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  #1  
Old November 21st, 2018, 20:36
XJ99JEEP XJ99JEEP is offline
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Sudden drop in fuel economy

I just had a sudden drop in my fuel economy that is pretty noticeable. I donít see any leaks in any fuel lines or anything. Is there a chance that my O2 sensors finally took a crap or that my map sensor is bad. Any opinions that could lead to a resolve?
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Old November 21st, 2018, 22:11
lawsoncl lawsoncl is offline
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Re: Sudden drop in fuel economy

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Originally Posted by XJ99JEEP View Post
I just had a sudden drop in my fuel economy that is pretty noticeable. I donít see any leaks in any fuel lines or anything. Is there a chance that my O2 sensors finally took a crap or that my map sensor is bad. Any opinions that could lead to a resolve?

Possibly. Is the engine fully warming up? I'm thinking stuck thermostat which should be obvious on the dash or a bad coolant temp sensor which can be tested with a multimeter.
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Old November 22nd, 2018, 06:29
XJ99JEEP XJ99JEEP is offline
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Re: Sudden drop in fuel economy

Yeah the engine is warming up to normal temperatures just fine. Ill check the thermostat and see if thatís the problem.
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Old November 22nd, 2018, 06:30
XJ99JEEP XJ99JEEP is offline
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Re: Sudden drop in fuel economy

Oops, I’ll check the coolant temp sensor
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  #5  
Old November 22nd, 2018, 08:56
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Tim_MN Tim_MN is offline
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Re: Sudden drop in fuel economy

A sudden drop of 30-40% in gas mpg's should make one suspect the O2 sensor circuit: fuses, wires, wire plugs, sensor faults.

Checking the thermostat and coolant temp sensor is also a good idea.
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Old November 22nd, 2018, 11:17
lawsoncl lawsoncl is offline
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Re: Sudden drop in fuel economy

I would suspect a 99 XJ would throw a code for a bad O2 sensor, but possibly not. If memory serves, the CTS on a 99 is the one in the thermostat housing. Resistance should be pretty high when cold, 5-10kohms. Once warmed up, it should be fairly low, around 100-300 ohms or so. When they go bad they often just open up and show really high resistance or open circuit and don't change.
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Old November 22nd, 2018, 11:20
lawsoncl lawsoncl is offline
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Re: Sudden drop in fuel economy

How-to reference for checking the CTS
https://www.autozone.com/repairinfo/...00c152800926a7
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Old November 23rd, 2018, 19:13
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crazymaci crazymaci is offline
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Re: Sudden drop in fuel economy

check for vacuum leaks,particularly on the intake side, i had a little rubber cap crack out and it dropped to 5mpg
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Old November 23rd, 2018, 21:08
BALTANAKT BALTANAKT is offline
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Re: Sudden drop in fuel economy

Take a look at the coolant temp sensor, O2 sensor.
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Old November 24th, 2018, 10:52
JeepNoob JeepNoob is offline
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Re: Sudden drop in fuel economy

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Originally Posted by crazymaci View Post
check for vacuum leaks,particularly on the intake side, i had a little rubber cap crack out and it dropped to 5mpg
I'd be leaning more towards this. Any fault with the ECT or O2 sensor is likely to result in a check engine light with a corresponding code.
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Old November 24th, 2018, 17:36
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Re: Sudden drop in fuel economy

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I'd be leaning more towards this. Any fault with the ECT or O2 sensor is likely to result in a check engine light with a corresponding code.
In fact, not so much. Typically they will get weak and do not throw a code until they are well and truly failed.

The O2 sensor is the most logical and most likely suspect. The Coolant Temperature Sensor is the next most likely suspect. Besides, testing them is simple.
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Old November 25th, 2018, 17:03
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devildog0 devildog0 is offline
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Sudden drop in fuel economy

Which sensor feeds the computer data for fueling? The one in the thermostat housing or the one on the driver's side of the head?

I've been battling bad fuel economy and have gotten my numbers up to 16 mpg from 12 mpg. I'd like to squeeze out a little more and my coolant gauge has been a little flaky which is fixed by messing with the sensor towards the rear of the head. It feels like the insulator where the plug hooks to the sensor is rotating back and forth a little bit.

This is on a 95 4.0

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