Last night while driving on the highway, the temperature gauge kept creeping up until it reached the L in normal. The outside temp was in the 40's and the heater was on. When I would come to a stop, the gauge would quickly move back to the middle position. I have never had a cooling system problem with this vehicle, and this is the first time I have experienced this type of temperature gauge behavior. I will add that the oil pressure gauge has also been reading lower than normal, with occasional jumps back to its usual middle location.

Does this sound more like an instrument cluster problem than an actual overheating issue? I can't see why the temperature would go up while moving on a cold night, and back down when I stop. It seems like that is opposite of what I would see if there was a coolant flow issue. BTW, this is a 1994 with 351/E4OD. It has around 7k miles on an engine rebuild which included a new radiator, water pump, thermostat, etc.

Thanks!
Dave

 

It could be a bad connection on the dash gauges or an engine ground. The lower radiator hose has a spring inside to prevent it collapsing from the suction of the pump when the engine is turning higher revs.
regards
rikard

 

Ok, I checked over the cooling system today and everything seems fine. No leaks, coolant level was normal, lower hose does have an inner spring, etc. I started it up and brought it up to operating temperature and the gauge was steady at the R in normal. Bringing the rpms up would cause the gauge to move slowly to M, but not beyond that. A test of the hose temperatures showed the upper at about 195 and the lower at about 125. The problem last night was only present when keeping a steady speed, and I have not taken it for a test drive yet.

Could a failing thermostat cause a hot-while-driving issue? It seems like if it was not opening all the way, that would restrict flow, keeping the cooling system from being able to keep up at higher rpms (when the engine is generating more heat). This just seems unlikely, since the temperature would drop so quickly once I came to a stop. If the cooling system was not able to keep up, it seems like it would not recover so quickly. I know there were several options for thermostat temperatures, but I do not exactly recall which one I used (195?). What is recommended?

I guess I need to determine if the engine coolant temp is actually getting too hot. I may swap out the coolant temp sensor for a new one, but is there a way to take a reading directly from the sensor to see if the problem could be at the gauge? Any other thoughts or suggestions?

Thanks!
Dave

 

I am going to pick up an aftermarket coolant temp gauge, though is there an easy way to install it in addition to the existing setup? I would prefer to have both, at least while trying to determine if the problem is the stock gauge, sending unit, or it is actually overheating. Would taping the new gauge sending unit to the upper radiator hose give me an accurate enough reading to see if both are following the same fluctuation pattern? Maybe a T adapter to mount both in the same location? I know neither of these are ideal, but this is just a temporary test. Thoughts?

Dave

 

I'm searching for my non-computerized dad who has a similar problem with his 1997 f-250 heavy duty/ 460. It gets hot (or the guage say it does) going down the road. If he puts it in towing mode, the temp instantly and rapidly returns to normal. He has to add just a little coolant to it every once in a while. Shop said it was getting water in the manifold, pulled the heads wihich were fine and put it back together. Problem remains. Now they suspect the radiator.

The top half of the radiator seems hot, the bottom seems cooler. Thermostat and water pump are newish.

Ideas?

 

I am always suspect when an engine appears to suddenly and rapidly dissipate an excess amount of heat. I just don't think coolant temperature can decrease that rapidly.

Over the weekend I dug my way down to the temp sensor. I noticed a couple of small coolant drips, one at the thermostat and one at a hose running to the throttle body. I tightened both of those up and then removed the old temp sensor. As the sensor started getting loose, a little air rushed out before some coolant pushed out as well. I am not sure why there was air there, but if it was trapped there, that may have been the entire issue.

I ended up installing the aftermarket gauge and sensor, which reads exactly as I would expect. Heats up to about 200, fluctuates very slowly a few degrees up or down depending on driving conditions. My oil pressure gauge is also reading back where it used to be, which is a few letters higher than it had been reading for the past few weeks.

I am now sure that the engine was not over-heating, but I am not sure why the gauge was reading off. Trapped air? Possibly, but why would that suddenly occur? The very minor coolant leaks/drips? I doubt it. My best guess is something with the instrument cluster, but I will need to mess around with it to see. If I happen to have it back apart for some reason, I may install the old temp sensor and stock gauge just to see how it is reading.