Auto transmission - Change the fluid or let it be?

My mom has a 2007 Civic Ex that she bought new and she rarely drives. The car sits in the garage and it is mostly used as a back up car by me or my brother.

I have been doing most, if not all the maintenance on it and it drives really well. I have done every oil change, valve adjustment, all new shocks/bushings, etc. It currently sits at 240k miles.

I borrowed it yesterday so my son could use it to take the driving test and on my way there I realized I never changed the transmission fluid. Ever.

I have left over Honda auto transmission fluid from when my wife had an Acura. Should I change it? or am I opening a can of worms by doing it at this point?

Transmission shifts fine.

Drain and fills. No more than that. Change a portion of the fluid to freshen up some of the critical properties it could use and dump some of the trash in the pan. 

Personally I would touch it. If it ain't broke(yet)....

How many miles? My VW guru advised me to not touch the fluid in our Jetta we bought with 100,000 on it. He felt that the suspended clutch material in the fluid was only doing good. I never touched it in the next 100,000 and the transmission still worked fine.

Smell it - burnt or brown color not red?  

I would just keep it topped up to proper levels.  

ddavidv said:

How many miles? My VW guru advised me to not touch the fluid in our Jetta we bought with 100,000 on it. He felt that the suspended clutch material in the fluid was only doing good. I never touched it in the next 100,000 and the transmission still worked fine.

i posed the "suspended clutch material" question to my guy at Tremec and he said "the only way that's possible is if the filter is so packed full of trash that the fluid is bypassing it entirely." he also said "You can not hurt a transmission by giving it fresh fluid and a clean filter."

and Bobzilla is correct, drain and fill is the way.

for example, my 2010 Odyssey holds 12 quarts.  I can get 3 quarts out by pulling the drain plug.  So, with pretty much every engine oil change (7500 mile interval), i drain 3 quarts of old fluid and replace with 3 quarts of new fluid.  so my ATF is, on average, somewhere between 22.5K and 30K old.  currently at 250K on original trans.

I've heard, from reliable folks, that have had trouble for changing high mileage fluid (due to the suspended clutch particles) so I know that is real. 
I personally believe in changing fluid in a new to me high mileage transmission, and has always helped, not hurt. So I know that's real, too. Despite being 180 out!!!

But, by 07, and Toyota? Life time trans fluid was a thing; if it ain't smelling/looking bad, and ain't shifting funny, leave well enough alone, I think. 
I never do just a flush. Bad for filters. If you do anything, drop the pan, change the filter. Top off with new fluid. YMMV

I generally leave them alone. My transmission guy says to do one or the other. Either change it on a schedule or never touch it. If it's been 100k on the fluid in it, new fluid could break loose all the trash hiding in corners and cause problems. He won't service a high-mileage auto without evidence of previous routine service. 

Then the drain and fill concept:

my brother-in-law was a Yamaha motorcycle mechanic and he believed in turkey basting out brake and clutch master cylinders with a top off to proper levels. 
 

Right? Wrong? Thanksgiving gravy tasted funny?

This is timely for me as I was just thinking the same thing. Our '18 Mazda 3 is just about 60k miles and 5 years, IE, no more drivetrain warranty. We plan on keeping the car a long time so I feel like going ahead and changing the transmission fluid and brake fluid (possibly power steering fluid as well) at this age would likely be a good idea.

I have done the drain and fill with a filter change on cars as high as 190k that have never been touched and it has only ever had positive effects. Made my dad's "lifetime fill" E39 drive an order of magnitude better.

I would definitely not do a flush on a high mile slush box though, they only seem to cause problems (dislodging all the deposit gunk into the valve body is always what I suspected happens). Drain and fill only.

The believable theory i heard on the "changing high mileage trans fluid will kill the trans" was the new fluid "cleaning" the sediment and debris, breaking bits n pieces of it free and those pieces clogging the filter. 
 

so maybe clean the pan and filter, then after new fluid check/change the filter again?  I imagine if there is that much debris in the trans that its pretty much a goner. 

bobzilla said:

Drain and fills. No more than that. Change a portion of the fluid to freshen up some of the critical properties it could use and dump some of the trash in the pan. 

I am leaning towards following Bob's suggestion above ... just drain, measure and replace what came out. Possibly 2 qts. looking at the size of the transmission.

You guys are killing me being all over the place lol.

This transmission does not have a pan, so I guess there is no oil filter to be changed? It looks like the one in my Acura, in which I would just measure what came out and refill 3x.

At 60k, I'd definitely change it.  It's somewhat overdue, but not 15+ years overdue.

I've heard both schools of thought.  On my own cars, I'm pretty nuts about ATF changes, I typically do a 3x drain/fill cycle and then do a drain/fill once per year (works out to be every 15-20k). However, I'd be worried about opening up a can of worms on an '07 with 240k that's never been changed (especially if it's working well).  On my Saab with 131k, I'll probably do a single drain and fill and see how it goes.  The fluid looks OK, and my guess is that it had been serviced at some point in it's life (I bought it off the original owner).  I only do a drain/fill, never a flush.

