Here is one that started a while back. I was driving thru deep water and truck died. Threw back multiple codes, mostly misfires and catalyst damaging misfires. This was attributed to faulty wires/coil pack. Got those changed and did new spark plugs (Bosch OE finewire Iridium, 7MM Bosch Plug wires and AB coil pack). Left the shop and truck ran beautifully. Plenty of power, no stumbling at idle, all seemed well. Until fill up time. Before this event I could rely on seeing 220 miles on the trip meter at half tank (mostly highway). After the ignition work I am lucky to see 160 at the half tank mark driving the identical route, this has been going on for several months now, was hoping it would sort itself out (duh). So now I'm left with tons of questions, did the tech leave something undone behind the manifold? Is a plug wire arcing? Could I have clogged a CAT during the misfire event (I had to limp it over 20 miles home after it finally started after puddle jumping). Did the O2 sensor (or 4) go tango uniform during the misfiring? Did I break a valve or piston ring during limp home? I know this must seem like paranoia, but the drop in economy is real, and dramatically lower than before the event. I've poked around in the computers, no codes. Monitored the ECU real time data with nothing appearing out of the ordinary. Any ideas what I could be missing? Anyone else ever see this situation before? Next step is selling and getting LR3, but I love my disco and really want to sort this out. Thanks in advance for the assist.

 

My first thought - Your fuel gauge wasn't ever working properly. You're used to getting 220 miles at 1/2 tank?! I eek out 260m with the light on...

Dave

 

Start basing you MPG the old fashion way, way more accurate if you fill your tank and later devide your mileage by the gallons you burn.

 

Yes, did that too. I topped up tank and ran dry. Did this long ago, and did it again after the economy plunge. Its not an indication error. Wish it was.

 

That being said, a half tank on the guage was never really a half tank. I just used it as a reference. I could get 350 from top to dry before, using same trick now i'm luck to get 300.

 

Your front O2's have been neutralized.
Replace the fronts, they are the ones that control engine fueling.
I assume you have already tried cleaning the MAF, if it got wet it needs to be cleaned.

 

Does your check engine light come on when you first turn the key?

If so,
then the light works.

I would think you would have a few codes.

 

I have already cleaned the MAF, was thinking it was going to by O2 probe related, thanks! Will change out the front two probes and see what happens.

 

So the front O2 probes have been changed with new, while they were out I bore scoped the cats, both looked perfect. First tank of fuel yielded 13 mpg highway, not great but still an improvement. Going to replace the MAF sensor next, don't like the look of it, can't get the thing completely clean. Any other thoughts on this one?

 

So, I don't think you ever mentioned fault codes, do you have the numbers if so what are they? Not wise to be guessing. As for the MAF, buy one with the exact barrel model that are on your. Any cheap replacement will usually throw codes before it warms up.