dropping MPG 4.0 ZJ - JeepForum.com
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post #1 of 9 Old 03-14-2019, 03:57 PM Thread Starter
04300exc
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dropping MPG 4.0 ZJ

I am baffled- I have a 1995 Jeep ZJ 4.0, all stock, 230,000 mi. My fuel MPG has dropped from 17 mpg to 13 mpg over about the last 3 tanks. I have not found any fuel leaks. Engine compression is 115-125 psi. ALL plugs are WHITE! (lean). Manifold vacuum is 15 in, and I cannot detect any leaks (I think vac should be 17-21???). Fuel psi is 41 psi at rail. I Purchased a new O2 and will put in today. There is raw fuel smell when I start vehicle in the morning.

Any help would be greatly appreciated.... I am baffled!

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post #2 of 9 Old 03-14-2019, 04:02 PM
riverzj
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Quote:
Originally Posted by 04300exc View Post
I am baffled- I have a 1995 Jeep ZJ 4.0, all stock, 230,000 mi. My fuel MPG has dropped from 17 mpg to 13 mpg over about the last 3 tanks. I have not found any fuel leaks. Engine compression is 115-125 psi. ALL plugs are WHITE! (lean). Manifold vacuum is 15 in, and I cannot detect any leaks (I think vac should be 17-21???). Fuel psi is 41 psi at rail. I Purchased a new O2 and will put in today. There is raw fuel smell when I start vehicle in the morning.



Any help would be greatly appreciated.... I am baffled!


Fuel smell could be issue but dropping mpg usually points to wheel bearing or brakes eating it.


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post #3 of 9 Old 03-14-2019, 04:06 PM
CatSplat
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Could be a sticking fuel injector that's making one cylinder run rich causing the computer to lean out all cylinders to compensate.
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post #4 of 9 Old 03-14-2019, 05:51 PM
kg6mov
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Also check the exhaust manifold for cracks, that can lean things out as well.


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post #5 of 9 Old 03-14-2019, 05:58 PM
redeye3
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my 96 had a similar problem. turned out to be a bad bearing on one of the idler pulleys. check them out. it would usually be the one bolted to the ac compressor mount bracket. it takes the most stress provided 95 have the same configuration
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post #6 of 9 Old 03-14-2019, 07:49 PM Thread Starter
04300exc
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How would you diagnose a sticking injector?
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post #7 of 9 Old 03-17-2019, 12:07 PM
GOLDWING
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I would use the torque app and a blue tooth obdII scanner and look at the long term and short term fuel trim readings. This will give you an idea what the o2 sensors are telling the pcm. You can also monitor engine vacuum which is my first guess.
As far as finding the leaking injector, if there is one:

GW

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post #8 of 9 Old 03-22-2019, 12:08 PM
8v-of-fury
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I always chocked my mileage up to just being a very poorly designed SUV. In regards to mileage that is. Everything else is spot-on and perfect I'd assume bad aero from factory, and i have made it worse. The I6 is under powered, and under geared. I've made that worse with knobby 10.5x31's.. the full time front axle is great for durability and reliability, but horrible for mileage.

It is so poor, that the 5.2 and 5.9 get better mileage than this 4.slooow :/

'98 ZJ 4.0/242, Locked d30/d35, 31x10.5x15, 2" BB w/ Bilstein 5100's
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post #9 of 9 Old 04-20-2019, 03:49 PM Thread Starter
04300exc
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I thought I would share an update - I put in a new O2, that raised my mpg from 13 to 15 mpg. I then replaced the exhaust manifold to a header design (old manifold was cracked and definitely leaking).
While apart I replaced the MAP sensor, coolant temp sensor and air temp sensor. I checked injector spray patterns....not horrible so left alone. Put it all back together and the result is a consistent 18 mpg on the highway. Town driving about 16 mpg. I hope this helps others if they have the same issue. Thank all for your comments and help.
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