Idle adjustment hex screw (with or without jumper)

[Booney1golf]

Good afternoon everyone.

I have an 85 911 3.2 and I have read that I can adjust the idle with the hex screw on the throttle body (I think that is what its called). When I turn this indeed it will adjust the idle but it does not stay at what I adjust it to.

I have read on this forum that you cannot adjust the idle as it is done electronically and also I have heard some say that if you adjust the screw you must have a wire jumper for the adjustment to take effect.

I have taken my car to two different shops in the Houston area to ask them to see if they can make any adjustments to my car to make it idle / rune better.

I really can't tell a difference after they charge me alot of money.

Thanks to you all for always being there to help us non-wrench types.

Best regards,
Daniel Womack
Booney1golf
1985 Porsche 911 Carrera Targa

[jbrinkley]

you mean two different places charged you and didn't tell you anything?
aside from that, what problem are you trying to fix?

[Argo]

Here you go ... http://forums.pelicanparts.com/showthread.php?threadid=188748&highlight=jumper+wire+idle

http://forums.pelicanparts.com/showthread.php?t=351514&highlight=idle+adjust+jumper

Argo
88 Targa

[nesslar]

I just recently successfully did this with my '86. The jumper was not necesasry. You heard me right.

Get the car warmed up and then at idle turn the hex nut counter-clockwise so the idle will be increased a bit. I turned mine a little more than one revolution and that raised my idle to 800 RPM from where it was before, about 650, which was just too low. I DO believe adusting too high might cause problems though, roughness, etc.

To me, the only real problem seems to have been "listening" to some of the things I previously read, like "can't be done on your car", and "you must get idle the same with the jumper and without". That was impossible for me, and frankly totally unnecessary as I soon learned. I could raise the idle with a jumper in place, but upon removal of the wire, the idle would fall.

Anyway, mine is fine, and after several hundred more miles of varied flyin' around, no problems at all and still idling beautifully. I have even tested it since, by just turning the hex nut left and/or right, noting the change in idle, then setting it back to 800 RPM. No jumper involved.

I noted a little "cough" once when I had the idle to about 950 RPM, so obviously that is too high. Returning to about 800 RPM has been faultless.

YMMV, but I doubt it. I mean mine is "living" proof.


Anyone else had luck with no jumper? Surely my car is no fluke?

[Booney1golf]

Good evening again.

I can get mine set at around 850 RPM's but after a little while it will drop to maybe 600 and the car will shake (vibrate more than normal) until it goes back up to 850 or whatever I set the hex screw to.

No matter what RPM's I set the idle to start with, it will drop 200 or so after a little then run rough for a while before going back up to my setting.

If I screw it all of the way in the car's idle evens out and the car runs good but when you first start out it acts like it is choking or not getting enough air or fuel (maybe both). However once the RPM's are up around 1500 she runs like a scalded dog.

Thats why I was hoping the jumper would help my car to maintain the idle speed that I set the hex screw at.

Very confusing for a novice thats for sure???

I won't go into my frustration with the various shops I have had my car at in the area

It seems like if you want something done right you have to learn how to do it yourself. Thank God for this site.

Thanks for the time you all always give me!! It is really appreciated. I am determined more than ever to learn how to do all the repairs / upgrades to my own car.

Best regards,

Daniel Womack
Booney1golf
1985 Porsche 911 Carrera Targa - AND I THOUGHT GOLF WAS ADDICTING

[Nine9six]

Do a search on "hunting idle" This has been discussed many times, with a wide variety of causes.
Hope this helps...

[jbrinkley]

you need the motronic manual seen here http://www.amazon.com/Bosch-Fuel-Injection-Engine-Management/dp/0837603005/ref=sr_1_1/105-3507175-7075614?ie=UTF8&s=books&qid=1190895023&sr=8-1

Remember that there are many things involved, icv, afm, unmetered air, o2 sensor, dme, chip, off idle switch, etc.

[nesslar]

Good advice, Jerry. It seems to me we need to hope that shops will stop sticking it to Booney1golf. With that, I'll offer one more possibility. Again, warm engine, etc.
1). Turn the nut in. Achieve your 600 Rpm.
2). Jumper the ports B and C with a piece of #12 insulated wire bared at the ends. I use a 2-inch piece shaped into a "U". It's not any trouble getting that black cover over to the left off, should have two wing nuts with washers. Be careful not to drop those sneaky washers. Take you time wiggling the cover free.
3). With the jumper in place, turn the srew out about a turn, maybe a turn and a half. The idle should rise, shoot for a little better than 750.
4). Remove the jumper. The idle should fall.
5). Now turn the screw out for idle just over 750 (another turn + ?) and you will have adjusted through both exercises to what should now be a steady 800 RPM or so. This is the only way I can figure to match the two adjustments.
Note! Be careful not to turn the nut out too far counter-clockwise! I don't know how far out it will go without popping outa' there! That of course is part of my disclaimer, as you are on your own. Seems to me there probably must be five or six turns available?? Maybe someone knows, and maybe someone at Pelican (or here) can count threads on a new part (throttle screw) to ease your mind. Still, you shouldn't need to turn it out too far anyway.
I hope you have success.

