Old Wives Tales - Changing Old Fluids Creates Problems

I've occasionally heard this old wives tale; that if you change the transmission oil in an old vehicle, you might create more problems than it solves.

Now, obviously this isn't the case for engine oil, but I've heard it in about just every other fluid on a vehicle. New coolant loosen up deposits and can lead to headgasket or water pump failure. New trans fluid can create slipping. New brake fluid can lead to premature leaks around calipers.

Anyone believe this stuff? And if there is some credence to the myth, what is it about new fluids that can create issues?

The only time it'll cause an issue is if the part in question is already about to fail. If new fluid cleans some dirt out of an already-shot seal, it might start leaking. Things like that.

IMO, if changing the fluid made a problem appear, that part was already shot.

I definitely opened up a brake line leak when I replaced brake fluid on the Roadmaster challenge wagon. It was a very rusty line tucked up on top of the frame. A few hard pumps during the bleeding procedure turned into soft, squishy pedal feel. Sure enough there was a mid-ship brake fluid leak.

That line sucked to replace, but I'm glad it happened in my driveway instead of needing a tow.

I've never had an issue with changing trans fluid and I also agree, that if it does have an issue, the trans was on its way out anyways.

Now what you really don't want to do on an old trans is a flush, that will definitely cause some issues.

Transmission could be two stories though. Extra 'grit' in the old fluid could modify the friction and keep a worn trans from slipping when it should have died long ago. Changing the fluid didn't create a problem, it ripped the crusty old band-aid off. Not very common.

On the other hand, a pressurized flush could break loose deposits that otherwise would have been happy where they were. There are now less 'turbulent' ways to flush almost all the old fluid out of trans without disturbing anything. It's best to just replace whatever fluid you can by dropping the pan, and do it often enough that the fluid never gets bad.

Also, mixing different types of fluid can wreak havoc. One fluid's properties may counteract another's. Always be consistent with the fluid type during changes.

SyntheticBlinkerFluid wrote: Now what you really don't want to do on an old trans is a flush, that will definitely cause some issues.

This. People being people (meaning stupid) think that changing fluid = flush. I refuse to flush a new trans. forcing fluid, under pressure, backwards through a system IMO is not hte best option for longevity.

Ive flushed transmissions with all diffrent levels of fluid degredation. The worst was a mid 90s buick a teenager had been gifted by his grandfather. The fluid was tar black, likely the original fluid over 200k miles. 2 years and over 20k miles later the trans still works fine.

I did the diffs and tcase in my truck when i got it and it had 190k on all the fluids aside from the trans and oil. No issues there either.

I change fluids for a living and these tales are why people wait for there transmission too start slipping before flushes, rear ends too be full of metal and making noise before fluid changes.

Bobzilla wrote:

SyntheticBlinkerFluid wrote: Now what you really don't want to do on an old trans is a flush, that will definitely cause some issues.

This. People being people (meaning stupid) think that changing fluid = flush. I refuse to flush a new trans. forcing fluid, under pressure, backwards through a system IMO is not hte best option for longevity.

A modern flush machine does none of what you just mentioned. This is exactly what i mean by people on the internet misinforming people.

A modern flush machine uses a pump that matches the fluid pressure your own transmission pump is putting out.it doesnt move fluid backwards or at higher then normal pressure. Its called a fluid exchanger.

Aircooled and I freshened up the transmission fluid on my wife’s old 2003 Explorer (with 212,000 miles) a few months ago.

When we removed the pan, we found that Ford had installed a nice big donut shaped magnet inside the pan right under the fluid return point. Honestly, we weren’t sure what it was at first as it was covered with so much oily metal dust that it had the appearance, springiness, and texture of velvet. I thoroughly cleaned the magnet (the rest of the pan was nearly spotless) before we put everything back together.

The old school counter guy at NAPA gave us a bunch of E36 M3 about how we’d just signed the transmission’s death warrant and should have left everything alone.

I’m skeptical…we did a good job of not allowing debris to escape…obviously, the magnet is effective at capturing new debris…the old fluid looked pretty good but smelt burnt so I have to think swapping fluid was helpful.

I suspect that selection bias is promoting this theory…it’s reasonable to imagine that people with failing transmissions often swap the fluid in the hopes of a quick, easy fix. When the fix doesn’t work, we’re left with a correlation between swapping fluid and transmission failure and our less than optimal educational system leaves many without the critical thinking skills necessary to differentiate between correlation and causality.

dropstep wrote:

Bobzilla wrote:

SyntheticBlinkerFluid wrote: Now what you really don't want to do on an old trans is a flush, that will definitely cause some issues.

This. People being people (meaning stupid) think that changing fluid = flush. I refuse to flush a new trans. forcing fluid, under pressure, backwards through a system IMO is not hte best option for longevity.

A modern flush machine does none of what you just mentioned. This is exactly what i mean by people on the internet misinforming people. A modern flush machine uses a pump that matches the fluid pressure your own transmission pump is putting out.it doesnt move fluid backwards or at higher then normal pressure. Its called a fluid exchanger.

It would help if the dealers would quit calling it "power flushing the transmission" instead of just "changing out your transmission fluid".

