Originally Posted By: BetaBear
I see that your fuel mileage has gone down, but what about power? Have you noticed any lack of power?
A few things can result in a drop in MPG. Have you noticed any brake squeal lately? Could be that the slides need to be lubed.
I've owned my ZJ for a while, and have put about 50k on it since I got it (Has the AMC 242 4.0L). I used to be able to get 23mpg highway no problem. Then I began to notice a drop. I'd driven the 1000 mile trip from wyoming to IL 3 times, and each subsequent time, mileage dropped. I noticed on my OHC that cruising mpg was going down as well.
Fast forward a while. I move to TN. mileage is horrible now, at about 18-19mpg highway. One day I notice an instant drumming noise that almost sounds like a tractor trailer using the jake brake, only muffled.
I traced it down the a bad front U-Joint that caused the problem. You couldn't see it visually, and I couldn't get it to have any play when I moved the front drive shaft and yolk. Turns out only 1 of the bearing caps had gone, so the rest was fairly solid. I had been hearing a metallic ping noise sometimes when shifting back and forth between drive and reverse.
Not saying it's U-Joint related. But it seems if you've checked the engine and powertrain for cause and no findings, drivetrain/chassis should be your next location.
I also shimmed out my D35c back to spec. Sucker was getting really hot because I had .019" of backlash in the rear end. Wore the teeth down polished on the end. Nasty wear pattern, but no spun bearings. shimmed it down to .005" of backlash, and helped quite a bit with mileage along with the U-joints. That had to do with a lot of my mileage due to power loss in the drivetrain. I never noticed the gradual loss in power until I fixed these issues.
BTW, how are the o2 sensors running? the scope should show that the bank 1 sensor has a short sine wave, and the bank 2 sensor should switch at about half speed.
Also, what was vacuum at?
Another issue on the 4.0L is the cracked manifold that happens to virtually all 4.0L jeeps. A crack at the Y pipe which has a large enough crack can result in excessive oxygen getting to the oxygen sensor. Oxygen sensors are reading fine, but read excessive o2, thinking you have a lean condition, and dumps more fuel in to compensate.
What's fuel trims look like? or does you scan tool have fuel trim capability?
Good luck with the Jeep
Power does seem to be down a little. The U-Joints have been replaced and theres no vibrations so I'm ASSUMING that they are ok at this point.
Vacuum was 16 and steady @ idle. Snap the throttle and it went to 0 then 22 when I let off the gas. I've checked other cars and they had the same vacuum at my 3100ft elevation. Revved up and holding the vacuum went to about 19 and stayed there w/o dropping. Cats not plugged.
In 2000 Jeep got away from the tin headers that crack and went to cast iron manifolds with duel exhaust that come together at the rear of the oil pan. These rarely crack, no cracks in mine........I'd hear that as I hate exhaust leaks.
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Fuel trims stay near 0. Usually +2 or - 2.
The o2 sensors are constantly changing and I cant keep up with them. .300,.420,.680,.220,480 etc.
Last night I walked past the empty jug of gear oil. Looks like I used 85w140 last time. That wouldn't make that big of a difference would it?
This is terrible!
