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1999 Suburban dropping gas mileage suddenly with new engine

13 messages,  Last post on Apr 20, 2008 at 10:14 PM

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#4 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshooter] by rockman59

Apr 05, 2008 (4:02 pm)

Replying to: rodeoshooter (Apr 05, 2008 12:48 pm)
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.

#5 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshoot by tidester

Apr 05, 2008 (4:09 pm)

Replying to: rodeoshooter (Apr 05, 2008 12:48 pm)
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

#6 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rockman59] by rodeoshooter

Apr 05, 2008 (8:07 pm)

Replying to: rockman59 (Apr 05, 2008 4:02 pm)
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.
 
Thanks,
 
Al

#7 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshoot [tidester] by rodeoshooter

Apr 05, 2008 (8:11 pm)

Replying to: tidester (Apr 05, 2008 4:09 pm)
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

#8 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshooter] by steve_ HOST

Apr 05, 2008 (9:16 pm)

Replying to: rodeoshooter (Apr 05, 2008 8:11 pm)
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?

#9 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshooter] by tidester

Apr 05, 2008 (11:41 pm)

Replying to: rodeoshooter (Apr 05, 2008 8:11 pm)
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

#10 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [steve_] by rodeoshooter

Apr 06, 2008 (8:51 am)

Replying to: steve_ (Apr 05, 2008 9:16 pm)
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

#11 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshooter] by steve_ HOST

Apr 06, 2008 (9:04 am)

Replying to: rodeoshooter (Apr 06, 2008 8:51 am)
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.

#12 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshoot by remermn

Apr 18, 2008 (8:24 am)

Replying to: rodeoshooter (Apr 05, 2008 9:19 am)
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

#13 of 13 Re: 1999 Suburban dropping gas mileage suddenly with new engine [rodeoshoot [remermn] by rodeoshooter

Apr 20, 2008 (10:14 pm)

Replying to: remermn (Apr 18, 2008 8:24 am)
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

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