An Idle Car is the Devil’s Workshop

In the area that I live I am not what you would call native. I live in a small town in one state but I am originally from a large city in a neighboring state. Being something of an outsider to this place I have noticed many things that are common here, that are not as common where I am from. One of the things that I have noticed to be so common is endlessly idling vehicles.

Here in our little town it is very common to be in any parking lot and find an empty vehicle running in one of the stalls. I’m sure that this is a common thing in many places; it just seems more common here than in other places where I have lived. Perhaps it is a cultural tradition and in talking to some of the natives about this situation many of them say it is because of the cold harsh winters. The problem with that theory is that you see it almost as often in the summer as you do in the winter. I think that perhaps the biggest reason this doesn’t seem so noticeable in the big city is that if you let your vehicle run unattended, some ne’er do well might decide that they need your car more than you do. This is probably less of an issue in a small town.

With gasoline being as expensive as it is, and with vehicle maintenance being such a nuisance at times, why would anybody want to burn so much fuel and put so many hours on their engines and get essentially nothing in return? Is idling a vehicle really necessary in order to warm it up on a cold morning? What kind of wear and tear occurs when a vehicle is not warmed up? What kind of wear and tear occurs when a vehicle is idled excessively? All of these are valid questions.

Let’s start with the issue of warming up that engine on a cold winter morning. Many people will let the engine idle for 10 or 15 minutes, to then drive the vehicle 10 or 15 minutes to work. The logic that many of them use is that it is bad for the vehicle if you don’t give it a chance to warm up. Some say that you have to let it run so that the oil can circulate through the engine. They say that idling the engine and letting it warm up gets more oil to critical engine components that would be starved for oil if you were to just get in and drive. This is not true.

The fact of the matter is that even when ambient temperatures are very low, it only takes a few seconds for the engine to build oil pressure and get oil flowing to all of the critical parts of the engine. The oil pressure actually builds easily when the engine is so cold so pressure wouldn’t be the issue anyway. The oil does flow more slowly when the engine is cold and it does take a bit longer to get to every place where it needs to be but we are still only talking about a few seconds longer than it would normally take at higher temperatures.

A crankshaft bearing with scuff marks made by the crankshaft
rubbing on the bearing.

The real issue here related to excessive idling and engine oil is the fact that when the engine is at idle the oil pressure is at its lowest. Engine oil pressure is responsible for supporting the crankshaft and the camshaft when the engine is running. Normal oil pressure on a warmed up engine running at 2000 to 3000 rpm is about 60 psi. At idle this pressure falls to about 30 psi. When engine oil pressure is low, such as it is at idle, then there is a greater chance that the crankshaft will actually touch the bearings that are wrapped around each crank or cam shaft journal. As an engine wears under normal conditions these engine bearings will gradually become thinner, but they should ultimately last forever. When oil pressure is low it increases the rate at which these bearings wear out. When an engine has very high mileage and the bearings are already somewhat worn, excessive idling can be even more damaging because engine oil pressure can fall to something as low as 5 to 10 psi. This is because the tight tolerances between the thicker or newer bearings and the shaft journals cause the oil pressure to run higher.

Some people say that a car uses more fuel if you shut it off and restart it, than it would if you just let it idle for a few minutes. This is completely untrue. In the old days some cars might use a bit more fuel on startup then they would if they idled for a few minutes but even then it was only under certain situations. Some people also say that you can wear the starter out if you stop and start the vehicle too much. Of course something is going to wear out if you use it. By this logic then we should just never drive the vehicle ever because it will wear out the tires and the engine. Assessing how quickly a starter wears out when a vehicle is stopped and started rather than idled is pretty much impossible.

The biggest issue in relation to fuel consumption and an idling engine is that a car that is running at idle is getting zero miles per gallon no matter how efficiently it is burning the fuel. If you complain about bad fuel economy from your vehicle, or you complain about how much it costs to fill ‘er up, but you idle your car regularly, you are only making matters worse. They say that the way you drive your car affects your mpg’s and this is completely true. Many of us would never drive our vehicles in a way that would compromise fuel economy, or cause harm to the engine (no matter how miniscule), yet many people will idle their cars at least 30 minutes every day.

