I've got a '99 RAM 1500 sport 5.9L and I'm having to refill the overflow bottle to the "max" line about every 2 days. It appears the leak is on the passenger side at the timing cover to block junction just above the ignition coil...as near as I can tell.

I replaced the waterpump and radiator about 10k miles ago.

I verified the truck has the factory plenum and appears to be leaking some oil inside. I ordered up a Hughs repair kit today. Would the leaky plenum have anything to do with the coolant leak issue or is that totally unrelated?

 

Totally unrelated

 

I'm not sure of your plenum. Just relaying what happened to me when I did my water pump. After several miles a of driving I noticed anti-freeze leaking. Not as much as you stated, but it turned out I didn't get my bypass hose right. Had to tear everything back off to fix. But hey the all the bolts came off easier!

 

Check all the hose clamps and make sure they are all tight. It's possible one could be leaking but that's exactly where mine started to leak.

 

If you're leakin' at the thermostat cover, when you replace it, don't use any additional sealer. Other than a new thermostat gasket, put 'er on dry n torque ta factory spec. I've been doin this on old style V-8's for almost 30 years now without any leaks. An old school mechanic taught me that 1. I'm guessin' the coolant causes the porous gasket to swell n seal.

 

borrow a coolant testing kit from advance auto, pep boys, autozone, oreillys, and place the system under pressure and look for leaks.

a common leak point is the the gasket between the timing cover and block.