Lukewarm heater output

5.3 in an ‘88 XJ-S, lukewarm heater output. Back flushed heater core, flows garden hose fully on with heater valve still in circuit. Engine temp consistently at bottom of “N” on gauge. New rad, hoses, thermostats (88C), coolant. Same temp output from heater whether in auto on manual full heat. Both heater hoses feel the same temperature, but cooler than rad hoses.
Anyone had experience with the coolant rails clogging or thermostats not blocking off bypass ports causing this?

I had a somewhat similar problem, my issue was related to the heater valve.

After I had drained the coolant there was an air pocket in the heater core, and even though the hoses seemed to be the same temperature it was mainly due to engine room heat. My solution was to hold the valve closed for aprox. 20 seconds (in the up position) and then let go, hoping that would build up enough pressure and thereby ‘flush out’ the air bubble and resume normal circulation.

Already been through that; new heater valve and flushed the core and hoses/ tube twice, then bled multiple times. There is just not a lot of flow. I want to verify that both thermostats are closing off the bypass tube when hot; if they don’t, flow would be reduced. I may try a circulation pump…some SUVs use them for rear heaters.