Startup RPM

[Barracuda]

Hey Guys,

Just an observation, but I was wondering if anyone has experienced the same thing or knows what this could be...

So when I start the car first thing in the morning, the RPM sits at about 1200 during it's startup phase to warm up the engine (I think that's why it does it - correct me if I'm wrong). I always wait until it goes down to about 800RPM before I put it in gear, don't know why, I've just heard it's good to do that.

Lately, however, when I start the car first thing in the morning, it just goes straight to 800 which is what it's normal idle RPM is... The weather has been even colder lately, too.

Does anyone know why this would be the case?

Thanks in advance!

[nomadx38]

When the temperature is too cold (under 10 degrees from what i can tell) the engine doesn't do a warm up of the O2 sensors (which is where it revs at 1200rpm).

I think it's designed like this so not to do any damage to the engine as cold temperatures mean the oil is much thicker and harder to pump into the pin holes in the cylinder heads. Cold starts are always bad for an engine and if you are revving a stone cold engine at 1200rpm for 2 mins with no oil it's all bad news.

Also the cylinder block itself would have seriously constricted from the cold weather and if the block is not gradually warmed up with the pistons they could seize.

If you look at the revs they go from 800rpm down to 600rpm after about 2 mins, but you can tell by the fuel consumption that it is running slightly rich - like a choke

In a nutshell do not be concerned - its quite normal

[Barracuda]

Ohhh! That makes perfect sense! Because when I put the car in the garage overnight (which is obviously warmer) it does go to 1200RPM when it starts. But when I leave it outside (where it probably gets below 10 degrees) it doesn't.

Thanks heaps, Nomad! Very informative and interesting! Much appreciated!

[EDM12T]

I have a question in regards to cold starts, when i apply my brakes on a cold start its like it switches the cold start off and the revs then drop to 800rpm, once i let go of the brakes it goes back up to 1200rpm, no idea why... anyone else facing this?

[Barracuda]

I have a question in regards to cold starts, when i apply my brakes on a cold start its like it switches the cold start off and the revs then drop to 800rpm, once i let go of the brakes it goes back up to 1200rpm, no idea why... anyone else facing this?

That's really odd... I usually hold my foot on the break while it's at 1200RPM and it never makes a difference. :\

[VWplo]

I have a question in regards to cold starts, when i apply my brakes on a cold start its like it switches the cold start off and the revs then drop to 800rpm, once i let go of the brakes it goes back up to 1200rpm, no idea why... anyone else facing this?

yeh ive had this happen to me aswell

[nomadx38]

It has something to do with the brake circuit controlling the turbo.

The revs don't actually change (well only a slight bit) but the exhaust / engine note changes quite dramatically. I think what happens is when you hit the brake it automatically opens the waste gate on the turbo and it vents the boost which changes the sound of the engine/exhaust.

Again pretty normal

[VWplo]

It has something to do with the brake circuit controlling the turbo.

The revs don't actually change (well only a slight bit) but the exhaust / engine note changes quite dramatically. I think what happens is when you hit the brake it automatically opens the waste gate on the turbo and it vents the boost which changes the sound of the engine/exhaust.

Again pretty normal

but there wouldn't be any boost on start up and idle would there?

i know its normal though

[EDM12T]

It has something to do with the brake circuit controlling the turbo.

The revs don't actually change (well only a slight bit) but the exhaust / engine note changes quite dramatically. I think what happens is when you hit the brake it automatically opens the waste gate on the turbo and it vents the boost which changes the sound of the engine/exhaust.

Again pretty normal

Good to hear mate, relieved now. Ever since i had noticed it, i never layed a foot on the brake again until it had warmed up as i was thinking it was somehow stopping the cold start.. Thanks for that.

[amazeer]

There is no boost pressure at idle. Yes the turbo will be spinning just like a fan freewheeling in the wind but it is not making any boost. If you hold your foot flat to the floor with no load, you will still have no boost. Stomping on the accelerator in neutral you might momentarily hit 1 or 2 psi boost, thats it. The brake circuit has no effect on the turbo or waste gate. Put your foot hard on the brake with your foot hard on the accelerator. You are still making boost. Unless the new fangled funky polo has some kind of electronic influence.

The wastegate controls maximum boost pressure. When your inlet is seeing the desired maximum pressure the wastegate opens to bypass some of your exhaust gas around the turbine wheel. When you take your foot off the accelerator, there is no load on the engine, it is not making any boost, the wastegate is closed. When you snap the throttle closed there is a massive reverse shockwave that goes back towards the turbo compressor wheel and tries to stall it. Its that shockwave bouncing between the turbo and the throttle body that causes the flutter sound.

A small momentary change in idle when pushing the brake pedal could be caused by the brake booster stealing a little of the vacuum. If there is a leak somewhere somehow in the power brake diaphragm then it would be worse. If only 100 or so rpm then I wouldnt be concerned.

The difference in the size of a piston at a 20 degree darwin morning compared to a 0 degree canberra morning is less than 0.05mm. The cylinders have only shrunk 0.025mm. You have more clearance in cold start than hot. In any case the engineers know about thermal expansion and they wouldnt make the clearance that tight (would they?). These days they are making them looser than ever to reduce friction.