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Old August 18th, 2004, 15:46
Land Yacht Land Yacht is offline
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Sudden gas mileage drop

92 unmodified XJ 4.0 HO with no symptoms of problems.

Gas milage has decreased suddenly and I have tuned it up in the last 6 months replacing the plugs, distributor cap & rotor cap, cleaned the injectors, ect., ect. & so on. All basic stuff.

The only mod is a 180 thermostat, installed well before this.

The factory exhaust manifold is cracked pretty bad. Could this be the problem?

TIA for any help.
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Old August 18th, 2004, 16:07
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CharlesS CharlesS is offline
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Re: Sudden gas mileage drop

A badly cracked exhaust manifold equals loss of back pressure in the exhaust system and a loss of low end torque. How much of a loss....depends upon how bad the exhaust leak. This will make you accelerate harder to achieve the same as without the exhaust leak.
The 180* t-stat could contribute to the loss of mpg if the engine warmup enrichment circuit does not end when it should.
Another thing that can hurt mpg is a bad O2 sensor.....

Hope this helps troubleshoot your problem.

Charles
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Old August 18th, 2004, 16:56
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red91 red91 is offline
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Re: Sudden gas mileage drop

Quote:
Originally Posted by CharlesS
A badly cracked exhaust manifold equals loss of back pressure in the exhaust system and a loss of low end torque. How much of a loss....depends upon how bad the exhaust leak. This will make you accelerate harder to achieve the same as without the exhaust leak.
The 180* t-stat could contribute to the loss of mpg if the engine warmup enrichment circuit does not end when it should.
Another thing that can hurt mpg is a bad O2 sensor.....

Hope this helps troubleshoot your problem.

Charles
I'll chime in with the 02 sensor. Next you'd better take care of the cracked exhaust manifold. You'll end up burning your valves eventually if you don't fix it. Then you have real problems.

The other thing to do, maybe before any of these is to reset your computer by disconnecting your battery cable for 30 min. Then it should figure out it has new plugs, and everything else. But if you have 100k plus and have never changed the 02 sensor. Do it asap.
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Old August 20th, 2004, 10:54
greenday9d1 greenday9d1 is offline
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Re: Sudden gas mileage drop

I got a great exhaust manifold for really cheap off ebay, if that helps at all. I also had a crack going around the entire pipe down at the bottom.
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