Carrera idle yo-yo

dorschman · dorschman's Garage

Went out to start her this morning and the idle yo-yoed between about 300 all the way up to about 1500 repeatedly. There was more smoke from the exhaust than I am used to seeing as well (usually a little puff on start up and that is it).

I read previous threads about temperature sensors and some people having trouble with this on a frequent basis in cold weather but the car has never done this before.

It was 22 this morning. I came home and tried again (about 40 out) and had the same thing.

I can give gas and pull the car out of the yo-you but as soon as I let off it either falls low enough to stall or just continues the yo-yo.

A couple of months ago it would not start and hold idle- I replaced the idle control valve and that seems to solve it.

I honestly have never felt the car has idled super smooth but have never had a problem like this before.

Any ideas?

thanks for any help.

JMA-RUF · 12-22-2005, 02:29 PM

Vacuum leak? Check all vacuum hoses/connections and re-torque manifold nuts to 18# and see if that cures the symtpoms. If re-torquing the nuts helps, then you may have a leaky inlet manifold gasket(s).

What color smoke? Black or blue'ish white?

dorschman · dorschman's Garage

hard to tell the smoke because it was twilight both times- I would say more of a blue/ white- smelled bad too- not like normal exhaust- hard to describe- smelled "dirty" and when I came in the house I reeked of it. I will get to the hoses and nuts this weekend.

thanks

DeutschMark · 12-22-2005, 06:11 PM

Double and triple check all vacuum lines and connections. Its probably a good time to replace the smaller lines also. Sounds like your idle mixture is off due to a vacuum leak and the ICV is trying to compensate. The same thing happened to me.

Try a search using "carrera+mixture" and you'll find lots of information.

p911dad · p911dad's Garage

In a little publication by Porsche from 11/84 called DME Test Plan for 911 Carrera, page 25 cites:CO level > specifications(% by volume) - with maladjusted CO, cold engine will begin to surge the greater the CO level deviation(from approx. 2.5% by volume).

CO level
The booklet goes on: CO level adjustment- engine must be at operating temperature! CO level is adjusted on the air flow sensor from the bottom. Use special tool 9156. (see repair manual for removal of plug from adjustment bore)

Adjusting values: at idle speed of 800 + - 20 RPM with electric equipment switched off and measured in front of catalytic converter: 0.6 to 1.0%(oxy sensor disconnected).

(install new plug in bore after adjustment)

Note: For CO level adjustments the engine should be run warm inside of the shop building-engine hood closed.
Make CO adjustment quickly after the engine has reached operation temp. 90 C or 195 F.
Make sure engine intake air temp is between 15 and 35 C or 60 and 95 F.

Check CO level after test drive is not necessary Reason: After a test drive the air flow sensor temp(temp sensor I) will be equal to the outside temp, ie, a different temp as that during the adjustment. The control unit is programmed, that the engine mixture will be leaned via a temp. sensor signal change.

So, possibly your CO adjustment has drifted out of spec for some reason.

gabenheimer · 12-22-2005, 08:35 PM

After checking the vacuum lines, you may try unplugging the oxygen sensor. If the computer does not get the correct signal from the sensor it will keep attempting to adjust the mixture and can cause idle fluctuations. When the sensor is unplugged, the system uses a fail-safe mode with a default mixture setting. This was a problem with my '86 Carrera. Once the sensor was replaced, the idle improved dramatically.