This has always been a great place to come for good information and I'm
hoping this time won't be any different. ;)
I can generally get around 12-13 mpg with pure interstate driving (no
big headwinds or anything) from my '99 K3500 with the 7.4L engine.
This past week I needed to drive about 250 miles on mostly interstate
with an empty car-hauler type trailer, pick up 20 school desks (total
weight couldn't have been more than 400-500 lbs.) and return home.
Before leaving, I checked all fluids and everything looked normal.
About halfway into the trip I filled the tank and noted we had been
getting a little over 11 mpg. This is exactly what I expected at that
point, so the trouble must have started after.
Arriving at our destination, the first thing I noticed was that the
engine was idling a little slow and a little rough. Also, there was a
serious lack of acceleration from a standing start. I thought maybe
we'd gotten some bad gas. When we stopped to load up, I opened the
hood and there was a strong electrical smell and a slight odor of
gasoline. This past winter we had a mouse build a nest on top of the
plenum, and my wife thought it smelled more like mouse nest to her,
like maybe we hadn't gotten it all out. I left the hood open to let it
cool better and air out some.
After we loaded everything up and got it tied down, we took off. No
change in performance. On the way home we DID have a bit of a
headwind, but nothing too serious. We stopped to make sure everything
was riding okay and fill up. Now we're getting about 7 mpg.
This truck would ordinarily pull this trailer like it wasn't there,
even with 400-500 lbs. on it, but now the transmission would shift down
every time we got to the slightest grade.
The last time we filled up, we were getting just over 6 mpg.
The "Check Engine" light doesn't come on and I don't have a scan
tool.....yet. I've ordered one of these units that hooks up to a
laptop so I can check all the sensor readings and see if there are
codes stored that don't activate the CE light. I don't actually know
if that's possible, but I've been told it is.
On a near-full tank, I put in two bottles of fuel system cleaner
w/Techron - not a double dose, but each bottle only treats up to 13
gallons or something and I've got a 34 gallon tank. There was probably
about 26 gallons in it when I added them. I've run it some since then
to give the cleaner a chance to work through the system. I still get
the same odors from under the hood - electrical and gasoline.
I also have a slight coolant leak from the intake manifold gasket which
- thankfully - is leaking exclusively to the outside.......so far.....
I'm figuring on replacing the intake manifold gaskets, which will
require R&R of the plenum, which will give me access to the fuel rails
and injectors and wiring.
I'm open to any and all suggestions here. One thing I DID do was
disconnect the battery cable for a few hours in an attempt to reset the
computer. No change in performance or symptoms occurred.
Thanks to anyone who has any ideas.