MPG are horrible!? even for 5.4l!

[jlo82585]

Transmission slipping, perhaps?

I don't think so, it sure doesn't feel like it and from what I have read the trans on the expedition is pretty much bullet proof but the engine seems to work pretty hard as the RPM's rarely get below 2k when im moving.

since your dropping the tank you may want to consider replacing the fuel pump as a complete unite being your mileage is up there unless you know if and when the pump was replaced.

the fuel level sending unit I bought is the complete float / pump assembly so that's in the mail and will be done as soon as i get it

It may be worth changing the upstream O2 sensors. (nearest the engine) These are used by the pcm to tune the engine. You can have your exhaust checked for back pressure to determine the condition of your cats.

Do you keep your tires properly inflated?? Rolling resistance can have a big impact on fuel mileage with a heavy suv. I like to keep mine on the higher side to help reduce rolling resistance but well within the range of the tires specs. Tire wear and comfort are possible disadvantages but you can monitor tread wear and alter pressure as necessary.

wouldn't a bad O2 sensor set off the CEL? and I thought bad cats would improve mileage as there is less restriction ( provided it blows the material out of the exhaust ) but either way im having the truck straight piped next week. As for tires, as soon as I can find a good tire that has good off / on road specs im getting them.

 

[splintrcel]

What are you using to calculate your mpg. The onboard computer doesnt really give an accurate reading. It is lies! It usually says your getting way worse on high way then you actually are.

 

[jlo82585]

the onboard computer for the fuel display is currently non-functional as my truck does not have a functioning fuel level sending unit so I have been taking the miles on the trip odometer and dividing it by the gallons I put in at each fuel stop.

 

[FordandPolaris]

Bad O2's can read bad voltage (the way they communicate their data with the comp) without setting the CEL. They are not easy to change though, I've tried a couple of times and cannot break them loose.

Bad cats usually plug up, and so they restrict flow more. Very rarely does a cat blow apart.

 

[BAD RONALD]

Bad O2's... are not easy to change... I've tried a couple of times and cannot break them loose.

I haven't used the stuff but guys swear by this penetrating oil, esp for frozen O2's.

Welcome to MouseMilk.com

 

[toms89]

Narrow band O2 sensors such as used in the expy can only read rich or lean, they cannot actually tell what the air/fuel ratio is. The pcm maintains stoich (14.64 to 1) by constantly toggling back and forth between rich and lean. This happens very quickly when the O2 sensors are new or in good condition. As they age and/or possibly get contaminated they may become slow to respond and this affects the pcm's ability to tune properly.

Catalytic converters, if contaminated (coolant, oil, ? ), run rich for too long, overheat, etc..... can become clogged. This happens progressively overtime and they become more and more restrictive to exhaust flow and may be difficult to detect until it gets to the point it causes driveability issues or sets cel.

Yes they may eventually burn up and break apart if overheated long enough, and your vehicle doesn't stop running first, but generally not till long after they restrict flow.

 

[jlo82585]

well after the gotts mods and a tune up ( oil and air filter ) i am now getting 10 MPG in town so its an improvement. I am having the truck straight piped tomorrow so that should help too, as well as new tires late next week..
I never thought about the way O2 sensors work but I also have to ask is it a common problem for the O2 sensors to go out or give bad reading without tripping the CEL? And as for cat contamination, if I was passing oil or water through the exhaust wouldn't I be having a misfire or CEL (due to the O2 sensor) on one of the cyl? Im still hoping plugs, tires, and fuel filter will correct the issue but it still has 2MPG in town to go and that's a stretch for those fixes...

 

[toms89]

The on board diagnostics with the pcm is designed to aid in diagnosis. It is by no means perfect. Only when certain parameters are met, or sensors exceed a given value will it set a cel and store the code. Does this mean it detects every issue every time and know whats wrong with your vehicle? No. I really wish it would tell me what's happening with my expy half the time but unfortunately it does not. I have had severe misfires before but as far as the pcm was concerned everything was just cherry.

Seems many put a lot of faith in the pcm as I have seen this question asked frequently. After seeing the software and experience tuning my expy I feel otherwise.

Yes if the pcm sets a cel it has detected an issue but it does not necessarily follow that no cel equals no issue.

Been over 20 years since I took logic class and can't remember the term for this but it reminds me of that class every time I see this question posted.

 

[toms89]

Found this via google:

What the Home Mechanic Needs to Know about O2 Sensors

Probably explains it better than I....

 

[FordandPolaris]

Straight piping will not help for two reasons. One is you won't keep your foot out of it, and the other is that the engine will have no back pressure and therefore low end torque will suffer. MPG's goes with that.

I would second the notion that O2's will go bad before they throw a code. My MPG's dropped off noticeably with no CEL a few years ago, and only last year did I start getting O2 sensor circuit CEL's. They are the last part of my system that needs to be replaced, so they have to be the culprit as nothing I have done has really resolved the fuel mileage issue.

Also no, if oil and water is getting into the cats, it will not cause a misfire. This stuff is way past the spark plugs. Tom has explained this to me before, and if the O2's go completely bad (through being fouled or other means), the computer has a crisis mode tuning table that it falls back on. Basically the truck still runs fine, the MPG's just suck. Instead of burning money having the muffler cut out, have the guys change the upstream O2's and be amazed as your truck gets better MPG's.