It depends how often the service has been done. Usually if it hasn't been done in a long time and there's no problems they say it's best to leave the old fluid in it. Sometimes changing the transmission fluid can cause new problems. You should have it checked at a transmission shop.
〉 Answered on Feb 23rd, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines
Check out our expert's advice!
Here is a great article from Autotechrepair:
Engine and transmission flushes are being sold left and right, but are they a good idea?
A New "Recommended" Maintenance Item...
In the last few years you were probably asked, or told, by you
dealer or quick lube place that you need an engine or transmission
flush, because the engine oil or transmission fluid is very dirty. They
will tell you that it is recommended that you have it done because your
engine or transmission will last longer if it is flushed clean. In that
they are correct, a clean engine and transmission will last longer. But
is flushing the best way to get a dirty engine clean?
What Is An Engine Or Transmission Flush?
Flushing is the high pressure forcing of fluid back against the
normal flow of the fluid. In other words if the normal flow is left to
right, the flush would force the fluid right to left. This is
accomplished by connecting a machine that will force special solvents
back through the engine and transmission. The idea is that by forcing
cleaning solvents backwards through the system, it will get all the
junk and garbage that has formed over time and "flush" it out of the
system. In theory this may be sound, but in actual practice, it's
The Dangers Of Flushing...
Flush machines do what they say; they force high pressure cleaning
solvents back through the engine and transmission and clean out some of
the accumulated junk that has formed. Now engines have small passages
and galleries through which oil or automatic transmission fluid flow
and there are one-way valves that keep the fluids from backtracking for
whatever reason. By using an aggressive cleaning procedure like
flushing, large chunks of accumulated sludge are broken off and forced
backwards through these galleries and valves and, more often than not,
lodge tightly and block them. This cuts off the normal flow of the
fluid and causes lack of lubrication in an engine and abnormal or no
shifting in a transmission. The results are expensive repairs, or more
often, engine or transmission replacement.
Who Recommends Flushing As Maintenance?
The shops that want to sell you the engine or transmission flush
charge anywhere from $49.95 to $99.95, not including a new engine or
transmission. Those are extra. And they state quite emphatically that
it is recommended that it be done. But who actually recommends that it
be done? I checked with GM, Ford, Chrysler, Nissan, Honda and several
other new car manufacturers and not one recommended an engine or
transmission flush as routine maintenance. In fact, they specifically
don't recommend it at all!! The new car dealerships that do sell them
use the implication that since they are the dealer that it must be the
factory that recommends it. And if they do say the factory recommends
it, they are flat out lying to you.
The only ones who do recommend flushing as a maintenance procedure
are the companies that sell the flush machines and the shops that buy
them. The flush machine manufacturers state quite clearly in their
operating manuals not to use their machines on "high-mileage vehicles".
That simple statement proves that flushing is not a safe procedure. It
also absolves them of any responsibility of any damage that may occur
due to the use of their equipment. This leaves the shop wholly
responsible for anything that happens and the cost of correcting the
damage that occurs.
I know this since I recently appeared as a witness in a lawsuit
where a person was sold an engine flush that destroyed his engine.
The fact is, if you do frequent engine oil and filter changes and
service the transmission every 15,000 miles there is no need for a
flush. I have customers that change their oil every 3,000 miles and
they don't need to use fancy oils and filters, and after over 100,000
miles, the oil comes out almost as clean as it goes in. They have
regular transmission services and their transmission still shifts like
new, even with well over 100,000 miles on it.
If you have neglected regular oil changes and you want to do some
interior engine cleaning, get the oil and filter changed and replace
one quart of motor oil for one quart of transmission fluid. The
transmission fluid has a high detergent content that will clean the
engine without damaging it. Do this every 3,000 miles and you will
clean the inside of the engine slowly and gently.
If you do get a flush, I recommend you do it when you can afford to replace the engine or transmission.