Cold starts and your engine coolant temp sensor

[zerosoma]

So, I got to thinking...

On fuel injected engines...the people that let their cars warm up in the morning, are they really wasting more gas than the people who hit the road right away? Considering the coolant temp sensor if not showing warm will inject more fuel into your engine causing it to run rich...

as far as mpg is concerned, what actually wastes more gas - warming your vehicle up for 10 minutes or hitting the highway with your accellerator depressed immediately? thoughts?

 

[Donald]

Originally Posted By: zerosoma
So, I got to thinking...

On fuel injected engines...the people that let their cars warm up in the morning, are they really wasting more gas than the people who hit the road right away? Considering the coolant temp sensor if not showing warm will inject more fuel into your engine causing it to run rich...

as far as mpg is concerned, what actually wastes more gas - warming your vehicle up for 10 minutes or hitting the highway with your accellerator depressed immediately? thoughts?


I would guess that the amount of extra fuel injected when the engine is cold is small, and assume it tapers off slowly.

 

[JHZR2]

Im no expert on vehicle controls, but my impression is that the era of rapidly heated O2 sensors helps this somewhat. My newest cars close loop within seconds, literally, even in the coldest temperatures, I assume because the O2 heater gets it working and so it can help prevent running excessively rich.

At no load heating doesnt occur very fast, but fuel flow is also super low.

Its a huge waste to idle a vehicle to warm it up. If getting into a cold car is such an issue, the user should consider a block heater on a timer.

Slamming the go pedal on a stone cold car to me is more an issue of thermal shock, thermal expansion rates, and the thermal gradients which arent good for the engine. One doesnt want to idle excessively, but also wants to minimize the thermal gradients when starting to move.

Once the engine is combiusting in a stable way, I say go.

 

[Lead Shoes]

Originally Posted By: zerosoma
So, I got to thinking...

On fuel injected engines...the people that let their cars warm up in the morning, are they really wasting more gas than the people who hit the road right away? Considering the coolant temp sensor if not showing warm will inject more fuel into your engine causing it to run rich...

as far as mpg is concerned, what actually wastes more gas - warming your vehicle up for 10 minutes or hitting the highway with your accellerator depressed immediately? thoughts?


I give mine 2 mins or so to warm up, but my un wrapped header and WAI help a lot in winter to warm up the car; I bet the mpg difference between the two methods will be pretty small but it is obviously subject to the vehicle and how long you let it idle on cold starts (too many variables).

 

[surfstar]

idling gets you 0 miles per gallon

 

[SteveSRT8]

^^^Exactly right.

And there is no shock to the engine to drive off sedately, keep the revs down and use small throttle openings for a couple of miles.

The big shock is when the first combustion event takes place against a frozen piston!

 

[SLCraig]

The simple answer as far as I'm concerned is, with the heat on, and my car idling, the engine would literally never hit operating temp. It has been so cold here the last 2 days when I drive for the first 5 ish minutes in the morning. Every stop I make the temp gauge drops and does not go up unless the engine is under load.

Idling to warm up for gas usage would be bad, because it would never hit operating temp. As I drive, the temp gradually increases. Takes a solid 10+ minutes though.

 

[asand1]

Originally Posted By: zerosoma
So, I got to thinking...

On fuel injected engines...the people that let their cars warm up in the morning, are they really wasting more gas than the people who hit the road right away? Considering the coolant temp sensor if not showing warm will inject more fuel into your engine causing it to run rich...

as far as mpg is concerned, what actually wastes more gas - warming your vehicle up for 10 minutes or hitting the highway with your accellerator depressed immediately? thoughts?

When idling you get 0mpg. When driving you get something better than 0mpg. When driving you warm up faster and get even better MPG sooner. What's the confusion? Just drive already.

 

[Clevy]

Idling will consume more fuel for sure.
The catalyst and 02 sensors get up to operating temp much faster when driving whereas a cold idling engine will inject more fuel to get these systems hot.
In all honesty idling a cold vehicle is all about driver comfort not about what's better for the engine.
In my opinion driving the vehicle is a far better way to warm it up then idling. Keep the revs low until the engine temp gauge is up to operating temp then give er. I can achieve freeway speeds very quickly and never rev over 2000 rpm,so for me getting in and driving only makes sense since I don't need to rev it high to achieve speed quickly.

 

[kr_bitog]

Originally Posted By: zerosoma
So, I got to thinking...

On fuel injected engines...the people that let their cars warm up in the morning, are they really wasting more gas than the people who hit the road right away? Considering the coolant temp sensor if not showing warm will inject more fuel into your engine causing it to run rich...

as far as mpg is concerned, what actually wastes more gas - warming your vehicle up for 10 minutes or hitting the highway with your accellerator depressed immediately? thoughts?

