What the ?$*# Gas mileage plummet

[cykaaro]

Have a '90XJ and believe it or not I get half decent gas mileage since runs 5.5" lift, 33's and none too light. The last couple weeks I've noticed all of a sudden my gas mileage has plummeted. Getting 450km/tank and getting a little over half of that now. I'm trying to trouble shoot why, but so far no luck. Anyone have any ideas?? I'm wonder if my TPS may be acting up or such. I know if I take off the ground from the battery and reset the computer, it'll go back up to where it should be, but seems after a couple tanks of gas, it goes back down again. No real clue what I should be looking at.
TIA!

 

[beakie]

thermostat?
last year when it started getting cold up here (in Ontario myself) I didn't notice my engine staying cold for a while, until the milege dropped so much I went on the hunt for the problem. Turned out my t-stat wasn't working and the engine never got into the proper temp zone... constantly pissing the petrol in to warm it up.
$13 later problem was fixed.

I found when my TPS went it caused engine problems, it was very obvious, jerking, stuttering and such.

I am sure some more ideas will pop up on here tho to help you out.

 

[Saudade]

O2 sensor on the exh manifold.

 

[RichP]

I noticed the new ZJ is getting poor milage, 15 mpg or so, but I also noticed that the thermostat goes up to 210 then opens and drops way down to the tic mark between cold and 210 then stays there which explains the code of 'too many cold cycles' when we put the snap-on modis reader on it on tues. It's snowing/raining here now and a bit too chilly but next opportunity it, tstat and cap, gets changed. It's opening correctly but not cycling as it should be.

 

[langer1]

I would also suggest something to do with tempature, t-stat or the engine temp sensors, you have two, one for coolent temp and one for intake air.
Either one can cause rich mixture.

 

[Matthew Currie]

I notice also that gas prices have risen sharply, and that, I'm told, is not just because of higher prices per barrel, but because distributors have finally run out of summer fuel, and are now charging for the new stock of winter fuel. I haven't checked my own mileage recently, but am told you can expect a measurable drop in MPG just from that.

 

[langer1]

I have found also the middle grades of gas give better MPG and often it's enough to save money.

 

[xjbubba]

Bad O2 (IE-"lazy") will cause rich running, and poor mileage.
Bad temp sensor (ECM temp input) can cause rich running, but will also cause higher idle. So if engine idle has not changed, I'd R&R the O2 sensor, especially if it has more than 50-60k miles on it.

 

[Pharaoh XJ]

did 94 HO have O2 sensors in the manifold too or is this just renix.
Cause I cant find anything plugged on my exhaust manifold ?

 

[RichP]

did 94 HO have O2 sensors in the manifold too or is this just renix.
Cause I cant find anything plugged on my exhaust manifold ?

All bets are off on yours LOL, I have no idea what an export to that continent had. Most XJ's for many years had one O2 sensor on the exhaust manifold and later another one by the cat.

 

[Blaine B.]

Higher octane fuels burn faster, right? So wouldn't your mileage down down.....however a small amount?

Also what does the intake air temp sensor control? I clean all of my sensors from time to time but wasn't really sure what it controlled. Does it go hand in hand with the coolant temp sensor to control the fuel mixture?

 

[Matthew Currie]

Higher octane fuels burn faster, right? So wouldn't your mileage down down.....however a small amount?

Also what does the intake air temp sensor control? I clean all of my sensors from time to time but wasn't really sure what it controlled. Does it go hand in hand with the coolant temp sensor to control the fuel mixture?

Octane rating alone should not affect flame speed. It only affects the energy required to light it off, with higher octane starting a little later. Once ignited, they should burn the same, so say the sources I've read anyway.

 

[Pharaoh XJ]

Seriously is the O2 sensor on the exhaust manifold or on the down pipe?
Can I spot it from the hood ?

 

[RichP]

On all our 97's and 98 the first converter is on the down pipe before it turns under the firewall.

 

[cykaaro]

Had to have an e-test done recently, and O2 sensor was replaced and passed with flying colours. Emissions levels are way under what the max requirement are. Engine always has been that way. Last weekend new 195* t-stat put in, and had to flush the heat core [totally clogged up so now finally have heat again!!] Cooling system is running exactly how it should. It typically runs just under the half mark, and at the hottest point gets a hair above the half mark It hasn't run like that in probably years.
As per idle, mine has always idled on the low side of what it seems most others have [and has been that way for the last 4-5yrs I've owned it]. The temp sensor I believe is fine. I have a second one put in and can monitor both so if one ever fails or give troubles I should be able to pick up on it. With the cooling system redone, I now notice a slight difference between the two temp sensors which before were always the same temp.

 

[Timber]

Xeleventybillion on the O2 sensor(s).
After I sank my engine in 4.5' of water, it ran like ass (I know--what else could I expect?). I checked everything under the sun (or so I thought) for almost two months. It turned out to be a bad O2 sensor. My symptoms were horrible mileage--~8-9 mpg--and a check engine light code which turned out to be misleading. Until I figured it out, I would end up disconnecting my battery about four times a week to get the CEL to go away and bring the mileage back up--temporarily, of course.

 

[xjbubba]

Seriously is the O2 sensor on the exhaust manifold or on the down pipe?
Can I spot it from the hood ?

It's quite possible your XJ is set up to run with leaded gas; consequently, you wouldn't have an O2 sensor.

I came across EPROMs for my Chevy ECM that did not support an O2 sensor, because they were intended for export to middle east countries.

 

[gray_penguin]

Xeleventybillion on the O2 sensor(s).
After I sank my engine in 4.5' of water, it ran like ass (I know--what else could I expect?). I checked everything under the sun (or so I thought) for almost two months. It turned out to be a bad O2 sensor. My symptoms were horrible mileage--~8-9 mpg--and a check engine light code which turned out to be misleading. Until I figured it out, I would end up disconnecting my battery about four times a week to get the CEL to go away and bring the mileage back up--temporarily, of course.

Do you remember what code yours was giving?

My 99 4cyl only gets about 17 mpg and I have a misfire on #2 code showing. I have replaced all the ignition parts and the injector on #2 and I still get the CEL coming on. I am wondering now if O2 might really be the problem.

 

[Timber]

P0171--too lean, bank 1. I really thought it was the MAP, and that's what most suggestions leaned toward due to the code thrown. A Bosch universal O2 (~$65 at Autozone) fixed it right up--and I mean immediately!

 

[cykaaro]

Going to put in the new O2 sensor later today when it's a wee bit warmer. DAMN! it's cold out last few days! so hopefully it'll fix my gas mileage. I'm thinking that because I'm getting some blowby it possibly may have fouled it? Also wass suggested to me it may be the MAP that is causing it as well. Any ideas on that?