[Ed Wright]

There is more to it than simply replacing the coolant temp sensor. I have fixed many like this over the years by cleaning or correcting loose/high resistance connections in that circuit, at both ends. As I mentioned, high resistance = lower than actual temp indications to the ECM. That is the reason for cold soak checking the coolant temp with your scan tool. Trying to work on an EFI vehicle without a good scan tool is like trying to work on a carb with no screw driver.

Another thing I forgot to mention is the tendency of those older TBI trucks to clog (with black. carbon/coke) the vacuum passage to the MAP sensor hose nipple in the back of the TBI, causing lazy response of the MAP sensor. If the ECM is seeing lower than actual manifold vacuum causes it to mis-calculate load, and reference cels in the wrong area of the V.E. Table, and making it run too rich. This is a mileage killer, for sure. A kinked, or leaking vacuum hose to the MAP sensor will cause that also.
I have seen many more low coolant temp indications cause cold start up problems in my shop than anything else.