Gas Mileage - Idling Cold vs Driving Cold

With engine management computers and fuel injection, vehicles no longer need to be "warmed up" prior to driving away. All you need to do is start the car, put it in gear and drive away...albeit a little slower and gentler. The car warms up best and fastest when being driven, not idling in your garage or driveway. This way, you aren't burning gas going nowhere...causing your lower mileage.

I know jdmccright is voicing standard wisdom on this issue and he's (and others are) probably correct. However, I wonder if driving a modern car when it's cold would waste more gas (in open-loop mode and stiff from cold) than idling it for a minute since idling takes very little gas to turn the engine. Anyone have any opinions or knowledge to share?

I think it's going to vary significantly Brian. It's going to depend on how long you idle vs. how fast you drive if you don't preheat, the temp outside if you're running cold. I don't see us getting a sure answer to this.

On Jdms note, I don't completely agree. Until we figure out how to make rings seal to cylinders the same way while both hot and cold, I think I'll wait the minute or two to warm up. Increasing cylinder pressure before the engine has come to a temperature where it can seal the combustion chamber properly doesn't sound like too great an idea to me. You'd be dumping a lot of your compression due to lack of sealing anyway.

cylinders nowadays are form/hot honed, meaning that they are honed while they are at operating temperature. So the cylinders aren't even circles until they are hot.

However, if you are in gear and barely giving it throttle, the engine isn't anywhere close to max peak pressure, so you might be okay as long as "a little slower and gentler" means "A LOT slower and gentler". But its going to be a while before i can talk myself into getting in and going.

To do the hot honing, the blocks must be heated in an oven. In doing so, the blocks must be very evenly heated. I wonder if they are ever really round afterward since the blocks are not evenly heated in operation, since the areas around the cooling jackets are going to be alot cooler than the areas of the block that are further away....

I guess that hot honing is the better of two evils.

I never claimed to be an expert...I'm more of a jack-of-all-trades. IMO, you'll get up to normal op temps faster when you are driving versus idling due to the higher rpm. Plus, the engine heat is dispersed throughout the rest of the system (transmission fluid via radiator heat exchange), so the sooner everything gets hot the better.

It is not a technical point...an idling vehicle without movement means zero mpg. That's why hybrid-drive vehicles shut the engine off when at a stop and rely on an electric motor to accelerate from a standstill (albeit at light throttle input). The technology eliminates that disadvantage by relying on stored energy.

To study it on a scinetific basis, one would have to monitor the vehicle's fuel flow rate and measure the difference in fuel consumption for both driving methods. Alternatively, calculations could be made but they would be more subjective since they would be dependent on the A:F ratio used, which changes as the engine warms up, and I don't wish to geek out at this point in the topic.

I never claimed to be an expert...I'm more of a jack-of-all-trades. IMO, you'll get up to normal op temps faster when you are driving versus idling due to the higher rpm. Plus, the engine heat is dispersed throughout the rest of the system (transmission fluid via radiator heat exchange), so the sooner everything gets hot the better.

It is not a technical point...an idling vehicle without movement means zero mpg. That's why hybrid-drive vehicles shut the engine off when at a stop and rely on an electric motor to accelerate from a standstill (albeit at light throttle input). The technology eliminates that disadvantage by relying on stored energy.


I think you've convinced me that you will get better mpg if you just get in and go. One thing that we do seem to agree on for sure is that we don't have the expertise to determine one way or the other, so this is still just guessing.

But for the life of the engine, I still think you're better off giving it a few minutes. Higher cylinder pressure means more force between the rings and the cylinder. if they haven't gotten to operating temp, they are likely contacting on a few points instead of on the whole cylinder, which can lead to premature wear of one or both surfaces.

I'll agree...there are too many variables involved that could influence the engine's output during that warm-up phase: initial temperature, change in compression efficiency due to "cold-warpage", fuel type, quantity, driving habits, rate of block heating from driving versus idling, amount of cylinder wall break-in/scouring. The calculations could be done but only on an ideal basis since many of the variables would have to be assumed.

One final thought I'll throw out, though I don't dispute the notion of non-circular cylinders when cold, the piston rings are there to help compensate for such irregularities. They spin around the piston (well...as much as they can when they're flying up & down) so as to minimize preferential orientation and thus minimize uneven wear of the cylinder walls. The more the engine is broken in, the more the rings and walls have "honed" themselves to the proper circular profile, thus creating an ideal, sealed chamber.

An interesting test one could do would be to run compression tests on cold versus hot engines and new versus broken-in engines. I'm not sure that the warpage would be great enough to show a measurable difference since you have oil present to plug those gaps. Would be an interesting test though...something fun on a weekend.

I'm all for minimizing engine wear, but most engine wear occurs during the start-up where there is little to no oil present on non-contacting surfaces...it has all drained into the pan. That is the primary reason for honing the cylinder walls, so that those tiny criss-cross grooves can hold oil. I agree that in a cold state, there can be warpage, but I'm not sure that you are getting such a significant reduction in chamber pressure due to blow-by since oil will be present to help fill those gaps once it starts flying around due to the crankshaft's rotation. Ah well, the world may never know...interesting topic for sure.

I was trying to get to the answer of which set of circumstances would use more gas. Idling at low gas consumption for a minute then driving when the engine was a little warm vs idling for 5 seconds (for example) and then immediately driving when the engine is colder and less efficient. I guess my point is that there may be little or no difference between the two.

I was trying to get to the answer of which set of circumstances would use more gas. Idling at low gas consumption for a minute then driving when the engine was a little warm vs idling for 5 seconds (for example) and then immediately driving when the engine is colder and less efficient.

I think JDM and I, after our long back and forth posts, have come to the dreaded conclusion of we don't know, and we probably will never find out?

I'll agree there...we just don't know for sure. Until then we all must make educated guesses based on what facts we have and our experiences. In my line of work, data talks and b-llsh-t walks. Thanks for the interesting discussion!