grantman18

I've done a lot of searching and I have done almost all of the recommended tasks to get my mpg's into the "normal" range. I ave a stock 2000 xj with 136k miles. Over the last 8 months (3500 miles) I have done the following maintenance/repairs:

Oil change (x2)
New air filter
New copper champion plugs
New cat-back exhaust
New water pump
Cooling system flush
New power steering pressure hose
new front pads/rotors
New accumulator and compressor
Replaced both upstreams O2 sensors with Bosch
Techron fuel system cleaner

Odd observations - 1. before the coolant flush, the whole system was really rusty and gross, even the inside of the hoses
2. When I replaced the brakes the inboard pads on both sides were totally worn where the outboard pads still had about a quarter of an inch left
3. A while ago I got 02 heater error codes that never came back after I cleared the codes.

Other than that my truck runs great, except for the 11 mpg that I get on every single tank, which is mostly city driving. I thought maybe my brakes were sticking but that doesn't seem to be the case. Could dirty coolant cause poor efficiency? Once I flushed the system the engine went from running at a tick over 210 to a tick under. Is there anything related to the 02 sensor Heater I could look at? Any tips or advice is appreciated, this mileage is killing my wallet.

OEJ

Well ur 02 heater code could still be an issue. When that happens the motor constanly runsin open loop. It never goes to closed because the heating element never reaches the right temp. Wigh is ususally about 600 degrees. So the 02 keeps searchin for gthe right temp so it can close andthus the motor goes out of rich fuel mode, if u will. That's the reason ur getin crappy milage. It always runnin rich.

The mistake u made is byin an after market 02. These motors are funny. I just went through this with mine. You have to get an o2 from the dealer. I was told that the signal was diff in the bocsh o2's.

grantman18

I just checked all of the unmarked fuses in the PDC and they were fine, so I checked the relays and they all operate properly as well. I suppose that the issue could be the Bosch sensors, but I doubt that's the case because the performance didn't change after installing them...it just stayed as terrible as it was before, it didn't get any worse.

This morning I performed the "PCM" reset by disconnecting the battery and discharging the capacitors. Supposedly this will flash the adaptive memory. I'm hoping that maybe the truck is just running off of bad habits from the old sensors.

If this doesn't fix the problem, does anyone have other solutions? The next step I guess is to run a compression test to see if I'm losing power because the cylinders aren't holding pressure. I have a 2000 so it could be the dreaded cracked head, even though my oil shows no signs of coolant. Any thoughts?

grantman18

I talked to my mechanic today and he was stumped. The only thing we couldn't rule out was a fuel system issue or a cracked exhaust manifold. Are there any tests for the exhaust manifold? I plan on renting a fuel pressure tester and running through the diagnostics for the fuel rail and injectors. Other than that, I'm all out of ideas. Anyone?