Here's what I do when looking for a K5:
First and foremost, get the VIN and run a CarFax. Get their monthly plan, it's well worth it.
1. Demand pictures before you even go and look at it. If they seller makes excuses and doesn't take photos, chances are there is a good reason - the truck probably has issues. Save yourself the headache and move on.
The pictures I ask for are:
> All sides of the truck, the hood, the roof
> Door panels
> Dash, gauges, glove box, stereo
> Both front and rear seats, including the mounting frames for the front seats (rust)
> Floors, removing floor mats
> Headliner
> Cargo area
> Tailgate when opened
> Hinges for tailgate, both from outside AND from underneath (rust)
> Inside rear wheel wells, pointing forward (rear passenger kick panels) and backwards (towards tailgate) (rust)
> Engine - top and oil pan (leaks)
> Transmission and transfer case
> Exhaust
> Tires
Yes, it's a lot of photos, but it can save you from making a mistake.
2. When you see it, check everything. All the lights, horn, wipers, radio, A/C and heat, air coming from correct vents, dome lights, gauges, windows, power locks, tailgate window key, all seat belts, seat recline, tilt wheel, 4x4 shift lever, EVERYTHING that performs a function.
3. Inspect underneath for any major rust. Pay special attention to UNDER the tailgate where the hinges mount to the body and in the rear wheel wells for the rear passenger kick panels which rust out.
With your hands, feel for any moisture in the carpet. Check the edges by doors, under the seats, and so forth. Water = rust. Stains can also be telling.
4. Feel the header BEFORE you start it. Is it cold? If not, then the seller probably started it right before you got there. That means this truck will not start in anything other than ideal conditions.
Check the coolant before you start it. Is it green/yellow/orange? Or is it rusty/black/oily?
Check the computer codes before you start it. You can do this easily, here is the list - I just printed out the "generic" section.
http://www.troublecodes.net/GM/
5. Start it. Mine takes 1-2 seconds to fire up. Quickly look for any smoke coming from the exhaust. Smoke means bad valve seals and a valve job for you. Put your hand on the air cleaner or valve cover. Can you feel the engine vibrating? If so, you have a tune up or mechanical issue. It should be pretty smooth at an idle. Smell the exhaust after it's warmed up. You should not be able to smell much of anything. If you smell gas, either the catalytic converter is not working, missing, or the engine is burning too much gas.
6. Drive it. Get it up to 65+ MPH (legally). Does it track straight? If you have to turn the wheel more than 1" to feel the direction change, something is wrong with the front end. Could be an alignment, could be ball joints or a number of issues. Make sure there are no high speed vibrations. Make sure it does not sway or drift or otherwise handle oddly (or unsafely!)
On your test drive, turn the radio off and out the windows up so you can listen for ticks/pops/rattles or other noises that would indicate trouble. LISTEN TO THE TRUCK. It will tell you what is going on.
7. Brakes. Do they squeak? Do they pull? Do they work? Does the pedal pulsate? All problems. Inspect the back side of each tire. Do the tires have little streaks on them? This is leaking brake fluid. Expect to replace parts or experience unexpected wheels locking up if drum brakes. Find a safe place with NO ONE BEHIND YOU and NOTHING AROUND TO HIT and do a 35mpg emergency stop. Does it stop in a straight line? Do any wheels lock up? Does it stop quickly (for a beastly truck!) enough?
8. Accelerate. Does it have good power? Does it shift smoothly? Check the mirror - is there gray (oil), white (coolant/water), or black (gas) smoke? If so, a tune up or major work may needed.
9. A dirty engine is what I call an "honest" engine. If there is a little goop/dust/dirt on it, great - the seller is not hiding anything. If the engine is sparkling clean (and the rest of the truck is not) or it looks like it has been cleaned VERY recently, the owner is probably hiding some sort of leak. Also check the tranny, transfer case, and differential covers for leaks.
10. Do a compression test. While a leak down test is better, a compression test can be performed quickly and easily, and will give you a good enough idea if there is a problem with the engine.
11. Transmission. If it's an auto, remove the dipstick and inspect the oil. Google for a chart of what it should look like (clear = good, black = bad). Does it smell like it's burnt? That is a sign of potential problems. Power steering fluid can also be inspected.
12. Tires. Check the date codes. Google how to do that, it's easy. If you do, you will find out those "great" tires are probably 10 years old. Check for mis-matched tires.
Check for uneven wear or cupping. You can Google tire wear and get some nice guides. That will tell you if it's out of alignment or needs parts.
Also check the shocks. If they are covered with rust, they probably need to be replaced.
13. I look under the hood and dash for missing/added wires. Did someone drill a hole through the firewall to hook up a 12v power line for his CB? Did he use a grommet, and fuse it, or is the wire going to cut through and start a fire? Are there hacked wires? Missing vacuum lines? I prefer unmolested trucks.
14. Lift kits. Many trucks are lifted. Be sure the job was done right. A loose bolt or bad parts can ruin your day. If the kit was not designed specifically for the truck, or was pieced together, there may be a problem with driving dynamics. Be sure you don't have a death wobble situation or the likes. Personally, I won't buy a truck with a lift kit, as I don't need one for my use.
15. Body alignment. Do the doors close without striking the body? Are the fenders perfectly aligned to the body? Or do they have larger gaps or touch the body? The truck may have been wrecked. Bring a fridge magnet and stick it to various body panels. If it falls off, there is Bondo.
16. Odometer. On pre-1989 trucks (I think) the odometer is only 5 digits. That means you will see LOTS of trucks where the seller says "Only 72k original miles!"
Bull! Check the carpet and seats. If they do not look brand new, the truck likely has 172k to 272k miles, probably the later and not the former... A CarFax can help expose that kind of mis-information (I call it lying.)
17. Recent inspection. Does is pass state inspection? What about emissions testing? When did it pass? Is there a sticker/form that proves it? If not, then the truck likely cannot pass inspection without work, which is why it is being sold.
18. Floor mats, dash mats and seat covers. Remove them to see what horrible problems are being hidden.
Personally, my experience has almost always been that the seller is hiding something from you. Hopefully it's nothing major, but usually I don't trust everything the seller says. If he can't prove it with a receipt, form, or visually prove it, assume it is NOT true. If he says the engine was rebuilt 30k miles ago, ask if he has the receipt. He probably doesn't. New tires? Look at the date code. New exhaust? Check for rust.
I find it FAR more informative to look over the truck WITHOUT speaking to the seller at the same time. That allows me to think, concentrate, and find out everything I want to know. Then if I have questions, I will ask later.
I hope that helps.
