6.0L Fuel Mileage

[[email protected]]

6.0L fuel mileage dropping over the last 2 months. I used to be able to fuel up and the range would be around 310 miles, now when its full it says 180 miles, It also doesnt seem like its running as well as it used to..The check engine light has never come on.. I do have a edge instight monitor that showed codes that I assume were pending codes since the check engine light never came on. The codes were two different ones saying fuel system lean but I cleared them and havent came back. Ive cleaned the MAF, Checked for vacuum leaks. I dont know what else it could be. Ive been told that it could be the cats or the o2 sensors but if that were the case why wouldnt it throw any codes?? Unless they are just starting to go bad?? Any one had this problem or any ideas?? Im about to just replace the cats anyway and hope that fixes it, the truck has 130k miles and still has stock ones. Any suggestions for high flow performance cats? has headers and is built motor with supercharger

[Stevens03]

I'd change the o2 sensors before changing the cats

[tyhuck]

Always start with the cheapest fix and work to the more expensive options. YOu dont want to replace several hundred dollars worth of parts to find it was a bad something-or-other that only costs a couple of bucks.

[69camarorag]

Really?........"built motor with supercharger" and your worried about mileage?

[tbhuge]

If both O2 sensors are going bad on the upstream, (bank 1 sensor 1 and 2), it wont throw a code because they are both responding slowly. I havejust experienced this in the past week with mine. Both my upstream sensors were bad as well as my mass air flow sensor. I also had zero codes. I have a friend who has a shop and he did a diagnostic on it. I also had just cleaned my mass air flow sensor right before winter. Cost me around $400 in parts this past weekend.

[[email protected]]

Thanks for the information. I'll change the o2's out and see if that fixes it.

[[email protected]]

I had my buddy do a diagnostic at his shop, No hard codes or anything. His tech2 scan tool shows the mV that the o2 sensors put out I have no idea what the mV's are supposed to be around to know if the sensors are bad, They all seemed to be pretty close to the same numbers they fluctuate a lot at idle but under load they were all between 700-850mV. Dont know if thats any way to tell.. also wondering if it could be fuel pressure? Would that have effect on mileage? Im wondering if my fuel pumps just getting weak and not getting the fuel pressure required. any ideas?

[tbhuge]

I am unsure what the O2 sensors are suppose to be reading, your buddy at the shop should be able to tell you. I just know that mine were both out so therefore mine did not throw a code, because they were both responding slowly and not throwing a variance between sides. I'm not sure how set up your buddy is for diagnostics, if he is just trying to help, but it isnt really his forte, you may want to look around at another shop to do a full diagnostic to figure out where the problem is before just throwing a bunch of parts at your truck. Shouldn't be more than an hour or two worth of time at whatever the local shoprate is. Up here it is $95/hr. So even if they worked on it for 2 hours that is only $190 to know exactly what is the problem. Otherwise you end up spending WAY more than that in just replacing a bunch of parts that are not ready to be replaced.