I have a 02 chevy Avalanche with aftermarket air intake and duel exhaust. Well let me start off from when I noticed the problem. In October I had taken my vehicle in for an oil change, they recommended I get a fuel injection clean, so I said okay. That same day I drove my truck around ALOT because I do use if for work. I noticed that my truck was sucking down the gas more than usual, infact I only got about 300 mpg when I was normally getting about anywhere between 390 to 420mpg. This has went on for a couple months. I had a tune up done, I had a transmission flush, I've checked my tires, brakes making sure the Ebrake wasn't stickin. I cleaned the air filter. I have done everything. Well recently the check engine light came on. I took it to Advance and put in on the diagnostics the code that came up was EVAP Emission Control System Vent Circuit Malf p0446. I don't know if this has anything to do with the poor gas or what else it maybe. If anyone may be able to help. I would greatly appreciate it. Thank you all sooooo much

I have a 02 chevy Avalanche with aftermarket air intake and duel exhaust. Well let me start off from when I noticed the problem. In October I had taken my vehicle in for an oil change, they recommended I get a fuel injection clean, so I said okay. That same day I drove my truck around ALOT because I do use if for work. I noticed that my truck was sucking down the gas more than usual, infact I only got about 300 mpg when I was normally getting about anywhere between 390 to 420mpg. This has went on for a couple months. I had a tune up done, I had a transmission flush, I've checked my tires, brakes making sure the Ebrake wasn't stickin. I cleaned the air filter. I have done everything. Well recently the check engine light came on. I took it to Advance and put in on the diagnostics the code that came up was EVAP Emission Control System Vent Circuit Malf p0446. I don't know if this has anything to do with the poor gas or what else it maybe. If anyone may be able to help. I would greatly appreciate it. Thank you all sooooo much
ONLY got 300 Miles Per Gallon!!! I assume you mean 300 miles per tank? Winter blend of gas can affect performance to a small degree. Is your vehicle a flex-fuel vehicle? Are you using E-85 for fuel? I don't have my referance manuals available right now but the P0446 MAY be just a loose gas cap or charcoal vent solenoid failure, I need my manual! I would suggest that you only use Top Tier Gas and you won't need to be concerned with cleaning the fuel injection system. Top Tier will also control deposit build-up on the intake valves.

Winter fuel blends reduce mileage by 4-8% due to the lower energy density of ethanol. But that doesn't explain the 25% reduction she is experiencing. If the shop that did the fuel injector cleaning offered any type of warranty on their work (usually 90 days or 1,000 miles) I'd head back and complain IMMEDIATELY. Tell them you've had sercvice done there and that now you have more problems than before. Speak to a mananger. Even if they say you're out of warranty, if you can show the gas receipts that'll strengthen your case for bad mileage since the servicing.

In general, fuel injector cleaning is rarely a good deal. Dealers and shops "recommend" it because it is easy money and rarely does any real benefit to the consumer. Most fuels have detergents that keep them clean. Most varnishing and deposits form from city driving and short distance driving where the engine is run enough to get hot and then shut off. I'd suspect that the cleaning gummed up something in the evap system and/or caused a stuck injector.

Are there any other symptoms? Sluggish acceleration? Exhaust smells like gas or other foul smell? Rough idle?

The if the vent circuit isn't working as it should, then the fuel vapor canister can't be purged and ends up becoming saturated or vice versa, it isn't being used at all, allowing fuel vapors to escape.

A real injector cleaning involves removing them and setting them up in a fuel manifold to feed them and operate the solenoid valves inside them. If this was done, then there's the possibility that one was damaged, not reconnected properly, or is still sticking. The onus is on the shop to make things right since their reputation is on the line.

Hope this helps!

I know this thread is older but I have insight to the problem you are having. But before I answer, what did you do to your exhaust? As in what set up and what diameter of pipe, how many pipes etc.