Sudden drop in average MPG
 

Since I've owned my truck, which has been nearly three years, my average MPG shown on the dash has been 15.7. However, since I replaced my vapor canister vent solenoid, it suddenly dropped to 15.3. The check engine light has remained off since the repair, so I'm assuming that I did it correctly. Also, the MPG average was at 15.7 both before and after the check engine light came on. The drop only occurred after I made the repair and the light went out. Anyone have any ideas on this? :confused:

My truck: 2005 GMC Sierra quad cab Z71 5.3L

Got me... Hopefully somebody might chime in with a theory. It has something to do with the atmospheric pressure on the fuel delivery system

4/10s of one MPH is very little...Just different driving patterns will do that.......Are you sure you have driven far enough to get an accurate reading?

Plus, I wouldnt rely too much on your in dash readout of gas mileage, they are not that accurate.......Fill the tank and do it the old fashioned way

You're right that it isn't very much. On a 100 mile trip at $3.39/gallon, that's not even 60 cents. I just find it strange that it dropped like that pretty much instantaneously. Over the ~3 years I've had it, I've seen it fluctuate but by no more than 1/10 MPG at a time. Even after the custom K&N air filtering system was removed (I bought the truck from my friend who wanted to keep it), it dropped a total of 6/10 MPG but slowly and only 1/10 MPG at a time.

Engines will use more fuel as the get older and things get worn.....could account for some of it. My 97 has 140,000 miles...gets about the same as yours, although I dont figure it out all the time.

Were you driving with the light on for a long time? I'm not 100% sure but maybe the truck wasn't going into open loop because the light was on. Now that you're getting into open loop mode, the car gets more power/less MPGs as a result ???

Just graspin' at straws here really. If anything, I would expect the MPGs to go UP now that your computer knows it can suck the fuel vapors out of the canister. You got me. I wouldn't worry about it.

I'm not super concerned about it, just more curious really. If it stays where it is or goes up, then all is well I suppose. If it drops again, though, I'll be a little more concerned. But to answer your question, yes - I drove it around for quite a while with the light on. Probably close to a year or so. I only did that because I knew what the problem was and that it was OK to drive it without fixing it for a while.

Just went down to 15.2 today. Ugh.

I would use a couple fill ups to manually calculate the mileage you're getting. There are no sensors that will give you that accurate of a fuel consumption on a stock vehicle, so don't take it with more than a grain of salt.

My guess is that your driving pattern has simply changed. If you're doing more local, start/stop driving, or if you even accelerate slightly faster now, you burn more gas and that amount of drop is completely within the realm of possibility.

Hope it’s okay to bring this post back from the dead...
 

Hey guys, I’ve got the same issue, same circumstances on the same exact vehicle- 2005 Z71 crew cab with 5.3L. I changed the canister purge valve and the fuel tank solenoid and immediately lost almost 4mpg. The only difference here is I coincidentally reset my average MPG the same day. I had pulled my camper and it was showing 12.2- prior to that, I was getting 17 mph average. After changing the parts and resetting the MPG, I’m showing 13.1 after about 200 miles. I drive like a grandpaw and haven’t changed anything at all besides the emissions parts.