If you regularly replenish the fluid of your coolant system (or oil lubrication system) due 2 a leak

… are you in effect, changing your coolant and/or changing your oil?

To be more clear, if your cooling system has a slow leak problem, let’s say you notice pink or green coolant “cake” around the radiator and/or you notice trace amounts of coolant on the undercover of your car—you’ve got a slow leak coolant problem. The leak forces you to replenish the coolant recovery reservoir every month (when previously, you’ve only had to refill it every 4 months.) You’re essentially replacing about a gallon or two of coolant every year.

Is this different than doing a coolant change or flush as part of regular maintenance?

Obviously, this is not to suggest that one shouldn’t get the source of the problem fixed…


Any closed loop system that uses a fluid where there’s a leak and replenishing is required will eventually replace all the fluid over time.


While if you have a beater where the oil or coolant “changes itself”, it is definitely better (for your car, though not the environment) than if you are always lax on changing your fluids, but it also means that some of the original, filthy, degraded coolant or oil never gets changed, along with acids, sediment, and other crud that remains in there. Plus the filter never gets changed that way, plus if you have a car that leaks oil, it’s likely that it gets run low frequently enough that you offset any “benefits” from this.

I’ve seen this technique work many times. However, it’s usually in the context of a vehicle where it’s not worth investing in a proper fix for the leak.

I wouldn’t trust doing this for coolant because the leaks either get bigger with time (leading to overheating and possible engine damage) or are causing engine damage already (bad head gasket leaking coolant into the oil).

When you change your oil properly with the oil warm, it runs out fast and presumably carries some debris out of the bottom of the pan in the quick flow. You don’t get that benefit from a slow leak.

No, it is not the same. Think of a slow leak as being analogous to the pores in a filter. It keeps the unwanted crap inside the engine and lets only filtered coolant out. Not a good idea.

I do hope you seriously plan to fix the problem.

I actually had a friend who did this with his oil . . . he changed his filter every 3000 miles and topped-off the fluid level with a quart of fresh at the same time, and he claimed that this made sense to him. I posted a similar question last Fall about this, asking essentially if it made sense to change the filter @ 3000 and go to 6000 mile oil changes. With oil and filters being so (relatively) inexpensive, I opted for a 5000 miles oil & filter change with full synthetic, as opposed to a 3000 mile oil & filter with mineral oil. Rocketman

BTW, he NEVER drained the crankcase, just changed the filter every 3000.

Remember the hype about the Franz filter in the 50s. This used a roll of toilet paper as the filter element and had to be changed every 1000 miles. But you could go 100,000 miles without an oil change, according to the write-up. On a 4 quart crankcase that meant changing the oil every 4000-5000 miles, but the sludge in the bottom of the crankcase would not be removed.

My father in law bought a small oil change suction pump (inserted through the dipstick opening) to change his oil. He could change the filter from under the hood. I explained patiently that the suction pump would not get the sludge out of the crankcase, but he would not listen. The car was totalled before it reached 100,000 miles so we will never know how effective it was. At least the oil and filter were changed.

Doc, did it look something like this?

I dunno, I think someone here can come up with a math formula to tell you how many years it would take to even get to 80%. Think of this though, take glass of muddy water. Then scoop a tablespoon out and at a tablespoon of clear water to it. See how many times you have to do it to get water clear enough to drink.

Ya guys bought toilet paper lately? Filters are cheaper.

It’d be easy to figure out how many tablespoons it would take to get out 80% of the gump if you know the concentration, but you cannot get to 100%.

Assuming a 16 oz glass, and assuming a 20% concentration of gump in the glass, that means there’s 3.2 oz of gump and 12.8 oz of water. A tablespoon equals 1/2 oz. Remove one tablespoon (a tablespoon is 1/2oz) and you remove 3.125% of the gump, leaving 3.1 ox of gump. Replacing the missing mix with clear water leaves 12.9 oz of water and 3.1 oz of gump.

The ratio changes exponentially. I’ll leave the simple regression to someone younger with more of their neural synapses still functioning, but it’ll probably run about 20 events to get to 80% clear water. Someone with still-functioning algebra skills, take this as a challenge…make me look foolish. Excel would be a perfect vehicle too. You’ll get a nice sweet curve.

I urge you young guys, keep up your math skills. you WILL lose them if you don’t. I openly admit that I should be able to do this formula in my sleep. But it would take me more work than I’m currently willing to do.

tsm is right: you will never replace 100% of the fluid by topping up after a leak. And you never replace 100% of the fluid when you drain the system and then refill. There is always some quantity of fluid that stays behind, adhering to surfaces for example.

An ancient Greek mathematician - whose name is well known but not remembered by me -said it this way: You stand some distance from a wall. You step half the distance toward the wall and repeat this action. When will you reach the wall? Never. Half of any quantity is still a number not equal to zero.

Ya guys bought toilet paper lately ? Filters are cheaper
Are you buying that real fancy stuff ?

That wacky Zeno?


Shanonia, I was thinking of removing 100% of the gump rather than replacing 100% of the fluid, but it amounts to the same thing.

I’ve heard the same illustration applied to football. If you get 50% of the way to the goal line with every play, you cannot ever get a touchdown.

@mountainbike‌ Yes, but in those days the ads were in black & white. Like the Fish carburetor, I don’t think it ever took off in great quantity; just cleaning out the canister and putting a new roll in every 1000 miles seems sort of messy.

He da man!

I almost fell off my chair when I googled the TP oil filter and discovered they’re still selling them.