UPDATE 04 jeep MPG decrease all of a sudden

Posted By: Dakota_Don

UPDATE 04 jeep MPG decrease all of a sudden - 09/16/09 12:04 AM

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

Posted By: 72challorange

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 12:28 AM

Is overdrive working?

Tom

Posted By: SALEM1912

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 12:54 AM

Just about any serious problem would put a check engine lite on. Any different driving habits,different fuel.

Posted By: Pacnorthcuda

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:00 AM

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.

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:01 AM

nothing like that has changed, I will see if the OD is working, which brings me to the TC also, I didnt think of the tranny..

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:02 AM

there are no codes or lights..I cant find any vac leaks either

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:04 AM

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?

Posted By: 72challorange

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:04 AM

Check rpms vs mph, is there a towing switch or od switch?

Tom

Plus check actual gallons used vs miles traveled not trip computer.

Posted By: Anonymous

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 01:19 AM

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.

Posted By: 70Duster

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 02:48 PM

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.

Posted By: Geezer

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 03:02 PM

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.

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 08:35 PM

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

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 08:36 PM

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..

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 08:38 PM

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.

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 08:40 PM

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..

Posted By: 70Duster

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 09:53 PM

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.

Posted By: 70Duster

Re: 04 jeep MPG decrease all of a sudden - 09/16/09 10:00 PM

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.

Posted By: Dakota_Don

Re: 04 jeep MPG decrease all of a sudden - 09/20/09 09:24 PM

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