so im looking at a 1991 cummins and was wondering is there any specific things i should look for or ask about?

 

Check blowby. Also dont buy one that someone else has modded.

 

I agree with 901stgen, check for blowby, and try to stay away from one that has the smoke screw turned in and the collar broke or the fuel screw turned in, thats a good sign of "abuse". Also try to get a 91.5 with a intercooler, but check it over for leaks. Oh and check the turbo for excessive shaft play.

 

Above all that follows, if you don't trust your own judgement, TAKE SOMEONE WITH YOU THAT KNOWS WHAT THEY ARE LOOKING AT!
A GOOD inspection takes at least an hour.

The LEAST thing that would concern me is if the injection pump has been tweaked on, but another sure sign of possible abuse is the HITCH and frame.
If it has a gooseneck, it's likely been pulled, possibly HARD.
A gooseneck and a clean, straight bed, with no dents or bows in the tailgate means people were careful, it also likely means they didn't abuse the truck.
If it has a receiver style hitch, look at the framerails for bowing or even repairs.

Front suspension also tells the story on how it's treated, get it to a shop you can trust and have them check the suspension, upper & lower balljoints, tie rods, pitman arm, idler arm, bushings, bearings and brake components.
All of this can be checked without removing a thing.

You can also do it yourself, with a simple heavy floor jack under the lower control arm.

Do a basic electrical check, high-low headlights, horn, turn, brake, blower motor, wipers, hazards, running/parklamps, dashlights.
Also look at the condition of the wiring in general, does it have a lot of repairs or add-ons? Splices? Under dash have a "rat's nest" of unknown wiring leading to nowhere?

Of any ONE thing that can determine buying one of these older trucks, the wiring is usually the most overlooked, yet possibly the most critical thing to look at.

Transmissions are a money pit, manual, automatic, it doesn't matter.
If they don't work, they aren't cheap, and if the word "Diesel" is attached, you WILL bend over.

A good cue to condition is, manual or automatic, if it "shifts funny", it's likely problematic.
If it is obvious it has leaked, it's already a big problem.

A Getrag that is quiet with the clutch disengaged, yet has a rattle if the clutch is engaged in nuetral, is likely suffering from bearing failure of some sort, if it "shifts funny", it needs rebuilt.
EVEN a throwout bearing isn't a "SIMPLE" problem.

An automatic that hesitates beyond the normal filling of the convertor (typical convertor drainback issue), has very dark and acrid-smelling fluid, won't shift into overdrive (if equipped w/overdrive), or is erratic shifting, likely at least needs major adjustment, if not rebuilt.

As far as the Cummins goes, high mileage engines will have blowby to varying degrees, but it doesn't mean the engine is junk.
Even an engine thoroughly coated in oil isn't necessarily a problem, it's WHERE it's coated and if the oil is fresh, or mixed with dust/dirt.
Higher up, oily/dusty grease, or "CAKE" just means that the valvecovers seeped over time and got a buildup of dirt and grease, a simple blowoff of the dirt/grease and new cover gaskets (along with a valve adjustment) will usually be sufficient.
I'd MUCH rather see a "CAKED" engine than a wet, oily one.

Lower oily residue on the driver's side tends to mean a diesel fuel leak, or, possibly a case leakage due to gasket, or dowel pin failure, which is time and money intense.

You should EXPECT a turbo to possibly need replaced if the vehicle has over 200,000 miles.
Some radial play is expected, just look for scrapes on the housing, or worn vanes on the compressor and exhaust sides.
A '91.5 (Intercooled 91) will likely have had an injection pump already, there was a recall on them, so it will most certainly have a replacement on it.

I have only ever seen one or 2 Cummins trucks for sale (That weren't to be bought for parts) that didn't fire off in a couple turns when I went to look at them.
BOTH had bad batteries.

The best trick in the world for selling a vehicle is HOW EASY IT STARTS.
It's usually the FIRST thing a potential buyer gets shown in a potential sale situation.
Don't fall for it. Inspect it. Carefully.

Caveat Emptor.

Mark.

 

Mark has some good info, but I think the main thing i would look for is the condition of the drive train. I bought my truck, and 4 or 5 months later the rear end blew up. Kinda my fault cuz I kept driving and it went 2 days before my rebuild kit came in but ya, drivetrain and electical stuff. My wiring situation is a mess, but everything but the cruise control works