GM and Ford call it the Idle Air Control which is the same as what Porsche calls the Idle Stabilization Valve.

[Jim Smolka]

Check the pipe / hose comming off the back of the Air Boot



Often there is a hair line crack in the rubber part. Option 1 is to replace the hose pipe. Another option is to have a new rubber piece crimped onto the pipe. the trick is that the old hose has is a 'step size hose' i.e., the input size is different that the output size. A local Hydraulic shop was able to made up a hose that worked and looks good too. Cost was around $10.

[PcarRacr]

OK...SO I had a Smog Pre-test done to see where I was at. I resolved the idle hunting by simply lowering the idle...now it idles smooth as glass all day long.

But here's the bad news...Went for the test and was feeling good because so far I have changed the ISV(IAC motor), Head Temp Sensor, Put a Stock Chip in it and the car has an O2 sensor that is only a year old. So I find out today that the car is worse than before!....the sad thing is that the car runs great...smooth as silk.

Here's the results.

%C02 %02 HC(ppm) Co(%) NO(ppm)
Measured 10.2 .1 177 7.92 106

Allowed 138 .85 1148

So as you can see its C0% is still off the chart. Before I made these changes it was at 5.91 which is still way high and the HC were measured the same as above.

Does anyone have any ideas? Is the 02 Sensor junk?

This car has spent the last 3 years of its life exclusively on the track but now that I have a new "race car" I want to put this one back on the street. I figured since the motor is stock and so is the exhaust I didn't figure I would run into soo many problems.


Thanks Again,
Chad Armstrong
85 911 V4 POC #192

[Jim Smolka]

O2 sensor could be bad. A cheaper fix is to splice a Bosch 3 wire sensor from a Ford onto the old O2 wiring harness.

[stlrj]

Or cheaper yet...I mean free, is to pop the cover on the air flow meter and recalibrate it to pass with a bit more spring tension and a tweek on the pointer.

Leave the 02 sensor alone, nothing wrong with it. It just can't do it's job when your mixture is so far out of wack.

[PcarRacr]

Thanks guys...I am taking back out to my mechanic today and we will try these things to make get the mixture right.

The car seems to run more than healthy so its just got to be soemthing not set right.

Chad