Lists of why maintenance is required (item specific).
I was thinking today, how many times people ignore the maintenance schedules for their vehicles. I thought I'd start a thread identifying specific maintenance items, and the REASON the periodic maintenance is critical. If you have others, post them, and I'll try to keep up the verified reasons in this main post.
1) Radiator fluid - Ethylene Glycol and Propylene Glycol actually don't deteriorate, but the additives get consumed. Basically, radiator fluid protects your engine by allowing heat to be transferred away from the very high temperature combustion chambers. Unfortunately, the properties of the base fluid are corrosive to some degree, and additives are included to protect your engine. For example, the "pink" fluid found in GM vehicles has a long lifespan, but not indefinite. The radiator fluid is actually corrosive to aluminum (think about how many aluminum parts this comes into contact with), and failure to replace the metal protecting additives through fluid exchange results in degraded gasket seals, and leaks. Also, 50% of the fluid is WATER. Water, alone isn't corrosive, but oxidation of parts can occur. Additionally, be sure to use distilled water, to avoid depositing minerals (which impede fluid flow, causing higher temps). There are a few other reasons to replace it as well, but these are the major ones. Follow your maintenance schedule, and if it's been many many years since you've changed it, you'll probably find a fresh batch of fluid will result in lower temps due to better flow (gunky radiator fluid doesn’t move through the system as well. This is also a reason a radiator flush is sometimes a good idea (it helps remove blockages). While you’re not likely to restore your system with a radiator fluid change, at the very least, you're stopping the progression of deterioration that may have already started. Change on schedule!
2) Engine Oil - Probably a million sites and as many opinions around on this one. A quick summary is that oil breaks down due to friction heat, and additive consumption (such as detergents). Oil is designed to suspend particles that would otherwise gunk up or damage your engine. Larger particles are removed from your oil by the oil filter. The small particles in the oil result from blow-by (fuel, vapors, carbon, etc), and from break-down. Dirty oil is doing it's job, BUT it has a saturation point. Oil can only suspend so many particles before they simply stick in your engine. Also, oil has detergents that sop up broken down oil (suspending the particles in the oil, as well). When your oil breaks down faster than the detergents work, you get sludge (simplified view, but it works for this purpose). Regular changing of the oil removes broken down, contaminated oil, and restores the detergents required to keep your engine's internal components clean. Because modern oils have detergents, they’ll even clean out gunk to some degree. Additionally, your motor oil’s capacity to “do the job” is not measured in miles or time (although both can be used as guides for change intervals). Follow your manufacturer’s recommended oil change cycles. Changing sooner is generally not necessary, as oil’s capacity to protect your engine under normal and severe conditions is accounted for in the maintenance schedule.
3) Engine Oil Filter - This filter removes larger particles from suspension in your oil. Good filters have lots of pleats and may filter smaller particles. Cheap filters will have fewer pleats. Removal of particles and material that could damage your engine is the filter's job. CAUTION: Adding engine cleaners, flushes, etc. may very quickly fill up your oil filter. If this happens the by-pass valve engages, and you're essentially circulating junk through your engine. This is also why you should replace the filter when you change your oil. If you have a “full” filter, particles that would otherwise be removed are now circulating in your system potentially doing damage.
4) Transmission Fluid. Transmission fluid breaks down over time due to heat exposure. The periodic and average temperature of transmission fluid over the lifespan determines the actual breakdown speed. Overheated transmissions actually turn fluid into a sticky varnish (depending on the fluid type), often called “Burning the fluid.” As long as you have not “burned” the transmission fluid, changing a part of the fluid actually extends the life of all of the fluid. Most manufacturer recommendations to change fluid by dropping the pan only results in a portion of the fluid being exchanged. This is normal and is accounted for in the manufacturer’s recommended interval. Failure to partially replace the fluid regularly can be mitigated, to some extent with full fluid flushes, though often, a critical aspect (the filter) is ignored. Do so at your own peril.
5) Transmission Filter – This small filter, actually pulls floating bits and pieces from wear and tear in your transmission and torque converter, out of fluid suspension. Just like dirt on your paint can cause scratches when you rub it, particles in the transmission fluid can quickly wear out clutches and other tight tolerance parts. If your transmission filter is clogged, doing a “transmission flush” isn’t going to change the fact that any new particles are going to continue damaging your transmission. Be sure to follow the manufacturer’s recommended schedule. Like an oil filter, this is an integral part in the protection of very sensitive components.
6) Break fluid, power steering fluid, clutch reservoir fluid. These fluids aren’t exposed to sheer forces, which would cause breakdown, but they can become contaminated from particles or wear of parts. Over time, these fluids get dirty. Dirt in the lines makes its way down into the components these hydraulic systems support. Dirt is grit. Grit destroys. Most manufacturers recommend partial replacement periodically to extend the life of your system.
7) Differential fluids – these fluids are of a higher viscosity. Basically, the weight of a fluid (such as 30 weight, or 75 weight) is an oil GRADE (nothing to do with specific gravity, for the science folks out there). The test, simply described, is like this. Take a specified amount of the fluid and allow it to pour through specifically sized holes at a specific temperature. Measure how many seconds it takes for the fluid to go through the holes. 30w oil takes 30 seconds in that test. 75w oil takes 75 seconds (indicating it’s “thicker). 20w oil takes 20 seconds, and so on. Oil thickens when it’s cooler, btw, which is why the temp is specified. Anyhow, differential and gear box oils use these thick, or heavy viscosity oils to protect heavy duty gears that are meshing, from damage. This stuff needs to be thick enough to protect from the grinding. Change according to manufacturer’s instructions OR… when you get water in your differential or gearbox. Water destroys the viscosity and may also cause frothing (air in the oil). Water and air do not make great lubricants, especially in the high friction environment of your differential or gearbox. Frothing also occurs when you overfill, hence the reason to make sure you’re not doing that, either! Heavy duty use (such as spinning a single wheel for 5 minutes trying to get out of a hole) may break-down the oil more rapidly, as well. Oil that is deteriorated from overheating or sheer damage doesn’t protect as well as the fresh stuff. Change on schedule!