No heat in 98 jimmy. Replaced water pump,

Have you backflushed the heater core? Check to see if both heater hoses are getting hot and if not flush it if possible. NOTE: most modern cars run hot water through the core at all times, but direct the inlet air through the core to supply heat by use of flapper doors usually operated by vacuum actuated devices. Vacuum leak, bad check valve to vacuum reservior, or bad actuator can prevent any heat.

heater hoses run from the rear of the block to the water pump. One hose into the heater should be hotter than the other because heat is transferred in the heater in the car. . The cooling system draws water from the bottom of the radiator ( water pump inlet)which is always cooler than the top hose and dispenses it throughout the engine and heads including the heater core. It is then pushed past the thermostat with all the heat from the engine to the top of the radiator where it is cooled down by the air passing through the radiator . Check that the air vent for the cabin is set to recirculated cabin air and not fresh air from the outside as it will require recirculated air to increase in temperature to work properly

sounds like the air blend door actuator motor is stuck on cold position.

The brown looking chiffon could be oil in the water. However, if you are sure the system is ok then I would check the actuator door motor on the heater and make sure it is allowing the heat into the car. Seems as if you have done as much as you can.

Flapper door inside duct work most likely stuck open not allowing heat to come thru the heater core.

Flushing the heater core is fairly easy just remove the 2 hoses going to the heater core and use a different hose if you have to and use a garden hose to backflush the blockage out..this may take as much as 5-6 times also check for vacumn lines missing or disconnected or breaks pinched ect... some where in the vicinity of the passenger side under hood by the a/c lines ect. good luck hope this helps

The heater guard actuator is located under the Dash area. It sounds as if you covered most all of the issues regarding system procedure... It could be as simple as one of the vacuum lines have come undone or the cable has slipped .

do the heater core coolant supply hoses both get hot at the firewall in the engine compartment? if yes then you have a defective temperature control blend door actuater motor.

Remove and raise upper hose at thermostat end and remove lower rad hose.
Fill rad with a hose and watch for flow from bottom rad hose and/ or bottom rad hose connecton.......water should fall though rad easily.
Honda rads are cheap.....any restriction....replace rad.

Remove thermostat....fill thermostat housing with hose and watch for water flow at bottom rad hose.
Flush good.

Remove heater hoses.......flush back and forth until a good flow is present "With heater control set to hot" if no or poor flow....replace heater core.

Stop leak can plug everything.

Also.......not likely causing over heating....but ....at your mileage,if you do not know if the timing belt has been changed.....CHANGE IT NOW!
Water pump is optional at this mileage.

I had problems with a similar truck,a 1998 GMC diesel after checking to make sure the control cables were operating the blend door and blowing the mess of the heater core,I replace the thermostats.Still no heat turns out there was a small amount of debris collecting in the fitting for the inlet hose at the thermostat housing after removing it and blowing out the lines the heat was fine.