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Discussion Starter #1
I've got close to 225,000 on my truck. No codes. I gutted the pre-cat a couple of years ago, and the main cat last week due to it rattling and found it to be coming apart inside. Runs good and drives good, but the mileage is really poor. Full synthetic oil change every 5,000 miles. I've got a scan tool with live data, but have no idea what to look for that would explain the poor MPG's.
Please don't about tire pressure, driving habits and how you get 30 MPG's out of your truck. I've always gotten pretty much what the advertised expected MPG's are for vehicles I've driven in the past. My last daily driver was an '01 Bonneville SSEi, an I got 18 around town and 28 on the interstate. Just like advertised for the vehicle. Over the 3 years that I've owned the truck, the mileage has decreased from 17, to around 13.5, but increased to 15 when I gutted my main cat. Since I've had the truck, the plugs have been changed to what is recommended here, air filter changed several times, regular oil changes, a few sensors changed to address CEL warnings to address problems as they arise, no real problems with the truck since I've had it. Occasionally I pull a small 4'X6' utility trailer with lumber and plywood in it. I love the truck and it has been incredibly dependable, but I sure wish I could squeeze a few more MPG's out of it. With the slight improvement in throttle response and MPG's from gutting the main cat, I'm considering a less restrictive muffler. Since the main cat only had the honeycomb flopping around in the center and none of the "filler material" around it, I'm thinking the muffler might be filled up with that foam stuff that usually surrounds the honeycomb, but I've been told that when that filler stuff deteriorates, the pieces are very small and just burn up or blow out over time.
Any suggestions would be appreciated. I don't "hot-rod" my truck. It's a valuable tool that I use for work, and I take care of it like I do all of my other tools. I want it to last a long time and serve me well. I bought the truck at 180,000 miles and I have no idea what maintenance and repairs were done prior to me owning it. Compression test was impressive, so I feel if there was ever a valve issue, it was taken care of. It was purchased new by a female in 2005, and traded in at the same dealership in 2014. The ownership then transferred from one dealership to another, then to a dealer auction place, then to the used car dealership that I purchased it from. Very little mileage put on the truck between 2014 and when I bought it in 2016, so I consider myself to be the second owner of the truck. Again, no codes, no rough idle, no strange noises, and I've addressed every little issue when I've gotten a check engine light and/or code.
Thanks guys.
 

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That is a pretty good guess about the cat material plugging the muffler.
I would change that out. Listen to sound clips to pick your muffler.
What sounds good to 1 person may not to another.
 

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Winter blend fuel which we have here up north really kills mpgs but not sure what blend they use were you live. That may have an impact. Colder weather also decreases mpgs. Also things wear out over time such as axle bearings, trans clutches which cause decreased mpgs. With all things added up can cause a noticeable change in mpgs. Tire conditions too. Such as worn tread. You didnt mention if All fluid changes have been made regularly either.
 

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O2 sensor can be lazy.

My precat broke open and never even set cel.
I thought rattle was in muffler at first.
Welded it back up and added a brace. Oem should have flex tube.
 

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Don't know about the 04 3.5, but when I changed out the O2 sensors at 100k miles in my 08 3.7 it made a big difference - right about two MPG.
 

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at 100k mi, I did plugs, O2 sensors, seafoam treatment, AF... Made no difference at all... Very well possible there are CAT problems.
 

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Wait, you guys are getting 14-15 MPGs??? My truck has been getting 10-13 for the longest time now... :LOL:
 

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Discussion Starter #10
Over the past couple of years, I've changed thermostat, O2 sensors, plugs, fuel and air filters, and have done all regular routine maintenance on schedule. When traveling on the interstate, I got 17 MPG's, but I set the cruise control on 80 and go.
 

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Over the past couple of years, I've changed thermostat, O2 sensors, plugs, fuel and air filters, and have done all regular routine maintenance on schedule. When traveling on the interstate, I got 17 MPG's, but I set the cruise control on 80 and go.
This may sound stupid, but trust me. throughout my years of learning in the automotive field. (also going to school for it currently) anyway, parts of the car can actually affect the gas mileage a LOT on a vehicle. it can be: a fog light housing missing or out of place, the hood not be properly adjusted, bug guards that are broken, bent sidesteps, bumper out of place, etc.
 

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Discussion Starter #13
When I was trying to figure out my rattle in my exhaust, I did get a code pertaining to the cat. I can't remember what it was, but after tightening all exhaust bolts - to the point of breaking a couple - and adding hangers to try to stop the annoying rattle, I got an error code basically stating something about the efficiency of the cat, I waited for the truck to cool down and I tapped the side of the cat and that's how I found my rattle in the exhaust. Exhaust rattles are very difficult to figure out, because the noise resonates the whole length of the exhaust because it's a tube. At the back of the truck, it sounded like it was coming from the front and then under the truck in the front, it sounded like it was comin from the back!! Frustrating!!
 

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After the first tire replacement on mine which dropped my mileage about 2 mpg, it has stayed pretty consistent between 22-23 mpg. Sometimes a little less depending on where I fuel (angle of approach to the pump can alter how much fuel I get into the tank) or if I run it faster than 65.Like most, closer to 21.5 in the winter 23. 5 in the summer. Mine is a 2.9 with auto, standard cab WT model. I just turned 162,000 miles on it.

Is your tire pressure correct? Colder temps make the tire pressure go lower....I pay more attention to mine in the winter just for that reason.
 

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Old Guy,
I liked the idea of replacing the muffler. Those have been known to plug up.
I would recommend going to a heavier wt. oil. If you are running a multi-grade oil. During your next oil change, I would change to a heavier grade of oil. Say 40wt. Or at least one that states ”for high milage engines”.
 
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I use 4w40 Mobile 1 oil and the Mobile 1 filter. Use it all year round. I like it.
 

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Why would heavier weight oil help? Not criticizing your advice, but just asking. I've always heard in out trucks, we need to stick with recommended oil so that timing chain tensioner and VVT actuator work properly. I'm not leaking oil anywhere that I'm aware of.
 

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I most likely do not need this weight oil but it is the oil Mercedes recommends. If it is good enough for it it is good enough for me. Ours takes 5 w 30 so it will be ok. I am changing mine today it will warm to 48 degrees about noon. I will do it then
 

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This may sound stupid, but trust me. throughout my years of learning in the automotive field. (also going to school for it currently) anyway, parts of the car can actually affect the gas mileage a LOT on a vehicle. it can be: a fog light housing missing or out of place, the hood not be properly adjusted, bug guards that are broken, bent sidesteps, bumper out of place, etc.
Ive heard a dirty car can even decrease mpgs. A cleaned and waxed car is definitely going to have less resistance which should increase mpgs.
 
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