Hot vs. cold oil change logic

Some of the old guys used to drain the oil, put the plug back in, fill the sump with diesel fuel or kerosene or ATF, run the engine at idle for a few minutes, and drain again. I've never had the nerve to do that, but I bet it sure would get the engine clean. Might also loosen a lot of crud better left stuck to where it's stuck. Another old guy would mix a spoonful of baking soda with a little oil and dump that into the case every so often. It would neutralize the acids formed when water mixes with oil and make the engine last a lot longer. Modern oils are much better at this sort of thing: cleaning and preventing corrosion. But as the engine wears it starts to get dirtier faster and changing the oil sooner will keep it lasting longer. The trick is to find a balance between the cost of oil and filters and the amount of extra life that oil/filter cost is buying for you.

Dan

With synthetic oil regular changes the car will out last you

Yes, but back then, oil was a lot more prone to sludging and there were some engine designs that had more sludging issues as well. So doing this every once in a while was probably a good thing.

I remember once seeing the valve covers come off of my neighbor's Valiant and seeing enormous amounts of goo inside there. They explained that because there wasn't any oil filter in the engine it tended to sludge up a lot so they just removed the valve covers and wiped it down. It's been a long time since anyone has had to deal with that kind of thing.

I don't think this actually worked particularly well, and since the undissolved baking soda is a little abrasive, it might not be a good idea either.

Yes, BUT modern oils are also better at keeping the combustion by-products in suspension as well. So the oil can be a lot dirtier without causing problems, compared with the oils of the fifties and sixties.

--scott

I think this has much in common with the argument about whether to keep your computer on all the time, or to turn it on just when you need it, as far as hard drive reliability and longevity.

For both arguments to have lasted as long as they have, the answer must be, "not a lot of difference." If there really were clear advantage to one way or another, the answer would have popped out long ago. So don't worry about it- six to one, half dozen the other.

Run the engine, warm it up good and drain/change it. If your thinking that it needs to set a while for the oil to all drain to the bottom of the engine. Consider that on most vehicles there is less than a quart of oil in the pan with the engine running. The rest is circulating in the engine. If you shut the engine off and wait 5 minutes all the oil will be back in the pan. (oh you may miss a teaspoon or two).

The sludge and crud will drain better warm as well. Want a demonstration of why? Go and coat your hands with some dirty oil. Now wash them using cold water. Doesn't come off very well. Now use warm water and you will find it works better. The heat allows the surfactants and detergents to work as designed, just like the detergents and surfactants in the oil will work better and keep the crud suspended in the oil. The oil filter does NOT catch it all, only what is in the oil as it goes through the pump.

Oh and just because you use a synthetic does NOT mean you cannot have sludge.

[...]

Engine oils really do hold solids in suspension. I've tested this, though not very scientifically. Get you a clean 5-gal bucket and dump your engine oil into it. You can tip it sideways enough to see the bottom and see that it's not instantly coated in crap. Let the bucket sit overnight and tip it again. Let the bucket sit a couple of days and tip it again. You'll probably be pretty impressed with the amount of crud that settles out of the oil. And it does not very readily come off the bottom of the bucket. If I change oil hot (5-7k, synthetic here too) I get far more crap at the bottom of the bucket than I do when changing the oil cold. The amount of oil that's inside of bearings and passages and the oil pump is pretty negligible compared to the sump volume, so allowing it to sit won't cause a lot more oil to drain out vs. leaving the plug off for five minutes while you change the filter. In fact, that oil shouldn't drain back, no matter how long it sits. What I do is drive it until hot, park it and pull the drain plug. Then do something else while it cools down a little. Usually 20 minutes to make it comfortable. Ford put my filter literally within and inch of the exhaust manifold, so touching that hot hurts. I spend that time doing other PM stuff, like checking tire pressures, cleaning out the cab, etc. Then change the filter, add oil, and fire it up. I also add some diesel fuel to my oil the day before changing it to help scrub the innards. Last time I had the valve covers off I didn't even have much varnish.

I agree... and the most important part is to have run the engine to its full operating temperature before draining the oil (without restarting the engine).

The other way is to leave the engine cool for an hour or so before removing the drain plug and get the best of the both methods, as well as reducing the risk of being scalded.

