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Chris
Chris, Shop Foreman
Category: Jeep
Satisfied Customers: 12739
Experience:  Jeep Certified Master Tech
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98 Jeep Grand Cherokee: Coolant Sensor..Taillight..V8..5.2l with 212K

Customer Question

I keep getting the illusive "Coolant Sensor Bad" & "Rear Taillight Failure" Errors on my 98 Jeep Grand Cherokee V8 5.2l with 212K with Original Everything. The light error I don't care about, as all my lights are on, but I know its part of a bigger problem.

I'm a Med student just trying to get through school, my knowledge of cars is limited, so simple direction and cheap fixes would be much appreciated! Right now, I have replace both the Coolant sensor on the engine block and the Coolant float in the coolant overflow reservoir = little change. While the light used to remain on all the time, now almost each time I start up the car, for a few short minutes, the Coolant error will flash, then it goes away. I have driven the car over 100 miles so if something was gonna reset it should have kicked in by now right? However, I am still only getting 5-7 miles to the gallon no matter how "ginger" I drive, and before this issue I was getting 14-19 MPG. All other maintenance on the car is up-to-date. I have been told this is because the system is being thrust into "open-loop mode" by this error? I DO NOT want to, nore do I feel I have the skill to perform the below fix (see link) or even if that will truly fix my low gas issue... there must be another, simpler way to get the car into closed loop mode = (BETTER GAS mileage).

http://webcache.googleusercontent.com/search?q=cache:http://www.jeepforum.com/forum/f13/coolant-sensor-bad-finally-fixed-175207/

Thanks
Submitted: 5 years ago.
Category: Jeep
Expert:  Chris replied 5 years ago.
Hello and welcome to just answer. The coolant sensor message you are getting has absolutely nothing to do with your fuel economy or engine closed or open loop mode. It is for the coolant overflow bottle only. If your "check engine" light is not coming on then the fuel system is most likely going into closed loop. The cause of your poor fuel economy is something else like worn plugs, dirty air filter, lazy O2 sensor, dirty fuel injectors, etc. That coolant level sensor is in the bottle to alert you if the coolant level gets low. If there is a problem in that circuit then you get that coolant sensor bad message. It is most likely a wiring or connector problem at the sensor in the bottle.

I hope this information is helpful and let me know if you need further assistance with this.

Chris
Customer: replied 5 years ago.
hi thanks for your quick reply,

"That coolant level sensor is in the bottle to alert you if the coolant level gets low. If there is a problem in that circuit then you get that coolant sensor bad message. It is most likely a wiring or connector problem at the sensor in the bottle. "

I replaced the entire coolant sensor that goes inside the bottle and the wiring also, though it looked fine, yet I still get the error almost every time I start the car. Before the error I got good gas millage. after the error, poor gas mileage. It was a Jeep Mechanic at Dodge dealership that told me the coolant sensor would send it into, he called it "limp home mode." So I am honestly confused. The engine light is not on. However a 4/5 months ago, before the gas millage went hinky perform a tune up like 2000 miles ago, had all the spark plugs, cables, distributor cap, etc changed, and all was fine. Then start of last month, boom, here comes this coolant error and down goes my gas millage into the dump.

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