Why is it OK to put cold oil into a hot engine?

[LTC8K6]

The coolant comes up to temp much faster than the oil. If you shut the engine off as soon as the temp gauge is up to the "normal" range, the oil will only be warm.

ZV

Who waits and watches the temp gauge? I just drive it around for a while if the engine is cold. :biggrin:

Or change it when I get home from work.

Fortunately, I really don't think it makes the slightest difference, hot, warm, or cold, as long as most of the old oil comes out. 😛

Changing it in the morning after sitting all night is good too.

I just prefer the oil to be at least warm.

 

[bruceb]

The RX-7 had an oil cooler also. If you live in an extremely hot area or do a lot of towing, it will help the oil and engine live a long time.

 

[Meghan54]

Our Lexus GX has an external oil cooler, as did our '03 Murano and even our old '02 Chevy Blazer. I tend to think you'll see an increasing number of vehicles factory equipped with external oil coolers over the next few years, just as the number has increased over the last decade or so. Guess increasing underhood temps, along with tighter tolerances inside the engines, will dictate this.

 

[manimal]

lots of big trucks with towing packages have oil coolers.

I threw one on my expedition since I was gonna tow a little. Didnt matter on that POS tranny lol grrr ford!!

 

[foghorn67]

Our Lexus GX has an external oil cooler, as did our '03 Murano and even our old '02 Chevy Blazer. I tend to think you'll see an increasing number of vehicles factory equipped with external oil coolers over the next few years, just as the number has increased over the last decade or so. Guess increasing underhood temps, along with tighter tolerances inside the engines, will dictate this.

I think so too.

 

[Pulsar]

Putting hot oil in a cold engine is not something to worry about from the perspective of damaging the steel. The operating temperature of an engine is far below what is required to cause microstructural change to the steel. Heat treatment of steel generally always starts with an excursion to 727°C, the temperature at which the microstructure changes to austenite, from which all other microstructures evolve. If you never get over the austenitizing temperature, you're not effecting real microstructural change.

Also, the temperature difference is not really enough to cause a problem due to thermal shock- this happens with glass because it is extremely brittle: it's microstructure is such that it has no ductility at room temperature. Modern (Post WW2) steel is very ductile. The coefficient of thermal expansion of steel is 7.3 x 10^-6 in/in K. So with a temperature difference 72K, the dimensional difference would be 0.000525 inches per inch. Well, well within manufacturing tolerance for any engine. A 1" dia camshaft would become 1.000525. Any temperature difference in a monolithic piece of steel will cause some amount of stress, but the difference here is so minute that the stresses are nearly zero- not even close to what you'd need to shatter something like putting hot glass in water.

1. The vast majority of metal in an engine is cast iron, or aluminum. Not steel.
2. The biggest problem with temperature differentials is not inside homogeneous metals like iron, but at the points where the cast iron block, aluminum heads, and aluminum front cover meet. The two metals have vastly different coefficients of expansion. This slight shifting causes leaks over time due to fretting of the cylinder head gaskets. In addition, components like the water pump and oil filter adapter are also gasketed or o-ringed. It's these surfaces that are damaged.
3. .000525 inches is 13 microns. It may surprise you to know that new engines are measured in microns, because the tolerances held during the machining process are actually that tight. In fact, the cylinder bore tolerances are even tighter than that.

Despite your inaccuracies, you are corrected in stating that cold oil and even cold water will not damage a *new* engine. New engines are tested for thermal shock by running at full speed, getting them glowing red hot, then feeding them water at the freezing point (or sometimes below the freezing point). This cycle is done repeatedly.

 

[Scarpozzi]

You typically pour it in the top over your crank,head, etc... That oil isn't being pumped through your engine block though. Most cars are turned off for 5-10 minutes before they're refilled. It'll just drop into the reservoir and warm up to 100+ degrees pretty quick...

 

[CraigRT]

I change it cold when I can (almost all the time). I have no idea why you would want to change it hot, it would take longer to drain since it isn't right there in the pan. Not to mention dealing with hot oil is just not fun.

I always do it cold as well.

 

[Throckmorton]

The coolant comes up to temp much faster than the oil. If you shut the engine off as soon as the temp gauge is up to the "normal" range, the oil will only be warm.

