sudden drop in fuel economy?

OK, here's an odd one. I've been somewhat reliably getting 15~16 mpg and then suddendly yesterday it dropped to about 12 (over 120 miles of same route as before). Today same thing. Only thing that I did was on Sunday I dropped a can of chevron fuel injector cleaner into the tank (I do that once every few months). Thing is that since I do that every so often I know what to expect: I usually loose about .5 mpg for the first half tank after using it (I fill up @ half tank) but then it goes back to normal. Scary thing is that it dropped more and it didn't recover. What is it? What should I check? I looked at my RPM's and they are nice and even. The engine sounds the same as it did before (for the most part if something changes in the way it runs I can hear it).

XJ has about 150K miles on it (not really sure as it's had the cluster swapped twice and it shows now 107K ).

wait, so you have you tried a fresh tank of gas? could just be a bad tank...

when was the last time you replaced the O2 sensor?

BrettM said:

wait, so you have you tried a fresh tank of gas? could just be a bad tank...

when was the last time you replaced the O2 sensor?

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I'm on my 2nd tank (half tank each time). I'll probably run this one to near empty and try again. 02 sensors? well.... shucks....
On a serious note, shouldn't 02 sensors throw a code as the odds of both of them going out at the same time are really slim and if it goes out it should really screw things up when it comes to mixture?

Kejtar said:

shouldn't 02 sensors throw a code as the odds of both of them going out at the same time are really slim and if it goes out it should really screw things up when it comes to mixture?

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Yes.
I don't presume to think you'd overlook a maintenance item as simple as u. joints, but my mileage went into the crapper right when both front axle joints went bad. Changing them gained me two m.p.g. average. Maybe it's something simple like that?

the funny thing is injector cleaner contains around 30-50% of diesel fuel inside the container. and high octane booster is nothing but diesel think about it what is diesel fuel also designed for other than fuel? lubrication. that is why they use it for octane booster less friction more hp

Timber said:

Yes.
I don't presume to think you'd overlook a maintenance item as simple as u. joints, but my mileage went into the crapper right when both front axle joints went bad. Changing them gained me two m.p.g. average. Maybe it's something simple like that?

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Those are OX joints and they have been greased less then 3K miles ago and they look solid (I did a visual inspection on them after the last wheeling trip).

Gotcha. Like I said, I didn't think you'd overlook something so simple.

Don't know what you have for an air cleaner, but it's worth a look. The simple stuff is sometimes overlooked.
My mileage went down significatly, when a rear wheel cylinder started to seep. I guess a shoe sucked up some juice and expanded some and I was dragging it a bit. I finally noticed the drag and heard the noise, but I could tell it had been fouled for awhile, before I noticed. Put it in neutral with the radio off and a window rolled down. See if coasts OK and if it makes any noises. Driving next to a wall often helps you to hear strange noises that are hard to hear in the open.
Bad gas is always a possiblilty, I get paid per diem and keep close watch on my odometer, mileage and gas bills. Every so often I get a bad tank and the mileage goes south for a tank or two. It takes a couple of tanks to get most of the wet gas out. Fuel injector cleaner is also good for bringing moisture into solution (the stuff that seperates and stays mostly in the bottom of the tank). Fuel injector cleaner often works just like gas tank dryer, I'm guessing the petrolium distilates work like alchohol.

8Mud said:

Don't know what you have for an air cleaner, but it's worth a look. The simple stuff is sometimes overlooked.

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Checked the filter Sat: it was clean.

My mileage went down significatly, when a rear wheel cylinder started to seep. I guess a shoe sucked up some juice and expanded some and I was dragging it a bit. I finally noticed the drag and heard the noise, but I could tell it had been fouled for awhile, before I noticed. Put it in neutral with the radio off and a window rolled down. See if coasts OK and if it makes any noises. Driving next to a wall often helps you to hear strange noises that are hard to hear in the open.

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Rear disks and they don't drag. In regards to noises? My rig is FULL of noises (although I think I can identify all of them still )

Of the two sensors the front one is the one the system really uses, I'm about to change mine now that the new engine is in, old one has 300K on it, original.
My mileage has been see sawing for the past two weeks, 18mpg on one tank, 20mpg on yesterdays refill. Working almost EXACTLY 50mi from home, 100 round trip tends to really be a no brainer for milage... 200miles and anything above 11 gallons I start getting antsy...downside is 35 of those miles are on the pa turnpike at 85mph in the morning run in....70 on the way home. I've also got some strange 'dead' spots when accelerating, now that it's warm out I'll be poking around with a meter this weekend and checking for any bent pins from the engine install..

