Get it scanned for the codes.

If it has a 14 pin ALDL and cavities "A" and "B" are occupied,
you can jumper those two and count check engine light flashes.

GMC Jimmy  - YES - The 94 was the last year for the old square body
design and was still OBDI Look under the dash around the steering
column. Find the 12 pin DLC connector. Using a paper clip, small piece
of wire or a commercially available "code reader", ground terminal "A"
to "B". There should only be five (5) terminals in this connector, four
along the top and one on the bottom. Above the one on the bottom (M), is
terminal A and B.

After grounding A to B, turn ignition key ON, do not start engine. The
MIL will start to flash. One flash, short pause, two flashes, long
pause, subsequent flashes. This is code 12 indicating the computer
diagnostics is working. It will repeat two more times for a total
of three code 12 flashes, then any stored trouble codes will display,
repeating each 3 times. Code 12 will flash again when all codes have
been displayed. Once you have the code either post it here OR go to

http://www.troublecodes.net/GM/ for a rundown of the codes and what they
mean.

GMC S/T - MAYBE - This was the year of changeover to the new body style.
That vehicle could be OBD I, OR OBD II depending on build date.

GMC Full sized - Yes - These were OBDI until 95

Count the pins on the ALDL connector
12 pins = OBDI
16 pins = OBDI.5 or II
OR look on the vehicle emissions label under the hood. It will tell you.

Most common failures - CPI, Fuel pressure regulator, Fuel lines (Nut
Kit), EGR (gets blocked open by carbon), Intake gasket leaking into
engine on the ends of the manifold.