An easier, cleaner way to investigate #2 is to measure the voltage at the ECU where the engine coolant temperature sensor is connected, watching it start off from cold start and drop as the engine warms. The voltage appears at the blue wire to pin 2 of the ECU. Pierce the insulation with a needle, or open the connector if you prefer.

Compare this voltage to this chart. More notes about this appear in the 7/9 FAQ, but the object is to prove the ECT or its wiring is the culprit before disturbing what might be crusty connections under the intake manifold.



The ECT for the 1990 (LH2.4 or 3.1) is black and wired differently, because it has two elements inside referenced to the brass housing and head, but it will work the same as a replacement for your single-element LH2.2 (blue) sensor. Not the reverse, though.

Yes, the chart is labeled for LH2.4, but the resistance curve of the blue sensor you have is the same, and the voltage conversion similar. If the voltage measurement seems daunting, you can fall back to the static resistance readings most do, but for those you must turn things off and disconnect the ECU.



Then again, if you'd rather pick up a wrench than a multimeter, it is a usual assumption a new-to-you 1988 Volvo could benefit from a good cleaning and new gasket on the intake side.

The sensor can be changed with the manifold in place using a 19mm deep socket on a 10" 3/8" drive extension if you take the AMM plumbing out of the way. Not much coolant is lost if you have the new sensor ready to install.
--
Art Benstein near Baltimore

"I lost one dream /It was in mint condition /In my sleep"