Im stumped, terrible gas mileage

[Slade]

i have a 2002 xls 4.0, about 150,000 on the clock. have been getting 10-11 mpg. new plugs, fluids, clean air filter, new fluids in the front and rear dif, transmission serviced, pcv valve replaced... anyone have any ideas. no real idle issues, transmission shifts ok. i should be driving an excursion for this kind of mileage...

one thing though, the truck sounds like it isn't idling down after startup. i know thats vague. you know what your truck sounds like when you start up and take off in the morning, but as it warms up it runs quieter? does that make sense? my truck sounds like that all the time. it runs fine, just doesn't quiet down and guzzles gas...

 

[RKelly2233]

Your talking about that loud fan noise on a cold start? Do a PCM flash at the dealer- cost $90 and it retrains your tranny to new settings. Also- do you have a heavy foot? In cold weather you gonna get worse gas mileage- the grade of winter gas is worse for your car, 2 somthing about the vehicle not being up to temp. I drive like a pVssy now and dont let my rpms get above 2K that often and I get 17 right now- and Ill easily get 21 on the summer even with AC on. Mine is 4wd btw

 

[Slade]

yes its the loud fan noise. no i don't have a heavy foot. mine is also a 4x4. so having the pcm flashed will fix the issue?

 

[Ronin8002]

fan clutch could be bad...causing your fan to be fully locked all the time. this causes a lot of drag and can reduce your mpg

 

[Slade]

swapped the fan clutch when i bought the truck. no difference.

 

[Ronin8002]

Dumb question but did you by chance replace it with a "Severe duty" fan clutch?

 

[Swp Phillies Fan]

when my EGR valve went bad i was only getting around 10 mpg. but my check engine light went on when that went tho. just an idea

 

[Robsbrd99]

Im kinda wondering if its idleing down at all after startup? It should idle just a bit higher and drop gradually. Was thinking it could have a vacuum leak or the throttle body needs to be cleaned

 

[ExPlOrErImPoRtS]

fuel filter replaced?

 

[skeeter123]

You have a tachometer, right? What rpm is it idling at?
Sure sounds like the fan clutch; did you swap it with a new one, or a previously-used one? I'd take it for a drive, turn off the engine, open the hood and try to rotate the fan by hand. Should be fairly easy to turn.
I have a 2002 XLT with 4.6L, get ~17 mpg combined, same as when it was brand new.

 

[Ronin8002]

My comment about the severe duty clutch was just because the severe duty models stay locked up most if not all the time.

 

[2002GreyHD150]

fan clutch, egr stuck dirty TB/throttle body elbow all possible reasons.

Tell us what you find.

Rob

 

[jrford]

I'd vote for a vacuum leak. The truck can idle higher than normal rpm, as its getting more air then expected the PCM should try and adjust delivery to compensate. After a while i would expect it to throw a code but maybe not, depends.

Another idea is the engine getting up to normal temps 195F? Dash Gage isnt good enough, need to find a way to measure it. If always cold it will never achieve something called 'Close Loop' which i where the best mpg comes from.

Also how long are the trips? My wife got 10mpg with a 1.5mile commute, thing never warmed up and never went into Close Loop.

So I too thought something was up. One tip someone mentioned was to fill it up and take it on a 100mile highway trip, or closest i could to keep the speed consistent, used cruise etc. Once i hit 100miles filled it up again 5.5gals and that gave me 18mpg. This is better over several tankfuls etc but for a quick diag it worked. Also if you have a trip computer reset the mpg and set the cruise to 65-70mph flat and level and after a few miles what does it read?

 

[tv4fish]

Slade - What rpm's are you showing when you're out on the highway at around 55-60 mph? That gas mileage is way off for the 4.0. I have an '02 EB and I get at least 16-17 even in the Winter!!

 

[Slade]

alright, let me see if i can address all the questions,

-i thought of the fan clutch, was one of the first things i changed, i don't think it was a severe duty.
-i am doing the fuel filter and servicing front and rear diffs this weekend
-will check the throttle body for build up, and elbow for a vaccum leak
-when i start it up, cold it idles a little above 1k and idles down to about 8 or 900
-we don't have too many 55-60 mph hwys that i have easy access too, at 70 i run just a hair over 2k
-im thinking there might be an iac issue too, weird idle surge and drop after startup, cold stall when changing gears...
-threw a code a while ago and did the dpfe.

 

[Robsbrd99]

I have a couple more ideas, like jrford said, the coolant temperature switch could be bad causing the computer to think cold cold cold and keep the fuel mix way rich. Also a sticking thermostat could cause the same thing. Seen a few people here with that problem. Does it warm up pretty fast or slow?

 

[Slade]

the temp sensor and or thermostat is a different idea. i will look into those as well. thanks for the ideas. with regards to how long it takes to warm up, it doesn't seem to take too long.

 

[irocdupi]

I get terrible mileage around town with my 4.0 94xxx miles, well maintained, new plugs, fuel filter, air filter, oil every 3k, new thermostat, all fluids changed including front and rear end. And after all i see as low as 11-12 on the winter, but some of my trips are quite short two to ten miles. Last time i took it on the highway I averaged 18 pulling my jetski up and down hills in upstate ny doing about 80 up and close to 90 down