Also in relation to overall efficiency is the fact that vehicle emissions are often higher when a vehicle is at idle compared to when a vehicle is moving down the road. Fuel control is optimized for rpm ranges that are off idle. The catalytic convert also has a tendency to cool down a lot, so much so that the chemical reactions that reduce the level of pollutants in the exhaust stream cease to occur and the exhaust becomes more harmful.

This decrease in efficiency also means that the carbon that is contained in the fuel does not get fully oxidized so it tends to just buildup in the combustion chamber, and in the piston rings, and on the valves, and in other places in the engine. When this happens, the carbon acts as an abrasive to slowly grind engine parts down and disrupt the smooth and efficient flow of air and fuel through the engine. Hydrocarbons will also build up in the crankcase oil causing it to lose its ability to lubricate internal engine parts. The buildup of carbon in the oil will require the engine oil to be replaced more often.

I think that most people that are spending a considerable amount of time warming up their cars, or the people that leave it running when they run into the Loaf & Jug, are either not thinking about it one way or the other, or they are thinking that they don’t want the inside of the car to get cold. How cold is it going to get if you shut the engine off while you are filling up with gas, or running into the store to get a cup of coffee?

Truthfully, letting a car idle really doesn’t warm it up very quickly, nor does it warm it up very thoroughly. The fastest and most effective way to warm up a car is to drive it. A car that requires 15 minutes to warm up the engine just idling can be warmed up just as much in about five minutes if it is just driven normally. This will not warm up the inside of the car but if your commute is longer than about 15 minutes then you will be plenty warm inside the car by the time you get to work. What about the rest of the car’s systems? When a vehicle idles for an extended period of time to warm up the engine, this does nothing to warm up the transmission or anything else on the car. Don’t these things need to be warmed up as well? No they don’t, or at least not purposely. Just get in the car and drive it like normal and all warming will be taken care of quickly and effectively.

Back when every car used a carburetor for fuel metering, it may have been advantageous to let the car warm up a bit on very cold days because the choke needed a chance to open a bit. The choke was a device that helped the engine to run with a bit of a rich mixture when it was cold. Sometimes automatic choke function was not that reliable and the car didn’t really run very well until the engine warmed up and the choke opened all the way. Some old carbureted vehicles had a problem with ice buildup on the venturi. Considering that the venturi works sort of like an airplane wing, we know an airplane wing is useless with a bit of ice on it, and the same applies to a venturi. Most vehicles had a system meant to prevent this kind of ice buildup, but they didn’t always work. After the engine warms up there is no problems with ice on the venturi. Modern vehicles (most cars built within the last 25 years) are fuel injected so they don’t have any of these problems that in the old days were avoided by letting the car warm up before driving it.

What about a diesel engine? Isn’t it true that the rules are different for diesels? I would ask, how could the rules be any different? A diesel vehicle at idle is still getting zero miles per gallon. A diesel engine still has a crankshaft that is supported by engine oil pressure that is best kept up. A diesel engine still burns a hydrocarbon fuel that when not fully oxidized in the combustion process leaves carbon deposits in the engine. Diesels are more efficient at idle than their gasoline counterparts, meaning that they don’t burn as much fuel compared to how much power they produce, but this rule applies to all operating ranges and not just idle. Nothing magical happens when a diesel engine is idling that makes it okay to do so. So why do the big diesel trucks idle at the truck stops so much? Probably to keep the driver warm while he is in the sleeper of the cab, or so that he can run his TV and microwave. Other than that the trucker does this for another reason. Quoting Tevya from Fiddler on the Roof that last reason is “Tradition!”

Since we live in a free country we can let our vehicles idle all day long if we want to. Everything that I have said here can be disputed in some way, or deemed unimportant by some, and that’s okay. Everything that I have said is true no matter how you spin it. If you don’t care about saving fuel or extending engine life, then you have just wasted the last few minutes reading this article. If you value your engine and you value your fuel almost as much as gold, then consider some of the things that I have said here and see if you can change some of your habits. Your car and your wallet will be glad if you do.