UPDATE 04 jeep MPG decrease all of a sudden
#469377
09/15/09 08:04 PM
09/15/09 08:04 PM
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Alton, IL
Dakota_Don
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hi all, the wife has an 2004 jeep gran cherokee with the 4.0. any way it was tunned up this past summer (june) oil, air, brakes,plugs, ect and we got according to the mpg gauge 19.9  per gal on the hwy, now all of a sudden it gets 14-15,  it runs great no lights, the tire psi it at 33psi. anone else have this problem?  i am not even sure where to start, maybe pull the plugs, fuel psi check.. air filter is a k an n and is clean like the day it was installed.. what else can i check before it goes to a shop for further diag.. thanks
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469378
09/15/09 08:28 PM
09/15/09 08:28 PM
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Joined: Jun 2006
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72challorange
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Re: 04 jeep MPG decrease all of a sudden
[Re: 72challorange]
#469379
09/15/09 08:54 PM
09/15/09 08:54 PM
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Southeast Pa.
SALEM1912
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Just about any serious problem would put a check engine lite on. Any different driving habits,different fuel.
Last edited by SALEM1912; 09/15/09 08:55 PM.
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Re: 04 jeep MPG decrease all of a sudden
[Re: Pacnorthcuda]
#469383
09/15/09 09:04 PM
09/15/09 09:04 PM
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Alton, IL
Dakota_Don
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Quote:
An O2 sensor thats gone bad can cause a sudden decrease real easy. Its one of the first things to change when this happens due to low cost and ease of replacement, assuming an error code isn't present to make you suspect something else.
is there a way to test the o2 sensor?? and can it go bad with out setting off a code?
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469384
09/15/09 09:04 PM
09/15/09 09:04 PM
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Joined: Jun 2006
Posts: 824
NH
72challorange
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Check rpms vs mph, is there a towing switch or od switch?
Tom
Plus check actual gallons used vs miles traveled not trip computer.
Last edited by 72challorange; 09/15/09 09:05 PM.
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Re: 04 jeep MPG decrease all of a sudden
[Re: 72challorange]
#469385
09/15/09 09:19 PM
09/15/09 09:19 PM
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Anonymous
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Anonymous
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On some vehicles the MAF sensor can go out of tolerance and not trigger a diagnostic code/light. If you over oil your K&N on some vehicles it will coat the heated sensor on the MAF and ssscrew your eyes out. If it's easy to get out, go ahead and remove it and clean it with alcohol, air dry. Be gentle. This is a problem where running diagnostics through the OBDII port is what is really needed.
Just a thought.
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469387
09/16/09 11:02 AM
09/16/09 11:02 AM
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Posts: 1,402
Littleton Colo
Geezer
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Quote:
Quote:
An O2 sensor thats gone bad can cause a sudden decrease real easy. Its one of the first things to change when this happens due to low cost and ease of replacement, assuming an error code isn't present to make you suspect something else.
is there a way to test the o2 sensor?? and can it go bad with out setting off a code?
If you have the low emissions version that they sold in certain states you have 4 O2 sensors and they will be out of tolerance after 100,000 miles and not trigger the check engine light. 
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Re: 04 jeep MPG decrease all of a sudden
[Re: Geezer]
#469388
09/16/09 04:35 PM
09/16/09 04:35 PM
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Alton, IL
Dakota_Don
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Quote:
Quote:
Quote:
An O2 sensor thats gone bad can cause a sudden decrease real easy. Its one of the first things to change when this happens due to low cost and ease of replacement, assuming an error code isn't present to make you suspect something else.
is there a way to test the o2 sensor?? and can it go bad with out setting off a code?
If you have the low emissions version that they sold in certain states you have 4 O2 sensors and they will be out of tolerance after 100,000 miles and not trigger the check engine light.
ok, i only have about 70k miles
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Re: 04 jeep MPG decrease all of a sudden
#469389
09/16/09 04:36 PM
09/16/09 04:36 PM
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Alton, IL
Dakota_Don
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Quote:
On some vehicles the MAF sensor can go out of tolerance and not trigger a diagnostic code/light. If you over oil your K&N on some vehicles it will coat the heated sensor on the MAF and ssscrew your eyes out. If it's easy to get out, go ahead and remove it and clean it with alcohol, air dry. Be gentle. This is a problem where running diagnostics through the OBDII port is what is really needed.
