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Tune -up done ,but MPG stil 15% below avarge

Hi.I have a 2008 chevy 4.8L express cargo van with 110K.
I have noticed a gradual MPG decrease over the 3 weeks from 10.5MPG to 7.5MPG,driving about 30-40miles a day.

I 've put a new air-filter and new cables and sparkplugs. That helped me to get to 9.0 MPG .

I’m not sure ,what else could that be.

I’ve noticed that car may sound like it is running at higher RPM at any speed from 15-70mph and up , but i’m not 100% sure about it. That may be the only change ,but i’m only 20% sure about that noise. Transmission seems to work fine and car doesn’t lack acceleration and power.

Thank you for your help.

Had a leaking fuel pressure regulator, If you pull off the vacuum hose and smell gas a possibility, my gad what bad mpg! have a temp guage and is it in normal range?

Temp gauge is always at normal range, car heats up at start as usual.
Where is a vacuum hose, what should I follow?

I have no repair manual for this car. I’ll get it soon.

A bad O2 sensor or other sensors can cause a richer than needed fuel ratio. But you have to get a diagnostic computer on it to see what the sensors are reading to see what may be out of whack. I’d bought a pristine Rivera with 100K on it and we took it on a 400 mile trip to try it out. I got about 15 mpg with it and was filling up continuously. The on-board diagnostics showed a lazy O2. When I changed that out and took it on the road the mileage immediately began climbing to a normal 27 mpg. So just that one sensor can make a big difference…

This van has a returnless fuel system . . . the regulator is part of the fuel module

But the tune-up WAS needed, because fuel economy went from 7.5mpg to 9mpg, not to mention it was due by mileage

Still lousy, so there’s more work to be done

if the van has higher rpms than it should, perhaps it’s not shifting into high gear . . . I believe this van has a 4-speed automatic. Since it’s the small V8, it should have a 4L60E

@Bing may be right, and it could be an upstream O2 sensor(s), but it could be something else

And he’s definitely right, in that hooking up a scanner would give you live data, which could be helpful

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2 questions:

How are you determining your mileage?

And what’s the weather like now, and how does it compare to 3 weeks ago?

MPG is being determinated by dash board computer and I also doubled check it with actual miles traveled and gas used . It was spot on .

The weather hasn’t change much . colder by 10F , still very warm weather.

I agree on checking the 02 sensor even if it doesn’t throw a code. It can start to degrade after 50k miles.

Sounds like you may have a fan clutch dragging/seizing up.

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Would that be causing 15% decrease in MPG ?

That would rob power from the engine, so you’ll use more fuel to compensate

If the car doesn’t overheat, could it still be fan clutch problem?

What does overheating have to do with it . . . ?

The fan clutch could be locked up, and the van was NEVER overheating

The implication is that the fan clutch could be faulty

People with trucks and SUVs complain about power loss and abnormal drag when the weather reaches 100F. When the fan clutch engages on a large vehicle it can draw as much as 20 horsepower. That doesn’t sound like much of a load on a 250 HP engine however your engine may have only 60 HP at 2000 RPMs.

The question is; are you listening to the fan roar or is the engine speed higher than normal?

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Yes. You need to know if the engine is actually running at a higher rpm than normal or not. A seizing fan clutch will make your car sound like a turboprop airplane.

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If its like the old fan clutches it means the fan is on full time even when its not needed to cool the engine, thus robbing fuel economy. Normally it would free wheel when not needed to cool the engine. I think you would hear a continuous roaring sound.

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Should i take the car to a dealer for diagnostics or a regular mechanic.
I don’t have a regular mechanic as I do all basic maintenance by myself.

A “regular” mechanic should have the tools to diagnose the problem. It doesn’t have to be a dealer. If you have a smart phone or tablet, you can get apps and a bluetooth OBD 2 code reader to allow you to data log the truck yourself. If you have no experience with this, it might be best to have a mechanic do this to solve your immediate problem with low mileage. But getting the tools can help you solve the next problem.

BTW, have you checked to see if the brakes are dragging? A slightly stuck caliper could add enough drag to affect MPG’s. Drive it for a few miles on the highway and check to see if one front wheel is hotter than the other - carefully! - a dragging caliper can get the wheel really hot.

Don’t go by the car’s mpg computer. None of them are accurate. The only real way to see what your mileage is is to set your trip odometer to 0 when you fill it up, then the next time you fill it up divide the number of miles driven by the number of gallons you put in.

Drive the truck for half an hour or so, make sure it’s at a normal operating temperature. Shut it off, then try to spin the fan with your fingers. It should move fairly easily. If not, the clutch is starting to seize.