[Tryumph]

Same problem, easy cure (at least for me).
This also happened on my '89 Trooper. Although I have the 2.8 liter V6, not the four-banger, it has the same coolant sensors as described here ... and one of them going bad was, indeed, the cause of the problem. The part was less than $15 (might have been less than $10) at an auto parts store.
The Trooper usually wouldn't start, and when it did, it ran horribly. And if it died, it was almost impossible to restart.
The way I finally diagnosed the problem (after repeated attempts to start the Trooper) was to pull the relay for the fuel injection, crank the engine briefly to get all the excess gas out of the cylinders, then replace the relay. At that point, it would usually start again.
As always, whenever removing a sensor on an older vehicle, particularly one in contact with water, it's a good idea to apply PB Blaster or some other penetrating oil for several days before you try to remove it. In my case, the sensor came out (relatively) easily, but you never know ...