Warm up your engine before driving in winter?
There is a huge debate on whether you should warm up your engine before driving in winter weather. A lot of the information arguing in favor of taking the time to warm up your engine in winter is simply outdated or just plain WRONG. It refers to a time when engines had far less thermal efficiency and needed warm up time for the carburetors to function well. Newer cars are specifically designed to warm up faster. The faster they warm up, the less pollution they produce.
Also, newer cars use much thinner oil that flows better when cold. Plus, fuel injection does a much better job of vaporizing the gasoline than a carburetor, so fuel injected engines simply doesn’t need the long warm up time. Here are some cold hard facts to dispel the cold engine starting myths.
Idling to warm up your engine is BAD for it
• At cold engine startup, combustion quality is at it’s worst. You’ve got a rich mixture and low turbulence so you get poor fuel vaporization and extremely low burn rates. Cold engine idling with a rich mixture produces blow-by with high water vapor content. The result is raw gas and water in the oil. Idling actually churns the raw gas and water into the cold oil, producing SLUDGE.
• Idling heats up your COOLANT, but it doesn’t heat up your OIL nearly as fast as you think. Oil needs to reach at least 150° before it can start burning off the raw gas and water. Idling can bring your COOLANT above 150°, but it can’t bring your oil to 150°. Putting your car in gear and driving is what heats up your oil and boils off the raw gas and water. You’re kidding yourself if you think you’re helping your oil warm up by idling the engine.
• The raw gas and water in the oil creates a worst-case scenario for camshaft wear. The gas in the oil actually removes the boundary lubrication on the camshaft lobes and bearings.
Cold oil develops proper pressure within seconds
It’s a MYTH that you need to warm an engine in order to get good oil pressure.
Fact 1: The definition of pressure is “resistance to flow.” When oil is cold, there’s PLENTY of resistance to flow. The oil is thicker and it doesn’t want to flow as easily as when it’s hot. Yet, your oil pressure light turns off within seconds after starting your engine. Ever wonder why? I mean, if all the myths were true; that you need to warm up your car before driving, it would take several minutes for the oil light to go out. Yet it doesn’t.
That’s because oil pressure rises to at least 7-15-psi. within seconds after starting. That’s the minimum amount of pressure an engine needs to keep metal parts separated. If you’re using the correct oil viscosity, your oil pressure will be around 40-60-psi. within 5-seconds. That’s enough oil pressure to lubricate bearings and hydraulic lifters.
Cold oil DOES flow in winter
It’s a myth that cold oil doesn’t flow
Fact 2: It IS cold and it IS thick. But that doesn’t mean it’s frozen
solid or won’t flow! I’ve seen many people claim that 10W-40 oil is solid at -20F. If that were true, we would be seeing exploding oil pumps every time winter arrived. But you don’t. Wonder why? Because oil isn’t solid at -20F.
The truth is, if you’re using the proper viscosity oil, it’ll start flowing within seconds after starting. Amsoil has a youtube video that purports to show the flow difference between conventional oil and synthetic oil at -40C. They chill both oils and pour them out of a beaker. The results are stunning! The conventional oil has the consistency of grease and that leads you to believe cold oil can’t be pumped. But that’s not really what the video proves. It actually shows that cold oil doesn’t flow well by gravity. Well, your engine isn’t lubricated by GRAVITY.
Instead, your engine oil is pumped by a positive displacement gear pump.
A Positive Displacement Pump, unlike a Centrifugal or Roto-dynamic Pump, will produce the same flow at a given speed (RPM) no matter the temperature or viscosity of the oil.
A Positive Displacement Pump is a “constant flow machine.” It pumps oil even in the coldest temperatures, no matter the thickness of the oil. As long as the gears are turning and the exit path isn’t blocked, the pump will move oil. If the oil pump couldn’t move cold oil, it would self destruct whether you try to let your engine warm up or just drive it off. Yet you don’t see exploding oil pumps in winter. Look at the oil pumps below and you’ll see that if the oil was frozen solid, the gears would just break off. They don’t. So let’s just stop repeating that stupid statement that cold oil doesn’t flow.
Driving a cold engine causes damage and makes it wear out faster
It’s a MYTH that You should warm up your engine in winter to prevent it from wearing out faster.
Fact 3: 90% of entire wear takes place on cold startup. But we’re talking about wear that occurs during the first 2-5 seconds after firing up, before oil pressure builds and oil flow starts, not 15 seconds later. Oil pressure is what keeps metal parts separated. As you can see above, you have oil pressure as soon as the oil light turns off–which happens within seconds.
