Automatic - Trans Flush vs Exchange and smell question

In the debate between which one should be performed,
Mazda 3 2010 2.0 automatic - Owners manual doesn’t recommend a service interval on the transfluid
Mileage is 117k driven it since new, Its about 50% Highway/15% Rural Country roads/ 35% Suburban/City area Find it to be not exactly city due to half the time is spent at constant avg. 50 MPH with occasional stop light and other half is avg. 30 MPH stop sign heavy area.

The transfluid was flushed at 75K, Fluid is brown so I know its about time to have it serviced. Shifting is normal, The only time its shifted rather hard is from 1st to 2nd last winter in subzero temps. My question is to get a flush or just exchange it. I know flushing forces more of the old fluid out but can damage the trans(or so they say), and an exchange more old fluid remains. Doing it myself is not an option unfortunately.

And just a curious question to smell I could best describe as fishy/metallic that comes off of the front/right of the car right after its been turned off for a couple minutes. The smell seems to originate somewhere by the right side of the radiator smelling it through the grill area. Coolant is changed regularly and hoses are fine, note that my sense to smell is rather strong (Can smell another cars driver smoking a cigarette sitting across stop sign). Nothing seems out of the ordinary besides the smell, cars driven many trips and miles since I recognized it. Just was wondering if anyone had any insight on it. The only thing I seem to dislike about the car is a lack of gauges, I don’t like having to rely on the idiot light to tell me I’m running hot if something were to ever occur.


Does this transmission have a pan that can be dropped and cleaned (and maybe even a filter that can be replaced while the pan is out)?

If so, is there a reason you’re not pursuing that option?

@JoeMario That’s what I was referring to by exchange, basically is it better to get the pressurized flush or to avoid that when getting it changed.

Thanks for that clarification. There is something referred to as an exchange machine, which apparently uses less pressure than traditional flushes. I can’t comment on the validity of that claim.

Whenever you go with a flush,

  • the filter in the pan never gets replaced (or cleaned).
  • the flush pressure dislodges the sentiment in the pan and causes it to circulate, where it
    can cause valves in the valve body to hang up. Then you have big $$$ troubles.

Also, many flush machines use “one-size-fits-all fluid” or additives which are supposed to allow fluids to be used in multiple transmissions. Don’t believe it. Too many transmissions today have very specific requirements and don’t fare well with those generic fluids.

Pickup any auto trade magazine which advertises transmission flush machines. The overwhelming pitch in those ads is about how a flush machine can help shops boost their profits. There’s a good reason some shops push them so hard.

@JoeMario Sorry to be unclear with that, but that sounds about right with the shop as it pushes their flushes on everybody. Going to look into somebody, or location that will drop the pan method.

Since Mazda lacks anything on it, what interval should it be replaced at in your opinion?
Planned on about 40-50k from here on out. The shop with the flush says 30k.

Those flush machines enable shops to charge the customer considerably more while delivering considerably less. It’s a “smoke and mirrors” machine. I feel confident that anyone who has serviced a few automatic transmissions found the black residue and steel dust after the pan was emptied. Cleaning the pan requires some effort using proper solvents. Flushing cannot remove that residue.

I agree 100% with Rod Knox since I never recommend transmission flushes. The reason is simple: The proper flush equipment is out there but it must be operated by a qualified technician. Finding both of them at the same location is a “perfect storm” which means it’s very rare. The “one size fits all” transmission flush machines are scary because transmissions vary greatly.

If a pan drop is done and the filter is replaced and then the tranny is filled with new fluid, it only replaces 30% of the actual fluid capacity of the transmission. The rest of the burnt tranny fluid is held in the torque converter, valve body, and tranny cooler.

So you’re not replenishing the transmission with new fluid, you’re contaminating the new fluid with the burnt fluid.

A tranny fluid exchange machine replaces all the burnt fluid with new fluid.

How this is done is, the inlet cooling line from the transmission to the radiator is disconnected. The machine is connected to the cooling line and the radiator. The engine is started and the pump in the transmission starts pumping the burnt fluid out into the machine while machine introduces new transmission fluid back into the transmission. This continues until the transmission fluid coming out of the transmission is the same color as the new fluid going in.

From the condition of the transmission fluid, you want to drop the pan and clean it, replace the filter, and reinstall the pan. Then have all the transmission fluid replaced with the exchange machine.


The problem is when shops use the machine without dropping the pan and replacing the filter.

@Tester, wouldn’t you do the exchange first, then drop the pan, change filter, then top off with fresh fluid? That way, your not running burnt crap through a fresh filter. I’m just asking.


All the particulates are sitting in the pan. You want to remove the pan and clean it to get rid of these particulates before doing the exchange. That way the new filter doesn’t get contaminated with the particulates.

Also, if you drop the pan first and find metal particulates there’s no reason to do the fluid exchange. The transmission has bigger problems.


Here’s something I’m curious about.

For years, dropping the pan, changing or cleaning the filter, and refilling every 30-50K was fine. (More often if towing was involved).

Then flush and exchange systems became available and so did the pitch “A flush or exchange is better because it replaces all the trans fluid instead of 30-40%.”

What made “drop the pan, replace filter, refill” no longer acceptable? If it isn’t, that’s new to me.

If you use a flush or exchange machine without first dropping the pan, then 30 to 60% of the fluid is in the pan and the machine sucks the fluid from the center and pushes fresh in from the side, so you are not removing all the fluid from the pan, simply diluting it continuously.

If you drop the pan first and clean/replace the filter and then refill with fresh ATF, then use the exchange machine, fresh fluid is drawn up through the filter into the valve body and torque converter, old fluid goes out to the cooler where it is intercepted by the machine and replaced.

If you do a drain and fill every 30k, you do not get all the fluid, but you don’t really need to. You are freshening up the fluid before it breaks down too much. As long as you keep to the schedule, the ATF will never get so bad that it causes damage. It won’t ever be brand new, but it won’t be totally worn out either, always somewhere in the middle, where it spends most of its time anyway even if you flush often.

Another alternative is to drain and replace the trans fluid 3 times with a short drive between each fluid drain. This substantially reduces the amount of old fluid remaining in the system. Honda recommends this practice for my 2005 Accord V6.

The only people I know that claim you only get 30% of the fluid with a drain and refill are the ones selling transmission exchanges or the machines to do them.

On any of my cars or vans, either dropping the pan or pulling the drain plug gets me about 50% of the stated capacity of fluid out.

I do it by mileage and don’t wait until it looks dirty and have never had any tranny problems on ant car I bought new or with less than 60000 miles on it.

The change came when transmissions started to be electronically controlled.

Instead of having a mechanically controlled spool valve in the valve body to control line pressures, that’s been substituted with computer controlled solenoid valves.

These solenoid valves are sensitive to any kind of gum, varnish, or contamination. So letting the transmission fluid deteriorate to the point where it turns to a brown color is when it’s time to do a
fluid exchange to get all the burnt fluid out.


Well we are not really in disagreement then, I just never let my transmission fluid get to that stage.