Millage (again)

My 92 XJ is the greatest motor vehicle ever crafted by the hand of man.
But at the risk of starting the great millage debate again... The millage kinda sucks. I get 12.5 MPG combined city / hwy. The speedo is calibrated.

It's all stock except for:
3" spring lift
30" tires
SYE
Neon injectors (made 0 millage difference).

I drive like an old man (because I am an old man). 55 on the hwy. It runs like a champ. The ignition stuff is all < 5k miles. The tail pipe is black even after long drives but they all seem to be. The only problem it has is that it bucks going up hills about once every 3 months.

You guys have any ideas to improve the MPG a bit? I'm not looking for the moon here but if I could get it to 15 or 16 it would make a big difference.

Marc

When was the last time you changed your O2 sensor?

Bucking, and/or stalling, and/or a 30-40% drop in gas mpg's usually is an O2 sensor issue. Check the wire plugs for corrosion or damage, the wire for damage, and test the O2 sensors. Typical O2 sensor service life is 100,000-150,000 after which they tend to become slow to react and cause you fuel mpg's to go down.

Thanks for the idea. the computer threw a code so I replaced the O2 about a months ago. It was an AutoZone special but it fixed the code. Maybe I should test it anyway.

Marc

Mile+age = Mileage

Perhaps you're right. The rig has 130k miles on it which doesn't seem too high. I've got a Toyota with 200k on it that still gets 35 MPG but maybe the jeeps older engine design suffers more with age.

To me it seems more like a sensor causing the computer to run the engine rich. I pulled a plug yesterday and it looked more of a brown color. I've saw expecting it to be more white. I'm not sure how the new alcohol fuels have effected plug color. I've replaced the O2, TPS and coolant temp recently while chasing other issues. Not sure what else to try.

I wonder about the exhaust having a restriction of some kind. Do catalytic converters ever get plugged up? How can you tell?

I've been thinking about running a differential compression test on it to see where it stands mechanically. I've only done this test on am airplane engine where you can control the engines rotation by man handling the prop. I have to figure out how it's done on an automotive engine.

Thanks for any thoughts.
Marc

How does it do at smog test time?
Pluging it in to a scanner can give alot of insight, if you have access to one.
An engine that young running only 30" tires should get great mileage.
Sensors, grounds, fuel and ignition system.

It does Ok on the smog test.

I keep thinking about getting a scanner but every time I go looking for one I get bogged down trying to figure out what to buy. Do you have any suggestions?

Marc

Most scanners I have used were in the 5k range. There are cheap ones in the 100-200 range that are pretty good. Anything snap on or matco are good.

marc davis said:

It does Ok on the smog test.

I keep thinking about getting a scanner but every time I go looking for one I get bogged down trying to figure out what to buy. Do you have any suggestions?

Marc

Click to expand...

If you have an Android smart phone you can pick up a BT OBD2 reader on amazon for cheap. There is a thread about it in tools I think.

I've read the app is awesome, but you have to buy the part that plugs into the car. Think it was like 200 bucks.

When was the last time you pulled out and cleaned the IAC

OBDII Bluetooth adapters are $19 shipped, but they're useless on a '92...

Put 30in tires on my 3.07 gears and lost 3-4 mpg. Threw 3.55s from an auto in it and got them back.

I replaced the IAC about 6 months ago. How often do they need to be cleaned?

Thanks for the idea of getting a BT OBDII but my 92 is an OBDI so it won't help. I've thought about getting an old SnapOn scanner. It's a little hard to decipher exactly what I would need to buy but maybe I'll just have to suck it up and do it.

My jeep has 3.55's in it.

Marc