Hello all. Been lurking here forever & finally decided to ask a question because I couldn't find any previous posts similar.

I've had my '99 5.3 SBSC 4x4 for about a year & a half, completely stock, roughly 60,000 miles. My mileage had been between 17.5 - 18.5 mpg, but it recently dropped drastically, to between 14.5 - 15 mpg. Any suggestions as to what might cause this? I have replaced the air filter (OEM type) & the spark plugs (I think I put in regular Bosh plugs). No 'service engine' lights or anything have come on.

I realize aftermarket exhaust and/or filter will help, but I want to find the cause of the problem first. Is it maybe the oxygen sensors? Or is it maybe just the **** poor gas we get during the winter?

Any suggestions would be appreciated.

thanks

 

I would start with a fuel filter, PCV valve, change oil/filter, and run some high octane injector cleaner. Those are fairly cheap ways to help/maintain gas milage. Use only AC Delco parts, and if feasable, run a synthetic motor oil like Mobil 1, Royal Purple, or RedLine. Anybody think of something I missed?

 

well, i have a 99' exsb with a 5.3 and am getting some where in the area of 15mpg on the highway. i have intake, muffler, programer, t-stat. i know i have a biger truck, but the milage shouldnt be that much of a difference.

when you take your gas millage, do you devide the number of miles you have dirven by the number of gallons you put into your truck, some people dont do this, but it is the correct way to get your mpg.

 

Thanks for the replies guys. Yes, I divide the miles driven on that tank, by the gallons put in. I'll definately change the fuel filter. Is it located underneath by the driver as in the previous generation trucks? I had always had good luck with Bosh plugs before, but the drop in mileage does seem to correlate with about the time I changed plugs. I'll try throwing in AC Delco instead. Any specific type I should try? Or just factory replacement?

Again, thanks for the help.

 

Zom,
Where are you in Michigan? I ask because there aren't many of us on this board from here.

 

J Smith

From the Grand Rapids area. How 'bout you? Is there much snow up there?

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Zom:
<strong> I'll try throwing in AC Delco instead. Any specific type I should try? Or just factory replacement?

Again, thanks for the help. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I was talking about the filters, and PCV valve when I said AC Delco replacement parts. If you noticed the loss in milage when you changed the plugs, what gap did you set your plugs at? Stock gap is 60, so 55-60 is where the gap should be with your plugs. If you want a better spark, use copper plugs. NGK TR55 is what the majority of the guys are running. They are only good for 25-40K miles, unlike the platniums which are good for 100K (so they say) but you will have better firing plugs, which should help. You can get the NGK plugs at your local parts store. Run a 57 gap on all your plugs, and you should be good.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Zom:
<strong> I'll definately change the fuel filter. Is it located underneath by the driver as in the previous generation trucks? </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Yes, driver side, closer to front, inside framerail.

 

Zom, welcome to the site. Just a quick word, if you change your fuel filter, be sure and unplug your fuel system fuse and run the truck out of gas. If you don't, you will get a nice suprise in your face. Also after you run it out, it's a good idea to disconnect your neg batt terminal before you start the work. Don't wory about lossing prime on the fuel system by running it out, it will prime just fine when you plug it back in.

 

Thanks for the advice, guys. I'll give the NGK plugs a try. Personally, I don't believe in the '100,000 mile' plugs. When I changed mine, it looked like they were about shot. I don't know how they could have made it another 40,000 miles.

And thanks for the heads up on running the truck out of gas. When I've changed the filter on previous vehicles, I would uncap the gas cap, but never thought about disabling the fuel pump & running it out of gas. I always made a mess.

Anyways, I hope those simple fixes make it better. I don't expect excellent gas mileage, but when it dropped so much, I was worried something might be drastically wrong.

thanks