Couple weeks ago I was driving home from work in about 90F weather so I was running the AC. About 30mins into the drive I had to pull into a gas station to fill up. As I'm waiting in line at the station, my engine is still running due to me needing the AC. I hear a random beep and see my CHECK GAUGES light come on. And to my disbelief the engine temp needed was approaching H. I turned the AC off and that slowly started moving the needle back down to the middle of the gauge. I eventually filled gas and went home. I figured this was a prime case of my electric fan not coming on with the AC. Since the engine was running at normal temp while I was cruising but started overheating while in Park. I didn't have much time to diagnose and the weather cooled off the following weeks so I didn't need the AC anyway.

So then we get to last week. I'm on my way in to work in the morning where the temp is in the 70s. I'm not using the AC in this case but about 10mins in to the drive the CHECK GAUGES light came on and I saw the engine temp needle approaching H. I double checked to make sure I didn't turn the AC on and it wasn't. I pulled over onto the shoulder, turned the engine off and popped the hood. Saw some steam coming from the engine. I checked the coolant level and there's nothing in the reservoir and I see water/coolant dripping underneath the truck. I tried to locate a leak in the engine bay but could not see anything. I waited about 5mins before turning the engine back on and driving to the nearest parts store. Bought a gallon of coolant and poured it into the reservoir. Waited until the needle was close to the middle of the gauge and started driving. Another 5mins later it's back up close to the H. I pulled over again and waited about 15mins for the engine to cool down some. Had to repeat this process every 5mins until I made it to work.

As I'm leaving work I decided to look at some things now that the engine was cold. First I checked the oil dipstick cause I was scared I might've blown my head gasket with driving on an overheating engine. Luckily the oil looked normal, nothing milky about it. Next I checked the coolant level and it was still at the level I filled it up to that morning. Tried to feel around the hoses for signs of leaks but nothing. I started driving home and just like that morning I had to pull over every 5mins to cool the engine back down.

So now I'm at the point where I don't know what part of my cooling system is failing.

• Is it the electrical fan? I was sure this was it but now the engine overheats even without using the AC.
• Is is the water pump?
• Is it the thermostat?
• Is it the radiator? The dripping underneath the truck seemed to be coming from the front of the engine bay. My best guess is that it's leaking from the radiator, onto the front bumper and then onto the ground.

Another thing I must point out is that I tried driving with the heater on blast that morning in order to pull the heat away from the engine, but the heater was blowing cold. It initially started off warm-ish but eventually would blow ambient temp.

I know there's still a bunch of things I need to diagnose, such as confirming where the leak is happening, that'll help me better understand what's going on but I haven't had the time to mess around with it. Figured I would just throw this info onto the forum and see if there's something anyone can think of that I would otherwise miss to check.

Thanks for your help in advance.

 

If you were simply adding coolant to the overflow reservoir, the radiator won't draw that in until the engine is cooling off. If you have a leak in the cooling system, it may not draw it in at all. When the engine is STONE COLD, look in the radiator, see where the coolant level is.

 

It definately helps to see where the coolant is seeping from.
Im about to deal with a temperature gauge that is flucuating.
I think I will be starting with the thermostat as I have no drips.
What engine do you have ? miles ?

 

Yeah, engine details? The overflow tank is less important than pulling the radiator cap and checking & filling that up when it cools off. This should have been done the first time. Don't do this when hot. Aside from hot steam and coolant shooting out, cold coolant going into a hot engine can crack the block. If you want to find the leak, get a radiator pressure tester and pump it up (when cold) to the PSI marked on your radiator cap. This will pressurize the cooling system and allow you to find the leak without the distractions of heat, noise and moving parts.

 

You can also use coolant dye & the appropriate flashlight to find the leak.

 

This makes sense. Didn't think of this in all the chaos. Exactly why I thought to post to get some suggestions that I may have missed. I'll take a look later today if I get a chance.

3.9 engine with ~165k miles.

 

Being an 03 3.9 you should have both an electric fan (comes on at 215 or with AC) as well as a clutch fan (starts engaging firmly around 208). The factory thermostat is a 192 I think, but 180 is a common replacement.

When it's hot, carefully squeeze the top radiator hose to check for pressure. It should be firm; if it's not, it's low on coolant.

These are plastic radiators, entirely possible for them to fatigue over time. A good OE replacement is a CSF Heavy Duty 2-row, #CSF-3516 . While you have it out, changing the water pump and thermostat is a great idea since everything is open and accessible. Water pump is generally due around 150ish before the bearings wear out and weep hole starts weeping. The fan nut is regular rotation I believe, and 35 or 37mm. A couple good whacks on the end of a 12" crescent should break it loose.

 

Pulled off the radiator cap this morning and poured about half a gallon of coolant in the radiator. That wasn't enough to fill it to the top. I'm going to buy more after work to top it off. I'm guessing that'll initially help the engine run at normal temp longer than 5mins. But with the leak ongoing I'm going to lose that too. I'll buy some dye to add as well and see if I can spot the leak.

 

Borrow/Rent a cooling system pressure tester from your local autoparts store. You can check 'em cold.

 

Stopped by to pick up coolant and had the same thought. Picked up a pressure tester and will try it out hopefully this weekend.