91 has no 25 amp fuse in the engine bay, so just listen when you let go of the key. If you hear the fuel pump run for a second after you crank and release, then your relay is okay.

Pull your plugs and see if they are wet after a no start. If so, it's a good bet your ECT, the coolant temp sensor for your injection, is bad. Sometimes you can get it to start with the throttle wide open. Sensor is under the #3 intake runner. If the plugs are wet, sensor is probably bad and you are running full rich at startup and gas fowling the plugs. Change the sensor, blow off the plugs.

If you have no EGR, you can get to the sensor with an extension, a deep 19mm socket and a universal. Tough, but do-able. You may break the old one on the way out. With EGR, still do-able, but requires the patience of a saint. Almost easier to pull the intake in that case.

I had the rough, stumbly bad/no start two years ago and replaced the ECT. Cured the problem. Aftermarket sensor only lasted two years as noted, and failed a couple of months ago. I put in a new Bosch sensor second time around.

DS