About Revs and Car Warming

[Danoff]

I drive a manual... which means I like to let it warm up before I start driving around. I treat 1st gear a little differently when the car hasn't warmed up because the revs are typically hanging out around 1.8k. So I won't give the car gas when I'm starting in 1st gear (or reverse), I just use the clutch. But that means that at some point while I'm driving, after the car has warmed up, I have to change my technique.

Instead of doing that, I simply wait for the revs to drop to something like 1.4k, indicating that the car is well under way warming up. However, because I'm an impatient person, I don't always do this, especially when leaving work.

When I leave work, I typically drive downhill. So instead of waiting for the car to warm up, I get it started with a little first gear, then put it in neutral and drive down the hill and out of the parking lot. As a result, I'm able to let the car idle and warm up, while simultaneously getting where I'm trying to go.

However, and this is where my question starts, I've noticed that while the car might idle at 1.8k when it's standing still, after I get past about 15mph the revs drop to more like 1k (practically a normal idle). If I hit the brakes and get down below 15mph, the revs jump back up and continue trying to warm up the car.

Why does my car do this?

 

[///M-Spec]

Your car is probably in a cold-start loop. The engine managment system is adjusting the car's idle to reduce cold start emissions. Many, if not most cars do this nowadays. The same thing happens in my BMW.


M

 

[Danoff]

Your car is probably in a cold-start loop. The engine managment system is adjusting the car's idle to reduce cold start emissions. Many, if not most cars do this nowadays. The same thing happens in my BMW.

How does idling the car at 20 mph emit more pollution than idling it at 0 mph?

 

[///M-Spec]

How does idling the car at 20 mph emit more pollution than idling it at 0 mph?

Honestly, I'm not an expert in this dept, but I'll tell you what I know. Basically, your car pollutes the most when it is cold. Cats don't work very well until they have some temperature in them so they do almost nothing when the car is first started. In addition, a cold engine runs a richer fuel-air mixture so it doesn't stumble and cut out, which adds more pollutants in the exhaust in the form of HC and CO.

Rising the idle helps the engine come to operating temp faster, so the cats can start working and the A/F ratio can lean out some and get closer to the ideal. If your owners manual (like mine does) tells you to drive away quickly on a cold start, this is part of the reason why.

I've always assumed that when the engine 'sees' the car is moving along, there's no need to mess with the idle --you are generating plenty of heat to warm up the engine just by drive along. If the revs drop to neutral while the car is in motion, it doesn't bother adjusting it because it knows you will soon select a gear and apply some amount of throttle.


M

 

[niky]

That's interesting... my car doesn't do that, but then, it's circa 2001 electronics.

Personally, I just start the car, check my mirrors and seatbelt, and slot it into gear and run it under power (staying at medium rpms). It gets the fluids warmed up more quickly than idling and coasting.

 

[Keef]

I haven't noticed my 96 Del Sol having any speed-related idle like that either. I have noticed that if I'm sitting still my idle dips slightly when I apply the brake, which is odd. The brake system isn't connected to the engine. Unless the brake lights require that much power from the alternator.

 

[Danoff]

Oh that reminds me of another idle blip I've noticed. When I start the car, the first thing I do is put the car in neutral and dump the clutch. But the first thing the car does is jump the revs to 1.8-1.9k. I've noticed that after I put the car in neutral, when I let the clutch out, the revs blip downward.

Why would releasing the clutch in neutral cause the revs to dip momentarily?

 

[inferno]

I haven't noticed my 96 Del Sol having any speed-related idle like that either. I have noticed that if I'm sitting still my idle dips slightly when I apply the brake, which is odd. The brake system isn't connected to the engine. Unless the brake lights require that much power from the alternator.

the power break/ABS pump is driven off the engine. no?

@danoff

how big of an engine is it? i dont know, you've got me there. it'd have to be a really small engine for the weight of the clutch/trans input shaft to drag the revs down. but it dosnt make sense for it to be that way on purpose...

 

[Jay]

Not entirely sure on the exact cold start idle speed when vehicle speed increases but I think M-spec is on track. There could be a whole variety of reasons Honda/Acura tuned the ECU to do that.

Oh that reminds me of another idle blip I've noticed. When I start the car, the first thing I do is put the car in neutral and dump the clutch. But the first thing the car does is jump the revs to 1.8-1.9k. I've noticed that after I put the car in neutral, when I let the clutch out, the revs blip downward.

Why would releasing the clutch in neutral cause the revs to dip momentarily?

But as for the rev dip when clutch is released:

When you have the clutch in you are disconnecting the engine from the transmission (pressure plate from clutch plate) when you put the transmission in neutral you are disconnecting the input shaft of the trans from the output shaft and other gear shafts.
So when you release the clutch pedal in neutral you are still locking up/connecting the clutch plate, input shaft, some gearing/bearings (depending on gearbox) to the engine, this mass that needs to now get Upto engine speed and is what dips your idle revolutions down.

I haven't noticed my 96 Del Sol having any speed-related idle like that either. I have noticed that if I'm sitting still my idle dips slightly when I apply the brake, which is odd. The brake system isn't connected to the engine. Unless the brake lights require that much power from the alternator.

Just noticed.
As inferno said the brake booster is getting its vacuum directly off the intake manifold so when you apply the brakes on idle that can have a effect on the idle revs.

@danoff

how big of an engine is it? i dont know, you've got me there. it'd have to be a really small engine for the weight of the clutch/trans input shaft to drag the revs down. but it dosnt make sense for it to be that way on purpose...

He said the revs dip momentarily (which isnt abnormal), not get draged down.

 

[retsmah]

My engine speed drops too when you put it in neutral and let out the clutch, like he said you are connecting the transmission to the engine which slightly increases load on it. Especially right as you let the clutch out since the gears have to be accelerated.

 

[SkylineObsession]

I almost always warm up my car for ten minutes or so or until the temperature gauge thingy is halfway (normal position). It seems to drive worserererer if i don't do this.

 

[niky]

Well, how much warm-up is needed is dependent on what car and what engine it is. Many older, carburated cars needed a lot of warming up before they'd settle down to a steady idle.

Many newer cars can get up to temp really quick, and quicker if you drive them a bit while warming up. Of course, this all varies from car to car.

 

[throttlehappy]

I almost always warm up my car for ten minutes or so or until the temperature gauge thingy is halfway (normal position). It seems to drive worserererer if i don't do this.

ten minutes isnt really necessary with the advances in modern oils
i warm my car up for atleast 1 or 2 mins

 

[SkylineObsession]

Yeah but my car noticeably runs worse if i drive it a couple of minutes after i've just started it, it is about 17 years old now.