Looks like you've got good advice so far. Where are you measuring your fuel temp? It should be between the lift pump and the IP to get the best measurement for the temperature of the fuel being injected.

I'm not exactly sure where you have your coolant tapped, but here's what I would advise...you may already have yours set up like this. Put a T in the coolant line where the coolant exits the head and goes into the firewall (to the heater core). The coolant from there goes right into your FPHE, then the heated filter (if you have one), then the heated fuel lines, then the tank, then to another T in the line near the aux pump behind the passenger-side headlight (you can put it just upstream of the pump and wire the pump to have key-on power to help you have better flow...I have no aux pump and get good fuel temps and good heat in winter at idle).

A faulty monovalve shouldn't affect your VO temps much at all, your VO loop should be in parallel to the heater circuit. If its stuck closed, all the coolant will flow through the VO loop, no prob...if its stuck open, it'll act just like the heater is on and will take a little longer for the fuel temp to increase, but it'll eventually get there. Mine works fine in the winter with the heater blowing.

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now