I have posted this problem in the 87-96 f series/bronco forum, but will post again here to hopefully shed some more light on the problem, here it is

I have a 1995 f150 ext cab 5.0 with 4 speed auto w/ electronic overdrive. the truck always ran right on the "N" in NORMAL on the temperature gauge. I drive the truck about seventy miles each day and it has always ran at a consistant temp. One day, took about 15 miles to warm up as opposed to the usual 5 mins in the drive way. Once it warmed up the temp kept fluctuatin between the N and the L in normal. Ford says this is perfectly fine, but with 120k miles on it and this being the first time it has done this, i do not consider this normal at all. here is a list of things i have checked/replaced but it is still fluctuating. It will heat up when i am in the throttle and cool down when I have my foot out of the throttle.
- checked gauge, grounded went to Hot, disconected went to Cold
- replaced temp sending unit for gauge, not computer.
- replaced thermostat and checked that it open at 195 before installing
- replaced fan clutch
- replaced radiator cap (13lbs)
- coolant was flushed about 2 mos. ago and appeared very clean when drained for T-Stat.

I do not know what else it could be. really driving me nuts. Whem it is running on the Hot, the upper hose reads about 160 with infered thermometer, when it is running Cool, it reads about 120 the coolant is actually warmer ,but that it the temp of the hose.

any help would be great

Thank you in advance and sorry about the length of the post.

 

What is the condition of your water pump? I wonder if it is starting to cause cavitation.

 

i just replaced it tonight and the old one did not look that bad at all. i replaced it because i was doing the radiator and figured it would be a good idea. plus it got a nice deal on the rad. 100 bucks for a 2 core.

after all said and done, the temp still fluctuates, i can not figure this out for the life of me.

thanks for the help and ill keep you posted.

 

I offer a few thoughts, somewhere your line is losing continuity and causing the gauge to wander. These can be tough to find since a little bit of corrosion is all that it takes.

Second, using a set of gloves, hold the upper radiator hose after it warms up. Flow should be uniform and not surge. You are looking for fluid flow through the hose - not burning your hand.

Finally, a leaking head gasket can cause havoc with the temp gauge, it just depends on how much combustion gas is being vented into the system.

 

i dont think the head gasket is to blame since i am loosing no coolant and smell none being burnt. i will run a pressure test on the cooling system tonight when i get home.

the upper hose appears to have uniform and even flow. one concern i do have is that it builds up some pressure before the t-stat opens. is this normal, i have never noticed it before on any of my vehicles.

as far as the line lossing cont. i dont think it is because i tested the gauge and it checked out fine. Hot when grounded, cold when disconected.

last night i put the old temp sending unit back it. now the gauge stays much closer to the "N" but still moves around a little. the new unit when installed read 108 ohms if resistance. the old unit at the same engine temp read 194 ohms. this would explain why the new unit held the gauge much higher, but wich one is correct. the engine was not run for about 6 hrs and it was 40* out so i think the engine was 40* as well. is there any other way to check which reading is correct?

thanks for all of the help.