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Low Engine Temperature in a 94 Ford Ranger

I have a 1994 Ford Ranger XLT with a 4.0L V6 engine (141,000 miles). Within the past 6 months the engine has overheated twice causing me to have it towed a dealer for repairs.

First Repair: Replaced thermostat, did a chemical boil on the raditor (it was mostly plugged), and performed a coolant flush. Water pump was inspected but was OK.

Second Repair: Replaced broken hoses.

Since then the engine temperature: (1) tends to run cool and (2) fluctuates frequently. At highway speeds (70 mph), it tends to run just barely in the normal range (but on the cool side). If I come to a stop, the temperature will usually increase fairly quickly until it reaches the hot end of the normal range, stay there for 2-3 seconds and then drop all the way down again. I never paid too much attention to the temperature gauge before these problems, but I thought I remember it staying fairly constant in the middle of the normal range.

My gas mileage has also decreased by a few mpg, but I don’t notice any other problems - nothing about the exhaust color or odor, tire pressure is OK, air filter is OK, the check engine light has never come on, and no discernible loss of power. Cabin heater seems to work fine and coolant levels are normal, both in the radiator and reservoir.

I replaced the thermostat assembly again last week and also changed the spark plugs and wires with only a minor effect on engine temperature - increased slightly but still oscillates.

Any ideas about what could be causing such large swings in the engine temperature?

You have air in the system, no doubt about it and I’d be willing to bed you had a mostly-plugged heater core if you had a mostly-plugged radiator. Air can get trapped behind a clog in the heater core and you’ll have a lousy time getting it out. First thing’s first, make sure the entire system is flushed, including making sure that the heater core is flowing freely.

If you have the luxury of an up-hill driveway or street, park with the nose of the rig facing up, that will help the air get up to the radiator neck. Let the truck cool down, the remove the radiator cap. Now start the truck and put the heat to maximum. Let it warm up and as it starts to warm up, you’ll notice bubbles appear in the coolant and when the thermostat opens, coolant will start flowing pretty rapidly. Put the radiator cap on and take it for a drive. Put some highway miles on it and it should work the air out.

Make sure that you have the proper coolant level in the overflow bottle, as well, and that the correct radiator cap is on the vehicle (if the rest of the cooling system was that bad-off, you might as well get a new radiator cap, too).

Erratic temp gauge readings may just be a faulty temperature sending unit (not to be confused with the temperature sensor). My experience is with the 3L V6, and on that engine it is right in front and on top (easily accessible). When this happened to my 1994 Ranger it was an easy fix, and the part was only ~$4.

Awesome. Thanks for the advice. I will try these ideas out and if it still doen’t fix the problem I will put up another post! Thanks again!

That hoses broke within 6 months of cooling system repairs bothered me. I don’t suppose a mechanic is obliged to note soft hoses while fixing thermostat, but a good one probably would. Unless OP was told hoses were bad and chose not to believe it.