throttle micro switch issue? with low RPM driveability issue.
 

84' Carrera 3.2

My car has always had this driveability issue in town around 2000 to 2500rpm. the throttle is either on or it is off nothing in between idle and 2000 rpm. It is challenging to say the least.

So...By hand I moved the throttle to try and get an rpm between idle and 2000 rpm, can't do it.

So I disconnected the micro switch plug, idle went straight to 2400rpm.
Next I jumpered the terminals in the plug on the wire side, the throttle went to normal and I was now able to get an rpm anywhere between idle and 2400 rpm but it would not maintain it and would cut in and out.

I can hear the micro switch click when I move the throttle slightly.

Does any of this point to that micro switch being faulty or is there something else going on here?

Is the micro switch adjustable?

Thanks
PK

Doesn't sound like the switch to me - I think that's working correctly in that it maintains idle control when it's closed. Sounds like you have a vacuum leak that is allowing too much air into the system, but not enough that the idle valve (ICV) can't compensate when the idle switch is closed.

On my '89, the brake booster venturi maze was a source of vacuum leaks.

HTH,
Chuck.H
'89 TurboLookTarga, 359k miles

x2

the switches tell the DME when the throttle is wide open or closed and nothing in between

I'm going to agree with Chuck. I had a minor case of the herky-jerky last year and after correcting all vacuum leaks it's gone.

When you pull the throttle lever off the idle micro switch the idle should bump up, but nowhere near 2k rpms. I don't have the number handy but I want to say it's 1200 or so, again don't quote me on that number.

Also there are two parts to the throttle lever on the side of the throttle body. As you apply throttle one part raises off the micro switch a mm or two before the second arm stars moving the plate in the throttle body.

Bentley manual does a pretty good job of walking through all of these adjustments.

Good look,
-J

I had weirdness with my idle initially which turned out to be a combination of several things, none of which I was able to pinpoint, but all of which improved over time as I attempted to correct them.

I would agree that the vacuum leaks could be a problem here, and they could come from any rubber or gaskets in the path: any vacuum line, intake gaskets and isolator, the rubber ring under the throttle body that joins the 2 intakes. many of the lines aren't special and can be replaced little by little using vac rated lines available from any auto store.

The PCV valve near the oil tank and associated rubber could be leaking. The smallish lines behind the TB could be suspect - easy to replace/fix at low cost.
any bushings and springs on the throttle bellcrank should be checked or replaced.
The goofy reducer on the vac line configuration that is behind the TB and goes to the vac booster should be replaced - especially the reducer part that goes to the metal pipe.

Test the ICV and the idle micro switch (sounds like you are satisfied with the microswitch already). make sure the O2 sensor is connected.
try to resist mucking with idle adj screws until you eliminate these variables with TLC. A lot of these things can be found by searching or PM me if you wan a link to some of my unprofessional fixes / posts.

The 84-89 Carrera has 2 switches on the TB, one is the idle switch the other is the WideOpenThrottle switch.

The more reliable switch is the idle switch, this one is a micro switch and you hear it click on and off as you lift the throttle off the idle stop. If the throttle is fully closed the switch should be closed with 0ohms resistance, use a good ohm meter to verify the switch. Then with just about 1mm off idle the switch clicks open, the switch has adjustment screws to adjust it.

But be aware that you can not and should not EVER try to set idle speed forcefully! The idle speed has very specific setup procedure that MUST be followed. The early cars (84-86) idled at 800RPMs while 86-89 idle at 880RPM and the idle speed is set (hardcoded) in the DME software. You can not just decide what speed you want by turning the TB bypass screw, don't even try doing that. So if you already mucked with this idle screw you will need to follow factory procedure to properly setup idle control.

You also said that when you unplug the idle switch RPM goes up to 2400RPM, when you do this you are telling the DME to switch from the idle maps to the PartThrottle maps and it's normal for RPMs to go up but they should not go up to 2400RPM but more like 1200-1800 so the fact that your car goes to 2800RPMs tells me you may also have some sort of air leak in the intake track or your base Idle Mixture is also off.

One more thing to try is unplug the O2 sensor and see if this helps idle and drive-ability, a weak or bad O2 sensor can easily command a over rich mixture. Just unplug the O2 sensor and see if things clear up.

The only proper way to set base mixture is with a WideBandO2 meter or a CO meter, without these you won't know what's really going on.

It's normal for these cars to idle at 800 or 880 (depending on year). and if you slightly open the throttle the idle speed jumps up to 1800 or so. You can not establish a speed below 1800RPMs if the idle switch is open and the engine is free running and not in gear, this is by design.

But this does not mean you can't run the car in gear at 1200RPMs, you certainly can. It's just a issue with the engine free running and not in gear.

Update

The ICV Valve was in the wrong way. The arrow was pointing up instead of down, Who would have thought. I turned it over in 5 minutes and Wala! fixed. I have struggled with this issue since I have owned it. But wait there is a little more. I do not believe the ICV is really doing anything. When I unplug it very little changes. IF I choke off the hose going to the ICV very little happens. I suspect the throttle plate is a little too open allowing most of the air to go through there instead of the ICV. Although it is running fine now.

Q: With the throttle plate too open at idle what are the repercussions of this?

Thanks for all you excellent comments so far.

