Hello everyone, I turned to here because I am trying to figure out what is going on with my 2003 Navigator. It's got the Ultimate 4x4 package, if that helps... Anyway, I had it overheat once a year ago climbing a mountain but chalked it up to air in the system from a recently ruptured heater hose. No issues until a few weeks ago when I was towing my brother's Chevy Cobalt on a Uhaul dolly. I got about an hour into towing it on the highway and the temps slowly climbed until I overheated. I use an OBDII reader and an app on my phone to monitor it. I was in the middle of Idaho (near Arco, so all desert) and it left us stranded. Got a ride into town and new coolant and water, then after filling it back up we were good for the next 4 hours until we got home. Granted, it was dark by then and nice and cool.


So this weekend, I had to go with my brother to Bozeman, MT to get the rest of his stuff (he's moving back to Boise). We had some issues going there but nothing too bad. Coming back we had a good sized hill Near Norris, MT that I though would give us trouble, but the hottest it got was 199º. We had maybe 1000 lbs behind us (trailer and stuff on it) and maybe another 7 or 800 inside the Navi. After the hill it went back down to around 190º and stayed steady until close to the Idaho-Montana border, which you slowly gain altitude. It climbed slowly and would never cool itself off. I stopped when it hit about 220º and it would cool itself off by idling. We went through this three more times over the next 6 hours, but also had about three hours where it was a steady 190-192º, including where I overheated the last time near Arco.


Here's what I at least kind of know... It would maintain the temp for hours and it was never a sudden climb in temps. Usually it would take about a half hour to get high enough for us to stop. I was towing with Overdrive OFF, so my RPMs were between 2 and 3 on most flat surfaces. It was obviously running hotter when it was warm out but the first time it did it near the border it was low 70s. We ran the heater on high and windows down and there were times the heater was blowing cool air despite coolant temps being 220º. I did have a badly plugged heater core but flushed it about 6 months ago and have had no issues with it. My thermostat is about a year old, and again, have had no other issues, only when towing (minus that very first time). My coolant mixture is pretty close to 50/50 and is pretty new stuff. I also tried as best I could to spray out my radiator, but I don't know how clean it got.


Any ideas on what to check? Part of me was thinking maybe my fan clutch isn't working right. My brother thought the fan sounded fine, but I thought it seemed quiet, like it wasn't running all the way. It was just so random that it was hard to diagnose, and I don't tow much so I have only dealt with this twice now. I know from the factory their supposed to be rated at like 8300 lbs towing capacity, and I was only towing about a 1000 (plus the extra weight in the Navigator itself). Any help would be greatly appreciated. This is normally my wife's DD and she gets stressed when stuff goes wrong (I have an A/C compressor going out and a power steering pump getting replaced this week, as well as an air suspension leak somewhere). I'm slowly chipping away at these problems but this one takes priority... Thanks! and sorry this was so long.

 

Strange that the heater was blowing cool when the coolant temp was elevated. Sounds like there may be a circulation problem. Is the water pump moving the coolant?

When it is fully warmed up, shut it off and go over the radiator carefully with your infrared heat gun looking for hot/cool spots. The radiator cores may be plugged/restricted. and/or the impeller on you waterpump may be corroded. Just saying.......

 

Maybe it's the temperature sensor itself acting weird. That cool air from the cabin heater sounds fishy... but a temp sensor reading high shouldn't cause that so I might be "blowing hot air" so to speak...

 

What engine do you have - 5.4L 3v ?
When you overheat are you dumping coolant out the overflow bottle?
Is the overflow bottle cap the OEM Ford cap?
The original Ford caps have a small pressure relief spring which is affected by heat more than the new large spring version.
The pic is the newer large spring cap - Motorcraft RS-527.

 

pdqford: I'll have to do that with the infrared heat gun and check for hot/cool spots. Maybe this weekend I'll take it to some of the hilly areas near me and see if I can get it a little warm and check it then. I have been wondering if the radiator itself is plugged and not getting proper circulation.


meeker: We were going to test that sensor while this was happening but I guess you're supposed to do it cold, and we were far from it so I will give that a shot. It's cheap enough to replace if it isn't testing quite right.


R&T Babich: It is the 5.4l 3v, and I do have the factory Ford cap on it. I've been kind of wondering if it could be cap related. When it overheats (230º+ temps) it kind of sprays out from around the cap. A few times when it was hot, the overflow bottle was near the top, and I know for a fact that with a completely cool engine that morning, I was in the "cold fill" zone.


Thanks for your replies!