No EGR on the '89 M103 Federal engines.

Coolant sensor at back of head? Two thermistors, one for the engine computer and the other for the ignition module. Can be checked with an ohmmeter to ground and the resistance compared to values shown in the sensor curve graph. Example: +20 degree C = 2.2K-2.8Kohm, +80 degree C = 290 - 370 ohm.