Low MPG, 2000 XJ, OBD-II solution, problem cause found

I think this is worthy of a new thread. Yesterday I worked with T3hk1w1 on his recent drop in MPG on his 2000 XJ. He did not have an analog volt meter...etc.

First off two mechanics had worked on this jeep and given up already. There were no OBD-II codes at all. The jeep ran fine, no symptoms other that sudden drop in mpgs. O2 sensors were new. TPS, etc was new. Engine was even new.

I tested the CTS, it was good. The MAP tested good, and the MIT was off some (reading about 190 F at 150 F), but not enough to be the sole cause of the MPG drop.

So we tested the upstream O2 sensor. The two white wires, grounds, were good. The gray O2 sensor was reading lean, .05 volts, locked in place, not moving at hot idle. The black wire, 12-14 volts the O2 sensor (HEGO) internal heater was only getting 3-4 volts. The rear O2 sensor was only .5 volts, not 12-14 volts. The final test was at 2000 rpm, engine fully warmed up, where we got a nice steady 0.45 +/- .02 volts out of the upstream O2 sensor, which is what it should do with a working O2 sensor.

At 2000 RPM in park, hot, the O2 sensor gets hot enough to work properly, so the PCM (ECU) thinks the O2 sensor is OK and working, and thus does not throw any OBD-II codes or check engine lights. We checked the 12 volt path to the O2 sensor's heater, and it gets to, and past the ASD relay and 15 amp fuse, but drops off, or out on the last leg of the wiring to the O2 sensors, which the computer can not detect!

So in slow traffic, stops, low rpm operation the PCM gets a lying steady lean signal from the O2 sensor, and swings the A/F ratio to the max rich table values trying to restore the 14.7:1 stoich, thinking the O2 sensor is working. When RPMs pick back up everything works fine! So long story short, loss of the 12 volts to the heater on the O2 sensors can cause low MPG with out throwing any codes even a 2000 year XJ.

Good work.

Have you found the reason for the drop off yet?

joe_peters said:

Good work.

Have you found the reason for the drop off yet?

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It has to be a bad connector connection, or busted conductor hiding inside the wire, which is hiding inside the harness. The wiring diagram showings nothing after the fuse except wire and the O2 sensor connector, QC. Since both O2 sensors are getting very low voltage, and different voltages, I am guessing it is the Y splice that splits the power to the 2 sensors that has gone bad, come loose, or broken the wire at that spot, which is no doubt hiding and buried in the harness, god knows where, in the last 6-8 ft of wire to the sensors.

I left him with the task of finding and hunting the bad wire spot, up to him. I also offered him the alternative of running new wire. I did the hard part that 2 previous full time mechanics could not find, LOL, with nothing more than a +20 year old low tech meter!

I will trace down and repair/replace the damaged part as soon as I have time, and will post up when I find it. It sucks doing work on your DD-it's hard to find down time.

Thanks for the update. What a PITA to deal with on your DD. I suppose you are just going to run a temp jumper?

joe_peters said:

Thanks for the update. What a PITA to deal with on your DD. I suppose you are just going to run a temp jumper?

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I told that is what I would do. Only problem is temp solutions that work, become permanent.:laugh3:

Ecomike said:

I told that is what I would do. Only problem is temp solutions that work, become permanent.:laugh3:

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That is so, so true! :roll:

It's temporary until it's proven to fix the problem. Then zip-tie it to the loom and it becomes permanent

T3hk1w1 said:

It's temporary until it's proven to fix the problem. Then zip-tie it to the loom and it becomes permanent

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Ahh, I always wondered what the zip ties were really for. I thought they were just their to slow down my repair work.:laugh3:

Zip ties are an affront to the true technicians best tool--duct tape!

Anyone have a wiring diagram for the O2 sensors? I'm a very visual person, and cannot figure it all out. The wires on the O2 sensor do not match the ones in the Haynes manual for my 1997 XJ.

Thanks guys, and sorry for digging up an old post

Buy a copy of the FSM, it's worth it's weight in gold. http://www.techauthority.com

As the wiring diagrams are part of a copyrighted work, owned by Chrysler, they won't be posted here.

Glad you brought it back up... bringing up good tech that is relevant is ALWAYS better than starting a new thread from scratch, IMO.

Makes me wonder if this is why my 98 only gets 11-12mpg (entirely stock!) - I replaced the upstream O2 sensor because the original was really, really bad. Stuck on rich reading and as a result I got whiplash whenever I drove it as it repeatedly started stalling out and then surging. DTCs indicated bad heater and signal stuck high, but was cleared up by replacing the sensor. I'll have to toss a meter on it and see if it's now stuck low... or just reading lazy.

The haynes is worthless... definitely buy an FSM for your year, you can generally find legit hard copies (with some greasy finger prints and dog-eared pages) being sold on ebay for 40-80 dollars per set. My ebayed 90 dollar set for my 98 netted me six different manuals covering all sorts of interesting things :read:

I was having this same issue, so I thought. I did some basic maintenance, plugs, wires, cap, rotor to no avail. Then I tried a fuel system cleaner from Lucas and guess what? I'm up at least 3-4 mpg! I still can't believe it! Not sure if it's a permanent fix but so far I'm 2 tankfuls in and I'm averaging about 15-16mpg in city driving. Much better than the 11 mpg I was getting when I bought the truck a month ago.

edit: The truck is a 99 XJ and is nearly stock except for a K&N drop-in filter and a DynoMax Exhaust.

chpmnsws6 said:

Anyone have a wiring diagram for the O2 sensors? I'm a very visual person, and cannot figure it all out. The wires on the O2 sensor do not match the ones in the Haynes manual for my 1997 XJ.

Thanks guys, and sorry for digging up an old post

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Check your PMs.

Dumb question time again: California or Federal emissions?

I'm guessing Federal from the earlier statements, but just want to be sure.