Re Installed a rebuilt pump and now the pump starts surging once I open up the flow to the max. Previously it did not.

Pump needs to be rebuilt or replaced, some of those pumps have a replacement surge valve inside, some do not

If you need further help, I’m available over the phone at https://www.6ya.com/expert/michael_2094e122cc46f24c

Remove the cap from the surge tank. Set the heater control temp to max (A/C in any mode EXCEPT max). Start the engine and idle until lower radiator to pump hose is hot. Allow the engine to cool to ambient, and then check/fill the surge tank and replace cap.

Valve stuck open from time to time
Lets keep in mind your talking a 1991
sitting for a few years or not.
Do a cylinder leak down test.
What was the reason the mechanic gave you ?

sound like an air leak, too much air coming in kills engine, try spraying like starting fluid by and seals, or do a compressed smoke test

Now if your old fuel pump was loosing pressure it tends to lean out Overheating your catalytic converter and melting it so exhaust gasses can't flow To test this is easy just remove the two bolts holding it on the engine pipe Start the engine if runs fine replace cat converter

yes it may be, but did you check the radiator for blockage? inside and out side, check to see if there is dirt on the fins. also check the cooling fan operation.
top hose get hot so that meant the thermostat is working ( it's opening ) try to bleed the cooling system again

You have fuel flow but the question now is are the injectors pulsing. Given the year of the truck my gut says your ECU, Engine Control Unit, has failed. Mitsubishi had issues with the capacitors in their early 90's cars, trucks SUVs. I have been thru this with two monteros and one 3000GT, very common. To check your injectors rent a set of NOID lites from Autozone our such. Remove an injector plug and itstall a noid light. Turn the engine over, if no flash, no pulse. Remove the ECU and carefully open it up. Look for the capacitors, there should be 3, look under the capacitors on the board; if there is a grease like substance, often time it has a green hew to it, then the capacitor(s) have failed. It may be possible to replace the capacitors or simply purchase a rebuilt ECU. Be ceratin the MD number matches on the new unit.
Please let me know your results and if this had assisted you.
Thanks, Tom

PROBABLY IT WAS NOT REBUILT OR PROPERLY INSTALLED.

Thank you for posting your question here on Fixya.com.

Have you cleaned or replaced the in-line filter found at the wall socket? How about the pressure valve found just after the wall socket...is it fully open, possibly allowing too much water to bypass the cleaner and return to the pool?

How about the main filter? If overall system flow is reduced because of a dirty main filter the booster pump would be 'boosting' lower than normal pressure so the net result at the cleaner is reduced as well. A cartridge filter is especially problematic here. It can look clean after rinsing it but after enough use the element needs to be replaced to restore original system flow.

Are there multiple return paths for water back to the pool that have valves? Can you throttle down a return to, say a spa for example, to force more water to the dedicated line for the cleaner?

If this answer does not fix your problem, please comment with additional details prior to rating the answer. Positive feedback is appreciated once your problem is solved!

John

Check the fuel pump pressure using a fuel pump pressure guage. It would not necessarily be unheard of to have a new, but bad, fuel pump. No or low pressure, then take the pump back and get another one.