Congrats on the find and fix - damn previous owners God bless their hearts. Mine replaced the ICV but neglected to test the DME drive for the ICV - it turned out to be bad in mine so I had to fix that myself - rare but not unheard of.

Forgive my response because once I research, learn and fix, I do a memory dump/forget easily. But if memory serves, the ICV is only needed when she is cold. So if your engine was warmed up, or the temps outside are moderate, I believe you will see less of a response to any tests you do - apart from some of the recommended ones that have you jumpering connectors or pinching lines. I cannot answer your throttle plate question either.

The ICV is used at all times while at idle. The idle speed has very specific procedure to adjusting idle and if someone has not followed the procedure things could be way off.

Let's say someone attempted to adjust idle speed by adjusting the idle stop screw on the Throttle Body this would really screw things up. The 84-86 DME has base idle speed built into the DME program code for 800RPMs and if you simply open the throttle plate via the idle stop screw to force a higher idle then the ICV code will simply try to asdjust the higher idle by closing the ICV till the ICV would reach it's fully closed limit and simply remain closed. This is very bad setup!

The correct way to adjust idle speed is to jumper B&C pins at the engine diag port, this is the pin round connector located on the rear fuse block tin. When you jumper these pins the DME commands the ICV to 1/2 open position, once the ICV is in the center position you can then set the base idle to 800RPM (84-86 cars) or 880RPM (86-89 cars). To set the base idle you normally just adjust the idle bypass screw on the Throttle Body till you are in the target idle speed. For the early cars I use a target base idle of 820RPM and later cars 900RPMs. If the idle bypass screw can't achieve this target speed then you may need to turn the bypass screw all the way in and then turn it out 3 turns after this you'll use the TB throttle plate stop screw to get the idle on target. Adjusting the TB stop screw is usually not needed unless someone messed with it. If the idle is way off following the above procedure I'd suspect an air leak and I'd rule that out before adjusting the throttle stop screw.

What happens often is that owners and mechanics without proper understanding of how idle works try to force the idle speed up, especially in the early cars with 800RPM base idle. The 800RPM idle IMHO is simply to low for these motors but the only way to bump it up properly is with a new DME chip that has base idle speed set in the program. Many tuners can easily give you different idle speed. In my car I have my base idle set at 920RPMs in my chip and I can easily alter base idle in any of the 84-89 chips. But the very best solution for early DMEs is to have them converted to run the 89 factory chip code, the 89 factory tune was the very best OEM tune for these 3.2L cars.

In closing: DO NOT try to just force idle speed without knowing the proper setup procedure. If you need more help understanding the procedure let me know.

Also see this thread
http://forums.pelicanparts.com/porsche-911-technical-forum/738755-idle-adjustment.html

nice job sal.
question, does the ICV close all the way once the idle switch is open?

When the idle switch opens the ICV parks half way. I think they did this so when you return to idle stop the ICV is already in the center range and thus fully ready to start idle compensation.

I have recently put the SW chip in. I attempted to adjust the idle by jumpering the B and C pins as instructed in Steve's handout but with the with the ICV around the wrong way I could not get an adjustment that seemed correct. Since correcting the ICV I have not tried to adjust the idle again so these are timely replies. Thank You. At the moment the car idles solidly at 800. I believe it is supposed to be at 880 from what I have read. At cold start up it jumps by 1000 rpm for about 6 or 7 cycles before it settles in. Coming off throttle to a stop light it will sometimes drop to 600 briefly before settling at 800. Not a big deal but a little annoying.

Thanks

Keep in mind Steve's chip could have idle at 880RPM, I'd confirm with him the idle speed setting for that chip.

oh wow.

One way to check the ICV is after the engine is warm, remove it and turn the vane to open or closed, then re-plug in the harness. The servos should move the vane to halfway open.

If you continue to have idle issues, then you might want to check fuel pressure.

And BTW it's "Voila" - a joyous expression of surprise. I have no idea who or where Walla is.

He told me does have the idle mapped to 880

If the chip has idle at 880RPM I would set the base idle at 900RPMs.

You have never head of Walla Walla in Washington State? You are missing out. Exceptional Porsche country. So when something is Walla that also means joyous expression.:)

Thanks

I will get at it today and let you know what happens.

PK

I thought Walla was an expression of the desire to wear muffintop jeans and t-shirts one size to small whilst shopping at Wallyworld.

84 3.2

Oh !@#$%^%$! the idle adjustment screw broke/stripped. IT is the flathead screw driver type or was. I was being very careful too. What now? I did a search but came up with nothing.

that makes sense.
i have been meaning to pull the ICV on my 88 BMW and do some tests on the ICV.
i think this is a very missunderstood device and often blamed for problems that are not its fault.
i think my B<W system and the porsche system are fairly close to being the same.

I wrote up testing the ICV some time ago, here's the thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/360766-bench-testing-carrera-idle-control-valve.html

On the throttle body it has 2 idle adjustments:
1 - the Air Bypass screw in the center of the TB
2 - the throttle plate stop screw on the left hand side

Which of the 2 did you break? The Air bypass is not turned with a screw driver, you use a small socket to adjust it (I think 8mm?)

No matter which one I recommend removing the TB from the intake, it's not very difficult and it will be far easier to repair on the bench.