Temerature sensors can mess with your mileage, I'm not real familiar with the 97, but much of the stuff is fairly universal. I know on the Renix the higher the resistance the colder the temperature (the colder the temperature the richer the mix). Connectors are worth a look, for moisture oil or whatever. Does the 97 have a MAT sensor? The MAT in earlier models gets covered in oil and goop. The MAP is always suspect when it's fuel related, especially rich. In my experience when the O2 starts to go, the motor often runs lean at speed and the idle hunts some.
The last couple of things that springs to mind are low compression and/or restricted exhaust. Both will lower your vacuum and make the motor work harder.
The last time through my harnesess (just for comparison, two weeks out of the shop) I found, one pin that had backed out of a ECU connector (iffy contact), two that backed out in the TCU connectors and a bent pin in the sensor harness. And oil in the TPS connector (really don't know how that got there).
Before I take it down to plug up to the test machine, I make sure all the perifials are clean and functioning as best I can. The shop gets paid by the hour and it adds up quick. Even with the high tech. support, they screw it up as often as not.

mines getting crappy mileage too, I can't figure it out. Every time I do a mod I get +2 mpg and then it gradually drops away after a few tanks, it drops lower than it was before.
electric fans netted me +2mpg for 4 tanks,
3" exhaust +2 for 3 tanks,
oxygen sensors +2 for 2 tanks,
header +2 for 2 tanks
tb spacer +2 for 3 tanks.
I have slowed down to driving at or below the speed limit, and my mileage is up to 16 from 11.5.........but it was 16 back when I swapped in the fans, and I was too busy enjoying my new power to drive like a granny.

oh and my fawkin ce light keeps comin on. somethin about purge system flow incorrect.
I know my injectors are clean, they looked brand new when I installed the header.
hopfully the temp sensor can help us because this thing is killing me at the pump.

see my post here.... maybe we can get the temp sensor questions put to bed... http://www.naxja.org/forum/showthread.php?t=81250

All I need to know is what the resistance reading is suppose to be a 210 for an OBDII vehicle.

On my 89 i was getting around 15, and it dropped to 8 or less ughh. It seemed to be a gummy cold start solenoid. Cleaned the throttle boddy and it was better but also found a leaky vaac to the MAP sensor. Got 18 on a long trip...so it worked 4 me..good luck

This is weird i just turned 150,000 on my 97 and my fuel economy took a sh!t too(10mpg), i cant figure it out i went over evrything i even ran a full tank of 94 octane through it and babied it , i got 13mpg nothing changed but 2 weeks ago i was getting 16mpg what is goin on??

I got the feeling the distributors are playing with the gas again, I've noticed this happen several times over the last couple of years. Just as a test get as far away from your local area, NJ even, and fill up there. NJ is alot more attentive to their gas quality than PA is, all PA cares about is the tax and correct metering on the pumps even if they are pumping water... NJ actually samples the gas and tests it...
Every time our 3 jeeps and the wifes olds go downhill on gas a fill up in NJ confirms it's not the vehicles, literally had that happen 5 times so far.

Timber said:

Yes.
I don't presume to think you'd overlook a maintenance item as simple as u. joints, but my mileage went into the crapper right when both front axle joints went bad. Changing them gained me two m.p.g. average. Maybe it's something simple like that?

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sorry to hi-jack real quick, but i'm just wondering how you explain u-joints changing the fuel mileage. i've never heard or even thought of that before. and i'm also working on horrible gas mileage problems on my 95XJ

They start to dry out and drag some. Its kinda like holding the brake slightly while driving. Added resistance = more work the engine has to do.

My 98 has been getting 13-14mpg since I got it used with 110K on the odometer. I wasn't sure if this was due to the 4.5" lift, or if it was normal. Sounds like I'll be checking the U-joints this weekend. It also seems to have been poorly maintained. I am hoping a tranny/differential/transfer case fluid changeout helps.

They start to dry out and drag some. Its kinda like holding the brake slightly while driving. Added resistance = more work the engine has to do.

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If they were that bad (bad enough to make a 4mpg difference) I think you would hear them, or notice that they were smoking or exploding.