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Bosivian 07-10-2007 03:36 PM

Tricky question for experienced tech.....
Please help me explain why I get 3mpg better gas mileage with the A/C ON!!!

I have a:
2004 Unlimited
21,400 miles
4.0L Auto
ratio 3:73
Chrysler says no recalls on my vin
fuel system just cleaned out by dealer.
new paper air filter
Everything is stock!

Here's the scenario:
Just bought this Jeep last week in CA and I get a CONSISTANT 14mpg highway with the A/C OFF and 17.5mpg highway with the A/C ON. I also get 9mpg city when I drive conservatively.

I just drove it cross-country putting more than 2000 last week.
I kept all the variables the SAME, including speed, RPMs, windows up, top on, cruise off, flat land, fuel octane and brand, etc. the only thing I couldn't control was wind which wasn't severe at all. I drove through about 11 tanks of gas changing one variable at a time and can not understand why I am consistently getting 3mpg more with the A/C compressor running. This logically tells me there is something wrong with either my fuel mapping causing it to run richer or some other drag being put on vehicle/engine. I have no trouble codes showing on the OBD2 scanner. I pulled the plug on 1st O2 sensor just to see if it would trip the check engine light which it did, so I know the PCM is working.

I have taken it to 2 different Chrysler dealers and they cannot explain it (flushed $300 ). They dismiss it and say I that wranglers don't get good mileage. I am looking for some advice. I know it’s not the most efficient vehicle Chrysler makes but I have friends with the same exact Unlimited who get better city/hwy mileage than I do.

Is there a sensor that is related to the A/C that would tell the fuel system to run richer or retard the timing?

Please let me know your thoughts........

debruins 07-10-2007 03:54 PM

well 14 is an ok MPG for a wrnagler, 18 is really good

do your friends have the unlimited(more weight) and auto(less effecient than stick)

about the a/c thing i have no idea

JCS05Rubi 07-10-2007 05:15 PM

At first I was thinking that the windows down w/ AC off (highway, not city) was the answer, but apparently not.

Only thing I can think of would be either a computer parameter tripped when the A/C is on, or somehow the clutch is locking when off, then spinning when engaged. Pop the hood and take a gander while the AC is off and while its on. See if you can tell a drag being put on the engine, or the belt slipping around the AC pulley, when its off.

Nicolas-Eric is the man with the electrical diagrams. Maybe he can take a look and see if there is a signal wire that goes from the AC to the PCM. That would tell us if there is some sort of auxilary mapping used the A/C is triggered.

Odhinn 07-12-2007 06:26 AM

Hard to say without seeing the vehicle in person and on a scanner. Sounds like the Computer could be throwing it into a run-rich mode when the AC is off. Possibly a sensor that is reading improperly but hasn't failed yet. I believe that all the things that would affect the gas milage on a TJ when the AC is on are computer controlled so that would be a good place to start IMO. Have a shop check for the proper voltages out of the sensors.

rebelBullDawg325 07-12-2007 08:57 AM

sounds like a good problem you got there. i aint even got AC to make my mileage worse, or better

Bosivian 07-12-2007 10:45 AM

Thanks Odhinn, I will try and do that myself as I do not fully trust a dealer/auto shop to tough through this with me.

Where can I get a list of sensors I should check and the specs as to what the ohms reading should be for them? Chilton, Haynes?

Can I test my sensors (O2, MAP, MAS, etc) with my ohms meter or should I purchase the more expensive OBD2 scanner that gives me the ohms readings of all the sensors?
The dealer can re-install the software on the PCM for $100. Is it possible the software is bad? I would think that would be very unlikely, although so are my symptoms!

If I can solve this problem of better gas mileage with A/C on, I would be willing to except the low city mileage of 9mpg, but as it stands now I feel there is something logically wrong.

Odhinn 07-12-2007 11:14 AM

You can use a meter but in order see what your Computer is seeing you will need a real time scan tool. This is a scanner that I have been eyeballing but Am not sure if it is real time or have any experience with this tool. If you end up taking it to a shop, find a good diagnostic shop that has a tech that still knows how to use an oscilloscope for the most part they usually diagnose a problem and not just replace parts or pull codes.

As far as a list of the sensors I would think your best bet would be a factory service manual and an electrical components locater book. Someone else here might have an online source for that info but I personally don't. Also check to see if the dealer has a electrical components spec list.

As far as the software possibly being bad. Anything is possible when an auto comp is involved. And yes there is something wrong don't let the dealer tell you anything different to get you out of their hair. Your AC will give you worse mileage when on, not off.

Hope it helps... tracking electrical and computer problems are a pain. Good luck

Odhinn 07-12-2007 12:38 PM

Well I finally examined the home page and saw that the scan tool does show real time.:D

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