damn gas mileage, whats wrong here

[danny Benavidez]

1. If you are buying ethanol mix, expect to lose up to 10% of MPG.

I got a 2001 and the above statement is very true

 

[twistedlizard720]

What about a steering pump going out or some other accessory the belt drives?

 

[Jays00XJ]

ummmmm.....this needs to be explored a bit. The engine should run smoothly in all conditions, all engine temperatures.

So it runs rough on startup periodically but ONLY when engine is cold? Never does this when the engine is hot? Do you ever notice extended crank times (having to crank longer than normal) when the engine is cold?

Do you currently have or have you ever seen a check engine light? If so, you have stored codes which should be pulled with a code reader or a scan tool for guidance, then post the exact code numbers here for comment.

Like once a month if that, on a startup either hot or cold, mainly hot, it will missfire (run rough), if I rev it a little it usually doesnt go away unless i do that for a minute or so, but if I go ahead and just drive it it goes a way in like 30 seconds or less. And when motor is cold she cranks up no problem everytime.

And as far as I owned the vehicle the check engine light was on for just a day and it ended up being my evap control system when I read it, basically my gas cap was loose.

 

[Jays00XJ]

so whats goin on?

 

[Talyn]

Like once a month if that, on a startup either hot or cold, mainly hot, it will missfire (run rough), if I rev it a little it usually doesnt go away unless i do that for a minute or so, but if I go ahead and just drive it it goes a way in like 30 seconds or less. And when motor is cold she cranks up no problem everytime.

Soak for heat soak on here. There is also another cold start issue which should be mentioned in several of the same threads.

 

[pajeepman]

Mine has sucked in the past few months also. Bone stock 2000 XJ sport with 225/70/16's. When I got it in April it was getting around 17MPG. Last tank it was 13.1 and has been around that for the last couple of months. No check engine lights or anything, It just got a new head on it last month, that didn't help it or hurt it. Sucks.:bawl:

 

[kastein]

Replaced the upstream O2 sensor on my MJ and gas mileage went from 13 to 17 (two tanks through it @ 17mpg so far.) And that's on winter gas, and doing an average of 80mph while on the highway.

EDIT: also, like I said before, pretty sure it was the original :shocked: it was in there REALLY tight, I couldn't even budge it with a foot long 7/8" box wrench till I braced against something and stomped on it. After that it turned very very stiffly till I dumped penetrating lube on it, then it came out very very easily. The difference was night and day.

 

[Talyn]

Soak for heat soak on here.

I meant search, not soak.

 

[joe_peters]

I meant search, not soak.

Why don't you just go and search your head!

 

[pajeepman]

Will a faulty o2 sensor trigger a check engine light? I guess you need 2 new sensors for upstream on a 2000. Any brand O2 sensors better than others? The build sheet form Chrysler said mine has California emission and rockauto.com has different ones listed for Cali and non-Cali emissions. Does it make any difference which one you get if you aren't in Cali? I don't really know how Cali emissions is different than non-Cali.

 

[birchlakeXJ]

Oxygen sensors can get "slow" or "lazy" over miles and therefore don't optimize fuel air ratio quickly which can result in a loss of gas mileage.

When they reach a point where they can't keep within programmed values or with other types of failues within the sensor, it is designed to trigger a check engine light but don't expect to see one when they are in the early stages of failure.

The "average" life expectancy of an oxygen sensor is between 100-150k. Somewhat dependent on how the engine is running. I've seen some original sensors go to 200k and still test pretty well for response time on an oscilloscope.

 

[kastein]

Will a faulty o2 sensor trigger a check engine light? I guess you need 2 new sensors for upstream on a 2000. Any brand O2 sensors better than others? The build sheet form Chrysler said mine has California emission and rockauto.com has different ones listed for Cali and non-Cali emissions. Does it make any difference which one you get if you aren't in Cali? I don't really know how Cali emissions is different than non-Cali.

Cali vs 49 state differences are in the exhaust manifold and catalytics I believe. If it says one or the other, you probably need the ones they specify. Bosch pretty much invented the O2 sensor, so I stick with them personally, since they are pretty fairly priced.

 

[Jeepguy03]

If the rough idle happens on hot starts only then like others said its the heat soak issue. My 2000 did the same thing and I had the dealer install the injector shield. Haven't had the problem since. I would also ditch those plugs. I ran those on my 2000 and it idled like crap and ran rough at times. With the coil pack ignition the plugs make a big difference. I put in the regular champion copper plugs and it runs nice and smooth.

 

[Talyn]

NGK is actually the proper plug for the 2000-2001 to help combat heat soak issue.

And if the rough idle happens from cold start it is a PCM reprogram. There is a TSB on that.