It's just got low compression. On VWs, even though the glow plug light doesn't come on with the engine warm, the plugs still get turned on and are on while cranking, so VWs don't have this issue when their engines start to get worn out. Volvo's glow system doesn't activate the plugs at all if the engine is above a certain temp threshold, so it can crank for quite a while when the engine is just warm enough that the plugs aren't operating but it has been sitting for awhile.
That is often what sends these cars to their grave -- won't start when warm due to worn bores. A few days ago I paid $35 for a pristine 1985 744 TD... you guessed it, it starts when cold but if you turn it off once it warms up and let it sit for half an hour it won't start at all. It was ready to go to the junkyard until I snagged it.
If your engine runs OK otherwise, I'd just install a pushbutton switch so you can turn the plugs on when you want to. You can run it alongside the factory relay system using a Ford starter solenoid. Pain in the ass but that's how it goes with these engines.
__________________
86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
|