So I have a 2002 Dodge Ram 2500 5.9 Cummins. Recently I have been having problems where it will run at normal operating temperature but when I start to slow down it will warm up. When I accelerate I hear a water rushing sound coming from the passenger dash side. I was told to replace my heater core and this could fix it. So last night I bypassed it and the problems are gone. I was loosing coolant but my floor was not wet or damp but when I would turn on the heater my vents would blow out a bunch of smokE/vapor that smelt like coolant bad. Am I wrong to think it’s only heater core problems or should I check something else.

 

Not sure what you mean exactly... Any engine temperature fluctuation is not related to the heater core. The heater temp could be though depending on blend door function. But all that said, any smell of coolant from the vents is an indication that the core is leaking.
I highly suggest you research the involvement of this job because its VERY involved. Per the FSM instructions you have to evac the AC system, pull the entire dash out, and remove the HVAC box. Crazy that people actually do this...
But there are plenty who havent either as you can simply pull the dash back enough to reach the core and cut the lines and replace it. Some refer to it as the short cut method...but it works. Here's a link to a thread about it: https://www.dieseltruckresource.com/...3hours-229112/

 

the reason I think the temp has to do with it is cause when air gets in the system it can cause the temperature gauge to read wrong. So if air is getting in at the heater core I was thinking that could be the problem. Since I bypassed it I have not had the temp problems. And I have researched it. As well as I’m going to school for heavy diesel mechanics and I am in an hvac class that I could do the evacuation in. We have the machines for it.

 

Read this>> https://www.dieseltruckresource.com/...3hours-229112/

 

My heater core finally went out and I pulled the dash et al as listed above, not a big deal if you take your time and realize the FSM isn't exactly very complete as to schematics etc. My truck had 4 bolts holding the hvac in place, the manual only shows 3. 20 year old plastic gets pretty brittle and you need top be careful and support everything to avoid damaging hard to replace parts. Took me about 8 hours total.
My core was leaking down the middle and drawing air into the system. The interior would fog up at shutdown and I got tired of smelling the sticky sweet coolant smell so I finally pulled the trigger on it and replaced the dash as well.

Been a year, no temp, heater problems since....