The GMC Owners Group

Vero8024 said:

i just got the gmc envoy and the transmission oil looks dirty so its safe to do a trans oil change? the truck runs great

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Yes the transmission should be serviced between 45,000 to 80,000 miles, it all depends on your diving habits and where the Envoy is located. If you do more hyway driving, your transmission has less shifts and fluid will stay cleaner. If you do more stop and go with short runs like less then 45 min, then your going to have to do more service to ensure your transmission fluid stays clean.

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 dangerous.

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. 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? If you check 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.

The fact is, if you do frequent engine oil and filter changes and service the transmission every 45,000 miles there is no need for a flush. I have change my engine oil and filter every 3,000 miles and I don't use fancy oils and filters, and after over 45,000 miles, the transmission oil comes out almost as clean as it goes in. I have regular transmission services and my transmission still shifts like new, well better then new even with well over 365,000 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 Marvel Mystery Oil. The Marvel Mystery Oil has a very low viscosity content that will clean the engine without damaging it. Do this every 600 miles before you service the engine oil and filter 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.

Good luck and enjoy your Envoy.