Rick's Free Auto Repair Advice

How long should you warm up your car?

How long should you warm up your car?

30-seconds to one minute (max) is all you need to warm up your car in winter

This question comes up all the time in winter and people get really jazzed about it. Most of the information advising you to warm up your car until the temperature gauge is in the middle is not only wrong, but damages your engine. Here’s why.

Fuel injection vaporizes gas much better than older carburetors, eliminating the need to warm up your car

Fuel injection atomizes and vaporizes fuel much better than carburetors. So it doesn’t need engine heat to accomplish vaporization.

Idling warms the coolant, not the oil

Let’s rephrase that. Idling warms the engine coolant but it does a poor job of warming the oil compared to actually driving the vehicle. If you’re concerned about getting good oil flow in extremely cold weather, driving with a light foot warms the oil much faster than idling.

Why idling doesn’t warm the oil

Oil’s job is to lubricate, remove heat and keep metal parts from touching. As soon as the oil light goes out, you’ve got enough oil pressure in the engine to prevent metal parts from touching. When an engine is under load, oil friction creates heat and oil flow moves that heat away from engine parts. But an idling engine puts very little load on the oil which means idling friction creates little heat. So idling doesn’t really warm your oil very well. In fact, it’s the least effective way to warm the oil.

Warming your oil warming by idling is even less effective in extremely cold temperatures. The oil picks up little heat in the engine before it drains back down into a cold sump. There it’s  exposed to frigid temps that remove whatever heat it just accumulated.

In extremely cold weather the oil in the sump can lose more BTUs than it picked up when circulating through an idling engine.

• newer engines use lower viscosity 0W-20 and 5W-20 oil that flows better than older 10W-30 oils when cold.

Idling loads the oil with gas

A cold engine needs a rich air/fuel mixture to ignite and overcome the heat loss produced by the cold metal combustion chamber. That rich mixture acts as a solvent and washes oil off the cylinder walls, so you actually get less lubrication during and idling warm-up.

Because some of the combustion is quenched by the cold cylinder walls, a portion of the rich fuel mixture doesn’t burn. That rich fuel/exhaust blow-by, along with the oil washed off the cylinder wall ends up in the sump where it churns into the oil. That fuel and water need to boil off in order to prevent sludge formation. But oil needs to reach at least 150° before it can start burning off the raw gas and water. It can’t reach that temperature while idling.

Again, idling can bring your COOLANT above 150°, but it can’t bring your oil to 150°. Only driving can heat it up to the point. So putting your car in gear and driving is the fastest way to heat your oil and burn 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 biggest downside to warming your car in winter is that you’re actually “lubricating” your camshaft and internal engine parts with oil that contains raw gas and water. Raw gas and water don’t lubricate very well. In fact, the raw gas in the oil actually removes the boundary lubrication needed to prevent metal to metal contact on the camshaft lobes and bearings. In other words cold idling accelerates engine wear. That’s whey idling you car in winter is BAD for it

Don’t confuse oil flow with oil pressure

FACT: In a stone cold engine, oil develops proper pressure within seconds

Oil pressure is what keeps metal parts from touching one another. Most oil pressure sensors are designed to turn off the oil light once oil pressure reaches a minimum of around 7-psi. Cold oil can easily reach minimum pressure within seconds of start-up. So you’ve got metal-to-metal contact protection almost immediately after starting a cold engine. What you don’t have is great oil flow.

Oil Flow versus pressure causes great confusion

Cold oil doesn’t flow as well as warm oil. There’s really not much dispute over that fact. The only question is “What the best way to warm up oil so it can do it’s job?” Well, it’s NOT by idling! Driving is the single best way to warm up oil.

Busting cold oil flow myths

It’s a myth that cold oil doesn’t flow

Cold oil is cold and it is thick, but that doesn’t mean it doesn’t flow. 

Amsoil has a youtube video that purports to show the flow difference between conventional oil and synthetic oil at -40C. Amsoil chills both oils and pours them out of a beaker. The results are stunning!

The conventional oil has the consistency of grease, while the synthetic oil flows out of the beaker. But what does that prove? NOTHING!

What you just saw is a comparison of the KINEMATIC viscosity of two oils. Kinematic viscosity measures the flow rate of a fluid under gravity. But all engines have an oil pump, so they don’t lubricate by gravity, they move oil with pressure. that’s called DYNAMIC or absolute viscosity.

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, a positive displacement oil pump with move oil.

If cold engine oil was solid and the oil pump couldn’t pump it, the pump, due to its very design, would actually self destruct. Yet you don’t see exploding

warm up engine before driving

Three types of positive displacement engine oil pumps

oil pumps in winter. Look at the positive displacement oil pumps to the right 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.

The truth is, if you’re using the proper viscosity oil, it’ll start flowing within seconds after starting and you’ll have enough oil flow to start driving.

MYTH: Driving a cold engine causes damage and makes it wear out faster

Fact: 90% of engine wear takes place during cold startup. But that wear happens during the first 2-5 seconds after firing up, before oil pressure builds to minimum. However, once the oil light turns off, you have enough oil pressure to prevent wear.

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. That’s because idling is the least effective way to warm engine oil. Driving is the most effective way to warm oil.

Engine 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 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

Idling a cold engine doesn’t burn off the raw fuel and water in your oil

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.”

Baloney

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 missed class the day the instructors talked about expansion and contraction.

Trust me, they didn’t; engineers understand expansion and contraction very well. 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? Were the corporate lawyers 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 get it to idle speed. Then they push back from the gate and start taxiing.

Having said that, there are exceptions: Some spray lubrication system 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.

Ok then, how long should you let your car warm up before driving?

Late model engines with direct fuel injection should be ready to drive in about 45 seconds. That’s about the time it takes clear the windows, buckle up, turn on the heat and radio. Then it’s time to DRIVE. If you’re uncomfortable with driving that soon, wait two minutes. But after that, you should put it in gear and go.

At that point you’ll have enough oil flow to fully lubricate both the high pressure fuel pump and camshaft. High pressure fuel pumps boost gasoline pressure to almost 2,000-psi. so they do need a few seconds of warm up time to get enough flow to the camshaft to prevent cam wear.

But on non direct injection engine, 15-30-seconds of warmup is plenty.

But don’t floor it. 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

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