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Here I'll discuss a few of the common service items and some typical service intervals. Many people ignore almost all of these and seem to get along fine, though I have absolutely no idea how that is.

Some people like to brag about how little money they've spent on their car since they acquired it. Most of these people are in for a big shock when stuff does start to break. 

Some people feel like they shouldn't have to spend money on their cars, because every time some little thing comes up they want to sell it and get something better/newer/german/american/bluer/faster/smaller/whatever. Mind you, little things often cost multiple hundreds of dollars to deal with, so this response is understandable to a degree.

Anyway, I want to go over some of the common things I try to deal with on my car. Most of these things are easily handled by the average backyard wrench-turner if he/she has a little time. 

Another important thing to recognize while we're going through these is that often there is a separate and shorter service interval prescribed if the components in question are subjected to what the manufacturer defines as "severe" or "extreme" service. Although the manufacturer's are well within their rights to define "severe" service for their own models, I think you'll see that for most people the "severe" service describes a large part of their typical use. 

Also, some of the stuff I will suggest will be what I would probably call 'preventative maintenance', but it's money/time well spent. The next time you're trying to decide whether you really /need/ to replace that part now while you have everything apart or if you can wait until next year, try to picture yourself stranded on the side of a highway in a snow storm in the middle of the night in the middle of nowhere - if you're honest with yourself, this new perspective should do a lot of persuading.

Let's start with....

It's just a reinforced rubber belt with (usually) square-ish notches on one side. The timing belt drives the cams that actuate the intake and exhaust valves. The belt is driven by the crank and normally tensioned using an automatic tensioning device, either hydraulic or spring motivated. 

Service intervals on timing belts are varied drastically from one manufacturer to another, from one engine to another, even from one application (same engine, different car) to another. You'll have to look up the service interval for your particular car. This should be in the user's manual.

The service interval for my 1998 V70 with B5254T engine is (depending on where you look) 70-80,000 miles which I guess is about 120000 kms. 

For the "severe" service interval it is recommended that the service interval be HALVED in situations where most of the driving is short-trip driving where the engine is being started and stopped often over the life of the belt (ie: most people). Halving is pretty conservative, but I would definitely not feel like I was doing it too often to take 20% off the regular interval. Oil, heat, water, coolant, etc, all shorten the life quickly. It's best to err on the side of caution.

This is typically a pretty expensive job to do as far as basic maintenance goes, as the access to the timing belt in today's transversely mounted FWD cars is very space constrained and there is very little room to manoeuvre. Also, in order to access the timing belt, anything in front of it will need to be removed as well, including the serpentine drive belt and often the crank pulley. If your car still has a mechanically driven cooling fan that will all need to be removed as well. 

It's much easier for the professionals to do this because they have large hoists and can safely get under the car and perform the procedures the way they were intended to be done. This is still a very do-able shadetree job though.

When you have the timing belt off already, it's a great time to check/replace the water pump, whose service normally requires removing the belt anyway (if your water pump is old, it might be worth it put that $80 pump in now for essentially nothing extra in labour, as opposed to doing the timing belt now and then doing the water pump next year, essentially paying for the labour twice). I don't know of any manufacturer's that list expected service life for water pumps. This will definitely not need to be done at every timing belt change though. 

Also, there are a number of components I call the 'rolling gear' which will be different for each engine and consists of automatic tensioners and guide pulleys (pulleys which spin with the belt but don't drive anything). It's also a good time to get this stuff swapped out. I've heard bad things about using aftermarket (not OEM, not from the vehicle manufacturer) stuff here. Again, I don't know any manufacturer's that list expected service life for these components, but this stuff will definitely not need to be done at every timing belt change.

In addition to all that, this is a great time to think about replacing seals. The timing belt runs on pulles which are attached to the rotating stuff inside the engine (cams and cranks) and where those rotating pieces protrude through the side of the engine, a seal is used to keep the oil and grease inside the engine, and water and dirt and other contamination on the outside. Seals also have a finite lifespan, and just like the water pump and rolling gear, everything 'in front' of them including the timing belt must be removed to get at them. Seals are the kind of thing that are normally put off until there is a problem, and as such, they are a great candidate for a cheap way to ensure that you'll get many more trouble-free miles out of your engine.

As I mentioned earlier, you have to take the serpentine belt off to get to the timing belt anyway. It's a good time to replace that belt (it should be a fairly cheap part - esp in relation to the cost of the labour to get all this stuff off and back on again!) and also to check out the automatic tensioner for that belt as well. You might as well replace anything that needs it/might need it soon, though these parts (serpentine belt, serpentine tensioner) are much easier to get to than the timing belt and anything behind it.

Non-interference engines are those with valves that don't cross paths with the pistons. Interference engines are those with valves and pistons whose path may cross (mechanically, as in collision) and cause a lot of damage in the process. I think almost all diesel engines and most modern gasoline engines are of the interference variety.

A broken timing belt WILL cause the pistons to contact the valves, and the engine WILL be ruined. It will cost thousands of dollars to fix it. I don't know how else to tell you how important it is to know when the timing belt was last done and when you need to do it again.

You should have the full story on the timing belt before you purchase a car. If the seller can't offer any proof that the timing belt was changed and when the service was performed, get a quote to have the service done and subtract that from the asking price for the car and GO GET IT DONE as soon as you take possession.