Hot vs. cold oil change logic

If you run an engine to its maximum temperature and let it sit overnight for 10 to 15 hours, won't a lot more oil drain into the pan vs. doing an immediate hot oil change and letting it drain for the usual 10 to 30 minutes? Some people say they wait much longer but I don't find it practical.

If I am correct, the main issue seems to be evacuating oil from the pan if the engine block is drained as best it can be. I'm not convinced that "stirring up" pan sludge by warming the engine will put it "back into suspension" if the oil and filter wasn't capable of holding it already. You're just spreading any (mobile) particles back into the engine channels. Then, you're left with a much shorter drain time (vs. overnight) to get it all back down into the pan. What's the real point?

It seems that any sludge stuck to the bottom of the pan is not going to move much, regardless of whether the oil is hot or cold ("cold" meaning ambient temperature; same temperature you pour new into the engine at). What is the real value of doing a hot oil change if it only allows a fraction of the time for oil to drain back from the engine block? The oil can only leave through one hole in the bottom of the pan, so shouldn't it be gathered close to that hole before you start? Why stir it up again if true sludge isn't movable?

When I pour new oil into an engine on a warm day, it seems to flow quite well, contrary to the notion that anything below scalding will creep like molasses. Used oil is thinner, if anything, so flow rates should be similar. I've exclusively done hot oil changes but I'm reconsidering. Hot vs. cold seems to come down to how well you can evacuate the pan, and how much MOVABLE sludge is actually there.

At worst, you'd be left with a thin film of "cold" oil on top of pan sludge that probably wasn't going to move anyway. At least that's my theory. Has anyone done serious experiments on this?

Jack

P.S. The ideal method would be to drain the engine AND pan overnight after the engine is hot, but that takes planning and isn't always practical. With any method, opening the oil cap to break suction should help it flow better.

Reply to
Jack
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I think the answer is to not worry about oil. Do you really care about the very end of the engine's life? Will you still be driving it when it's smoking? Will it still be road-legal? Will you not have a better job by then and be able to afford a car that isn't 20 years old? Even if that's all true and you really love your car, you can fix it with a cheap replacement high-milage engine. As long as you change the oil roughly OK and don't thrash it much the engine will almost certainly last longer than the body.

Reply to
nottoooily

Filters catch BIG particles. It's the small stuff we want out of the engine. All modern oils are ashless-dispersants that keep this stuff in suspension rather than letting it settle out, unless it's been sitting a long time. The sludge we want out of the pan is mostly precipitate from water/oil reactions, and it's tight at lower temps. Water gets into the case as combustion gases, which contain water vapor, get past the rings and condense in a cold engine. Short trips are bad for the engine because the water never gets boiled off.

Cold oil drains more slowly, and sticks to the inside of the pan as well, so that you don't get as much of the heaviest contaminants out of it. Waiting for engine oil to drain back into the pan after running is mostly a waste of time, since any good filter has an anti-drainback valve in it that traps oil in the galleries. Anything that will drain out of the workings will drain quicker if it's hot. So hotter oil will carry more of the crud that has settled in the galleries when you remove the filter and oil runs out of the case.

True sludge is less movable when cold.

In a neglected engine there might be significant sludge. In most that I take apart, there's not much sludge; mostly it's varnish, baked- on stuff that won't drain out anyway.

Opening the oil cap won't break any suction. The case is vented through the PCV into the intake manifold. It's not a sealed system. Never was.

Dan

Reply to
Dan_Thomas_nospam

snipped-for-privacy@hotmail.com wrote in news:1189270630.710553.191560@

22g2000hsm.googlegroups.com:

Thanks for the philosophy but this isn't rec.throwaway.society. I like to get top life from most things I buy. This is a 10 y/o Toyota 4-cyl. with

190k miles. It's doing fine and so is the chassis. I change the oil (synthetic) every 5k to 7k miles and would like to know if hot is really better.

I paid cash (new) and have no interest in monthly bills and higher reg. fees and/or insurance. Do you know how many people are strung out on credit for want of new things? The new car high is temporary and other people don't really care what you drive; even though it may boost internal pride. If it does the job, why let it go to pasture? A big boost in MPG would be my main reason for a replacement.

Jack

Reply to
Jack

Dan_Thomas snipped-for-privacy@yahoo.com wrote in news: snipped-for-privacy@y42g2000hsy.googlegroups.com:

As stated, I would make sure the engine is quite hot before letting it settle overnight. If what remains in the pan can't be dragged out by "cold" oil, the cold method is inferior. But I don't know that this has really been tested.

