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Tracking down cause of poor MPG.

Old March 11th, 2019, 12:15 PM
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Default Tracking down cause of poor MPG.

Generally speaking, how much time would it take an experienced mechanic to try and track down the cause of poor MPG.

I have a 2002 chevy express 2500 with a 5.7L and get 9.5 mpg / city with the van empty. Mainly very short trips, which I would think 9.5 is still poor.

This is beyond the scope of my ability and will have to hire someone to do it. Just wondering if it is worth the cost or just sell it and get another one since it's old and not really worth that much.

Last edited by Roger1; March 11th, 2019 at 12:27 PM.
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Old March 11th, 2019, 2:28 PM
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You are guzzling quite a bit. Since there are many causes for this, I can't say how long it would take without more information. It could be anywhere from a 5 min fix to a new engine.

How much on the odometer? Do you have a check engine light? Have you checked the usual suspects? (air filter, tire pressure etc.) Do you do regular maintenance?
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Old March 11th, 2019, 3:39 PM
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Originally Posted by Roger1 View Post
I have a 2002 chevy express 2500 with a 5.7L and get 9.5 mpg / city with the van empty. Mainly very short trips, which I would think 9.5 is still poor.
Stickered mpg estimates are usually based on "ideal" conditions. A 5.7L, 4 spd transmission, making several "very short trips" might only net 9.5 mpg (unfortunately). Be prudent and check the basics like mountainmanjoe suggested, but short trips don't allow the engine to achieve any real efficiency (and stop-and-go driving makes it worse). Unless there are other issues with the van or you need/want other features that you'll find in a newer van, I don't think I could recommend upgrading based on poor mpg alone. I average 12-14 mpg around town and about 17 on the freeway in a 2016 extended 6.0L. It's better than 9.5 mpg, but for the $20k to buy a used one you could buy about 8,000 gallons of gas. Alternatively, it will take over 230,000 miles before you "break even" on a newer van with a better mpg average (assuming you spend $20k on the van, got 14 mpg, and gas is $2.50 per gallon).
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Old March 11th, 2019, 6:02 PM
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A friend of mine used to do those MPG tests, and they're done on a Dyno. No wind, no weight drag. Considering the age, you probably can't do much better. The newer ones do better. My '16 w/6.0 averages about 12 to 13. And I have a heavy foot.
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Old March 11th, 2019, 6:13 PM
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The sticker MPG numbers used to be based on wind tunnel tests in unrealistic conditions.

I think the EPA MPG numbers are based on statistics.
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Old March 11th, 2019, 6:16 PM
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Over inflate your tires 5-7 lbs. Get a clean air filter. Check your tranny fluid. If it's dirty and smells burnt it's on it's way to death. Jack it up and see if your brakes are dragging. Disconnect the AC compressor for a while. It runs when you use the defrost. It takes a tol of gas to move a 5000 lb box down the road.
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Old March 11th, 2019, 8:16 PM
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Originally Posted by Roger1 View Post
Generally speaking, how much time would it take an experienced mechanic to try and track down the cause of poor MPG.

I have a 2002 chevy express 2500 with a 5.7L and get 9.5 mpg / city with the van empty. Mainly very short trips, which I would think 9.5 is still poor.

This is beyond the scope of my ability and will have to hire someone to do it. Just wondering if it is worth the cost or just sell it and get another one since it's old and not really worth that much.
for comparison....I get 10mpg, imperial gallon, city with my 6000lb 2500 pu. 18mpg 100% highway. Its mechanically in excellent condition. If the fuel trim is within +/- 10%. spark plugs don't look black.... rather a brownish white...it metering correctly.
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Old March 11th, 2019, 9:32 PM
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Hi Roger,

Couple questions, has the van always done this since you have owned it or has it changed over time?
What is the temp outside there?
How short are these trips?

Before paid a mechanic, I would troubleshoot it by taking a little road trip on a relatively flat piece of highway. Find someplace to have lunch that takes a tank of gas to get there and back. Then if you are in the 12-14 range you may be ok.
Like others have suggested, a tune up cant hurt.

John
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Old March 11th, 2019, 10:27 PM
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I had the same van. Mine was a passenger model.
I got 12 to 13 mpg city, and 16 hwy. I had michelin ltx 245 75 16 load range e tires. The factory tire size is 225 75 16. factory inflation is 50 psi front 80 psi rear. I ran about 76 rear since 80 psi is the max for those tires.
What tires are you running? what pressure?
When were the front wheel bearings last packed and lubed?
Are your brakes in top order? No dragging and all parts moving freely.
How old is the exhaust? My muffler started to go, and the new one made a little improvement.
When was the air filter last replaced? The manual says about 50 000 kms....However its worth it to do once a year.
Are the points under the dizzy cap clean? Right spark plugs? (iridium)
Have you checked for codes? Your o2 sensors may need changing. They help to control fuel mixture among other things.....
Also, the picture Joe posted is from fuel economy.gov Its a great website and the numbers are posted by owners, not the manufacturer. It is usually pretty close.
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Old March 13th, 2019, 10:06 AM
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Answers to everyone's questions.

It has 157K on it.

I've only owned it for about 7 months and have put less than 1K on it. I had been looking for some time for a van, but here in the Tamps Bay area vans are overpriced and I will only buy something that is a good deal. My plan was to buy something newer with less miles, but I snagged this one super cheap and figured it would be OK for the time being.

Air filter is only a few months old.
Tires 225 75 16 have less than 500 miles on them. Upon purchasing the tires I didn't check to see what air pressure was put into them.
Brakes have less than 750 miles and bearing where repacked at the same time.
Have not done a tune up.
No idea how old exhaust is. If I rev the motor it does make a crackling noise. We don't have any emissions laws here in FL
Have not checked for any codes / haven't had check engine light ever come on.
The overwhelming majority of my trips are 2-6 miles round trip.

At what speeds should the tranny shift at when driving normally up to 45 mph.

I will be driving to GA next month and can see what it gets HWY.

Last edited by Roger1; March 13th, 2019 at 10:09 AM.
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