4L Coolant Gage Fluctuation

[FordsFan]

Hi Folks,

My 1995 Explorer about 6 months ago began to exhibit a temperature gage fluctuation from approx. 1/4 scale to almost 3/4 scale during a 100 mile trip I was on. This was out of the blue, after about a year and many miles of rock solid reading at the normal 1/3 range. About a year earlier I replaced the water pump and thermostat with new coolant and hoses. Recently I have replaced the Tstat twice, both temp sensor switch and sending unit once. I drained and refilled the coolant once, burped the system and all of this to absolutely no avail. My heater will blast very hot as well. When the gage reaches the 3/4 level it takes 5-10 seconds to descend to the 1/4 range but in a few minutes of driving at slow to high speeds it will repeat this iteration. Engine is unfazed, runs perfectly. I'm thinking about changing the T-stat once more. Stumped! This is BTW not my first rodeo nor my first 4L Ford. I have never seen this behavior. Would appreciate some well informed insight here.

 

[slsnut]

Hi Folks,

My 1995 Explorer about 6 months ago began to exhibit a temperature gage fluctuation from approx. 1/4 scale to almost 3/4 scale during a 100 mile trip I was on. This was out of the blue, after about a year and many miles of rock solid reading at the normal 1/3 range. About a year earlier I replaced the water pump and thermostat with new coolant and hoses. Recently I have replaced the Tstat twice, both temp sensor switch and sending unit once. I drained and refilled the coolant once, burped the system and all of this to absolutely no avail. My heater will blast very hot as well. When the gage reaches the 3/4 level it takes 5-10 seconds to descend to the 1/4 range but in a few minutes of driving at slow to high speeds it will repeat this iteration. Engine is unfazed, runs perfectly. I'm thinking about changing the T-stat once more. Stumped! This is BTW not my first rodeo nor my first 4L Ford. I have never seen this behavior. Would appreciate some well informed insight here.

Ok, solved my own problem here. After using my infrared temp reader on the manifold behind the T-stat housing and watching the temp go from 195-245 degrees at idle, I discarded my theory about this being a sensor or gage issue. I took out the second new 195 deg Super Stant I installed and put in one I had for years which was a 180 degree unit that was at least an inch and a half longer on the engine side. Refilled using the same procedure I had used 3 time before i.e. removing the sending unit on passenger side, filling radiator until coolant ran out of that hole then re-install, then burp the rest of the system, rad cap off with vehicle tilted up in my sloped driveway and voilla! Rock solid at 180-185 degrees. The sensor hole is just above the T-stat so this ensures no air is trapped behind it or has to bubble past the jiggle valve in the T-stat, delaying the air purge process.

Near as I can figure is that the longer probe on this T-stat reached further in side the engine cavity to get a more accurate sense of the temp. Nothing else makes sense. Have used Stant units for years, but in this application evidently they are incompatible. Who Knew?

 

[koda2000]

I'd bought a Stant t-stat for my '01 ST and ended up removing it and replacing it with a Motorcraft t-stat (twice the price of the Stant). I guess it's one of those times where it pays to stick with a Ford OE part.

 

[shadow460]

IMO if it's a Ford, there are certain parts that it only gets Ford variants of. The stat is one of them.
My 98 Sport runs between 181 and 192 degrees. At 3/4 of the way up it was over 210. The gauge is set up to stay at a certain point over a range of temps. From 171 to 192 on mine the needle barely moves, but get it to 200 and it shoots up like a rocket.
My voltage gauge fluctuates when the lights are on, even with a new battery and alternator. I'm not worried about it in the least.

 

[96eb96]

These 4.0 OHV engines are very sensitive to any air or t-stat issues. Even a bit of heater core backup could cause symptoms, because the heater core is the bypass (there is no bypass hose on these engines). There was actually a TSB to address coolant fluctuation, has to do with steam formation in the system. There is a special bypass kit that is no longer available. Some have bridged the heater core lines, does not affect heat.

Mine runs from 174-185. Confirmed with the scan tool and the gauge tracks it precisely under the Temp icon. Runs 30-40% of the way between C-H. This is with the Motorcraft Stat.

Pre-97 gauges actually are designed to reflect true sensor readings. I installed a motorcraft oil sender (and a resistor) from older fords and my oil pressure gauge tracks RPM precisely. In 97 they were turned into true dummy gauges. Most people get scared with gauges move so it cuts down on dealer time.

 

[shadow460]

Yep, that scan tool is invaluable.

 

[FordsFan]

Ok, solved my own problem here. After using my infrared temp reader on the manifold behind the T-stat housing and watching the temp go from 195-245 degrees at idle, I discarded my theory about this being a sensor or gage issue. I took out the second new 195 deg Super Stant I installed and put in one I had for years which was a 180 degree unit that was at least an inch and a half longer on the engine side. Refilled using the same procedure I had used 3 time before i.e. removing the sending unit on passenger side, filling radiator until coolant ran out of that hole then re-install, then burp the rest of the system, rad cap off with vehicle tilted up in my sloped driveway and voilla! Rock solid at 180-185 degrees. The sensor hole is just above the T-stat so this ensures no air is trapped behind it or has to bubble past the jiggle valve in the T-stat, delaying the air purge process.

Near as I can figure is that the longer probe on this T-stat reached further in side the engine cavity to get a more accurate sense of the temp. Nothing else makes sense. Have used Stant units for years, but in this application evidently they are incompatible. Who Knew?

Ok, after a 4 hour trip the following day the fluctuation is back! Worked perfectly for first 3 hours on freeway at 70 mph then began small fluctuations and got worse from there. Only this time the fluctuation occurs at lower temps and makes heater temps lower. It ranges from the line just above the C mark and 2/3 of the full travel then drops like a stone. Since this is a 180 deg stat, I'm not surprised about the lower temp. Still in a quandary here!