The counter argument to the “better warm up your engine” argument is this: If you believe that it’s best to let the oil warm up so it can flow better, then the last thing you want to do is let it idle? Why? Because it takes an FAR longer to warm up your engine oil at idle speed than it does when you drive it.
Coolant temperature is NOT an indication of OIL temperature!!
Read that again. Coolant temperature is NOT an indication of OIL temperature!!
Your engine coolant may be up to operating temperature, but the 5-6 quarts of oil in you pan is still cold. Idling doesn’t warm it. In fact, on a cold day, you’ll lose MORE oil heat by letting your engine idle than you will by DRIVING it.
READ THAT AGAIN
It’s not good to let an engine warm up in winter before driving
It may be better for your buns, but not for your engine!
It’s a MYTH that warming up your engine is good for it
Fact 4: Uh, no. It’s not good for the engine. When an engine is cold, the computer commands a rich fuel mixture. It has to because most of the “fire” in the cylinder gets quenched by the cold engine and cylinder head. That extra fuel washes oil off the cylinder walls, so your goal is to get it up to operating temperature as quickly as possible so the computer can reduce fuel.
Plus, all engines, even new ones have some amount of “blow-by.” That means a certain portion of the air/fuel mixture flows past the piston rings and into the crankcase oil. Blowby gasses contain unburned fuel, CO, CO2, water and oil. That extra unburned fuel and blow-by harms your engine two ways:
• The water and fuel mix with the oil to reducing its ability to lubricate and maintain pressure and flow
• Water and fuel in the oil causes oil degradation and sludge.
The faster you warm up the engine, the faster the computer will start cutting back on fuel. Faster fuel cut-back means less cylinder washdown and less fuel and water in the crankcase.
Idling a cold engine evaporates fuel and water from your oil
Myth 5: Warming up your engine in winter evaporates the fuel and moisture that gets into your crankcase oil.
Fact 5: Yeah, that’s true to some degree. But late model vehicles don’t heat up enough at idle to burn off that extra gas and water. Again, the quicker you drive, the faster the engine heats up. The worst thing for your engine is to let it warm up for 10 minutes, then drive 10 minutes to work and shut it off. That leaves way too much fuel and water in your crankcase.
What does your owner’s manual say?
The nay-sayers point to issues like: “metal contracts when cold and needs time to warm up” and “engines are like old people whose muscles need to limber up when getting out of bed.”
While it’s true that metal parts contract when cold, but to think that engines should sit and idle to warm them up would be to believe that the engineers didn’t design engines with that in mind—or maybe you think they missed that day in engineering class. Trust me, they didn’t; they understand expansion and contraction. If they were worried out metal expanding before operating your vehicle, they would TELL you to let it warm up. In fact, the lawyers would INSIST that they tell you to let it warm up. No, they’d devote the first 20 pages of the owner’s manual with big WARNINGS telling you not to drive a cold engine. But they don’t. Are the engineers stupid? Did the corporate lawyers go out for martinis when it was time to review the owner’s manual?
Ever boarded a plane on its first trip in the morning? Do they start the engine and let it idle for 10 minutes before taking off? Nope. They start the engines and immediately run it up to idle speed. Then they push back from the gate and start taxiing.
Having said that, there are exceptions: Some engine designs DO require a longer warm up period and the instructions in your owner’s manual always take precedence over my general advice. If your owner’s manual recommends a several minute warm up, FOLLOW THAT ADVICE.
How long should you let your car warm up before driving?
That depends on the type of engine you have. Late model engines with direct fuel injection should warm up for about 45 seconds. That’s enough time to get good oil flow to fully lubricate the high pressure fuel pump and camshaft. Those high pressure fuel pumps boost gasoline pressure to almost 2,000-psi. Driving them right away can cause excessive camshaft and fuel pump wear. On other engines, 15-30-seconds is plenty.
How to drive after you’ve started your engine in winter
Just because car makers recommend driving after starting doesn’t mean you can put the pedal to the metal. Start the engine and let it run while you buckle up, turn on the radio and defrosters. Then put it in gear and drive GENTLY for the first 3-4 minutes. At that point, you’ve got good oil flow and pressure.
• Avoid jack rabbit starts and heavy acceleration for the first 3-4 minutes.
• Avoid heavy acceleration.
Hammering the pedal on a cold engine, especially an engine with a turbo, can DESTROY the turbo and the engine bearings.
©, 2018 Rick Muscoplat
Posted on by Rick Muscoplat