ZV

Interesting... Bob says it can take 30 minutes for oil to get up to 200F http://www.bobistheoilguy.com/motor-oil-108/

 

[Zenmervolt]

Interesting... Bob says it can take 30 minutes for oil to get up to 200F http://www.bobistheoilguy.com/motor-oil-108/

That seems accurate to me. The coolant is usually up to temp in less than 10 minutes and the oil will take much longer. My car runs 4 bar (~60 PSI) of oil pressure at idle for a good 15-20 minutes before there's enough heat in the oil for it to even start dropping towards the hot oil pressure of 2 bar (~30 PSI) at idle.

Granted, that's the 951 which is running 15w40 so it runs slightly higher pressures than modern cars tend to, but you get the idea.

ZV

 

[tonyfreak215]

Honda B16as have an oil cooler.

 

[Throckmorton]

That seems accurate to me. The coolant is usually up to temp in less than 10 minutes and the oil will take much longer. My car runs 4 bar (~60 PSI) of oil pressure at idle for a good 15-20 minutes before there's enough heat in the oil for it to even start dropping towards the hot oil pressure of 2 bar (~30 PSI) at idle.

Granted, that's the 951 which is running 15w40 so it runs slightly higher pressures than modern cars tend to, but you get the idea.

ZV

So at what point can you drive your engine hard, redline it, etc? Usually manufacturers say once it's at "operating temperature" but all you have to go by is coolant temperature.

 

[jlee]

So at what point can you drive your engine hard, redline it, etc? Usually manufacturers say once it's at "operating temperature" but all you have to go by is coolant temperature.

I drive pretty cautiously until oil temp is at least 100f and coolant is at or very near operating temp (CTS-V). In my MR2, I judge temp by pressure...once I'm below 20psi at idle, I'll hammer on it.

 

[chucky2]

I seem to remember an old thread on BITOG that possibly referenced Noria that said something to the affect that particulate starts to leave suspension in the oil within 2 minutes or something like that after shutdown.

Obviously, I have so many qualifiers in there that that may be complete BS, but I remember at the time reading it and adjusting my oil change routine because of it. Now I prep for the oil change before going out, take it out for 20-25 minutes (errands, cruise, whatever), and then bring it back, park it, and immediately drain. Getting the most oil out with the most particulate/contaminant in it would seem like the best approach, as long as there are no ill affects of colder air rushing up into a hot engine.

Chuck

 

[tonyfreak215]

I drive pretty cautiously until oil temp is at least 100f and coolant is at or very near operating temp (CTS-V). In my MR2, I judge temp by pressure...once I'm below 20psi at idle, I'll hammer on it.

What would happen if your oil was up to temp/pressure but your coolant temp was still cold?

I know you don't want to push a cold engine hard but that is usually due to lubrication reasons. What about coolant?

 

[Pulsar]

What would happen if your oil was up to temp/pressure but your coolant temp was still cold?

I know you don't want to push a cold engine hard but that is usually due to lubrication reasons. What about coolant?

Nothing. Likewise, cold oil does nothing. After the first 15-20 seconds, the oil has already developed a skin between adjacent metal parts. In fact, cold thicker oil develops a thicker skin that keeps the metal aparts apart more easily. The only issue cold oil poses is lack of flow, but obviously oil systems are engineered to handle that.

Again - new engines are tested thoroughly using thermal shock tests on dynos.

http://www.roush.com/our-capabilities/testing-and-development/dynamometer-testing.aspx

Their thermal schock setup puts -25C coolant through the engine after it's been run till the block is red hot. That's a pretty common setup amoung all the engine manufacturers.

 

[tonyfreak215]

Nothing. Likewise, cold oil does nothing. After the first 15-20 seconds, the oil has already developed a skin between adjacent metal parts. In fact, cold thicker oil develops a thicker skin that keeps the metal aparts apart more easily. The only issue cold oil poses is lack of flow, but obviously oil systems are engineered to handle that.

Again - new engines are tested thoroughly using thermal shock tests on dynos.

http://www.roush.com/our-capabilities/testing-and-development/dynamometer-testing.aspx

Their thermal schock setup puts -25C coolant through the engine after it's been run till the block is red hot. That's a pretty common setup amoung all the engine manufacturers.

