I have a 2005 GMC Sierra 2500HD that is getting poor gas mileage. Yes, it is a truck, and yes I expect bad gas mileage but 7.5 miles/ gallon is horrendous. I pull s 4 horses trailer sometimes, so no comments about getting a smaller car please. A year ago it was getting about 14 miles/ gallon+ and something has caused it to get significantly worse. I have replaced plugs, wires, had the intake cleaned and new air filter. I have had the catalytic converter checked and everything is good. I have switched to only Shell gas to eliminate the cheap gas. Anyone have any other ideas.
Just finished driving one of those things up British Columbia’s Coquihalla highway and back. Pretty cool truck…not the best at cornering lol. I would change the fuel filter…its possibly dirty/ineffective. The good thing is that is a small repair ($30 I think) so its a low opportunity cost. The other thing I would try is depending on where you park your truck you may be leaking from the fuel line and not know it. I park on gravel, so leaks arent always obvious. Put some cardboard down under the length of the truck that you grab outta a dumpster or whatever, and see if its wet in the morning after it was parked overnight.
You may have a “lazy” oxygen sensor that’s not functioning properly, but is not bad enough to cause the “check engine” light to come on. In that case, the truck’s computer will revert to a “default” program which causes the engine to run rich, significantly worsening fuel economy.
Another possiblility is a bad coolant temp sensor, which will cause the same thing.
Both the oxygen sensors and the coolant temp sensor can be checked at the dealership.
Maybe a sticky thermostat.
I change 'em every 4-5 years anyway.
If your 02 sensor is so bad your truck will only run in open loop your going to set a code,same thing with a coolant temp sensor. I don’t agree you need to see a Dealer to get these things checked (the car has built in diagnostics to monitor these things,it is not 1960 anymore). An Independant with a good drivability tech that knows how to run his scanner can tell you exactly what is happening with your vehicles fuel management system.
Not to argue but my Buick was getting about 12 mpg and never set a code. Changed the O2 and it immediately within 5 miles on the highways jumped to 29 mpg. You can confirm the lazy O2 or temp sensor though with a computer diagnosis for $100 which should have been done before the other work.
A lab scope would be my tool of choice to confirm the switching rate on an 02 sensor. With the scan tool you can see where the fuel trim sits, this would be a good clue as to if a problem was trying to be compensated for.
If you know what to look for and an accurately measure time you can use a scanner (or a DMM) to verify switching, but it is harder. You can OHM the temp sensor for the PCM and compare to a expected value to tell if it is out of range. Fuel pressure ought to be checked also. All of this stuff can be checked and nothing needs to be guessed at
Follow Circuitsmith’s advice since the thermostat is a fragile and antiquated cheap little part that no one has improved in years but that has an asymmetric effect on the rest of the engine if it fails, especially now.
I have had this exact same problem with my truck. Unfortunately it was the 99 to 2002 body style and the 2005 that you described so it may not apply to you. I took it to the dealer and they fixed it for a pretty hefty price. I don’t remember exactly what they changed but it runs great now. Dealer mentioned it was a common problem for my model years.
Best of luck!