Frost plug locations for 1991 gmc 3500hd

Should be under the passenger side dash.Silver square box with3-4 plugs.

Ok, may i suggest that you check your HEATER COIL, you can located it by pulling up " HEATER COIL" for your specific vehicle, this is what your coolant travels thru while hot and air passes thru this coil to produce the heat to warm your vehicle and typically the heater coils are located on the passenger side. Now as far as the leak location, check heater coil connections it could be the coil itself. GOOD LUCK!

There is no dip stick on some of these models, to check the transmission fliud level you will need to check from bottom of the car and to add there is a red cap on the top of the transmission.

there is a small cap bolt on the passenger side of the ****** located in the area of the axle going into transmission, the car needs to be at operating temp and lifted level with engine running remove the plug and fluid should drip out, if not add fluid with plug out a pint at a time until fluid starts to drip out . note add a pint and shift in all gears and back to park and check for dripping fluid if dripping install cap. it is full.when adding ,after each pint run shiffter into all gears first. good day

The drain on your evaporator core housing is most likely plugged. Condensation on the evaporator core drips down into the plastic housing and drips onto the ground through a hose on the firewall on the
passenger side.

Find the small hose on the firewall in the engine compartment. You should be able to pull off that
hose and see if its stopped up. If the hose is ok, you may need to remove some of the ac ducts
under the dash to get to the evaporator core housing to unclog the drain hole.

I have no doubt that thats the problem.

u have a clogged evaparator core drain... u need to have the car put on a auto lift an have the small drain hose for the evap box the hose is located in the engine compartment on the passenger side fire wall

the drain tube for the drip pan under the condenser is plugged up. the drain tube is located outside the fire wall on passenger side of truck. (small black rubber hose). you can take a long plastic tie strap and insert into the hose to clear any trash out of the line without any damage to the hose. hope this will help!

on the rear of the engine on passenger's side connected to the upper radiator hose.
these honda had problem with the idle control valve, it stuck open cause the engine to surge

passenger side bottom of engine bulkhead/firewall, small tube, u will have to crawl under the truck to see it.

A cracked Intake manifold. Check the manifold in two places, on the front cross over behind the alternator and the base of the heater hose connector on the rear of the passenger side.

i think you mean freeze plugs, any way there will be three on each side of engine block below exhaust manafolds. to get to the one on the right side clear to the rear of engine, the starter will have to be removed. if its the one in the middle, motor will have to be jacked up and motor mount will have to be removed to get to it. also there are freeze plugs on back of engine block, behind flywheel if one or both of them are leaking trans. will have to come out to replace them. also chk. the rear of intake were intake, and head, and block come together, same in the front could be leaking there also. as far as your truck not wanting to start i will need a little more info to help with that, like is it gas or diesel? if its gas does it have spark? can you hear fuel pump run and relay click? i will try to help the best i can but i need the info.