944 lux starting issue?

[jwindwood]

Guys
On cold starts, should my 1985 944 Lux "choke" (As in raise the revs until warm)?
It doesn't seem to do this. I hadnt noticed before because it's been warm, but very cold this morning and although it started, the revs were at the normal level and it shook and rumbled for a coupleof miles until it started to warm up.
I know that the temp sensor at the front of the head can affect running, but its been very recently replaced. (I have the receipt.)
Any ideas, or is this normal?
Thanks guys.

 

[vitesse]

Yes,it should ramp up to a warm-up idle speed-around 1000rpm on our 924S's controlled by a thermotime switch in conjunction with the extra air valve under the inlet manifold & then drop back to normal idle speed as it warms up.

 

[jwindwood]

Thanks. So next question is what can I do about it?

 

[robwright]

The engine speed will not increase when cold. The DME temp sensor tells the DME that the engine is cold and it makes the mixture richer as if you were choking the engine. Same result. They can shake a bit when cold. This will improve with new engine mounts.

 

[vitesse]

!st thing is to check all the items involved for correct voltages /resistances at start conditions-I can't remember them-I look them up in the Haynes manual or Clarkes Garage on the web-you might have a loose Bosch connector,bad earth ,faulty sensor-particularly the cold start thermotime one .The sensor you see fairly easily with a Lucar connector under the inlet area RHS of engine is the temperature gauge sensor-the one you need to test has a Bosch plug connector.

 

[vitesse]

Well you may well be right,Rob,the fact is both our 924S's-which have the same engine & system as far as I know ,have an increased idle speed at cold start because of the DME increasing the mixture in conjunction with the extra air from the ISV or EAV-which is returned to normal once warm by the DME temperature sensor.This is to reduce stalling & assist pulling away when cold.However ,I bow to your superior knowledge.[]

 

[sulzeruk]

There is an air bleed thermo switch type thing on all the cars. It opens when cold to enhance the idle speed. It is the thing with 2 small tubes poking out the top that goes into the 90 degree right angle pipe that comes out the back of the head. It should close when hot and pass when open. That is where the pipe that connects to the bottom of the throttle body goes. If knckered, can also be a reason that they can smoke a bit when cold.
Still available form Porsche!
Alasdair

 

[vitesse]

We are now talking about vacuum tubes ,I think-does it matter which way round they are fitted,Alisdair?-I only ask because on the e-bay car I found one disconnected when I was going through all the initial checks & felt they had been on the wrong way round when compared with our other 924S-since then,having copied the other 924S,the cold start has been as I describe.Trouble is as a long experienced DIY'er,these things don't crop up that often ,& you forget & also find that simple things like connections aren't covered by manuals.