A close friend who can no longer ride (illness) asked me if I wanted to buy his 2009 Trek Fuel EX 9. This bike has been hanging in his garage for 9 years and was only ridden for a couple of hours after it was purchased. What are the potential problems in buying a bike that has been idle in his garage for years?
Its entirely possible the bike is good and safe to ride with a bit of air in the tires and shocks/forks - this would heavily depend on the climate in the garage it was stored in and condition it was in before being hung up.
Presuming the bike looks in good condition with little or no surface rust, the following are the things that might need attention apart from what a regular service would entail (regular service is things like chain clean and lube,tuning brake check etc.
Tires/tubes - Personally I would be putting on new tires and tubes. They may be OK, but the rubber will have hardened and not be as grippy as tires should be. If they look OK, check them after each ride for a few rides to be sure they are OK.
Brakes - Check they are not seized and work reasonably well. Seals could be gone, so look for leaks around the calipers, Keep in mind if it has not been used for a while, leaks may not show up immediately. Presume the pads and discs have been contaminated.It may just be dust and good clean will fix it, but in a garage that dust may contain oils. Budget for new pads and if needed give the discs a really good clean and put new pads in Depending how its stored, a brake bleed may be needed. If Dot fluid I would suggest a bleed regardless as DOT is hygroscopic (Absorbs moisture) and after 10 years it will probably be contaminated.
Shocks/Forks - Best to give these a full service with new seals and O-Rings. If you don't want to go that far, a basic service is highly recommended. These are expensive bits to replace so its cheap insurance. Check for pitting and corrosion on the stanchions and shock damper body. They will be ridable - for a while - with pitting, but the cost of replacements on such and old bike makes it uneconomic.
If there is surface rust on things like the chain, things like a new chain and new gear cables may be in order. These are low cost items so should not be a concern. Apart from these components, a bit of surface rust largely cosmetic.
The suggestions in the other answers to your question are all excellent points. ,Many relate to the overall safety of the bike which focus heavily on the tires and hydraulic brake system and these should be addressed first of all. Minimally, the tires inspected and inflated to the riding pressure of between 45 to 65 PSI for the stock 26 x 2.25 inch rubber.
The brake system clears my suggested pre-ride inspection when rotors appear planar and are free of damage or rust. The pads are intact, positioned correctly and when the brake lever is squeezed, the pads come into contact with the rotor at the same time. I'd take the time to clean both pads and rotors with rubbing alcohol and use fine grit sandpaper on the pads to prep them. Thouroughly bleed the brakes as soon as possible.
The '09 Trek Fuel EX 9 has left piles of positive reviews in it's wake, features Shimano XT front drivetrain, SRAM XO rear end, and Fox suspension. It cost over $3600 new a decade ago and still holds a good value today. It is a sweet, sweet ride and if the storage atmosphere was even just average....well, I'm excited for you! While 26 inch wheeled bikes are fast fading away, this baby will shine just fine in this sunset of 26ers.
My plan would be to thoroughly clean and lube the entire machine. A bike shop's complete tune up....repacking the wheel and pedal bearings, rims trued, chain cleaned and lubed, made to appear fresh out of the box. Shifter cables and housings would stay if free of damage and rust, but I would dismantle, clean and lube them (Tri Flow IMHO). Lightly wax the frame after a good wash. Regarding the suspension: inspect pivot points and lube--it would take a fair amount of rust with pitting to get me to replace any bolt or bushing right away but they'd shine clean and lubed. Shocks would have service kits budgeted for and put on order. In the meantime, air them up, new oil saturating the wipes, set the sag, pre-load, and fiddle the rebound speed. After 20 or so fairly tame miles of a shakedown ride or two, I'd put that baby to the test collecting Lucky Charms (green circles, blue squares, black diamonds). Enjoy!
Mattnz above mentioned some very good items.
Having gone through this situation, I usually check the tires for cracking problems, and pump up the tires to see if the tube is still good.
Next, I check the chain for problems related to dryness and gunk. Inspect the braking system to see if it even brakes.
Then, see if your friend is willing to let you take this bike to a local bike shop for a check on what needs to be tuned up. Doing this will keep you riding on a safe bicycle.
The biggest potential problem may be the price. If after you pay for all necessary service listed in other answers can end up with total cost around 50-60% of new bike of similar class it would be great thing to consider. However many people would stick to the price they have paid 10 years go and reduce it by 10% because "it is as new". But they do not factor in service you still have to do, and the fact that while some components were top of the line at the time bicycle has been manufactured, now days they are not, and similarly equipped bicycle can be bought for fraction of the price.