Yes, warming up while stopping is a waste of gas, 20 seconds is enough before start hitting the road.

 

[Smokescreen]

I use the fast idle after cold starts to my advantage. I don't touch the throttle and letting the higher idle move my car out of the garage and down the street. Once high idle kicks down I then equalize with throttle. Its the best of both worlds, my car warms up as designed and I don't waste fuel. I am just glad I live in an area where I can fast idle for a couple of miles if I wanted to without being hounded.

 

[JHZR2]

Originally Posted By: SteveSRT8

And there is no shock to the engine to drive off sedately, keep the revs down and use small throttle openings for a couple of miles.

The big shock is when the first combustion event takes place against a frozen piston!


There sure is shock. The issue is thermal expansion and heat capacity (how many kJ's it takes to heat up the metal). Doing it faster rather than slower isnt necessarily a good thing in the long run.

There is a reason why we dont recuperate our engines, which would give much higher MPGs... and that is it.

Yeah there is a big one for the first even on the cold soaked piston, definitely. But again the het capacity causes a heat quench which is why combustion isnt necessarily fully steady for the first few cycles. Its all about the rate that stuff heats up, as well as the delta across the engine.

If these werent concerns, we wouldnt warp heads.

 

[Donald]

I do wait for the oil pressure to show on the gauge. But we are talking seconds not minutes.

 

[bigblumer]

For the Kia: I let it warm up for about 2 minutes or so, just to make sure the fluids are circulating. I notice it most in the power steering. If I jump in and go, the pump whines and its tough to steer right away. If I let it sit for two minutes, I don't have a problem. Maybe its time for a PS fluid change this spring...

For the Outlook, I start it up for the Mrs. in the morning, mostly to let the seat heaters warm up so the leather isn't so cold. The other morning it was -14, and the seats were cold enough to give you an Ice Tan (what southerners would call Frost Bite) if you didn't let the seat warmers do their magic.

 

[brandini]

I have a scangauge and I usually try and let the coolant get to about comfortable room temperature before driving (~75 degrees). Then I keep it under 2.5k until fast idle stops, and under 3 until the coolant gauge gets to center, at that point i'll turn the heat on.

 

[HTSS_TR]

We have no cold start only cool start, if starting the engine at 50-60F is a cool start, in So Cal so I don't worry about it. I drive away within 2-3 seconds after engine started.

 

[eljefino]

I like the way you think. I think to expand on it it goes like this:

-- A zero-degree cold start needs 2x as much fuel as a warm start.

-- Going 50 MPH on a stone cold engine needs 2x as much fuel. But the injector pulse width is much longer. It's not going to warm up faster than idling with air blowing around in the engine compartment, and all that cold ram-air hitting the intake.

-- So give the engine a running start, as the whole injector pulse that's "wasted" at idle will be recouped, and then some, when you hit the highway.

I don't agree, I don't disagree. But my car works for me. I don't need to justify warming it up in this weather.

 

[dailydriver]

Originally Posted By: SLCraig
The simple answer as far as I'm concerned is, with the heat on, and my car idling, the engine would literally never hit operating temp. It has been so cold here the last 2 days when I drive for the first 5 ish minutes in the morning. Every stop I make the temp gauge drops and does not go up unless the engine is under load.

Idling to warm up for gas usage would be bad, because it would never hit operating temp. As I drive, the temp gradually increases. Takes a solid 10+ minutes though.


It is not even getting above 0*F during the daytime there lately, is it?!?

I would probably have the front of the radiator COMPLETELY blocked off if I were there now.

 

[itguy08]

Turn and burn has been my theory. We dont get the extreme cold of our Norern neighbors. Worked for the past 600k or so of my driving life. As long as you have oil pressure, drive the darn car. If you want, take it easy for a while that's fine but with modern cars you don't need to.

I say it warms up quicker under load. You have a closed thermostat so all the heat is trapped in the block to speed warmup. Modern cars get into closed loop quick so wasted fuel is minimized.

Use whatever works for you - probably makes no difference either way in the end.

 

[Tuberocity]

Seems crazy manufacturers don't design vehicle systems to match the weather conditions in which they are sold. Here in Ca, my auto heats up quick, cold or hot weather, (of course it never gets much below 30 here & that's rare) and is ready to be floored in less than a mile of driving. Eastern autos should be simularly equiped or designed to deal with freezing temps. Maybe a switch under the hood for winter driving, and flip it back for summer? lol Sounds like a good idea, but then this would cost the manufactures, and of course the consumer.