why is my idle going up and down

[Hoang928]

why is my idle going up and down while it is around 30 degrees and below



81 porsche 928

[rhjames]

Euro or US spec car....?

it makes a difference between the two----CIS or L-jet injection






--Russ

[Danglerb]

Same for engine hot or cold?

How much is it moving?

Is it going up and down continuously, or slowly dropping to a lower speed and staying there?

Auto or 5 spd?

[NoFlat6]

Id say its running rich.

[Danglerb]

My wild guess is idle switch, but it could be the thing thats like an ISV the early cars used.

[Hoang928]

my car is a us spec with a five speed

the rpm is moving up from around 4000 rpm and down to around 1000 rpm really quick while the engine is cold and hot and it does that continuously

thank you for repiles

[surfdog4]

if it's an '81 L-jet then it could be the decel valve..fortunately it's an easy check...pull (and cap) the vacuum line going to it and see if your idle settles down. (assuming of course that the vacuum line is connected at all or is connected to the correct port on the throttle)

I had the "hunting idle" problem but i'm not sure i went to 4000 rpm...still this sounds very similar and this is an easy test.

The decel valve is basically an air connection between the air box side and the manifold side of the throttle body..I think it's labeled "vacuum limiter" on the parts diagram. It's the flying-saucer shaped thingy attached to the side of the throttle body..PITA to get to but you can reach the vacuum contol line for it fairly easily

[Danglerb]

One of these ...
0-280-160-302

Vacuum Limiter, 928 (1980-82)
$195.00

[Hoang928]

thank you

[surfdog4]

if it is that, you can eliminate it all together and plug the throttle ports..the downside on the 5 speed is the rpms will drop instantly when you lilft off the throttle to shift. I decided to attach some hoses to the manifold and air box ports on the throttle body out to the top of the engine and plugged them there (on certain BMWs with L-jet, they mount the decel valve where you can get at it)...then i just bought a used decel/limiter from 928 international (1/2 price sale, but I haven't installed it yet). This way i can hook it up without removing all the intake system (again) and see if i like it better with or without.

My old decel valve actually held vacuum so i thought it was OK but with just a little vacuum it apparently leaks and hence the hunting idle.

Good luck!

[Danglerb]

I ran an old vacuum limiter through the parts washer the other day, no idea if I killed or cured it yet.

[surfdog4]

i wonder if i killed mine with a heavy hand on the mighty vac..it was holding vacuum but seems like it must have been partially opening with a weak vacuum..anyway we'll try this new used one i just got from 928 international

[Hoang928]

do you guys have picture of this vacuum limiter


thanks

[Danglerb]

Its a 5" diameter flying saucer sideways next to the throttle on the non linkage side.

[rhjames]

Originally Posted by Danglerb

Its a 5" diameter flying saucer sideways next to the throttle on the non linkage side.

looks like a brake vacuum booster, only it's about 4 inches in diameter, located on the passenger side of the throttle body. it will have two large hoses attached between it and the throttle body,

it is used to limit "throttle down" while shifting.

Porsche discontinued using these on the later cars.







--Russ

[devilinblack]

Not sure if 81 still uses the K-Jet, but could this also be a rich condition? I know there's a 911 thread about a lot of people needing CIS adjustments when the weather turns cool because of the cooler and denser air, but I guess that would lead to a lean condition with more oxygen?

[rhjames]

Originally Posted by devilinblack

Not sure if 81 still uses the K-Jet, but could this also be a rich condition? I know there's a 911 thread about a lot of people needing CIS adjustments when the weather turns cool because of the cooler and denser air, but I guess that would lead to a lean condition with more oxygen?

81 Euro is CIS (K-jet), US spec is L-jet.....








--Russ

[rich]

Disc on the left

[Hoang928]

is it also called distributor vacuum retard line