Flushing out oil-filter-change myths

Q : I have a 2001 Chevy Silverado extended cab, 4x4, 5.3litre engine with an automatic transmission, 330,000 kilometres mileage. At around 150,000 kilometres I took the truck to my backyard mechanic and requested the transmission oil and filter be changed (first time).

He said he would change the oil but not the filter, as a new filter may allow an increase in oil pressure in the transmission and damage seals.

Fast forward to 330,000 kilometres. I park the truck in my garage and it seldom gets colder than -8 C in the garage at any time in the winter. I start the truck and let it warm up for a few minutes and when I put the transmission in reverse there is a delay of 10-20 seconds before the truck will begin to back up at idle.

This symptom is new this winter and doesn’t happen after the truck has been driven for a few minutes. I have checked the transmission oil and it’s still red and does not have a burnt smell. The fluid is at the full mark.

I have been told by several people that you should never ever change the transmission oil in high-mileage vehicles.

On several website blogs and forums it appears that 75 per cent of the people say if you change the oil and filter, your transmission will blow up within 2,000 kilometres and the rest say it doesn’t matter. Some say to put in a transmission additive, but pretty much all of them say never get the transmission flushed at that mileage.

Can you provide some advice in this situation?

Tom

A: Let’s dispel some myths first. Changing the filter when you change transmission oil will not increase the fluid pressure inside the transmission, nor harm it. If the filter was so plugged that it restricted oil pressure then your transmission already has serious mechanical problems inside.

Next, it is OK to change the oil in high-mileage transmissions? Many people have transmission problems such as you are experiencing with reverse gear and try changing the oil to correct it, but it doesn’t.

The problem isn’t with the new oil or filter but with some fault already in the transmission that causes the failure. Note, most vehicles now do not need routine transmission oil changes.

The oils are better and so are the clutch materials. However, if you tow or haul heavy loads, the manufacturers usually do recommend transmission oil change intervals.

Regarding transmission flushing, this process puts high detergent cleaner through the transmission and it can cause varnish or sludge to loosen. This can sometimes cause a valve to stick in the transmission valve body and the transmission doesn’t shift correctly.

I recommend you avoid flushing unless it is part of a complete transmission overhaul and is used to clean debris out of coolers and lines.

Additives can be useful for older transmissions. The seals can harden with age and do not seal as well. Transmission additives include a “seal conditioner” that will soften the seals, but they will not correct broken seals or worn clutches.

The delay in reverse engagement of your transmission could be caused by low transmission oil pressure or a problem with the seals on the “low/reverse” clutch piston.

If oil pressure is low, it may be a faulty pressure regulator solenoid or sticking pressure control valve.

These can be corrected by removing the oil pan and valve body, but if the clutch piston seal is faulty, then it is time for a transmission overhaul.