Pelican Parts Forums - surging off idle

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- - surging off idle (http://forums.pelicanparts.com/porsche-911-technical-forum/389411-surging-off-idle.html)

usually when the idle speed is in need of adjustment, it's because it's too high or too low. so adjusting the speed screw back to the normal range would not adversely effect the mixture. the mixture is also adjustable within a certain range that the o2 sensor can handle. not doubting you, but there is some leeway within reason. instead of a jumper, i use a round black relay with the top popped off. plug it in and press the top contacts together with your finger.

John,

Yes I have seen the relay used as well and that works fine.

So if you are saying you set the idle by first pluging the relay into the socket and holding it down and then proceeding to set the idle with the black relay contacts held down then we are on the same page. This procedure you describe is exactly the same thing as jumpering B&C pins and then setting the idle. It's just that you did not mention the black relay in your first post and I wanted to be sure Henry did not proceed to set his idle without B&C jumpered.

The point is you really can't alter the idle without altering the chip, my car idles at 920RPMs because I changed the chip program. I was just trying to explain that you can not just pick your Idle RPM it is pre-built into the chip. All you do is get it in the ballpark with the ICV parked at it's center position (50% open).

Quote:

Originally Posted by john walker's workshop (Post 3732686)

Thanks for the post! I did not unplug my O2 when I jumpered b and c! This might be part of my problem.

I would also like to know the CO adjustment procedure with a digital voltmeter since i do not have access to a analyzier. When you have the time of course!;)

Quote:

Originally Posted by scarceller (Post 3732487)

I'm giving you the idle adjustment procedure right from the Motronic manual titled "911 Carrera DME Testing Plan" this manual covers the Motronic system and howto fix it.

Here is the page on setting the base idle:

http://forums.pelicanparts.com/uploa...1201529315.jpg

Please be aware that you CAN NOT alter the idle by setting the idle screw! The DME controlls the idle in the Carrera and the only way to change the idle is to alter the Chip. I know this for sure since I burn these chips often and my 84 Carrera idle is set to 920RPM because of the 20/21 Web Cams I had to increase the idle. I did this by re-programming the chip.

The 86 USA chip has it's idle pre-set in the chip to 800RPMs. The idea for seting the base idle with B&C jumpered is to get the base idle in the ballpark. I ussually set the base 25RPMs higher than the chip idle. So for your car dissconnect the O2 sensor (O2 MUST be disconnected) jumper the B&C pins and set the idle to 825RPM or so then pull the jumper and the DME will command the ICV to pull the idle down to 800RPM.

When you jumper the B&C pins it tells the DME to center or park the ICV at it's center point (50% opened). This insures that the ICV is half opened and has headroom to pull the idle down or up.

Many folks think you can somehow alter your idle by just adjusting the idle set screw but this is not possible! The idle set screw is only used to get the idle in the ballpark when the ICV is parked or centered.

Also, notice that the Procedure for base idle set should be done along with the CO setting but setting CO is a little more detailed and I'll assume you are OK here so just try to set the idle as described.

If you really want the details on a simple CO set procedure that can be done with just a Digital Volt meter and the existing O2 sensor I can post it later. You can get the CO in the ballpark with just a digital volt meter. But the best method is with a wideband O2 sensor, I have one of these installed in my car but they are expensive.

Hope this helps you better understand your idle.

Henry,

For CO setting do you have access to a wide-band O2 device? I ask because there are 2 ways to set CO: one with just your stock O2 narrow-band sensor and a Digital Volt Meter. The other with a wideband O2 and just setting the AFR to 14.7, this second approach is far more accurate than the first.

Just want to know what you have available?

But don't even worry about the CO setting till you get the idle fixed. So if your idle is not holding at 880RPM +/- 20RPM then no sence trying to fine tune the CO levels. The CO level adjustment is a very fine tunning done on the Air Flow Meter (AFM) it just deverts very small amount of air around (by-passing) the Barn Door in the AFM. It is very tiny adjustment and really does not make sense to do if the idle is way off from the 880RPM. You need to nail down the Idle issue first.

good info/stuff......thanx!

Henry,

Take a peek at this thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/358902-uneven-idle-low-tickover.html

Here is a good example of what can happen if the intake gaskets get partially sucked in. This fella had same issue as you describe (Low RPM down in 600RPM range) he had very large vacuum leak. He also claims starter fluid did not really find it.

Just thought I'd share the thread.

Sal,
thanks for that link...I hope I don't have those gasket issues... I'll see if I can work on the idle project this weekend... I may even take the ICV out and bench test... I have to fight my son for the other car.. he's 6-3 and 190 now so that is an issue. ;-)

Quote:

If you start to crank the idle screw 'IN' you richen the mixture and then the ICV is commanded by the DME to 'OPEN' to lean it back out. Now if you continue to crank the screw in the ICV will bottom out and fully open, at this point it is out of headroom and if you still continue to crank the screw in the idle will change.

Same holds true for cranking the screw 'OUT' leaning the mixture. This will cause the ICV to fully close in an attempt to richen the mixture and eventually it will bottom out on the other side (Fully Closed).

Screwing the idle screw in or out has no effect on the mixture. This merely allows more air to bypass the throttle valve. All air, either going through the idle circuit or through the ICV valve, is metered air.

Mixture will only be affected by adjusting idle mixture screw on the AFM, or by vacuum leaks in the intake downstream of the AFM,

ianc

Ianc, as always thanks for the input. I'm with you and I see your point, the mixture does not change but you do affect the idle RPM if the ICV is parked (B&C jumpered).

But if you decide to adjust the idle screw WITHOUT B&C jumpered, for example lets say you start turning it clockwise you lower the by-pass air but this causes the ICV to open more to compensate and if you continue turning the idle screw you will eventually bottom out the ICV. Do you agree?

Thanks for pointing out that the mixture is not really changed. I was just trying to explain that you can really screw up the ICV and cause it to bottom out in one direction or the other if you simply decide to start messing with the idle screw WITHOUT jumpering the B&C pins. This was the only point I was trying to make. I really should not have used the terms Rich and Lean in my description.

Thanks.

Quote:

Originally Posted by ianc (Post 3733714)

Quote:

if you decide to adjust the idle screw WITHOUT B&C jumpered, for example lets say you start turning it clockwise you lower the by-pass air but this causes the ICV to open more to compensate and if you continue turning the idle screw you will eventually bottom out the ICV. Do you agree?

Yes, that is it in a nutshell. You're aiming for the idle value programmed into the ECU when you turn the idle screw. With a warm engine under ideal idle conditions, there should be no difference in speed with B&C jumped or not. This indicates the car wants to idle at the preset 880 (or 800) RPM. The ICV will adjust it if it should fall below or move above this for any number of reasons, but only within the amount it is able.

ianc

Ianc,

Thanks for confirming the procedure, as always you are a wealth of information on this board.

Quote:

Originally Posted by ianc (Post 3734042)

Quote:

Thanks for confirming the procedure, as always you are a wealth of information on this board.

Ha, I've picked up a thing or two over the years, but I'm not sure I'd go that far. :)

Thanks for saying so though,

ianc