Horrible fuel mileage

Mattchev · August 17, 2015

I recently bought a 08 Sierra 2500 6.0L. There are a few issues with the truck and I'm wondering if they might be connected. First when I start it up there's a pretty loud clicking noise that sounds like the valves that goes away once it's warmed up, second until it's warm it sputters a little from time to time and bogs a little if I step on it, third I'm getting terrible fuel economy 24L/100 km or 24m/g. The only things I have done so far is put fuel injector cleaner in two fills and motor flush when I changed the oil

Bob D · August 17, 2015

.... terrible fuel economy 24L/100 km or 24m/g...

Better check your math. Looks more like 9.8 mpg.

How many miles?

Time for a tuneup, clean throttle body?

sahls01gmc · August 17, 2015

The ticking is normal. Could use a throttle body clean up by what you are describing. Plugs and wires in good shape?

You're getting around 10mpg's....that's not terrible. At best I get 14mpg's (with a tailwind doing no more than 65mph).

newdude · August 17, 2015

Don't ever do an engine flush. EVER.

Plugs, wires (AC Delco don't waste your $$ on aftermarket) clean the throttle body and check for codes. Average should be anywhere from 10-14mpg combined driving on an HD with the 6.0.

sahls01gmc · August 17, 2015

There's a reason I drive my 35mpg G6 to work everyday....and not the truck!

Mattchev · August 18, 2015

I didn't do the math I just saw it on another post that the number should be the same. Don't believe everything you read I guess lol. It just hit 200,000 and I was wondering about the plugs and wires. My last truck was a 2000 Silverado 2500 6.0L and it seemed much better on gas and did not have the ticking sound. It was at 322,000 and just starting to burn oil

BlackZ71Silverado · August 18, 2015

A truck with 200,000 miles?

That fuel mileage isn't bad for the HD gasser, not bad at all. If MPG's are a concern, you definitely bought the wrong vehicle.

The engine flush was a terrible idea! You probably broke up all of the old oil sludge, now that is in there floating around waiting to clog any and all passages that were not clogged before. A really bad idea on a VVT engine.

Don't be surprised if your oil filter ends up clogged. If you get low oil pressure, check the filter first.

Hope you don't have any issues after that flush but it's highly unlikely with that much mileage. There was probably some build up that is now floating through the engine clogging oil passages and whatever else....Good luck with it, hope it hold up.

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Mattchev · August 18, 2015

It's not that I'm worried about fuel economy. I bought the truck to pull my tool trailer and it works great I was just surprised when it seemed so much worse on gas then my old 2000 with the same size motor and then with the ticking and the bog before the engine has warmed up I thought there must be something wrong. I'll do another oil change right away and hopefully anything floating around comes out. And it's km I'm in Canada

Badass69 · August 18, 2015

You may need to do the basic tune up stuff first.,,,,, but I am going to take a stab and say you have no check engine light correct? Now even though you don't check things like fuel pressure.... your O2 sensor feedback, mass airflow sensor feedback and so on. Empty that truck should do better than that..... hell my 05 gets around 16-17 l/100km at 110 km/hr on the highway with 4.10's and a 4L80E. Your truck has the 6L90E and either 3.73 or 4.10 so either way the mpg should at least be on par with my 05.

Plus if it's stumbling you are correct as something is wrong with it even if you have no indication on the dash. Do plugs and an air filter if the truck needs them and work from there....... but my money is it's something deeper rooted than that.

It could be a myriad of things wrong. Take my friends 04 3/4 ton 6 liter truck as an example. His truck pinged like no other on any grade of fuel and once I tuned it with EFILive it just got worse. So while datalogging it the next day for him I noticed exactly what his issue was on the scan. His mass airflow was completely non responsive...... it read but took forever to pick up changes (Like accelerating) and was reading too low. Check engine light never once came on because it was still not out of range for what it could read...... but was dead wrong in what it did read. I grabbed a new MAF I had kicking around and voila...... truck has been fine ever since and doesn't ping or do anything else stupid.

For what you need to fix you need some test equipment.

Gonna go out on a limb for the Americans reading this and his numbers are probably kilometers..... so 200000 km is like 120000 miles.....

Ticking could be anything from a valve train problem to a simple exhaust leak. If the leak is bad enough and ahead of a front O2 sensor that could contribute to your terrible economy.

I had an 09 2500HD extended cab short bed 4X4 6 liter with 3.73's. It got around the same as my old 05 still does..... 16-17 l/100km with a canopy, roof rack and about 1500 lbs of tools on it... but that was at 100 km/hr. Faster and it was closer to 20 l/100k

Mattchev · August 18, 2015

Thanks 94Vmax that's more like the information I was looking for. No there's no check engine light on. I was wondering about the MAF cause I had a similar problem with my 2000 but it would hardly run when it was cold and that turned out to be the intake manifold gasket but with that I had all kinds of messages coming up

notbob · August 19, 2015

I bought an '01 GMC 1500 5.3L with 147,000 miles on it about 5 years ago. It used to tick bad on startup for a minute or two and, if I ran it at higher than 70mph on the interstate. It also gave a puff of oil smoke on startup often.

At some point I started using "High Mileage" oil and now I never have either of the previous issues. Whether the oil type alone made the difference, I can't say, but at 320,000 miles now, it runs better now than ever. I get around 17-19 hwy and 15 city. I use Shell regular gas almost exclusively, too. I change oil every 3,000 - 5'000.

My intuition says the oil type really made a positive difference, and at any rate it sure can't hurt.

I love this truck! and, I was always a Ford guy before. This is a wonderful motor.