After tune-up, MPG PLUMMET!?! - JeepForum.com
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Go Back JeepForum.com > Models > Jeep Grand Cherokee & Commander Forums > ZJ Grand Cherokee Forum > After tune-up, MPG PLUMMET!?!

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Unread 07-17-2011, 11:01 PM   #1
congerz83
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After tune-up, MPG PLUMMET!?!

I just changed my plugs (CHAMPION Copper) Air filter, fuel filter, wires, cap and rotor and put some Lucas injector cleaner in the tank. My MPG went from 16.5 to 13.9! My best was 18.1. I've also noticed that my tank capacity seems to have dropped as well. I normally fill up shortly after the ligh comes on. My last fill ups were, 22.26/17.7/18.13/19.33. Is this a common problem with the ZJ?

I did notice my plug gap was smaller than the plugs I removed. Could improper gap cause such a drastic drop? The old plugs seemed to be the same Champion Coppers as the ones I put in. Is there a better plug anyone reccomends?

It's a 4.0 with 153,000 on it.

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Unread 07-18-2011, 01:42 AM   #2
Crazy4Wheelin
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I lost mileage in my zj as well... not sure why. I believe I pulled coppers out as well and we are slowly loosing mileage. I'm thinking that my next tune up i'm going to try different plugs. Maybe platinum or bosch +2s? Anyone know how they run in ZJs?

Idk about the smaller gap especially if its the proper gap the vehicle calls for. I know on most cars, the smaller proper gap causes the car to run better and get better mpg.... but then again, we are talking about a jeep. Lol

I figured it was just my motor since it has about 210k on it. After I did this tune up, the family and I took it to NYC for a week and dealt with some harsh and hot driving conditions. Figured that had something to do with it as well.

Thanks for posting up. I oddly like when someone has the same problem I do. :-) if I find anything I will deffinately post up.
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Unread 07-18-2011, 03:19 AM   #3
ratmonkey
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I bet you didnt gap the new plugs did you? Never just put plugs in from the package.

Also, reset the ecu any time you do a tune up.
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Unread 07-18-2011, 04:15 AM   #4
mudruna
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yup reset the ECM by pulling off the negative cable so it can relearn it's new parts. And also you should check your spark plug gap, they all need to be the same and within spec to make your ignition system happy.
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Unread 07-18-2011, 07:51 AM   #5
congerz83
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Quote:
Originally Posted by mudruna View Post
yup reset the ECM by pulling off the negative cable so it can relearn it's new parts. And also you should check your spark plug gap, they all need to be the same and within spec to make your ignition system happy.
I did neither. Can you save me the search function and let me know what to gap them?
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Unread 07-18-2011, 08:04 AM   #6
timmay5309
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You need a guage to gap them, most auto parts stores have them right on the checkout counter. There should be a sticker on your radiator support that will tell you the spark plug gap that shows the emissions info. Always check the gap before installing them. They are banged around in shipping, by the parts people, etc... Unhooking the ECM and gapping should do the trick. How frustrating to do a tune up, and have it do the opposite effect, grrr!
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Unread 07-18-2011, 08:53 AM   #7
congerz83
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Quote:
Originally Posted by timmay5309 View Post
You need a guage to gap them, most auto parts stores have them right on the checkout counter. There should be a sticker on your radiator support that will tell you the spark plug gap that shows the emissions info. Always check the gap before installing them. They are banged around in shipping, by the parts people, etc... Unhooking the ECM and gapping should do the trick. How frustrating to do a tune up, and have it do the opposite effect, grrr!
Especially being that the #1 plug was a complete PITA. I'm reading it's .035. How long do I need to leave the cable disconnected?
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Unread 07-18-2011, 08:56 AM   #8
1stblack97ZJ
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Quote:
Originally Posted by congerz83 View Post
Especially being that the #1 plug was a complete PITA. I'm reading it's .035. How long do I need to leave the cable disconnected?
10 minutes or so
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Unread 07-18-2011, 12:36 PM   #9
congerz83
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Ok, gap set, computer re-set. I'll post results.
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Unread 07-18-2011, 03:47 PM   #10
timmay5309
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I know what you mean on that # 1 plug. I cracked the new plug putting it in there on my first tune up. I didnt realize it till removing everything i put in, that being the last thing of course. Frustrating on how after doing a tune up, it runs worse that before...
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Unread 07-18-2011, 04:42 PM   #11
ctak24
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Quote:
Originally Posted by Crazy4Wheelin View Post
I'm thinking that my next tune up i'm going to try different plugs. Maybe platinum or bosch +2s? Anyone know how they run in ZJs?
Copper conducts electricity better then Iridium or Platinum.
Also, the +2 and +4 plugs are more hype than anything. They still utilize only one ground electrode.
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Unread 07-18-2011, 04:47 PM   #12
Crazy4Wheelin
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Thanks for the info. I ran bosch +4s in my saturn and the burned to hot... started frying the rings so I put platinums in instead of the standard copper and it ran perfect.

I checked the gap on mine before I put them in and they were correct. Always reset computer and still have same problem. Any other suggestions besides what has been mentioned?

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Unread 07-19-2011, 12:18 AM   #13
gothchick
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Might want to also make sure the plug wires a firmly seated on the plugs and the cap... And from the cap to the coil. Might be arcing somewhere..
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Unread 07-19-2011, 09:28 AM   #14
Derph
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Check firing order.
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Unread 07-19-2011, 09:31 AM   #15
JS97ZJ
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I use chamion platnuims, and saw an increase of 3mpg over standard copper. And they last much longer.

Throw some fuel injector cleaner every time you change the oil keeps the junk from bulding up. Proformance and economy will remain steady in between tune ups.
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