cold Idle Circuit

mlang88 · 10-17-2009, 12:16 PM

does the 944 have a cold idle circuit? when I start my car for the 1st time it idles at norm temp idle. just wondering shouldn't it have a fast idle till the car reaches normal temp??

Thanks
Matt

jpk · 10-17-2009, 02:09 PM

For starting, the DME retards the ignition and cycles the fuel injectors a bit longer to richen up the mixture. As it's warming up, ignition and fuel gradually go to normal as the DME reads higher temps from the temp sender.
There is a fixed amount of air that bypasses the throttle all the time (the sealed air bypass adjustment) and for cold idle, there is an auxliary air bypass controlled by the DME. That's all it takes. The DME reacts fast enough to let it idle at normal speeds when it's cold.

mlang88 · 10-17-2009, 02:30 PM

thanks john,

I could tell it was running rich just thought it should of had a higher idle.

matt

John_AZ · 10-17-2009, 02:43 PM

Hi mlang88,

Early Porsche 924 and the first 1983 -1985/1 944 had the auxillary air valve. It was replaced by the ICV/ISV-idle control valve or idle stabilizer valve whatever you want to call it.

The DME temp sensor ($7-$17) resistance is monitored by the DME. This part fails and causes idle problems. Clarks has a test.

If you think your ICV is dirty, you can clean it.

Read "idle valve test" on texasblake site and other good (document) information is there if you read ALL topics.
Home - Porsche 951 - texasblake.com

Did you ever get the AC fixed?

GL
John_AZ

mlang88 · 10-18-2009, 09:29 AM

Thanks john_Az

Haven't done the a/c needed a break I'm going to get to this weekend. I had to do the oil cooler seals and instal the good oil cooler I got from hondaduster. this weekend I had people over so I did the rims (other post) and I'm going to wax her out.

mlang88 · 10-18-2009, 12:43 PM

John, so are you saying it should have a higher idle at 1st start till it warms? I cleaned my idle control valve and tested it when I had the intake off, all good but never checked my DME temp sensor.

jpk · 10-18-2009, 01:28 PM

I loaned out my Bosch fuel injection bible so I can't confirm, but I don't recall seeing anything about increased idle speed on cold start. Based on what my own car does, the idle at cold start is pretty much normal; if it's higher it's only by a couple hundred RPM (maybe 1000 RPM instead of 800) I think the early car was pretty much the same; I don't recall it ever idling over 1000 rpm.
The DME temp sensor is a negative bias; It reads higher resistance at cold temperature and as it warms the resistance is reduced. If there is a short in the circuit (either in the sensor itself or the wiring) the DME will "see" a lower resistance and think the engine is warmer than it really is. likewise, if there is a break in the wire (open circuit) the DME will think the engine is cold all of the time. I think the spec is >3000 ohms cold and something like 600 ohms hot. Clarks may have the exact numbers. BTW - the DME temp sensor is the one with the 2 wire connector (like the injectors). The single wire sensor is for the dash guage.

John_AZ · 10-18-2009, 02:50 PM

Quote:

Originally Posted by jpk

For starting, the DME retards the ignition and cycles the fuel injectors a bit longer to richen up the mixture. As it's warming up, ignition and fuel gradually go to normal as the DME reads higher temps from the temp sender.
There is a fixed amount of air that bypasses the throttle all the time (the sealed air bypass adjustment) and for cold idle, there is an auxliary air bypass controlled by the DME. That's all it takes. The DME reacts fast enough to let it idle at normal speeds when it's cold.

Quote:

Originally Posted by mlang88

John, so are you saying it should have a higher idle at 1st start till it warms? I cleaned my idle control valve and tested it when I had the intake off, all good but never checked my DME temp sensor.

mlang88.

(Not sure of which John you were referring to)
jpk's information is accurate as how the DME controls the idle. The DME does not increase or decrease the idle-it stabilizes the idle from cold to normal operating engine temp. My '87 is set for 950rpm and the '88 is set for 1000rpm. It does not fluctuate during warmup.
"The DME reacts fast enough to let it idle at normal speeds when it's cold" jpk

GL
John_AZ

mlang88 · 11-01-2009, 08:55 AM

o.k when I first start up it idles at 1000 when it warms up its at 1200-1250 what it the right idle for a 1988

John_AZ · 11-01-2009, 09:10 AM

I looked on the passenger wheel well in the engine compartment.
840 +-40 rpm for my 1988.

My tack reads 1000 at idle on my 1988 924S
The tack reads 950 at idle on ly 1987 924S

Steady, no increase.

I suppose I could lower the rpm at the throttle body by 50 or so rpm, but I don't like to change anything if it is running great and passes emissions (both cars this year).

Small vacuum leak?

GL
John_AZ

mlang88 · 11-02-2009, 07:02 AM

Well I went and got a new DME temp sensor. tested them both with the same temp water in a cup. the old one read lower ohms than the new one but not by much maybe .15 ohms. the vacume lines are new I just replaced them so I dont think vacume leak. mabe the tach is off I know the spedo is off like 5+mph and the gas gage only reads 3/4 when full

John_AZ · 11-02-2009, 08:10 AM

I looked at Clarks Trouble shooting page and maybe you need to rebuild the TB-throttle body?
Troubleshooting - Engine

I could not find a kit on Pelican. So...
Porsche 944 Throttle Body Reseal Kit.

Have you seen these videos on the AFM?
Air flow meter fix / refurb

Since you did the vac lines, the TB may be the vacuum leak ???

John_AZ

running_cold924 · running_cold924's Garage

bruce arnn has you covered for the TB rebuild 944TBK

mlang88 · 11-02-2009, 09:20 AM

I checked and tested the AFM its good. when I had the tb off I cleaned it and the shaft didnt have any play in it. the idle is stable at 1200, just high. it hasnt hit 1250 with the temp send change