RX Reven' wrote: I suspect that selection bias is promoting this theory…it’s reasonable to imagine that people with failing transmissions often swap the fluid in the hopes of a quick, easy fix. When the fix doesn’t work, we’re left with a correlation between swapping fluid and transmission failure and our less than optimal educational system leaves many without the critical thinking skills necessary to differentiate between correlation and causality.

This. Nobody makes a peep when they swap old fluid for new and the transmission keeps working fine. But when fresh fluid clears out the crud sustaining a slushbox on its deathbed, everyone hears about that.

I swap out the fluid and filter on every automatic transmission i get. Never had trouble.

Same with all other fluids.

dropstep wrote:

Bobzilla wrote:

SyntheticBlinkerFluid wrote: Now what you really don't want to do on an old trans is a flush, that will definitely cause some issues.

This. People being people (meaning stupid) think that changing fluid = flush. I refuse to flush a new trans. forcing fluid, under pressure, backwards through a system IMO is not hte best option for longevity.

A modern flush machine does none of what you just mentioned. This is exactly what i mean by people on the internet misinforming people. A modern flush machine uses a pump that matches the fluid pressure your own transmission pump is putting out.it doesnt move fluid backwards or at higher then normal pressure. Its called a fluid exchanger.

Yes this. I repair and refurbish BG flush machines for a living and I don't see how any other forces fluid backwards through the system.

Perhaps some of it could be in the Manufacturers vs Aftermarket producers of all types of fluids.

I was just reading about controversy over Toyota WS and how some people thought it wasn't a quality product, and others were worried about Valvoline MaxLife ATF not being a proper replacement.

This is how I understand it:

Joe Dumbass has no concept of preventative maintenance and doesn't maintain his vehicle, the transmission starts to misbehave.

"I should have that serviced, which I believe will solve the problem since I never get it serviced."

Neglected dying transmission continues to die.

"The transmission service broke my transmission!"

BrokenYugo wrote: This is how I understand it: Joe Dumbass has no concept of preventative maintenance and doesn't maintain his vehicle, the transmission starts to misbehave. "I should have that serviced, which I believe will solve the problem since I never get it serviced." Neglected dying transmission continues to die. "The transmission service broke my transmission!"

I believe this is pretty darn truthy.

I generally chalk those kinds of stories/myths up to not understanding how the system works. Exchanging fluids reveals existing problems not creates them.

I used to go through that logic all the time with networks overseas (mainly because I liked it when my internet connection worked.) Just because it "works" doesn't mean it's working properly or was setup correctly.

"rebuilt" several transmissions by changing the fluid and filter.

not sure how long they lasted.

The0retical wrote: I generally chalk those kinds of stories/myths up to not understanding how the system works. Just because it "works" doesn't mean it's working properly or was setup correctly.

This applies to all sorts of stuff. "Volkswagens don't have heat" comes to mind, old VWs have great heat if you didn't throw away the thermostat and flaps that choke down airflow to the cylinders and heads when cold which diverts more airflow from the fan into the heater boxes. The engine also doesn't run hot if all the cooling tins and engine bay seals are there and you aren't running 50 weight oil in it that's mostly bypassing the oil cooler because the engine regulates oil temp based on viscosity.

Household wiring also comes to mind, it's insane how many outlets out there have the polarity flipped because it still works. Then there's E36 M3 that will burn your house down that people do like flying splices and stuffing old fusesboxes full of 30 amp fuses.

I would second the not flushing and add not using the BG cleaners. I worked as a warranty engineer for a German auto manufacturer for about 8 years and we saw a high correlation of transmission failures that could have a BG flush performed about 20k miles prior. We would find the rear most hear set extremely overheated/burnt.

With that exception I have heard the same as above for old trans fluid preventing slip but without any evidence. Never heard the brake fluid one but pretty much all fluids are incomlressible so old brake fluid and new in my option are just as likely to pop a line just new fluid has less water absorbed and boils at a higher temp.

In reply to RX Reven':

Every pan I've ever dropped has had a magnet in it. I always clean them off like I was taught to in shop class. The parts store guy is a doofus.

The felt look on the magnet is largely clutch material and ok. When it looks like a porcupine you have issues

I changed the trans fluid on a '00 Mustang (4R70W) I had bought off the original owner. It had 103k miles on it and on the original factory fill. Fluid had a brown tint to it, but didn't smell burnt and the trans shifted perfectly fine. I drained the torque converter (Ford was nice enough to install plugs on them for some time) and dropped the pan and replaced the filter. The bottom of the pan had a good coating of material and the magnet had a nice thick layer of grey goop. I refilled with Castrol Mercon V and had no problems whatsoever until I sold the car 3 years later. It shifted perfectly fine, no shudder, nothing. I've serviced a few other cars with some miles on them since, and no issues whatsoever. There are too many variables to guarantee it, but from what I've heard or read most ppl don't have issues doing this granted the trans behaved nicely beforehand.

As for coolant and brake fluid, those I would automatically replace regardless. Old brake fluid can potentially absorb moisture over time and cause lines to deteriorate and calipers to seize.