Worst thing you can for your engine IMHO is to only run it long enough to get the oil warm prior to draining it (as many do). Some even recommend you do it that way. :)

There isn't room in the engine for three quarts of oil to "circulate." There might be a pint in the galleries, and the oil in the filter stays in the filter after shutdown. The galleries are no more than a half-inch in diameter, mostly smaller than that, and in the crankshaft they are 3/16" at most. If there was only a quart in the pan the pickup would be sucking air half the time as the vehicle bumped around.

filters catch BIG stuff, like 10 microns or bigger. The smallest stuff stays in the oil, accumulates there, and thickens it.

Dan

FWIW, all I've ever felt is necessary is to drain the oil, take 5 minutes and inspect the undercarriage, put the plug in, change the filter, prefilling it if possible, and add fresh oil. I figure the .05 quart of dirty oil left behind isn't that big of a deal since it's only been 3-5 months since the last change anyway.

I can't be bothered to wait 45 minutes or 6 hours to wait for 100% of the oil to drain - 99.5% of it is fine. I also don't worry about the temperature.

Ray

see my previous post - I should have added that my wife's Beretta is a

17 year old car and I've been doing it that way on that car for 10 years

- the body is going, but the motor still runs fine.

I do it on my race car. I do it on my 2001 Trans Am. I think you're overthinking a simple oil change.

Ray

OVERKILL? From a guy who's debating how to change oil like it's brain surgery, I think you're missing the point.

If you change your oil and filter on a regular basis, you'll get 200,000 miles from just about any car today using any oil and filter.

BUT.. the people who use the cheap oil and filters are also the ones who never change it.

Ray

Don't make a mountain out of a mole-hill. Either way will get MOST of the old oil out. Personally, I prefer to change the oil within one hour of a hot shut-down. When I pour the hot oil out of my catch-pan and into jugs for recycling, I notice that hot oil really does leave a lot less film on the catch-pan than room-temperature oil does, so it must also leave a lot less film on the inside of the engine oil pan. And yes, that's with SYNTHETIC oil.

And is ridiculous overkill. 90% of the oil distributed through the engine drains back to the pan within the first 2 minutes after shut-down (which is why 2 minutes is generally accepted as long enough to wait to get a valid dipstick reading). Waiting 10 to 30 minutes is, within the limits of practicality, as good as waiting overnight.

With any method, opening the oil cap to break suction should

The engine isn't sealed so opening the oil cap doesn't do anything. It can pull plenty of air in through the PCV make-up air breather.

Having pulled the pan off of engines run to *very* high mileage but with modern oils and decent change intervals (7000 miles or less) I can assure you that unless the engine had a problem like the Toyota inadequate PCV system and ensuing sludge, you'll find NOTHING in the pan. Modern oils just don't let gunk accumulate on the bottom of the pan like 60s vintage oils did. If you're going to find any accumulation, it will be up on the top-end around the valve train, where the oil is much hotter, there's less volume and flow of oil, and the PCV system is pulling combustion gasses across the hot components. Even that proved to be quite clean (spotless, in fact) when I replaced the valve cover gaskets on my wife's car (Chrysler 3.5L) at 200,000 miles after a life of using synthetic oil.

Jack,

Next oil change take a 100 mile ride on the interstate then come home and change the oil. It will pour out like boiling oil. I once changed oil after a 360 mile trip back from Ohio--- talk about hot oil!

Never had sludge in 309,000 miles and I'm not worried about it. I change oil between 3-4000 miles, had the pan off twice in its life & the inside was clean as bare metal can be.

happy motoring.....

harryface

2005 Park Avenue 61,289 1991 Bonneville 309,899

And if you really think that through to it's logical conclusion, an argument could be made that changing the filter only on every other oil change as recommended on many applications would reduce the chance of even a momentary dry start even more. In addition, a used filter, as long as it is not clogged to the point to where the bypass opens excessively, probably filters better than a brand new clean one.

Lee Richardson

I felt the same way until I changed the oil on a 4640 John Deere and got an extra gallon out of it by leaving it drain overnight. Six gallon bucket was four inches down on the side when I left it at sundown and almost overflowing in the morning. If I get any of that action from an auto it's worth doing - I still don't worry about the temp though.

The only passenger car that I know of that mentions extended drain intervals is the newer Vettes - they have baffles in the pan and take ~10 minutes to get all the oil out.

Otherwise, I've found that if the car holds 5 quarts of oil, if you add

5 quarts and fill the filter, you're good to go, so there's no 1/2 quart of dirty oil still in the motor.

Ray