Silverado deteriorating fuel mileage

1metalguru

Silverado deteriorating fuel mileage

Unread post by 1metalguru » Sun Jan 10, 2010 5:16 pm

Hi:

I have an '07 Silverado crew cab short box 4x4 with a 4.8l Vortec engine. The truck has about 120000Km or about 75000mi on it. I bought it used about 6 mos ago.

My fuel mileage was never that great, about 17-8 mpg around the city, but it has steadily declined over the last 6 months. Last 2 fills were just over 12 MPG! And that's Imperial gallons, about 25% larger than a US gallon! There are no codes to be seen, it seems to still run ok, ie lots of power and no misses or anything. To be honest, I live in Canada, and the last month or so has been pretty cold, so I expect a bit poorer mileage in winter, but this is ridiculous. My friend has an '06 5.3l, (although not a 4x4) and he gets 20-22mpg in the city! It's not like I drive like a wild man either, I'm pretty conservative...

I'm thinking plugs, I don't know if they've ever been changed, but shouldn't the factory iridiums get over 100000 miles? Also, a parts guy suggested O2 sensors, anybody have an opinion on this? They are pretty pricey, I want to make sure they will fix the problem before shelling out $3-400 for new ones... Fuel filter has also been suggested, but I can't see how this would give poor mileage. Anyway, I cannot find a replacement part # which suggests there isn't an external fuel filter on this model.

Thanks in advance, guys...

Meccboy

Re: Silverado deteriorating fuel mileage

Unread post by Meccboy » Sun Jan 10, 2010 7:16 pm

Thats right. There is no filter to replace. It is internal to the fuel pump.
Spark plugs would be a good idea even though they are 100k mile plugs. Make sure air filter is clean.
O2 sensor would show a code, or cause running problems if 1 or more where bad. Dont really think that is your problem.
An intake air temp. sensor or coolant temperature sensor are possibility. Either can be reading inaccurate, but not enough to set a code.
Of course be sure you are not in 4WD too much of the time.
Fuel systems on todays vehicles are extremely accurate and sensitive to problems. There should be a code set if mileage was that bad.
Possibly injector dirty causing it be open too much and casue a rich condition.
As you can tell, there are several possible reasons. A scanner reading live engine data may be needed to look at all sensors to determine what might be happening.

1metalguru

Re: Silverado deteriorating fuel mileage

Unread post by 1metalguru » Sun Jan 10, 2010 7:35 pm

Thanks for the quick reply.

I forgot to mention that I have a 3" magnaflow catback single exhaust and 4" air intake with 10" conical gauze type filter. Filter is clean.

I have a computer trip monitor that has some realtime gauge functions. I have looked at the intake air sensor and coolant temp sensor readings, they appear to be correct, as do the MAP sensor. I cannot monitor O2 sensor voltages, AFR, or things like knock sensors with this unit, however.

I have used 4WD a few times over the last month when there are storms, but rarely and for only a few minutes at a time. My commute to work is fairly short, so it is possible that the engine spends much of it's time in open loop. I have checked that, and it warms up fairly fast, usually just a few miles and its up to operating temp. But, this has been the case ever since I bought the truck, so I doubt that has much of an effect.

Is there any free or reasonably priced software out there that could monitor the sensor individually so I can troubleshoot this problem better?

Any other causes you can think of?

User avatar
butchkaz
Site Admin
Posts: 173
Joined: Wed Nov 18, 2009 6:13 pm

Re: Silverado deteriorating fuel mileage

Unread post by butchkaz » Mon Jan 11, 2010 1:13 am

I read through this post.
OK, well aftermarket items can cause odd readings with the factory software looking at MAF and o2 sensors. I might suspect the exhaust.
Being a dealer tech, dont have much experience with the afermarket.
Spend all my time just trying to keep up with GM :roll:
Be Sure To Check The Related Topics below For More Help.

  • Similar Topics
    Replies
    Views
    Last post