1989 W350 running cold in Missouri

[762frmafr]

So, a little back story. I bought my truck in South Carolina. When I drove it down there, temps were around 70-80* F. The truck has an aftermarket temp gauge that shows actual temperature. Driving in SC, the engine would get to 180* or a little past then you could see the that the thermostat was opening cause the temp would drop to 120-140*. Then it would raise to 180* again then drop to 160* then fluctuate between 160 and 180. So I drive the truck home the 700 miles. Everything was fine. Couple hundred miles later it is now winter in Missouri and the water pump goes out. I replace the water pump and radiator cap and fill back up with a 50/50 coolant mix. Now in the morning it takes a long time for the truck to warm up. When it does get to 180* the thermostat opens and the temp drops below 120* to almost 100* and takes forever to get back to 180* if it even does. Once the thermostat opens it usually just goes back up to 140 or 160. This is even with 2/3 of the front of the radiator covered with cardboard. Should I replace the thermostat? If so, where is the best source for a OE Cummins thermostat? Also, I have been doing some searching and came across this... http://www.mishimoto.com/dodge-5-9l-cum ... 94-98.html I am thinking about that thermostat, but I don't want to have to change it again come summer time. Opinions? Suggestions? Will it hurt anything not coming up to temp?

[762frmafr]

I broke down and bought an OE Cummins Thermostat. Called the local Cummins dealer and gave them the engine serial number. The thermostat they sold me was part number 5292738. Haven't put it in yet but since I didn't get a reply on this thread, I guess we will see if it works.

[dazedandconfused]

I've had horrible luck with the parts store thermostats on everything. My Dodge has one and it gets warm just not warm enough.

[ellis93]

I've never bought an aftermarket thermo that worked in my truck and I went thru 3 different brands. The last and final thermo was cummins.
All the aftermarket stuff had the wild temp swings until you drove it for an hour or so,or were towing. Really annoying.

[762frmafr]

UPDATE: The above mentioned Cummins Thermostat has cured the problem. I put it in and bled the system of air and topped it off with coolant. Took it for a test drive, and with the radiator still 2/3 blocked with cardboard, the needle is STEADY at 160* after opening.

[dazedandconfused]

Good to hear.

[762frmafr]

As a side note, the heater blows WAY hotter than it did before.