Just a thought.
I havent oild the k and N yet, but i will clean the MAF, hopefully that will help..
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Re: 04 jeep MPG decrease all of a sudden
[Re: 70Duster]
#469390
09/16/09 04:38 PM
09/16/09 04:38 PM
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Alton, IL
Dakota_Don
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Quote:
First, check for the proper gap on the new plugs. If OK and if you still have them, I'd put the old plugs back in for a tank or two and see if the mileage increases. I've experienced on two occasions that the wider (worn) gap of the old plugs actually increased fuel mileage. If you dont have the old plugs, try increasing the gap .010" on the new plugs and see what happens since the plugs are so easy to remove and replace on that engine.
Also, if the tune-up was done at the dealer, make sure they didn't reflash the computer according to a TSB (even though you didn't ask for it). Been there done that on the same vehicle, same engine and I had to take it to another dealer to get that mess straightened out.
well i dont have the old plugs any more, I gapped the new ones according the the manual, it got great MPG from june till just about a week ago.. but i will pull them and look at them too.
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469391
09/16/09 04:40 PM
09/16/09 04:40 PM
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Alton, IL
Dakota_Don
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Quote:
Quote:
First, check for the proper gap on the new plugs. If OK and if you still have them, I'd put the old plugs back in for a tank or two and see if the mileage increases. I've experienced on two occasions that the wider (worn) gap of the old plugs actually increased fuel mileage. If you dont have the old plugs, try increasing the gap .010" on the new plugs and see what happens since the plugs are so easy to remove and replace on that engine.
Also, if the tune-up was done at the dealer, make sure they didn't reflash the computer according to a TSB (even though you didn't ask for it). Been there done that on the same vehicle, same engine and I had to take it to another dealer to get that mess straightened out.
well i dont have the old plugs any more, I gapped the new ones according the the manual, it got great MPG from june till just about a week ago.. but i will pull them and look at them too.
well come to think of it the thing did idle funny one day, took it to the deal and they said the batt was bad.. not sure if they flashed the PC or not, but if they did would that cause the bad MPG??
and i thought the plugs were a pain with the coil packs on them..
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469392
09/16/09 05:53 PM
09/16/09 05:53 PM
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PA
70Duster
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Quote:
well i dont have the old plugs any more, I gapped the new ones according the the manual, it got great MPG from june till just about a week ago.. but i will pull them and look at them too.
Sorry, I misinterpreted your original post. I thought the mileage decreased right after the plugs were replaced. Since the mileage was fine for awahile after the new plugs were installed, I doubt the plugs or the gap are the issue.
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Re: 04 jeep MPG decrease all of a sudden
[Re: Dakota_Don]
#469393
09/16/09 06:00 PM
09/16/09 06:00 PM
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PA
70Duster
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Quote:
well come to think of it the thing did idle funny one day, took it to the deal and they said the batt was bad.. not sure if they flashed the PC or not, but if they did would that cause the bad MPG??
I took my '04 4.0L GC to the dealer for a brake rotor recall under a class action lawsuit (which was another fiasco) and discovered after driving for awhaile afterwards, that I lost about 4 MPG. I lift the hood and see a sticker indicating the computer was reflashed (I didn't ask them to do it). To make a long story short, it took another computer reflash at another dealer to get the mileage back to normal. If you are into long stories, I can go into more detail, but this is a quick summary.
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Re: 04 jeep MPG decrease all of a sudden
[Re: 70Duster]
#469394
09/20/09 05:24 PM
09/20/09 05:24 PM
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Alton, IL
Dakota_Don
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Update, well i could not find anything wrong with the jeep, plugs looked great, sensors looked good, TB looked good and clean, so the wife nd i took it for a long drive sat after noon, and while she was driving i played with the OD, seems like it wasnt engaging even thou the light said it was..
anyway buy the end of the day it was getting 20.3 MPG with 5 people and one was approx 300LBS
so guess the OD has a problem, seems to be working fine now..
thanks for all the suggestions..
Don
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