I have poured used "cold" (70F+ ambient temp.) oil from a drain container to see how much gets left behind. Whether hot or cold, the remaining film is not hugely different in my tests. It seems that only a few extra tablespoons (if that) should remain in the engine pan using the cold method. I assume the internal shape of a sump is designed for optimum downward flow. You could pour in a half quart of left-over new oil to help push it out.

But it's hot when you park it and most of the draining into the sump occurs when it's hot. That's why I don't see the point of warming it up again and giving it much less time to drain all over. The "cold" method makes use of time and patience that people lack when they are thrown into a task.

Still, it's HOT when the bulk of the draining occurs after you park it. Time has already done most of the work. It collects in the pan/sump overnight, waiting for you to remove the plug. Gravity has done its work inside the engine block for much longer than most people can wait in dirty clothes.

I agree there might be some pan sludge carried off better with hot oil, but the question seems to be how much. Below, you say the stuff that sticks is mostly varnish.

If sludge stays suspended in the oil and doesn't settle out overnight, the main difference should be viscosity and flow rate from the pan the next day. I could wait an extra 5 or 10 minutes vs. scalding my hand on the oil and filter. A lot of this is about convenience due to engine layout.

That seems to side with my cold change theory, unless sludge would tend to gather without hot changes.

Well, I do notice an extra rush of oil when I loosen the cap, though it may be temporary. I also blow into or slap on the opening and some extra oil comes out. Rocking it has some effect also.

Jack

Reply to
Jack

On issues of oil types, oil changes, and oil filters, you will get little on this newsgroup but personal opinions.

Hard data, rigid testing, is hard to come by.

My opinion is that a warm or hot drain is better than a cold, because the viscosity is reduced and the oil and dispersed particles will flow out of the engine more easily.

Start hot and let it drain overnight, if you wish. That should be the best of all worlds.

And I change at 3-4,000 miles, not 5-7,000. In this way I never seem to have varnishing of the engine parts, sludge, and - most of all - premature engine wear and failure. Again, this is opinion and practice, not hard science.

Oil changes are damn cheap compared to engine repairs or replacements.

Reply to
hls

Your putting way to much thought into changing oil,,, crank it up ,let it run about 5 minutes,shut it off ,pull the plug and let it rip,replace the filter...do this every 3,000 miles,thats all you need to do,,,if the engine burns up, it would of went anyway,,chill out a little,,there's alot more things you could be sweating,,,if you think your engine is full of sludge, change your oil every 500 miles a couple of times.

Reply to
challeno

"hls" wrote in news:4NCEi.3751$ snipped-for-privacy@newssvr14.news.prodigy.net:

That's what tends to be said but I'm challenging it because the engine block is irrelevant if the oil has already drained from it (when it was hot and thin). The whole debate can be limited to what happens when you try to drain the oil pan after the engine blocked is as clean as it will get.

Definitely true but I find that impractical. I like to get automotive work done in one shot. I also don't like to leave a car undriveable overnight in case of emergency.

Since the oil would drain (initially hot) into the pan for hours overnight, the whole question can be isolated to a detached pan you run experiments on. All you need is a good amount of used oil and a heater that mimics engine temperatures.

Assuming you have an oil pan from an old vehicle to test:

1: Is drainage just a matter of viscosity and flow rate, meaning a longer wait for (air-temperature) oil to drain from the sink? For me, waiting an extra 5 or 10 minutes is worth it vs. dealing with scalding oil and a hard- to-reach filter.

2: Is there loose sludge (not varnish) on the sink walls that only hot oil will remove? If cold oil also carried it away, just more slowly, time would be the main issue.

3: If you change your oil often enough, is sludge a concern in the first place? "Hot oil theory" is based on the idea that the oil is washing or dragging away stuff it couldn't hold in suspension. From what I understand, if your oil/filter isn't suspending stuff you've waited too long anyhow. Any water in the pan should boil off when you next drive a decent distance, so I don't see water as an issue. Acids could be, though.

4: If all the nasty particles are indeed suspended in the oil/filter, what difference should there be aside from waiting longer for "cold" oil to drain?

That's how I'm looking at this.

Jack

Reply to
Jack

At risk of sounding too practical. That stuck stuff in the bottom probably takes many changes to accumulate, and some of it will never be removed by draining. Would an even better idea be to drain it any old way, then occasionally take the pan off and scrub it out? Possibly the time you save doing quick changes will cancel out the time it costs to clean the pan.

