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Why would my MPG plummet?

Powerman

NAXJA Forum User
Location
Colorado
First off, this is not a "oh I wish I had better MPG" thread.

For a long time I was running bigger tires and high gears. Over the summer I put on 33s and 4.88s. I was happy to bring my MPG up. Stock I could run 18s. With the bigger tires I was in the 14s. After regearing that brought it to 16. Haven't looked at my computer in a while but when I did I was down to 13.4 MPG! I keep tabs, I know what my MPG has been.

So I replaced air filter, did my plugs even though they were not bad, ran a can of BG44K through it, and tried to stay out of the throttle. ZERO change, 13.4! The thing is, my engine runs good. It's a 2000 with 130K and I don't have , or have had any problems. I have a cold start issue, but I've always had that. (think it's the fuel pump check valve)

So what could cause such a drop in MPG and still have my engine running smooth? As far as I know, none of my sensors have ever been replaced. I have also been meaning to lift my valve cover. They seem to be making a bit of noise. But the MPG thing has happened sort of all of a sudden. Any suggestions??? Thanks
 
family TJ said:
Valves are non adjustable on the 4.0 so need to look at them unless your checking for a stuck valves or broken spring.

At least on that issue, there seems to be a bit more clatter. I've never had it opened up, but you can't tighten the rockers or anything? If not that saves some work.
 
PolishX said:
did you change the speedo gear ?

Good question, but yes.My MPG wasn't that far off before and I just compensated. But I did get it right when I regeared and my speedo is right. I regeared 6 months or so ago so I had good history what I was getting.
 
OK, will a failed O2 probe always give a CEL. Cause I was wondering that.
 
Also check your vac. lines for cracks.. typical on HO XJ's...

Also, suggest...replace fuel filter, run some Lucas oil stabelizer in the engine to help quiet valves and pump up the lifters..
 
Didn't think about vac leaks. I actually know of a place I should look. I will check that out tomorrow.

I don't have a replaceable filter. It is in the fuel pump. But I can throw some Lucas in. Thanks
 
Powerman said:
OK, will a failed O2 probe always give a CEL. Cause I was wondering that.
Not necessarily one about the O2 sensor, though. I had a P0171 (system too lean, bank 1) and it ran like total ass--I was getting between 8-11 mpg. I can't afford to throw money at problems, so I took my time testing every sensor I could (took weeks). I finally gave in and bought a Bosch universal upstream O2 sensor ($55 at AutoZone). My mileage went right back to normal, and it runs like before. Actually, the mileage is maybe 1-2 mpg worse than when it all began, but that was back in October when temps were higher and the winter-blend gas wasn't here yet.
 
When I replaced the bad O2 sensor on my 91, I wasn't getting a CEL. I did see a 1-2 mpg increase though..

Plus, my cat was bad. I replaced them roughly at the same time; the Cat was first and it brought my mpg up from abysmal to bad. When I did the O2 sensor, it went from bad to not too shabby.

:D

I don't remember the actual figures, but it seems the Cat gave me around 2 mpg back, and the O2 sensor brought it up to around 14 mpg (mostly city driving.)

:)
 
I keep my tires up. My XJ sat for 6 hours. I pulled the line off my vac resivoir and I still had vacume. Am I wrong in assuming I don't have a vac leak? I have 2 O2 probes on my 2000. How do I test them to see if they are working?
 
XJPhoenix said:
When I replaced the bad O2 sensor on my 91, I wasn't getting a CEL. I did see a 1-2 mpg increase though..

Plus, my cat was bad. I replaced them roughly at the same time; the Cat was first and it brought my mpg up from abysmal to bad. When I did the O2 sensor, it went from bad to not too shabby.

:D

I don't remember the actual figures, but it seems the Cat gave me around 2 mpg back, and the O2 sensor brought it up to around 14 mpg (mostly city driving.)

:)
X-2 A rich fuel mixture will kill 8 out of the 9 lives of the Cat in a hart beat. :greensmok
 
PolishX said:
depending on the year isnt there two 0- 2 sensors , one on the cat and one on closer to the engine ?

I thought that in 2000 they began using something like 3 sensors?...I know it was more than 2...
 
Powerman said:
Over the summer I put on 33s and 4.88s. I was happy to bring my MPG up. Stock I could run 18s. With the bigger tires I was in the 14s. After regearing that brought it to 16.Haven't looked at my computer in a while but when I did I was down to 13.4 MPG! I keep tabs, I know what my MPG has been.
Double check my screwball math, but I come up with a 16.25% change in mileage. Switching to winter blend fuels (aka oxygenated fuels), and operating in lower average ambient air temps, I had at least a 10% drop in mileage. The change may be greater than 10%, I didn't moniter too closely last summer to determine my best average mileage. In that light, another 6% is not all too dramatic.

So when did you record the 16 mpg, and the 13.4 mpg?

Regarding the vacuum question: I have a confirmed leak through my EGR (90 Renix), yet the reservoir holds vacuum. I guess that means the reservoir circuit works?

BUT, you can do a lot of diagnostic with a vacuum gauge. Own one of those?
 
Hypoid said:
Double check my screwball math, but I come up with a 16.25% change in mileage. Switching to winter blend fuels (aka oxygenated fuels), and operating in lower average ambient air temps, I had at least a 10% drop in mileage. The change may be greater than 10%, I didn't moniter too closely last summer to determine my best average mileage. In that light, another 6% is not all too dramatic.

So when did you record the 16 mpg, and the 13.4 mpg?

Regarding the vacuum question: I have a confirmed leak through my EGR (90 Renix), yet the reservoir holds vacuum. I guess that means the reservoir circuit works?

BUT, you can do a lot of diagnostic with a vacuum gauge. Own one of those?

My friends at work mentioned Winter and the gas, but I have never noticed this kind of drop before. I am not real anal about MPG, but I generally know what I'm getting just from flipping through my computer. The 16s came through Summer. It was consistent. My driving doesn't change that much. If it does change it's because I took a trip or did some wheeling. This is the lowest I've had and I've reset and tried to bring it up but nothing. I've had my XJ for 4 years now in Colorado and even with the bigger tires and higher gears it was never this bad. So to me this is unusual, but again I really don't have a clue cause my XJ is running good. If something was off, then it would be running like crap.

What do you sugest I check with a vac guage? Do you know how to test O2 probes?
 
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