[jbrinkley]

The idle screw comes right out and theres an o ring that should be cleaned.
It isn't hard to get a perfect running motronic system. But you have to take apart stuff and replace/fix as needed.
Taking it to a shop for an idle/mix fix is only as good as you system is.
It will compensate for air within it's limitations.
After that it's downhill.
If you take it to a shop and tell them to adjust idle that might be the only thing they do.

[john walker's workshop]

be sure the little micro-switch on the left side of the throttle housing always gets depressed completely when the throttle is closed, so the computer knows the engine is in idle mode. you can hear a tiny click if you work the throttle by hand, just off of the closed position.

[nesslar]

Right on. Beyond just trying to adjust as discussed here, I'd agree there may very well be more to the problem.

[Booney1golf]

Thanks Guys. You all amaze me everytime I have a problem there are so many responses and people willing to help me.

You all gave me several things to try this weekend.

I showed a Buddy of mine my question and all of the responses that you all gave me to try and he was so amazed he told me he wanted to buy a Porsche to be a part of this great Fraternity.

Thanks again and best regards,
Daniel Womack
Booney1golf
1985 Porsche 911 Carerra Targa - AND I THOUGHT GOLF WAS ADDICTING

[mtbguy]

I was having this same problem and lurking on the thread- thanks everyone!

[ZOA NOM]

I would warn against guessing and trial and error troubleshooting on the motronic system. There are several adjustments and many more variables involved in a smooth running 3.2. The jumper is used to bypass the ICV, which will affect the true idle setting. If you simply adjust the idle with the throttle body idle screw, you will be changing the air/fuel mixture and possibly leaning it out which could cause damage to your engine. My advice in these motronic rough/hunting/erratic idle threads always begins by establishing a known good CO mixture setting using a gas analyzer. Without confirming this, you are guessing where your air/fuel mixture will be. Once you establish the CO mixture, eliminate all external sources of vacuum leaks. THEN work on Idle Control Valve, Airflow Meter, etc.

[mtbguy]

In my case, I have already had a mechanic replace the air temp sensor, clean the idle control valve and set the CO with a analyzer.

I thought that it would be safe to tweak the idle setting without damaging the engine (my idle speed was around 1050 and now is approximately 850-900 after me following the above procedure). I also had a mechanic tell me that it would not be possible to harm the engine by tweaking the idle setting, unless I made large (500 rpm) changes. He told me that air has already been accounted for/measured by the AFM when it passed through it.

So whats the truth about this?

[jbrinkley]

true, the icv is past the flapper. icv lets air past a closed butterfly, that's all.

zonas is correct, messing around without a WB or analyzer will accomplish nothing with worn out/broken components.
It's very easy to learn and understand everything about this motronic. A book or two and you're done.
if you do it right the first time it will last for years without ever touching it

[mtbguy]

I just bought the Probst Bosch fuel injection book- it seems to be the "Motronic Bible" but I have yet to read it. Thanks for the info........

[ZOA NOM]

Originally Posted by mtbguy

I also had a mechanic tell me that it would not be possible to harm the engine by tweaking the idle setting, unless I made large (500 rpm) changes. He told me that air has already been accounted for/measured by the AFM when it passed through it.

So whats the truth about this?

This is technically true, but only applies if all else is functioning correctly. There are three sources of metered air that enters the engine; past the flapper, around the flapper (CO mixture slug), and around the butterfly (ICV). These all work together to provide steady idle, and to change the volume of air ingested under varying load. The only purpose of the flapper is to send a voltage to the DME, which it uses to determine the duty cycle of the waveform it sends to the injectors. The CO mixture adjustment is simply a flapper bypass slug that allows adjustment of the volume of air for a precise stoichometric ratio at idle. If that slug is adjusted to full open, and the idle adjustment is turned all the way in (restricting airflow to compensate), you could have a serious a/f ratio problem off-idle that you don't realize exists until you start to detonate. Add to that the possibility of any other source of unmetered air into the intake system, and you can see how it can get ugly if you have no baseline. Bottom line is that the Motronic system, like jbrinkley says, is very durable and reliable if adjusted correctly. It's the variables and codependency of the adjustments that will have you chasing your tail.

[Lorenfb]

"The jumper was not necesasry."

He's right-on! Use of the jumper are for those that rely-on/quote the Bentley manual!

[ZOA NOM]

The jumper is certainly not a requirement, but it eliminates a variable in the equation. It's there to allow an easy method of setting the idle, provided the other variables are known. Bentley didn't install the plug where the jumper goes, Porsche did.