Please help! Temp gauge is erratic on highway

[sarcastic_cook]

I have a 2002 Mazda B2300 Duratec w/5spd. I have been fighting this issue for a while. It started out of nowhere. When driving on the freeway with the temp at normal it starts to fluctuate erratically. The needle will jump from normal to the "C" (the line below the normal bracket sweep) then creep back up to normal or a little above. It will set there for a few seconds before it starts again. It will do that over and over. If I keep the engine under a load for a period of time the needle will spike from normal straight to the "H" (line above normal bracket sweep) and turn on the check gauge light. When that happens I just keep going and after a moment the needle will do a full sweep to "C" followed by it creeping back up to normal.

Everything else works like it should and the coolant is clean. The engine engine does not actually get hot.

Things I have done since this started: changed water pump, temp sensor on back of head, thermostat assy, new coolant and checked for blown head gasket with combustion leak detector.

After doing all that I still have the same issue but the needle is not as erratic as before. I am baffled by this and not sure which direction to go in next.

Thanks in advance for your help

 

[JamesD]

The temperature switch (that controls the gauge) is likely bad. Or you have a short in gauge or the wire.

 

[Soledad]

I agree with JamesD, it's likely a short in the gauge or the temp switch wire on the temp sensor/swith you replaced at the back of the head.

Just thinking out loud here but I don't think the temp sensor in the middle of the head between the spark plugs would cause this problem since it sends the head temp to the ECU to infer coolant temp. However, a check with Torque Pro or Forscan might tell/show you what's going on.

 

[hank857]

Check you rad cap, I had a similar problem did the tstat, changed the sender to no avail so finally checked the pressure cap and it was bad.

 

[Soledad]

Yeah, that's very true. Those degas bottle caps do get weak over time. If it were weak and letting pressure escape you'd see some telltale "white mark" puking on the bottle.

 

[sarcastic_cook]

The radiator cap is fairly new but is definitely worth taking a closer look at. Soledad thanks for mentioning torque pro, it never occurred to me to use that to watch temps.

 

[AllanD]

a VERY COMMON reason for erratic temp guage readings is due to a faulty engine ground...

TYPICALLY this is a result of a problem with the grounding wire that connects from the rear of the drivers side cylinder head and goes to one of the mounting bolts for the wiper motor...

As a note if you have a factory cassette radio with the green florescent display
intermittent connection of this specific ground wire can actually damage the radio...
(It actually causes permanently reduced volume when playing cassette tapes
because it basically "Blows up" the tape head pre-amplifier circuit)

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