over a few recent trips I noticed my Disco had 10 mpg. New wires, plugs, and air filter made no change. My long term fuel trim was at 10.2 indicating dumping fuel to accommodate for a lean condition.

no O2 codes were ever thrown and the voltage was fluctuating within spec.

Other owners said was an air leak. I found no leaks.

i made a guess and put in new O2 sensors anyway.

Instantly my long term fuel trim dropped from 10.2 to a steady 3.1 and fuel economy increased from 10.2 mpg to around 13-14 (lifted D1, 255-85-16s, winch, bull bar, etc so heavier than stock).

moral? O2 sensors may be **** but not throw codes. If you have bad mpg it's a simple fix.

Also: easiest way to change is to remove the coil pack and you can lay over the motor and use both hands to reach the plugs. I have no idea how people change them from underneath.

 

Ive been having catastrophic MPG for a while too, will be giving this a go soon. What did the O2 sensors set you back? and how long did it take you to swap? Thanks.

 

They were $106 shipped off Amazon.

I removed the cool pack and plug wires and could easily reach both sensors to unplug and plug in the new ones. I have no idea how people reach up from below and do it blind.

i lay an old blanket over the motor and just laid on my belly across it.

mine also came out of the exhaust easily but I can see where that $25 tool would be handy.

took maybe an hour with a smoke break

 

Sweet, thanks for you info

 

It was pretty easy removing the coil pack - four nuts, but they are small. Keep one of those magnets on a stick handy. You really have to lean up over it to see all four of them - a 1/4" drive works with an extension. I can't remember the size of them - 10mm maybe? If you can get them to where they are loose enough to reach with fingers so you can keep a hold of them. Lots of nooks and crannies for them to drop into. Make sure your plug wires are numbered! Once you remove all the wires you can see better to remove the coil pack.

Once the coil pack is completely out of the way you'll be able to look on top of the bellhousing and see where the 02 sensors come in from each side. I installed them into the exhaust first and then pushed the wires up to the top of the housing, then got back on top of the motor laying across it from the radiator and could reach them both. They clip two ways: into each other, and also into a metal hanger thing that keeps them in place. You can slide them off the hanger and then clip/unclip them together. They unclip by pinching the back side of the one in the wiring loom - it raises a piece of plastic that catches two divots on the 02 sensor clip. You'll see it when you get yours.

I put them in the exhaust first to keep from twisting the wires by screwing them back in if they were clipped first.

Its still kind of a pain in the rear but not nearly as bad as doing it blindly by just reaching up from the bottom.

 

Couldn't agree more. Put in exhaust first THEN clip.
In my other post I noted I just located o2 sensor extension wires that appear to match our type, if you have the 4 wire. However, they're listed for Hondas and Acuras. Come in various lengths up to 36 inches. Plug them babies in and never have to make that crazy reach in there again. Again, I have not confirmed them 100% but by picture they're an exact match. I'll know by the end of the week but they look spot on.

​​​​​​​http://m.ebay.com/itm/Extension-Harn...801?nav=SEARCH
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So my long term fuel trim has jumped back up to 11.2. That's higher than it was before I replaced the 02 sensors (and the the LTFT had dropped to 3.1).

It has been only city driving, 2 miles each way, for the past week.

On Discoweb the consensus seems to be an air leak causing the 02 sensors to think I'm running lean but I've been over every inch of rubber tubing and can't find anything. I also couldn't find a shop with a smoke tester to test for a vacuum leak but I did the cigar trick - nothing showed itself as leaking (I know that's not a fool proof way of testing).

My fuel pressure is only at 30. I have a gauge on it now to see if it drops over a few hours but it didn't drop immediately (usually the sign of a leaky injector). One theory is that the fuel pressure is too low causing a lean condition so dumping fuel resulting in higher LTFT?

So I figure the more eyes the better - anyone have a guess as to why LTFT would be so damned high? New 02 sensors, new plugs, new 8mm wires. No apparent vacuum leaks (small possibility there is one but I think it would be quite small).

When I put on the new 02 sensors the LTFT dropped to 3.1 and I bumped up my MPG over highway driving by 3-4 mpg. Over the past week the LTFT jumped up to 11.2.

The MAF seems to fluctuate when using the OBDII reader but that's no guarantee (it wasn't throwing bad 02 sensor codes).

I never throw any code other than an ABS rough line thing though my ABS fuse is pulled because I know my ABS is faulty.

Once warm it idles smoothly around 700 or so (as I recall).

 

You're focus shouldn't be on the LTFT, should be on the STFT only. If you're seeing better fuel mileage in city driving then the O2's solved that little mystery. Now why I say that.
4 miles round trip a day is hardly out of the ECU's choke routine (run rich until operating temp) in either 2 mile direction, you should be riding a bicycle. If you really want to mitigate a vacuum leak smoke'n a stogie aint gonna cut it, you have to get dirty. Don't know if you have new head gaskets yet and if so who did the work but I can tell you from experience beyond the Rave that if you don't square the intake to both heads especially if milled before you button em down you just wasted your time. The plenum doesn't use a gasket either, that's factory. Well guess what, after many thousand heat cycles how long before you see a couple thou warp between the two? Are your MAF numbers moving while the truck is idle? It shouldn't. The air moving through should be very steady and if not you either have a vacuum leak or your have an intake/exhaust valve not sealing or the MAF is pooping out. I could go on and on. Your gonna have to do some serious detective work in order to rule in/out what, if anything, is not up to par. To me that's the best part of owning one of these old gals. Compression test would be a big help in so far as at least you'd know the internal breathing condition of the hard parts. Another point on the guts of the motor is the cam, they notoriously are known for thin annealing and wear quickly through subsequently effecting the motor's inhale/exhale efficiency. An aftermarket cam (US made) is the cure. So STFT/LTFT really isn't a player until the motor condition is well known unless you've owned it from day one and have been religious about it's care and feeding.

Your fuel pressure is only a pound or two low, not really a player yet. If it was in the 20's it would be time to replace filters and then pump.

 

Thanks. I've been told the STFT is nothing to worry about and to focus on LTFT. From what I've read high LTFT could indicate either a vacuum leak or a bad fuel pump (or bad MAF).

Trust me, I know. However I live on barely 2 lane roads (most are 1 lane) very heavily travelled, with immediate drop offs into ditches - there's no white lines on either side and no room. It bothers me like crazy (I rode 10 miles ech way for years in CA, but in WV it's impossible). My temp gauge does read half way (I do let it warm up and it lives in a garage that's heated) and my I do go into Closed Loop. I do need to get it bck out on the highway and see if the LTFT pops back up after a couple miles - it was at 3.1 on a 300 mile RT immediately after new 02 sensors but after a week of back and forth driving jumped to 11.2

I'll check that next and see.

The entire top end was rebuilt 20k miles ago, including new cam. I've had it about 4k miles and it runs great other than the bothersome poor mpg. The PO was very skilled, made a lot of custom suspension parts, documented the entire top end rebuild, etc. That's not to say that gaskets and whatnot couldn't be leaking, but from a rebuild standpoint he wasn't an amateur.

Thanks for the additional advice.

 

LOL...... I'm glad your into it. I could barely breath I was typing so fast. IF it was in my mitts I'd still do a compression test just to see the numbers, it's not because I don't trust others. How many miles on the rest of the motor? Does the oil smell like fuel? Another issue I've seen in the past is the FPR. That can either have a diaphragm/hose leak or fuel deposit crust on the needle not allowing enough to pass back to the tank. It's the most expensive piece of fuel kit naturally.