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bad fuel mileage

2K views 14 replies 10 participants last post by  dauntless89 
#1 · (Edited)
Im having problems i have a 97 cummins 4x4 5 speed 3.55 gears i cant seem to get over 14 miles to the gallon I drive decent it has a intake my egts hardly ever get over 350 degrees going down the highway has a new air filter fresh oil change anyone have any ideas
 
#6 ·
When you are driving 55 mph what is your RPM? It is possible your torque converter is not locking up. It is also possible you are not getting into overdrive.

I would check your fuel filter and pre-filter. That is just good preventative maintenance. I bet the real issue is those tires.

One other real possibility is you are getting better mpg than you think. Oversized tires show LESS mileage than you really put on. Although you may think you are getting 14 mpg perhaps you are really getting 18 mpg. If the speedometer gear was not changed your speedometer should be off.
 
#13 ·
How many miles does it have on it, the stock muffler could be getting stopped up. Weak lift pump or worn over flow valve will cause low power which in turn will cause poor mpg. Do you have fuel press or boost gauges. Go to user CP and fill out your sig.
 
#14 ·
I seem to be getting pretty low mileage compared to pretty much every one else as well. But most of my driving is city.
 
#15 ·
One time, when my fueling/exhaust was stock, I towed about 5,000 lbs 35 miles or so. My normal 20mpg was unchanged on that half-tank (I refuel often to avoid pump-side heart attacks), but every tank after that dropped by 1 mpg for about 3 tanks, at which point I could tell performance was suffering. This was right around the time my local stations were switching to winter-blend diesel, and I assumed this was just the fuel. I grumped, sucked it up, and pressed on. A couple weeks later, I was playing with my boost reference line to see how much boost my stock engine would build, and romped on it pretty hard as part of that process. I noticed it seemed to run stronger after that. I also did that "testing" was at the start of a full tank, and when I refilled, I was magically back at 20mpg.

Here's why:

Wikipedia said:
Diesel exhaust contains relatively high levels of particulate matter (soot), consisting in large part of elemental carbon. Catalytic converters cannot clean up elemental carbon, though they do remove up to 90 percent of the soluble organic fraction[citation needed], so particulates are cleaned up by a soot trap or diesel particulate filter (DPF). Historically, a DPF consists of a Cordierite or Silicon Carbide substrate with a geometry that forces the exhaust flow through the substrate walls, leaving behind trapped soot particles. As the amount of soot trapped on the DPF increases, so does the back pressure in the exhaust system. Periodic regenerations (high temperature excursions) are required to initiate combustion of the trapped soot and thereby reducing the exhaust back pressure. The amount of soot loaded on the DPF prior to regeneration may also be limited to prevent extreme exotherms from damaging the trap during regeneration.
 
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