I have an '03 Silverado 2500 4x4 6.0 with 143K miles, which I use mainly for towing my 25' (5700lb dry, 7300lb loaded) travel trailer. I live in Washington State, so I have to navigate hills a lot. I have noticed lately, my truck is struggling to pull my trailer up hills, and that goes for anything not level, or downhill. Also my fuel economy has significantly dropped from about 14 mpg to around 8 mpg. There is no noticeable knocking, no oil leaks (although it burns about 1/2 quart every 3K miles), and it starts without hesitation with just tapping the key. Oil pressure is usually around 40, climbs to 60 under stress. engine & tranny temps both seem nominal. I'm wondering if the engine is just getting tired and nearing an overhaul, or if the tranny just needs flushed, or both.

 

When was the last tuneup (spark plugs, wires, etc)? That's the first thing I'd consider. I've got an '06 Ram 1500 w/Hemi with 259K miles and it pulls just as good as when it was new.

 

most power related problems...start with the fuel pressure first. if you have a scan tool...what are the fuel trim readings at a 4th gear acceleration...if its adding more than 10% when loaded... its a sign the fuel pressure is low...check the fuel pressure with a gauge.

 

im with tec, check your fuel pressure. my 02 was actually running with 20 psi but if i had to give it gas to take off or pass someone it would just fall on its face

 

the facts are pretty vague at this point.

ok could be engine not trans, or trans not engine, or even computer that talks in-between them.

trans? low trans oil can cause issues (such as slipping). heavy hauling up hills "requires" (suggest) a trans cooler. do you have one? is your trans fuel buring and plaquing your trans?

ODBII scanner? you have one they are $20 plug it in tell us if there are any codes!! ($20 to $2000 - but $20 will do but make sure it displays trans codes for your car/model). surely you would not have one and pay a dealer to fix it, or try to fix such an issue without one??

YOU SAID MILEAGE PROBLEM. that means you should hurry. if you run your car with a mileage problem for a long time, if it's engine related issue, it could cost you your engine if not fixed (this is a good reason never to skip emissions inspection btw). but if it's the trans and slipping and not the engine - well it'd be better to not drive it at all if it's slipping - might damage the trans.

sorry i could give any more insight than that at this point