Hello all, first time on this forum, hopefully someone can help.

Here is the situation. Just recently acquired a 2001 Navigator 4x4 with 130K on it. Had it checked out and all was good, and the thing runs great with no issues other than the AC compressor as of now. Friday was the first time I was pulling my 32' travel trailer, which is 6900 pounds dry, and a little under 8000 pounds with our stuff in it. The capacity on the Navigator is 8800 pounds, with the 5.4 Liter 32V motor, so I am almost 1000 pounds under capacity. The truck was towing PERFECTLY.....it was actually much better than any other truck I have owned for towing, probably because it is so heavy....not sure.

Anyway, about 45 minutes into the trip, driving 65 miles an hour with the overdrive OFF @ 2500 RPM the whole time, the truck started overheating. Pulled over, added a little fluid (the antifreeze was a little low, but not bad....), let it cool off, and started back on my way. There were no noticeable leaks anywhere, and all fluid levels were maintaining. Oil pressure was good, there was nothing else i could see that was making it do this. Another 15 minutes into the trip, it started again. Pulled over and let it cool off. From that point on, I turned the overdrive back on and drove much slower so I could manage the RPM's (to keep it around 1500) and the transmission shifting as to not damage anything. From that point on I was able to keep it from overheating until I got to the campground.

Did some research, and the only thing I could really do at the campground was to change the thermostat, in the event that was a problem. Apparently those are prone to failure in those trucks. So I start back home on Sunday and I drive 65 MPH with overdrive ON so I could keep the truck around 1600-1700 RPMS, and there was no problem, with the temperature in the normal range for over an hour of the trip. Pulled into a rest stop and thought I would try it with overdrive OFF (as I should be towing) - and within 10 minutes of getting back on the highway the temp was danger high so I stopped again and let it cool down, driving the rest of the way with overdrive ON so I could keep the RPMS down again, and all was well.

Here I the things I checked for, and all seem good:

1) Both main lines coming to and from the radiator seem to be close in temperature and normal.
2) Both lines going to and from the heater core seem to be the same as well
3) No leakage in the cooling system at all from what I can tell
4) Brand new thermostat - operating correctly
5) Fan is working properly, sucking air into the radiator as designed
6) Replaced the reservoir tank cap in the event it was letting air into the system
7) Drive belt is in good condition and all functions of the motor are working correctly and withing range from what I can tell outside of the AC compressor (clutch - pulley is spinning freely)
8) All fluid levels are good and maintaining proper levels

This is mystery and I can't get a good answer on this. If it were the water pump impeller fins, I would see overheating in normal driving conditions, at least that what I would think..... I drive distances frequently with the truck and never had any issues at all.

Any recommendations??? Please help!!

Thank you!!

 

I would guess higher RPM would be more coolant moving.


Only thing I can think of is a restriction in the cooling system. (Debris - sludge or??) This would keep the temps the same in the hoses, just the amount of coolant moving would be less than ideal. If the system has not been drained and refilled since you bought it may be it has never been drained and refilled. Now would be a good time to drain and refill. Especially remove the lower radiator hose to get anything laying in the bottom of the radiator out of there.


As a side note: towing that kind of load at lower RPMs may not be the best for the automatic transmission. The automatic transmission fluid needs to be moving to get cooled. Lower RPM does not accomplish that goal.


>>>>>>>>>>Action

 

As long as the transmission is not hunting for gears leave OD on. No reason to spin the engine faster if it is not needed. My owner manual says nothing about disengaging O/D when towing.

I would suspect blocked radiator fins (bugs/dirt/etc.) or the inside is plugged up somehow. Perhaps a PO used that dreaded stop leak to "fix" an issue.