My BIL had a '16 CR-V that he bought with 18k on it a few years ago.  It just rolled 165k the other week, and I asked him if he'd ever change the rear differential fluid.  He said he hadn't.  I was at the Honda dealer picking up fluids and asked the interval for his rear diff, and it's every 30k.  Their advice was that if it was never changed at 165k, just leave it.  Their reasoning was the same I saw above about the clutch material being suspended in the fluid might be helping it at this point.  He ended up trading in the car, but it's an interesting debate.

Besides extended oil change intervals, not knowing the intervals on the changes of the other fluids is what scares me the most about buying an older used car.  When buying used, if you can see the service history in Carfax, it's a big plus to me.

Most of the problems come from people taking high mileage transmissions and having them flushed. THAT is a trans killer. The process of flushing with the solvent many of them use will loosen up a lot of varnish that can block tiny passages or clog screens or any number of things. A slow refresh with a drain and fill does not do this. 

ATF has a really hard job and in my opinion one of the hardest. IT's being used as a hyudraulic fluid, a coolant and a lubricating oil for gears and clutches in the trans. Most ATF will sheer when it starts getting older, dropping the viscosity drastically which, with what it needs to be doing, that change will affect a lot of things. Maybe not enough you feel, or happen overnight but it will increase wear and slippage with time and use. 

In reply to Slippery :

there is a filter, its just not meant to be changed. To change that screen you have to split the case IIRC on those. It's not like the paper-like filters in the old TH350/C4 etc. It's just a fine mesh meant to capture debris. With the age, it's likely also holding a lot of varnish. One thing the new fluid will do is to help disolve the free floating varnish again and hopefully increase flow and pressure. 

To the op: you probably know, but Bobzilla would be the advice I'd lend the most weight to 

bobzilla said:

Most of the problems come from people taking high mileage transmissions and having them flushed. THAT is a trans killer. The process of flushing with the solvent many of them use will loosen up a lot of varnish that can block tiny passages or clog screens or any number of things. A slow refresh with a drain and fill does not do this. 

ATF has a really hard job and in my opinion one of the hardest. IT's being used as a hyudraulic fluid, a coolant and a lubricating oil for gears and clutches in the trans. Most ATF will sheer when it starts getting older, dropping the viscosity drastically which, with what it needs to be doing, that change will affect a lot of things. Maybe not enough you feel, or happen overnight but it will increase wear and slippage with time and use. 

In my particular case, would you just take it to a trusted shop and tell them "Just drain and fill, do NOT flush" since it only has 60k on it? We purchased at 33k miles about 4 years and I suspect it was a lease/rental return based on year miles. It was an '18 that I got in June of '19. So it had maybe 1.5-2 years of service before that?

In reply to z31maniac :

I won't lie, I hate flushes. The chances of something bad happening go up astronomically over simple fluid changes. Now, there are certain manufacturers that this is the ONLY option (looking at you audi) as it has to be hooked to specific machines to cycle fluid in and out. But for everything else, I'd look at what the manufacturers recommendations are for time/distance and use that as a starting point. Honda (or at least Acura) recommended 30k mile drain and fills for trans, transfer and diffs. They know you're not going to be changing all the fluid but refreshing it a little at a time. 

Drain and fill, as long as you A.) are using the correct fluid, even if that means spending silly dollars buying unicorn blood from the dealer and 2.) correctly refilling to proper levels are never a bad option. This allows you to refresh the fluid in the trans even if only a little at a time, extending its useful life. 

Do y'all have dedicated measuring cups for your transmission fluid? Seems like replacing the exact same amount that came out is pretty important. 

Datsun240ZGuy said:

Then the drain and fill concept:

my brother-in-law was a Yamaha motorcycle mechanic and he believed in turkey basting out brake and clutch master cylinders with a top off to proper levels. 
 

Right? Wrong? Thanksgiving gravy tasted funny?

As in not bleeding the lines and just swapping out the fluid in the MCs? That would be wrong. The fluid in the lines would remain mostly old, it doesn't circulate (except in some very odd racing brake systems), just moves back and forth a little. I would say you'd get a better result than this if you saved the old MC fluid and put it back in after bleeding the lines with fresh fluid and leaving the MCs near empty.

In reply to thatsnowinnebago :

Cheap plastic gallon pitcher works well. You know it's 4 qts, mark the line, empty and refill with fresh. 

In reply to thatsnowinnebago :

I use one of these from HD

A flush is just a fluid change.

When we do a drain and fill, we find the transmissions total capacity and drain and fill the transmission enough times that we use that much fluid.  IE say the trans holds 7 quarts, when you drain it, four quarts come out.  Refill the trans, run it through the gears, drain it again and refill.  

A flush uses twelve quarts, period, and it does the same thing except a machine (just a diaphragm separating two halves of a container) adds as much fluid as comes out of the trans cooler line.  The fluid leaving the vehicle pushes the new fluid in.

Changing old fluid might have damaged some 50s transmissions but this isn't the 50s anymore.  What wears isn't the frictions anymore, thanks to torque management, and now clutch to clutch engagement.  What wears are the solenoids and valve bodies, and if there is crud in the fluid, it will score the hell out of the bores.

I wonder if people who say OMG don't change fluid ever bother to reset the trans fluid adaptives in the computer, which may also require a shift pattern relearn.

Just another support for using the exact fluid for whatever you are doing. Too many one size fit most trans fluids out there. Whos idea was it to say Dex6 is ok to use for Dex 3? NOPE.