Eddie Hotrod

Hello all

I've recently acquired a 1984 3.2 Carrera, which runs beautifully, but has an idle problem. It idles at a steady 900-1000rpm, but if you rev it, as the revs drop back down, it seems to dip below the idle speed for a split second and then return to the 900-1000 rpm range. It does it constantly, although it's a little better when warm. Sometimes this causes it to stall, which can be a little embarassing in traffic, other times it keeps running, but does make it difficult to drive at slow speed, and can be a problem when slowing down quickly. I've check that big T-shaped valve on the throttle body (idle stabilizer?) and it looked ok, although I can't be sure. Has anybody else had this problem, and any ideas how I can fix it? Any help much appreciated.

BER

First, you can not assess the condition of the Idle Stabilizer Valve (ISV) by looking at it. You should remove it and clean the inside with spray carb or brake cleaner. The Bentley Manual has a procedure for checking to see if the ISV is functioning properly.

Have you adjusted the idle? There is a specific procedure to follow on the 3.2 Motronic engines, so it's not as simple as turning the idle by-pass screw. The Bentley manual and the small Technical Specifications Book for your model car both have the procedure for setting the idle.

Bruce
'84 Carrera
'77 911 (euro spec)

mducharme

Hi guys, If you figure out this problem please post it. I have the exact same thing on an 84 3.2 cab
Mducharme.

86targawhite

I have a similar problem. The car idles at 700-800 RPM but when I come off the throttle it stalls. Oxygen sensor?

mducharme

Hi there, Maybe in your case its the oxygen sensor (lucky). I wish my problem was that simple. My o2 sensor is brand new along with the temp. head sensor (up grade), plugs and wires, all new filters, no vacuum leaks ( I can find) even cleaned out the idle control valve. No change still searches idle. Although it doesn't stall anymore since I adjusted the idle control screw.It was set 4 complete revolutions to the left, turned it back to 2 1/2. I hope this helps the original post. It didn't cure my problem but it might cure his. good luck
M ducharme.

scottb

I have a fluctuating idle problem on my 84 when it gets warmed up. I've replaced the O2 sensor, the CHT sensor and the idle control valve. Problem persists. It's been suggested to me that my mixture might be too rich. I'll have my mechanic look at it next time it's in the shop.

MikeF

Just for kicks, try disconnecting your 02 sensor and go for a drive. Also, have you changed anything on your car (exhaust system, etc.) without modifying the chip accordingly?

Just a thought.

Lorenfb

The problem you describe is decel idle undershoot.
The following should provide some insight:
1. The Motronic F/I normally shuts the injectors
off on decel when the RPM is above 1200 and the
idle switch is closed (foot off gas pedal) and
on again below the 1200. This should cause
a small undershoot (50 RPM). This is normal.

2. Undershoot greater than 50 RPM results from
a number of things;
a. the air screw in the throttle body being in
too far,
b. a very lean mixture at idle, air flow meter
screw out too far,
c. the idle switch not closing,
d. a sticky air value,
e. an ECU chip mod programmed incorrectly,
f. and a possible O2 sensor forcing a too lean
operation.

Review the above and check out this website
(systemsc.com). It has some good tech info
about Porsches, you might find it useful.

Good luck.
Lorenfb@aol.com

Eddie Hotrod

Great responses guys, it looks like there's a few of us with the same problem, so we have more chance of finding a cure. O2 sensor is out in my case, because I have a UK car and it doesn't have one. I'm going to check out the things in Lorenfb's post, then I'll let you know the results. Can you just let me know where the airflow meter screw is, and where and how I might fix a sticky air valve.

Thanks all

Eddie Hotrod

I've cleaned and checked the operation of the idle stabilizer and cleaned the throttle and air flow meter (as far as I could reach inside) and no difference. I'm pretty sure the air screw is set right, so I'm quite interested in the air flow meter screw - but I can't get to it. How can you adjust this screw without taking the meter off the car?
Help please.

Lorenfb

The idle mixture screw is on the bottom of
AFM. It's not easy to get to w/o taking the
unit off. Some units have a plug in the
access hole.

If it still has a plug, then no one has
adjusted it and it's probably OK. Focus
on the other areas, e.g. fuel pressure,
hot temp sensor S/B about 300-400 ohms.

Make sure no one has put a special chip
in and that timing and mixture adjustment
setting in the ECU is all the way CCR.
It's the rotary switch in the box.

Good luck
Lorenfb@aol.com

scottb

I had my car at the mechanic yesterday. He adjusted the mixture. So far, so good.

Eddie Hotrod

I've done my mixture too, it's better but not perfect. The search continues.

eion

Eddie et al.,

I have another possibility to throw into the mix - bearing in mind I have a 964. I do, however, have the Workshop Manuals for the '84-'89 cars, and I'm bored enough to have read them a couple of times.

Firstly, a UK (or any ROW) car won't (shouldn't/mightn't) have the plug on the air flow sensor that needs to be drilled out (and then replaced, according to the manual!) on the US cars.

Without looking at the car (or being guru-familiar with Carreras), it's hard to say what diagnostics/adjustments in the Manual apply to you... PM/email me and I *cough* may be able to do some scanning. A subscription to AllData might also be worthwhile to check for Technical Service Bulletins pertaining to the car. Certainly for *my* car, there are a number of TSBs relating to this kind of problem (well, it *is* an early 964 ).

One last suggestion, which I may well get laughed at for (I don't know how the DMEs varied from the Carrera to the 964) - but if a system adaption is possible for your car, do one. Basically this involves disconnecting battery power for 20 minutes or so and then letting the car sit at idle for 10 minutes... and feel free to laugh at me if it's a totally stupid suggestion.

Hope this helps...

Eddie Hotrod

Another thought, I've set my mixture to 5%. Is this correct for an '84 Carrera 3.2, Euro spec?