Strange overheating problem....

It just started yesterday, and the again today.

1998 Dodge Ram 1500 5.7, it only overheats in the mornings, it has been cold lately, 28-32 degrees, it runs fine on the highway, a little warmer than normal, but not bad. Come off highway and the temperature really goes up. On the return trip home 50-60 degrees, still runs a little warm but does not over heat.

Its not low on fluids and I run a 50/50 mixture for radiator fluid.

New radiator six months ago, and I am changing out the thermostat today. It does not have an electric fan

My other thoughts is it could be water pump related?

Any other suggestions/thoughts?

5.9 right? Being a 98? I would suspect possible fan clutch, or junk in front of rad, but you just had that out....

The thermostat is a good place to start.
Test the freezing point of your antifreeze.
Be sure the radiator is full, don't trust the overflow bottle, there may have been, or maybe still is air trapped in the cooling system.
Make sure the AC condenser or whatever cooler might be in front of the radiator isn't interfering with the air flow through the radiator. bent fins or dirt accumulation in the fins will cause your problem. Gene

How is the heater, good hot heat?

Thanks guys,

"5.9 right? Being a 98? I would suspect possible fan clutch,"
Yes 5.9 (lol) I was think fan clutch as well. over 300,000 miles and never replaced.

Poorboy, I have checked all toughs things Thats is what leading me to believe that the thermostat may have gone bad.

Rhinodart, How is the heater, good hot heat?
Good point, I will check it out.

I didn't get a chance to pull the thermostat yesterday, will do so today. It just seems strange to me that it is worse during the cold mornings then the warmer afternoons.

Air pocket? After changing the rad, did you jack the front way up or park it on a steep incline to burp it?

Failing that I would pressure check the cooling system before replacing the fan clutch. Pressure losses through even something as simple as a leaky clamp can all sorts of issues.

Sorry for the late reply guys, I got called away for work.

I pulled the thermostat and tested it, works fine. pressure tested they system and radiator mix (my friend Al, let me borrow his testers), no leaks and fluid is at a 50/50 mix. Checked the inside of the cab for leaky heater core, no leaks. It really had me scratching my head.

Decide to check all the other fluid levels. Pulled the oil dip stick....down well over two quarts, not good. Decided to change the oil and filter, drove it three days no overheating, So, sometimes its the things you are not looking at that cause your problems. Second lesson, start checking the oil level more frequently.