300M 3.5 serpentine belt replacement

What about the other belt?

Nope. 15mm combination wrench and a 13mm socket, swivel and ratchet is all that's needed.

The tensioners are jack screws that pull the idlers tight, go at them from the bottom. Loosen the bolt that goes thru the center of the idler pulley with the 15mm, back off the jack screws with the 13mm.

Remove the upper radiator crossmember for access - a little of a PITA, but not as bad as it looks. It is advisable to replace the tensioner pulleys (with bearings) periodically. They do go bad - usually making noise before anything bad happens.

Also - I don't know if you considered this or not, but the timing belt must be replaced around 105k miles - not an option unless you want to risk damage to the engine (piston/valve interference). If that happens to be due, consolidate some of the labor by doing the t-belt, t-belt tnesioner, water pump, and the accessory belts and their tensioners at the same time.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

What is the purpose of this kind of setup? Do any V8 engines go through this?

Ken

The car is a garage queen. Just cracked 20,000 miles on the odometer. Shouldn't have to worry about the timing belt for another decade or so. The wife puts about 3k miles per year on the car. Steve

Ken, Which particular aspects of "this setup" are you asking about? The way the belts are tensioned, or the water pump being integrated into the t-belt drive line, or the accessory belts being hard to get to?

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

Bill,

Sorry for not specifying. What I should have asked is about Interference Engines. Seems like there are plenty of them for six and four cylinder engines. What is the reason for designing an engine like this?

Ken

Ken,

When an engine is designed, there are power gains to be made by increasing compression ratio. One of the determining factors that goes into achieving higher compression ratio is minimizing the volume (air space) captured in the combustion chamber (defined as the space enclosed by the top of the piston, the exposed cylinder walls, and the cylinder head (with valves). This same space that you are trying to minimize also gets invaded by the valves as they open (projecting into the combustion chamber). If you move the piston up further to reduce the volume, it moves closer to the valves that are trying to open as far as possible for good breathing. So - bottom line - when optimizing the performance, along with optimizing the shape of the chamber boundaries for proper flow for performance and emissions, you eventually will hit a point where the valves and pistons hit if they are not synchronized (i.e., timing belt breaks). Normally (with good timing belt), that is not a problem as the valves are closed or only partially open when the piston is all the way up (at "TDC" - top dead center).

The manufacturer can do things like machine divots (reliefs) into the tops of the pistons to allow higher compression while making room for the valves to be open with piston at TDC, or add more valves per cylinder to allow good breathing with lower valve opening height - but such measures can add cost and/or degrade emissions.

So, basically, everything is a compromise. The manufacturers bet that there is more gain to their bottom line by giving higher performance, while minimizing their costs even if it increases maintenance costs or risks of damage (which is probably a safe bet since 90+% of consumers are ignorant of the risk anyway) - the old "What the customer doesn't know won't hurt them" philosophy.

Bill Putney (To reply by e-mail, replace the last letter of the alphabet in my address with the letter 'x')

Sounds like what Ford did to their customers with Piston Slap. I won't be purchasing another Ford Expedition again. I wonder how many customers Ford lost because of this and if they even know.

Thanks again for another great explanation.

Ken