Running hotter

CAR TOM

MEMBER
1997 DODGE NEON
2.0L
4 CYL
2WD
AUTOMATIC
151,400 MILES

I have the car listed above with a SOHC engine. My radiator and water pump has 32,000 miles on it as does the timing belt. Since day one, the temperature gauge on the dash has read the same showing 1/4 from cold mark, but as of last week I noticed that while driving at highway speeds the temperature gauge has been rising to 3/4 mark on the gauge. The issue is that the gauge then drops to the regular 1/4 mark, then goes up again to the 3/4 mark, then cools down again to the 1/4 mark.
Then I noticed with the engine hot after driving I went to gas up, I shut the motor off to fill up which took maybe ten minutes. When I got in the car and turned the key to the accessory position the gauge pegged to extreme hot scale on the gauge, but once I turned the motor on, within ten seconds the gauge settled at normal 1/4 warm on the gauge. What could be the culprit?
Thank you.

3 Replies

CARADIODOC

MECHANIC
33,723 POSTS

I have the solution, but I don't know why this happens all of a sudden. The same thing happened to my '88 Grand Caravan when it had around 300,000 miles. Mine would move up and down about a half dozen times, then settle down. The cause is the temperature sensor for the dash gauge is a little ways away from the thermostat. It takes some time for the heat to migrate over to the thermostat and make it open. By that time, the gauge is already reading hotter than normal.

Once the thermostat opens, coolant starts to circulate, then the cold coolant in the radiator rushes into the engine where it causes the thermostat to close again. This repeats until all the coolant has reached normal temperature.

The solution is to remove the thermostat and drill a very small 1/16" bleed hole in the center plate. A lot of thermostats, especially those for import vehicles, already come with that hole and a little "dangle" hanging in it. That hole is too small to adversely affect the operation of the thermostat, but it lets just enough coolant flow when the thermostat is closed to get the hot coolant to it sooner. By the time the gauge reads a normal temperature, the thermostat will be opening, and those high and low swings will no longer occur.