Sorry, I wasn't clear enough. I meant driving the car hard with fully warmed up oil but cold coolant.

 

[Zenmervolt]

Sorry, I wasn't clear enough. I meant driving the car hard with fully warmed up oil but cold coolant.

Well, for one thing, that's not ever going to happen because the coolant heats up so much faster than the oil. The only way to do this would be to drain the engine of all oil, heat the oil to operating temp outside of the engine, pour the pre-heated oil into the engine, and then start the engine and drive. There's no way anyone's going to do that.

As a thought exercise though, the reason we wait for coolant to reach operating temperature before driving hard is that it allows the engine's temperature to stabilize and is a reliable indicator that the metal parts are sufficiently up to temperature. Remember, temperatures inside the combustion chamber are 3,500 to 4,000 degrees Fahrenheit as soon as you start the car. The harder you drive, the more energy each combustion cycle has and taking it easy for the first part of the drive allows the metal components in the engine to expand more slowly. Running hard on a cold engine puts extra stress on things like head gaskets and piston rings. While doing so once or twice probably wouldn't hurt anything, making a habit of it would wear out the engine faster from the increased stresses of more aggressive heat cycling.

ZV

 

[LTC8K6]

My Brother's 2012 Grand Cherokee shows the oil temp, coolant temp, trans temp, and oil pressure on the EVIC. I think an experiment is in order.

Maybe start from cold and record each one every minute for 10 minutes?

 

[LTC8K6]

I suspect that with an oil cooler the oil will remain relatively cool unless the engine is working hard, and will probably cool back down again when the load is reduced.

 

[tonyfreak215]

Well, for one thing, that's not ever going to happen because the coolant heats up so much faster than the oil. The only way to do this would be to drain the engine of all oil, heat the oil to operating temp outside of the engine, pour the pre-heated oil into the engine, and then start the engine and drive. There's no way anyone's going to do that.

As a thought exercise though, the reason we wait for coolant to reach operating temperature before driving hard is that it allows the engine's temperature to stabilize and is a reliable indicator that the metal parts are sufficiently up to temperature. Remember, temperatures inside the combustion chamber are 3,500 to 4,000 degrees Fahrenheit as soon as you start the car. The harder you drive, the more energy each combustion cycle has and taking it easy for the first part of the drive allows the metal components in the engine to expand more slowly. Running hard on a cold engine puts extra stress on things like head gaskets and piston rings. While doing so once or twice probably wouldn't hurt anything, making a habit of it would wear out the engine faster from the increased stresses of more aggressive heat cycling.

ZV

Sorry, this is a threadjack, but the reason I ask:

I was racing my Spec Miata to get my comp license up at Roebling Road back in Feb with a track temp of 45 F. Since my car is a FL car I don't run a T-stat and have a huge radiator with other cooling system upgrades.

The car's coolant and oil was properly warmed up while in staging and during the warm up laps, but once the session started (and I got out of the draft) my coolant temp plummeted. I tried blocking the radiator but with the temp swing later in the day (got up to 80), I was never able to get it right.

The car still runs great and there weren't any bad effects. I'm just curious if it would actually be a problem to run a motor like that.

 

[LTC8K6]

http://www.audizine.com/forum/showt...-temperature?p=8252900&viewfull=1#post8252900

Looks like someone has already done it for an idling engine.

The oil isn't that far behind the coolant here and it rises pretty quickly for just idling.

I disagree on his warmup ideas, though. I warm up the engine by driving it easy for a few minutes.

Of course other cars may be different with the oil/coolant relationship.

 

[RU482]

crazy question for you all. My neighbot drains the oil, then cranks it over for a few seconds. Claims he gets another ~1/2 quart of dirty oil out that way. Is he crazy, or is this common?

 

[tonyfreak215]

crazy question for you all. My neighbot drains the oil, then cranks it over for a few seconds. Claims he gets another ~1/2 quart of dirty oil out that way. Is he crazy, or is this common?

Common. And it's not really a problem as there is still oil lubricating the parts.

 

[LTC8K6]

http://www.bimmerfest.com/forums/showpost.php?p=5628422&postcount=1

Another couple charts that split the temps of the coolant in the engine and the coolant in the radiator along with the oil temp.