I have a 1999 GMC 5.3L truck that has 70K miles and have had the mileage drop from 18-20 mpg to 15-16 mpg with no error messages. The engine has no noticable misses, running poorly, etc. The drop was not gradual. Any ideas on what could be wrong?
by 
teedoff
ANSWERS: 2
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Check your air filter. If it is clogged, then that will affect gas mileage since the engine can't breathe. Try a few fuel additives to clean out injectors, condition the gas, etc. Sometimes if there is a small clog in the engine, it will affect the mileage. I wouldn't expect it to drop that much but you never know... Another thought: spark plugs...ever changed them? This could be letting unburnt fuel run through the engine if they aren't all functioning properly. Normally, fuel mileage goes down gradually so I must admit I'm a bit confused as to what it could be in your case. Good Luck, Hope this helps
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If you have a tach. watch it as you accelerate up to 55 mph. It should be around 1800 rpm when you are in forth gear and the torque converter is locked up. Sounds like that perhaps you may have a trans problem. Not getting complete overdrive. As you state the engine is not running poorly that pretty much rules out ignition problems, May have a fuel pressure reg. leaking. To find out pull the vaccum hose off it and start the engine and watch the nipple on the reg., if it gets wet with fuel or fuel comes out then the reg. must be replaced.
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