Ok, this is my first post. Please help, I am out of ideas. My 2004 Ram 4.7 L is running hot at highway speeds (around 235-242). I have replaced the thermostat twice, (boiled both to make sure they opened), new fan clutch and fan, new radiator, had the water pump checked, checked "dummy gauge" with shops diagnostic (to see if gauge was bad.)

After all of this the temp still gets hot when driving on highway. The truck will run for hours with heat or AC on in driveway or around town, but as soon as it gets to highway speeds it starts to get hot. 3/4 mark on gauge. I am hoping that it has a air in the system but I doubt it.

If anyone has any ideas please let me know I'm at my wits-end. Hopefully someone has had the same problem and got it fixed.

 

Im sure you have checked the coolant level by now

All you have in the cooling system is the rad, fan, t-stat, water pump and heater core.

You have replaced the rad, fan and stat. You have checked the pump. If you have heat then the heater core is fine. AC blowing cold at idle means the electric fan is also working.

I hate to say it but the only thing you have left are the heads and head gaskets. I cant think of anything else.

 

Yep, Highway speeds is when it should be most steady. How about a compression check? It probably won't show anything, but might. I saw a hydrocarbon test where they stuck a strip of treated paper in the overflow tank to detect a cracked head or blown head gasket. I don't know where it was available from but it is a shot. Good luck...

 

I don't know how you checked out that water pump but I would take it off and check the impeler maybe its not turning properly at high speeds or pushing enought water at higher speeds. Allso if you have mixed the wrong typs of antifreez it will become thick and cause over heating.

 

I've thought about the heads, but it doesn't completely overheat, just runs at 230-240. It stays steady at that range, it doesn't smoke, the engine sounds great, and there is no water in the oil or oil in the antifreeze. So hopefully thats not the problem.

I talked to a mechanic today and told him about my problem. My new radiator doesn't have a radiator cap or fill hole on it, and I don't have a pressurized resevoir. So I'm taking the old radiator to have it boiled and put it back on. He said that the new one can trap the air in the system and not allow it to get out at all without a pressurized resevoir.

I am also going to check the coolant temp sensor today to see if this was the problem. I've seen cars with bad heads/head gaskets and I just don't think that is what this is. But I really appreciate the help. Please continue to give advice because I'm sure this will be two more failures today.

 

Ok another update... .I took the truck for a drive today and got it to read hot on the gauge, I took an infrared temp gun and checked the temp on the engine. At the outlet hose where the upper radiator hose attaches to the engine it read 211, at the lower hose it read 154-175. I checked it at various places across the front of the engine and couldn't find a spot anywhere that read higher than 211.

I was wanting to know if I was looking in the right spots or if there is an exact place I need to take a reading. (for instance where the coolant temp senors are) So anyone who knows the location of the sensors let me know. Also if there is another place I need to take a reading please respond. I'm hoping its just a faulty sensor, after I have spent nearly 500 on the truck.

 

I have a similar problem with my 2004 3.7L . I also had similar readings with an infrared gun. The temp of lower hose was 150. So far I have changed the water pump, thermostat, temp sensor, radiator cap, both hoses, coolant and had the radiator checked. I am beginning to think that although the temperature sensor is sending the proper value to the PCM ( Powertrain Control Module ) , the PCM is not sending the proper signal to the gauge. The only way to check this is with Diagnostic Readout Box at a dealer.