Temperature Gauge fluctuating - Ranger-Forums - The Ultimate Ford Ranger Resource


2.9L & 3.0L V6 Tech General discussion of 2.9L and 3.0L V6 Ford Ranger engines.

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Old 03-08-2015
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Temperature Gauge fluctuating

Ok Sorry if this thread has been posted before but i need to figure this out. SO today i bought a 1996 Ranger 4x4 off Craigslist, the truck ran great only thing i noticed was that the temperature was below the half way mark. I drove the truck back up to Jacksonville at the end of the trip i noticed that the temperature gauge was almost on the C, when i got off my exit and was stopped at the light i noticed that the gauge was climbing almost to the hot mark. When my light turned green the temp gauge leveled out but was fluctuating. I was thinking about replacing the thermostat, radiator, upper and lower hose and getting a new temp gauge direct, or order a new instrument cluster. Any advice or suggestions will help.
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Old 03-08-2015
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Recommend you don't replace all that stuff.

Normal for Range temp gauge after 10 min warm up would be just below 1/2, it might go just above 1/2 if hauling a trailer or on a long uphill grade.

Running 1/4 or below after 10minutes usually means thermostat is stuck open or someone put the wrong t-stat in, stock t-stat needs to be 190 to 195degF.

That is where I would start, I would also replace the temp SENDER as they are $6-$8.
3.0l will have TWO temp readers on the engine.
A TWO wire ECT Sensor, only used by computer
A ONE wire temp Sender, only used by the dashboard gauge

You want to change the 1 wire Sender

If you want to test the gauge and its wire, find the temp sender, disconnect it's wire.
Turn on key, needle should be at or below Cold
Use a wire to Ground the Sender wire, needle should go all the way up to Hot
If this happens then gauge is OK.

Also test the fan clutch.
Engine cold, start engine and run it for 1 minute then shut it off
Pop hood and spin the fan
It should spin easily with a good pull/push
After driving for 15-20min, temp gauge near 1/2, test again, should not spin easily


If thermostat is stuck open then engine can't warm up so gauge would stay 1/4 or lower, especially with high air flow thru rad and in engine bay from driving at highway speeds.
When you stop the air flow stops, so less heat can exit the rad fins, the fan is suppose to take over then and pull air thru the rad for the extra cooling.

Fluctuating temp could just be based on air flow thru rad, you would know better than me on that, it isn't instant but if gauge always goes up when stopped then I would say fan clutch is partially to blame.

Could be eaten away impeller blades on water pump, how does the coolant look, fresh or old.
Fresh could mean it was VERY old before putting vehicle up for sale, lol, if truck looks well maintained then that's probably not the case.

Last edited by RonD; 03-08-2015 at 06:39 PM.
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Old 03-08-2015
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i bought off the guy and he said the MAF went bad and replaced i looked at the truck and it smoked for a little, and i said no and left. A week or two later i see it on Craigslist again so i went to look at it again and it wasnt smoking at all. I checked the oil no chocolate milk color put a paper up to the exhaust and had him rev the engine and nothing came out so i thought to myself that it cant be a headgasket. on the drive back i would notice the temp gauge sit at 1/4 or less on the freeway, when i got off the freeway the temp went past the half and then the light turned green and when i gave it gas the temp went down. got home and shut it off and head a bit of bubbling but not a lot. Sorry for the life story
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Old 03-08-2015
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After engine was cold did you check the coolant level?
Was it full?

And what about the overflow tank, what was the coolant level in there after engine was cold.

Next time you are driving and engine is warmed up turn on heater to HOT, Fan high, defrost.
When you come to a stop and gauge climbs does heat get hotter or cooler(air in the system)
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Old 03-08-2015
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i checked it this morning and it looked low in the radiator, and in the over flow tank it was at the half way make were the clod line mark is at.
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Old 03-09-2015
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Then there is a pressure problem.

Modern coolant recovery systems are pretty easy to understand with a few pointers.

The Cooling system for the engine is sealed with a radiator cap, inside the cap there are TWO valves, yes two
The larger valve and spring hold pressure in the cooling system, it is usually rated for 14-18psi of pressure
In the center of the cap is a smaller valve, this allows coolant to flow back into radiator when cooling system has negative pressure, yes negative pressure.

Water pump is not really a "pump" it is a circulator it doesn't add pressure in the cooling system.
Physics 101, when you heat a fluid it expands in volume.
As the coolant circulates in the engine it warms up removing heat from the cylinders, as it warms up it expands and this expansion is what adds pressure to the cooling system.
When the pressure reaches 15psi(14psi rad cap) the larger valve in cap opens and allows hot coolant to flow in to the Overflow tank then large valve closes maintaining the 14psi pressure.
Not alot of coolant will come out, this is the HOT line on the overflow tank, COLD line is just what it says, that is the level coolant in the tank should be at when engine is cold, this line is there to tell you if you are losing coolant.

OK, so engine is warmed up and coolant in overflow tank is above Cold mark, you shut off the engine.
As the coolant in the engine cools down it shrinks in volume, as it does the 14psi pressure starts to drop, when it gets to -1psi the smaller valve opens and coolant is sucked back in from the overflow tank, this continues until all the coolant that went out comes back in.
So when you remove the rad cap on a cold engine it should ALWAYS be filled to the top, not even a little air.

If coolant is low then there are a few things to check.
First is the overflow hose, if it has a crack or leak then that will allow AIR to be sucked back in as engine cools, air is easier to move than coolant so air comes in.

Leak in the engine cooling system does the same thing, under pressure coolant will leak out and it may leak on to a hot engine part so you don't see the leak on the ground.
But as engine cools and pressure gets to -1psi that leak can suck air in, air is easier to move than coolant in the overflow tank.

Rad caps do fail, it is often wise to replace these when thermostat is replaced.

You can rent a cooling system pressure tester for about $15-$20, it is basically a hand pump with a pressure gauge and fitting for Rad cap opening.
On a cold engine you pump cooling system up to 20psi and then let it sit and see if pressure drops, if so you start looking for the leak, much easier to find on cold engine, lol.

Head gasket or cracked head issues:
With newer engine technologies we rarely see "milkshake" oil anymore, it does happen and is certainly a sign of a major issue, but because oil looks OK doesn't mean cylinder leak is off the table.
The Cooling system surrounds the cylinders, that is the whole point of the cooling system, to cool the cylinders.
So a leak in a cylinder will almost always be seen in the cooling system first.
A cylinder has 150+ psi compression when cranking, when running/firing it is close to 1,000psi
So any leak would cause some of that pressure to go into the cooling system.
As we know from above the cooling system has no pressure until coolant gets warmed up.
So if you start a cold engine and overflow tank starts to fill up or bubble then you have a cylinder leak, or if you remove rad cap on cold engine and start engine coolant should just sit there, if it starts to overflow or bubble then you have a cylinder leak.
Pressure is being pumped in from a cylinder.
You can search on this site for: glove test
It is a free test to see if there is a cylinder leak.

Last edited by RonD; 03-09-2015 at 07:17 PM.
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