Hey guys, new to the forum as of this morning. Went into panic mode last night about my gas mileage. I have a 2007 H3 and have had it since last August, 75000 miles, no issues until now. Last week my engine light came on, called onstar and they said my emissions system wasn't functioning normally.. I thought it is running good so no big deal ill get it fixed when i can. Last night I took my wife out for dinner. I drove round trip 75 ish miles. When I started I had a more than full tank of gas. Now I am sitting slightly above half a tank. Correct me if I am wrong but I think something is a little off there.. Any advice would be fantastic! Thanks in advance!

 

75 miles round trip and starting off with a full tank of gas, then something is wrong.
You could have a gasoline leak
The gas gauge could be malfunctioning and you don't have a full tank.

You mention you started off with a little more than a full tank ! A full tank is a full tank.
Did you top off before you went out to dinner ? Don't trust the auto shut off on the pump.
Did you smell any gas after you got home ?

I'm just spit balling here, but 75 miles(ish) and you should have used up no more than maybe four gallons of gas. This depends on how you drive.

 

My point exactly. I filled it up from E put in close to 20 gallons. i didnt drive it hard at all actually it was mostly highway miles.. i ran the codes last night and got 4 and they are P0300 general misfire, PP0455, P0442, P0449. Planning on replacing the spark plugs this afternoon hopefully that will fix the misfire. Would these codes throw off my gas mileage that bad? smh.

 

P0300 will affect your fuel economy. Most common causes are ignition(plugs,insulators,coils), fuel (injectors), vacuum leak (intake manifold), or faulty camshaft position sensor

 

You first said you had a more than a full tank of gas, then drove about 75 miles and then ended up with slightly above a half of a tank. How can codes and bad plugs cause such terrible mileage
Am I missing something ?

A half a tank of gas is about 11.5 gal. drive 75 miles and it comes out to 6.5 mpg

 

Hold on... Wait for it... I've been waiting for ALL the information too...I asked for a refill calculation(miles driven/gallons to refill), then stated, only the p0300(out of all the codes retrieved) could cause a loss in fuel economy. I'm still waiting too.

 

Alright. from the top. i put 20 gallons in the H before i left. drove round trip approximately 75 miles. When i got back my gas gauge read slightly above half. when i ran the codes originally i got P0300, P0455, P0442, and P0449. Today i replaced the spark plugs and ran all the vacuum lines that i know of found nothing. Added 1 can of Seafoam to the half tank of gas that i have now. Disconnected the Neg battery terminal then reconnected it and the engine light came on again. Had Onstar run a diagnostic test on it they pulled one code which was P0449. Any ideas?

 

Pulling off the EVAP solenoid today. anyone have any experience with this?

 

Before you start freaking out further, do the easy stuff. Wipe off the rubber gas cap seal ring make sure it is nice and clean, put it back on tight. Check that your intake manifold is not suffering from loose bolts (sometimes they are just finger tight). Have you cleaned your Throttle Body and MAF? You are due.

 

There's plenty of information on this site, or you can google it. Main thing is disconnect the canister and remove from the rig, then work on a bench. Read the following threads:

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