OK everyone, So I have a 99 ram 1500 4x4 with the 318 engine. My truck sat for 5 days while I was on vacation and when i came home today and drove it the water temp gauge would rise and fall. My gauge usually sits in the 175 to 190 range. Tonight when I drove it, it would go from 175 up to about 200 and then slowly drop back down to about 175 and then slowly rise back up to 200. When i left for vacation it was about 20 degrees and when I got back it was about 50 degrees. Coolant was low, I topped off the coolant and went for another drive and it did the same thing again. There is no coolant in the oil and no noticeable coolant leaks in the engine bay. I plan on changing the Thermostat first and then moving on from there. Any ideas what it could be? Thanks in advance

 

Sounds like the thermostat is working though...... try burping the cooling system again, and try and find out where the coolant is escaping to.

 

sounds like its still low on coolant. let it sit and idle with the radiator cap off for awhile and top it off as needed. also fill overflow tank to max with coolant.

 

I had that problem my thermostat was going wack. Put a new one in and everything was fine.

 

So this morning, i filled my radiator up through the cap while idling and then filled the overflow tank up to the max line. I let it idle for about 20 minutes to warm up and then went for a drive and continued to do the same thing with the gauge fluctuation. My truck isn't smoking bad and also the oil level is good and isnt milky so that can rule out a head gasket issue. Any ideas other than the thermostat?

 

My guess is that there's still a little bit of air in the system that's keeping the temp sensor from contacting the coolant, giving you the screwy readings. Sometimes it takes a lot longer than you think for air to get completely purged. I'd just give it a while longer to sort itself out. If you notice that at some point the coolant level has dropped in the radiator, that might be an indication that an air bubble has finally worked its way out, and that your gauge problem has gone away.

 

Mine did this same thing after I broke the coolant temp sensor when I did my head gasket job. I put the new sensor in and new antifreeze, but didn't purge the system. It took like a month for all of the air to work its way out. With the truck cool, park on a hill, open the cap and top off the radiator and let it idle for 10 mins and get the air bubbles out of the system. Now, that it's purged, the gauge stabilizes at 195 degrees.

 

Maybe your rear freeze plugs are causing it to slowly leak antifreeze, which in turn is causing air pockets in you system. Just food for thought.

 

tonight, I put in a new tstat, purged the system again and my problems were solved...thanks everyone for your advice

 

Glad we could help - I hope that's your last mechanical problem in 2011!