Does cold make the car rev higher at startup

[UKbob]

I started the S last night, which was probably the coldest start Ive done since July when I purchased, and the car maintained a constant 2k revs for 15(?) seconds or so.

As I reversed 15 feet to turn around, the revs slowly fell back down to 850 approx idle tick over.

Should the car do this in the cold? Cant think why or what else might make it continue to rev so high after starting the engine

[Tripps]

Its been a while since I've done a cold start but I seem to recall the same thing happening.

I'd guess some sensor is telling the ECU is needs to compensate and idles quicker... (All suposition as I'm inept under the bonnet!)

After fives or so minutes all used tor return to normal.

[WildfireS3]

Ah Rob you're back. Mine idles at around 1500rpm for a while, 5 min or so in the morning when I start up. Settle after the temperature gauge starts moving.

[shmern]

UKbob said:

I started the S last night, which was probably the coldest start Ive done since July when I purchased, and the car maintained a constant 2k revs for 15(?) seconds or so. As I reversed 15 feet to turn around, the revs slowly fell back down to 850 approx idle tick over. Should the car do this in the cold? Cant think why or what else might make it continue to rev so high after starting the engine

Isn't this a sign of a sticky stepper motor ?

Dunno, mine seems to do the same every now and again and a quick scan of the forums seemed to indicate stepper motors on V8's ?

Shmern

[Colin Blues2]

I think it's supposed to do this from cold to accelerate the warm up process and then drop back to the normal 'hot' tickover.

[UKbob]

WildfireS3 said:

Ah Rob you're back. Mine idles at around 1500rpm for a while, 5 min or so in the morning when I start up. Settle after the temperature gauge starts moving.

Aye Had some catching up to do, after spending too many working days PHing it

Im not even sure what a stepper motor is, so will happily opt for the warming up supposition.

[monkeypilot]

Remember the old bi-metal strip automatic choke back in the old days of carbs? Well your stepper motor performs a similar role in a "far more expensive if it goes wrong" type of way. Gives you a richer mixture on a cold morning.

[oldred]

UKbob said:

Im not even sure what a stepper motor is, so will happily opt for the warming up supposition.

Rob

Mine did this, it was the stepper motor. If you can imagine you are standing at the right hand side of the engine bay, look at the far left side of the plenum chamber (alright the big shiny ally thing on top of the engine) where it meets the bulkhead you will see what looks like a round metal device, probably about 1 inch in diameter and about 3-4 inches long (too bone idle to go out to the garage to measure it) with a plug on the end, that is the stepper motor. Disconnect the plug, unscrew the stepper motor and you will see it looks like a plunger with an arrow shaped end. What the thick bit does is to rotate (internally), the shaft with the arrow head has a screw thread on it so goes in and out, and this controls the airflow in the plenum. This comes into play when the air is cold (sort of like a choke) and when you come to a stop it sits above tickover for a few seconds and then returns to idle (i'm told this is to blow out any unburnt fuel in the exhaust) Take some carb cleaner and squirt it liberally into the thick part to clean out any muck. This should resolve the problem. If not they are about £70


When you refit it, either fit a new gasket or wrap some ptfe tape around it to ensure a airtight fit, oh and don't try to overtighten it.

Cheers,

Paul