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So my 95 Mazda b2300 was reaidng fairly cold, but the needle wasn’t pegged all the way down. Even after an hour of running it would do this. So I assumed thermostat.



Replaced it, and now the needle doesn’t even get into the range.



Thoughts?

I would check or replace the sending unit in the engine block next. I assume you used the correct thermostat with the correct heat range.

I used OEM 190 degree.

I can check out the sending unit, but this was a pretty quick deterioration and I wonder if one day the sending unit might read 25% less than it normally does??

My experience, many years ago, is yes, that can happen. The part is 13 yrs old and can deterioriate. It didn’t used to be very expensive or difficult to replace, so I would go ahead and do that.

Other choices may include check the actual radiator warm up temp, and verify that the new thermostat is OK and antifreeze is up to around the 190. If so, then you can choose to not repair anything and just know that the gauge is off. The danger is a possible overheating situation later and whether you would know that it is occurring.

My daughter went through two OEM thermostats in her Corolla once, and both were defective out of the box. Finally got a cheapo from Pep Boys to work correctly, after much frustration, so I don’t automatically rule out much, anymore.

Regardless of what the gauge says, is the engine reaching the correct operating temperature? How does the heat work?

If the engine warms up quickly to 190 or so, which we hope it will with a new thermostat, then either the sender or the gauge if bad. I’d test the sender.

How do I check engine operating temperature without the gauge provivded in the dash?

Heat is slow.

You measure it with an infra-red thermometer or some other device. If the thermostat is working correctly the engine should warm up quickly and you should have some heat in just a few minutes.

When I had the same problem on my 87 Ranger I hooked up my scanner to check for any trouble codes.

If the thermostat was stuck open, i would have gotten a code 21 “insuficient coolant temp”’ Instead I got an 11 code “system pass”

This told me that I had to check for a bad temp gauge or a bad temp sending unit.

To check the gauge i simply removed the connector from the temp sender & ground it to the engine. The gauge moved to full hot as it should.

8 bucks for a new temp sender & a couple of minutes to replace it & problem solved.

Note there ae 2 sending units one is a 2 wire and sends coolant temp info to the PCM/computer.

The one you want is a single wire.

If I remember correctly the location on your 2.3 is at the top rear of the engine on the driver side.

check the coolant level 1st; may sound counterintuitive but low or no coolant scenarios could yield zero info to temp sender unit.

Next, check the thermostat. Occasionally these are defective even when brand new. Check it 1st in boiling water. Put in cold water on stove and bring to boil—Use a candy thermometer to see that it opens at the correct heat range.