1999 Suburban dropping gas mileage suddenly with new engine

rodeoshooter · April 2008

I have a 1999 Suburban & in the last 2 months the mileage went from 18 to 13 on the hiway. It started on the 11th of Jan when I got 16.7 after driving 167 miles in ice and snow. Filled up and got 14 on dry roads with 45 degree temps. The engine was replace on 1 Jun 2007 and has 20,000 miles on it. I took it to the local dealer twice. The first time they claim the timing was retarded 6 degrees and claim the corrected it. I then got 11.5 mpg after their "repair". Took it back in on the 24th of March and they claim it is running perfect. Now the Dealer pleads stupid!

I would really appreciate any suggestions. I replaced the fuel filter and pump in Jan 07. The wires and cap and rotor were replace in Dec 06. The dealer states I might want to try replacing all of those again but states it would be a long shot. Replaced all of the exhaust system in June 05 and replaced right bank cat. converter a month ago.

Please help me. I drive about 30,000 miles a year and the gas will kill me at a 33% drop in mileage.

Thanks,

Al

tidester · April 2008

I don't know what the cause of your problem is but I wonder whether you're seeing any difference in either the amount or color of your exhaust. You're using 50% more gasoline per mile driven than you did before which sounds rather extreme.

tidester, host
SUVs and Smart Shopper

rodeoshooter · April 2008

No, I have not noticed a difference in the exhaust. I am currently running about 25-33% less on my mileage. It has recently passed the state emissions and the dealer has had it twice and claims it is running "perfect" even when I questioned them about the exhaust emissions.

What would it mean if there was a difference in the amount or color?

Since the engine is under warranty I would love to be able to have them replace it. Do you know it the "Superchip" will make a difference?

Thanks,

Al

rockman59 · April 2008

Are you using Ethanol now instead of regular gas? If so that could be the problem as Ethanol delivers much less mpg than regular gasoline. Have you checked the EGR valve? That is an inexpensive part of the engine that can contribute to a sharp decline in mileage. It is hard to believe that something major is wrong with your engine if you are passing the emissions test in your state. Whatever is causing a 25 to 33% drop in mileage almost certainly would put your engine in non-compliane with the emissions standards.

tidester · April 2008

What would it mean if there was a difference in the amount or color?

It would mean that there is incomplete combustion. The "superchip" is supposed to optimize combustion and so it would make a difference if it were to malfunction.

tidester, host
SUVs and Smart Shopper

rodeoshooter · April 2008

I am not using Ethanol. I know it will cause a drop of about 10% in the mileage. No I haven't checked the EGR valve. I will have to find it first.

I did put 2 bottles of Gumm injector cleaner into a little less than half a tank of gas today. I also bought a spray to clean the Mass Air Flow out with. I use a K&N airfilter and reoiled it last October. It was suggested that the MAF might have become a little clogged.

I sat 1 on 1 with the service manager yesterday and he had suggestioned a new GM airfilter and other than going to a different dealer for help he could not come up with any suggestions. In fact he made a point of not offering any suggestions to my questions. He was what you would call a "artful dodger" in his attempts to dodge all of my questions. If I didn't know better based on his position at Appleway Chevy in Spokane, that he was a full fledged idiot! Heck I might think that anyway.

Sorry but he was quite frustrating in his lack of help. After paying for service I now know what it is to get "screwed" by the dealership. :mad:

Thanks,

Al

rodeoshooter · April 2008

Wouldn't incomplete combustion show up in the emissions?

I didn't notice much of a difference with the SuperChip when I tried it earlier. I reset the motor before I went to the dealer and now my SuperChip will not recognize the vehicle so I will have to wait until Monday to hear from SuperChip.

Will going to a synthic oil help with the mileage?

Thanks,

Al

stever · April 2008

So the MAF cleaner didn't help? That would have been my guess based on the oily filter.

Synthetic may give you 1 to 3% better mpg.

Does synthetic oil help mpg?

tidester · April 2008

Wouldn't incomplete combustion show up in the emissions?

Almost certainly but I did ask my original question before you told us your emissions results!

tidester, host
SUVs and Smart Shopper

rodeoshooter · April 2008

I haven't cleaned it yet. I am a wimp as we are still getting snow here. 4 inches yesterday. Flurries again today. When will spring ever sprung? I will try it today. Is the K&N filter not a good one to have or use? I have heard rave reviews about them.

Is the cost worth running synthetic for the 1 to 3% of mileage? I thought it would do better than that. I pull a 24 foot trailer during the summer. Would synthetic make it run cooler?

Thanks,

Al

stever · April 2008

Someone in that link I posted claims to get a 10% increase using synthetic, but I trust Shifty's estimates. I don't think it's worth it; regular oil is pretty good these days. I didn't use it during 20 winters in Alaska when my cars lived outside, and they cranked ok. But yeah, towing is hard on the drivetrain, so syn could help there I suppose. Less opinionated opinions

can be found in the Engine Oil - A slippery subject Part 2 discussion.

I'd rather just toss a paper filter every 15,000 miles than remove it and clean it and reoil it, and then have to worry about getting excess oil in the system and messing up sensors. I don't believe the claims for increased mpg and horsepower on those filters either.

Back to your problem, there are so many things that could be hammering your mpg, you need to be careful that you don't wind up throwing a lot of parts at it before hitting on the solution. That could cost a lot of time and money.

Take a look through the Got a Quick, Technical Question? discussion - maybe some posts there will give you some ideas to kick around with your mechanic.

I don't think you mentioned whether any codes have been thrown off. Maybe Schucks could pull them for you, but the dealer likely cleared them anyway. Was there an O2 sensor near that cat you replaced? May want to check the connections on it.

Cold spring here in Boise too.

remermn · April 2008

Hi!

I had a similar thing happen 3 or 4 years ago with my 99 Suburban.
Noticed my gas mileage had dropped and my exhaust smelled as if the truck was running rich. I replaced the 2 oxygen sensors ahead of the catalytic converter (there are also 2 sensors located behind the convereter however those only monitor the converters operation) Anyway, the 2 forward oxygen sensors regulate the fuel air mixture and as they age they can cause the engine management system to allow more fuel than is necessary into the engine. After replacing the 2 sensors my city mileage went from 10 to 13. Before replacing those sensors I did ask my mechanic what he thought. His reply was not to do it because there was no code for the oxygen sensors stored in the engine computer. So if you have not replace your oxygen sensors recently you might want to try it. Good luck.

Mike

rodeoshooter · April 2008

Mike,

I will give that a try. I will have my mechanic check to see if it smells to rich first. Chevy dealer told me the emissions was ok.

I was having some codes about the sensor before I had the right bank cat converter replaced about 40 days ago. I recently bought a chip and installled it and am still at 14 mpg on the highway. All the stations in this area are now selling 10% ethenal to us. So I guess the chip did make a slight difference since I was getting 13.5 on non-ethenal without the chip.

Thanks,

Al

1999 Suburban dropping gas mileage suddenly with new engine

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