AFM Problem??

[timsirk]

Ok, I need some advice from someone wise in the way of the idle.

Right now the car (85.0) idles about 600, maybe a touch more when warm...I just cleaned some oily stuff out rubber boot/thottle body today and reset the idle because it seemed a little high at 900. I really don't know what its suppose to be.

Here's the catch: when I don't let the gas off VERY easily, the rpm will drop down to around 400, the car will shudder a little, and recover to the idle speed I set it at. It always falls back around 400 even if the idles is set high. Oh, and another thing: I'm not sure I trust my tach...it feels like it reads low. Can I check it against the shift light, or does the shift light read off the tach??

My microfiche manaul is on backorder so I'm just playing a guessing game here.

Any suggestions?

[mideastmafia]

i have the EXACT same problem and i still havent been able to fix it.

i have heard answers that range from Aux. Air Valve, to Chlostrophobia.


SHAUN

[Scott R]

I have heard on several different boards that there really is no fix for this. It's the DME. And conditions being fed to it that cause it. I'll try and find the link, but a guy has caome up with a workaround that involved drilling something on the throttle body to get a different air reading to the DME. Some have claimed to fix by replacing the DME, didin't work for me. My idle is about 900, keeps the problem masked a bit.

[TheStig]

THATS THE SAME PROBLEM I HAVE RIGHT NOW!!!!!!!!!!!!!!!! HELP US SOMEONE!!!!!




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Porsche. Es gibt keinen Ersatz.

[andrew4266]

I had the same problem until I took apart the intake and cleaned everything out. It is also a good idea to clean the cylinder head. It's amazing how much gunk gets built up in there. I did all this to mine and never had a problem since, not to mention it restored lost power.

[timsirk]

Sorry to hear this isn't an easy fix, anyway you cut it. It does "feel" like an electrical problem though. I would almost bet you guys are right about the DME.

how do I know if my tach is right?? it sounds fast for what the tach shows ALL the time...come to think of it, almost every gauge in my car reads wrong. The tach is too low, speedo is to high

AND the oil pressure is almost always pegged out unless its idling then its still just one mark away from pegged. that might not be the gauage!! what a friggin mess.

Tim S


[This message has been edited by timsirk (edited 07-13-2001).]

[Ehab]

Alright, here we go ... if you guys look through the thread 'MAF details', I was actually talking to Ahmet about the AFM, and how to fine tune it.

Simply put, there are way too many variables that control the idle of a 944. I have an early one, so it might be a bit simpler than the later ones.

The idle speed is supposed to be 900 plus or minus 50, according to the sticker inside the fuel door.

The two main things you can play with to adjust idle are the air flow on the throttle body, and the amount of fuel, as set by the AFM.

The Shudder problem is intrinsic to the DME of the car, because as you let off throttle, it waits until the rpm falls to 900 before it starts to open the fuel injectors again, causing a lag in response, and an rpm undershoot to something around 700rpm.

The guy that figured out a solution to this actually reprogrammed the DME ... but that involves putting in new ROM chips into your DME, after burning the program in.

Two ways that have allowed me to reduce the shudder effect are:

1. There is a screw on the throttle body which determines how open it is during idle, if you screw that in a bit (1/4 of a turn), you will have a slower drop in rpm when letting off the gas, which reduces the undershoot. However, be careful, cuz if you push the screw in too much, you will open the throttle switch, and the DME will not regulate idle rpm anymore, you can hear the switch click past a certain point in opening the throttle.

2. There is an allen bolt in the AFM, to the far left before the intake tube. This is usually covered with a piece of plastic so people can't play with it, but I didn't have the cover, meaning that the PO has already screwed around with it. So experimented with that, and it actually also controls idle rpm through the Air/Fuel ratio ... I ended up using this option because that way I was sure that the throttle switch was working properly, and I didn't get any rpm variations with temperature as I used to when I used option 1. Turning that bolt in richens the mixture at idle, while turning it out leans it out. Since this bolt seems to control things only at idle, I played around with it (along with the throttle body idle screw) to tune the idle speed to my liking (900 - 1000 rpm) and get the least shudder possible.

Good luck.
E

[Lawrence Coppari]

Oil gauge should be pegged under normal operation except at idle. At track temperatures, mine drops to around 4.0-4.5 bars. All other times except at idle, it remains pegged. When cold, it pegs at idle. 15W50 Mobil 1.

[Magnus_944]

Have you seen this? It's about the shudder and a solution to the problem.
http://members.tripod.com/944dme/dme/shudder.htm
http://members.tripod.com/944dme/

/Magnus

[Ehab]

Yes, but the guy himself admits that the DME modification is too much trouble for all its worth, and that playing with the throttle body's second screw is a much easier way of doing it.

E