I doubt you'll get any actual answers from usenet. Its more in the nature of these people to offer annoying advice on how to avoid the problem than a solution on how to solve it ;)

Reply to
nottoooily

You go out and crank your car and let it run for 5 or 10 minutes after it has been sitting all night,,you won't be dealing with scalding hot oil after running your car for a little while,,,*_if_your_theory_was_correct_there_would_be_a_ sack*

*_full_plus_thousands_of_10_minute_oil_change_businesses_going_bankrupt._*
Reply to
challeno

Your exactly right about dropping the oil pan and scrubbing it out,,but the original guestion,I believe was about hot vs cold and how long to drain the oil.You can let your car sit for a week with the oil plug out and not one bit of sludge will come out of the pan than did the first 5 or 10 mins.

Reply to
challeno

challeno wrote in news: snipped-for-privacy@no-mx.nodomain.com:

Why would I just make it lukewarm and send dirty oil back up into the engine block again? The "cold" change lets it drain (initially hot and thin) all night. I would either do it at max. temp. (drain in 20 or so minutes) or max. TIME (drain into pan overnight, then drain pan).

I'm not really arguing against hot oil changes, just presenting the idea that cold ones may be equally or more effective. Either way I don't mess with quick lube shops or mechanics in general.

I just now did my first "cold" oil change. It was draining from the block all night and I drained the pan in 85F weather today. The volume that came out seemed the same as previous hot changes. It flowed at maybe 70% of the hot rate and actually went quicker since it was already gathered in the pan. I chased it with half a quart of extra (new) oil and let it drain out. Based on color, it seemed to do a good job of purging the remainder in the pan. This is synthetic oil which tends to flow better than regular anyhow.

Not dealing with the hot filter (positioned such that it always spills) was a big plus. I'm still undecided as to whether this should become a routine.

Jack

Reply to
Jack

Absolutely right, Challeno. It is not rocket science.

Change it regularly and frequently, and use a good oil filter. Then go to sleep and dream of Paris Hilton, or whatever. Your day is over.

Reply to
hls

challeno wrote in news: snipped-for-privacy@no-mx.nodomain.com:

Nothing personal, but that mentality is why I still steer clear of American vehicles. Some of us are into precision, not "like, who cares, man!"

Besides, the 3,000 mile oil change interval is not only more work, it's been invalidated as too frequent, at least with synthetics. Most owners manuals indicate a 5,000 or 7,500 mile interval for normal driving (soft commuting). I used to believe that 3,000 miles was smart, but if the oil can hold contaminants in suspension for longer, it's overkill. It creates more labor and adds hassle to long road trips.

Of course the oil makers and lube shops won't argue with 3,000 miles. Sell, sell, sell!

Jack

Reply to
Jack

challeno wrote in news: snipped-for-privacy@no-mx.nodomain.com:

You've skipped the details of my original question/comment. Having used synthetics and done most changes (hot) at 5k miles, I am not worried about sludge, I am suggesting that when you park a car after a good run, the oil has a lot more time to drain (initially hot) from the engine block overnight. It is a thorough drain vs. a 10 or 20 minute rush.

After the oil has gathered in the pan, the issue is whether the final step of draining it from the pan is that much different hot vs. cold. I don't think it is but more evidence is needed. This is really an engineering or physics question.

Jack

Reply to
Jack

"hls" wrote in news:KNGEi.2597$3Y1.1521 @newssvr17.news.prodigy.net:

Pardon my intrusion into rec.autos.slack! I was expecting more replies along the lines of Dan_Thomas'. I also had other maintenance that made it tricky to drive the car that day but it's not worth explaining.

Jack

"Time for a cool change..." (Little River Band)

Reply to
Jack

Why not park it after the drive, open the drainplug while hot, and let it sit overnight?

Reply to
Noozer

If you are using Synthetic and doing the drains that often it makes no difference whether you do it hot or cold or anything in between. There isn't any sludge in your engine that you need to worry about moving. If you were doing changes every 20K it might be something to worry about but then it would raise the question of "why worry", you are abusing it anyway if you are doing those long intervals.

Reply to
Ashton Crusher

There is no need to change it any sooner then roughly every 6000 miles.

Reply to
Ashton Crusher

If someone is really worried about sludge the best thing would be to park it when it's hot, let is sit overnight, drain it, then get some kerosene and shoot the kerosene into the pan thru the drain hole and put the plug in and let it sit for a bit, then drain it back out, then pour a half a quart of some cheap oil in to flush out any remaining kerosene.... or some variation on that kind of routine. Personally, if you think you need to worry about sludge you simply are not keeping up with your oil changes, change it every 6000/6 months and it will never have an oil related problem.

Reply to
Ashton Crusher

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