Hard starting, stall, poor gas mileage – Tech Tip
There are many things that can cause hard starting and stalling, but the key to solving this particular puzzle is the clue about poor gas mileage. One inexpensive sensor can be the cause of all these problems—the engine coolant temperature sensor (ECT).
When you turn the key, the computer takes a quick look at the ambient air temperature and the ECT. By comparing the two temperature readings the computer knows whether the engine is cold or at operating temperature, and whether it’s hot or cold outside. Based on those two readings, it decides what the air/fuel mixture should be.
A cold engine needs a rich mixture to start and stay running. As the engine warms, the sensor reports the warmer reading to the computer and it leans out the mixture. However, if the ECT isn’t working properly, the computer will continue to pour fuel into the engine. That’s where you get poor gas mileage.
The ECT is a thermistor that changes resistance based on temperature. To test it, you’ll need an ohm meter and a reference chart. Car makers usually install two coolant temperature sensors—one for the instrument panel gauge and one for the engine computer. Make sure you test the right one. ECT sensors usually cost less than $25 and are easy to install. If you’ve neglected changing the coolant in your vehicle, your ECT could be corroded or coated with insulating deposits.
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© 2012 Rick Muscoplat
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can u pls help me to locate this sensor in a 1994 legacy? thanks
It’s located on the right rear corner of the engine. There are two sensors–one for the computer and one for your dashboard gauge.
hi,
thanks a lot for the answer. are both sensors identical? my chk engine light is up n its struggling to start in the morning. which sensor sh i replace? thanks for ur help
I don’t know which is which. You’ll have to disconnect one at a time to see which one controls the temp gauge on your dash.
July-August 2009
I changed the sensor and have used 6 tanks of gas since the change. Mileage has improved an unbelievable 20%. It is still not exactly good but 20% is 20%.
Ian
I have a 1997 F150 Ext cab, with the 4.2 V6 and auto tranny. It gets bad mileage, 12-13 with about 50-50 hwy/city driving. No oversize tires, etc. In addition, it emits a light amount of black smoke from the tailpipe. Its not really visible except at startup, but the buildup on the tailpipe exit is substantial. Recently I moved the tailpipe exit to behind the passenger rear tire, whereas before it exited in front of the tire and the wheel was always black. At startup it blows the black smoke for a split second and runs rough for a second or two, then catches and seems to run fine. The truck runs GREAT, no check engine light. I have replaced plugs, cleaned the entire egr system, run fuel injector cleaner (1 bottle of the Techron cleaner)thru every 6000 miles or so. Could the ECT be causing the bad mileage, rough starts, and black smoke? Or should I look elsewhere?