Hard starts
In addition to the glow plug issues mentioned above, hard starts can be caused by air in the fuel system. Again, seeing how the truck starts when cold AND hot are key. A truck which starts right away, runs for a few seconds and dies, has air in the fuel system. This can be repaired inexpensively, but requires time and talent. A truck which requires extensive cranking to get started may have a glow plug problem, or a major air leak. Be wary. A HOT truck which doesn't restart after sitting for 20 minutes or so is suffering from injection pump heat soak, and is probably going to need a new injection pump. So, after that first extensive test drive, come back 20 minutes later and restart it to see! Check the level of fuel in the tanks. Most all trucks run good on a full tank, but only the ones with healthy fuel pickups run down to the E mark. If the tanks on the truck you are looking at are full, it may have a bad pickup sender. After all, who sells a truck with full tanks! Try switching tanks, and make sure the fuel level changes. Maybe the other tank is pretty low, and you can at least ascertain if that tank has a good sender in it.
Road test
When you start an IDI, it should fire within a few revolutions of the crank. Low batteries, bad cabling or a weak starter will not spin the engine fast enough to obtain a quick start. When the IDI's are first started and the water temp is below 130 degrees, the cold idle advance and cold high idle should be on. After starting, depress the pedal slightly untill about 1200 RPM and release. The high idle should maintain around 900-1000 RPM. During this time, the cold advance will be on and the engine will have more clatter than when it is warm. Allow the engine to warm up, and observe that the high idle kicks down. Depending on ambient temperature, this may be 5 to 20 minutes. This would be a good time to walk around the vehicle and check the lights, exhaust system, and look for leaks. Bear in mind that the factory gauges are notoriously bad, and poor indicators of actual engine conditions. Nominal readings for factory gauges are usually mid center position. If the previous owner has installed aftermarket gauges, and they work correctly, your in luck. A pyrometer (exhaust temperature gauge or EGT guage) is an invaluable gauge, and one which you should install as soon as possible, if not so equipped. If the truck has one, idle temps are generally around 260 degrees. Check the parking brake. A lot of owners ignore these, which isn't good. You may want to ask the previous owner if he used it. If he didn't, assume it is bad and it may stick on when you press it down. The brake cables may have rusted up. Applying it now, may render you unable to drive it, so evaluate that at a more appropriate moment. When you are ready to drive the vehicle, adjust the side mirrors so that you can observe the exhaust smoke as you drive. The smoke should appear as follows under the various driving conditions.
Normal cruising should be light haze to no smoke
Lugging should be brown or black
Normal acceleration should be light haze to dark smoke*.
Against compression should be no visible smoke
Glow plugs.
As previously mentioned, glow plugs are a major concern on the 6.9. The reason - the 83 to 86 glow plug controller design causes the plugs to burn out when it fails. If the glow plugs are Autolight brand, then your probably looking at pulling the heads to get them out. The 1987 6.9 has the 7.3 glow plug system which is not a problem.
