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Username Post: poor gas mileage        (Topic#154264)
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-15-07 07:18 AM - Post#1120198    

Over the last 2 months I've noticed an increase in fuel consumtion. Going back and forth to work 6 days a week about $40 bucks was the norm, over 10 days. This week, $70 in 6 days. I have replaced the plugs with AC, the old ones had the brownish whitish color which I think is normal. It would also rule out a flooding condition. I have read on here that O2 sensors can go without throwing a code. What about plug wires? Starts and runs fine, doesn't smell of fuel etc.
'96 5.7 210K, 126,000 mi. 4L60E xcab shortbox...
any advice?
'96 Silverado ext. cab. 5.7l


 
Bishop 
Frequent Contributor
Posts: 1488
Bishop
Age: 27
Loc: DMV Area
Reg: 01-06-07
03-15-07 07:27 AM - Post#1120202    
    In response to new96er

well for starters i would look at the air filter make sure it's nice and clean, check your tire pressure, i know that it can mess with your gas milage but i dont know if it will mess with it that much. How old are the wires? Are they brittle or hard? any spots where they have been rubbing agaist something metals cause then they might be arcing.
Not only so, but we also rejoice in our sufferings, because we know that suffering produces perseverance
Romans 5:3


 
Bad Bowtie 
Valued Contributor
Posts: 4536
Bad Bowtie
Age: 42
Loc: TX
Reg: 07-04-03
03-15-07 08:45 AM - Post#1120257    
    In response to Bishop

Fuel filter? Sea Foam treatment?
GMC. We are Professional Grade.

http://www.cardomain.com/id/2004VHO


 
chrisk1500 
Very Senior Member
Posts: 1817
chrisk1500
Loc: Saskatchewan, Canada
Reg: 10-30-04
03-15-07 08:50 AM - Post#1120261    
    In response to Bad Bowtie

Cat plugged?....
07.5 DMAX 2500 HD ext cab, 4x4, Edge EVO Race, 4" straight pipe, PPE tie rod sleeves, bushwhacker pocket flares, etc...

06 Escalade


 
tobey 
Senior Member
Posts: 241

Loc: E. Central Indiana
Reg: 12-20-04
03-15-07 09:39 AM - Post#1120300    
    In response to new96er

I have a 89 chevy truck with a 5.7 My gas mileage went downhill over time. From 23 to 15 on trips. My thermostat would not let the engine go to full temp. Also the TPS was bad, and everytime I gave it gas it was telling the computer I was flooring it. Even though I was driving it easy. The thermostat was telling the computer the engine was still cold, and made it run rich. Hope I helped with your question.There are are good answers to this post, and I hope you fine what is causing your problem.

 
jbknight311 
Frequent Contributor
Posts: 1011
jbknight311
Loc: Fort Worth, TX
Reg: 11-27-06
03-15-07 09:51 AM - Post#1120304    
    In response to tobey

Plug wires seem to last about 3-5 years I find. Even the life time warranty ones I buy. Cap and rotor about 100k miles would be about right. Plugs I replace very 50k just the way I do it. I figure they might last to 100k, but would be stretching it.

You can check the plug wires, if the ohm reading is about the same on each one then they would be good. I think it's around 18k, Cap and rotor would help fire the beast better. Fuel filter and such, normal turn up stuff. You can have a good reading wire that is shorting out to another wire or the engine, so look at each wire for any dark spots as well.

O2 sensors, use to be, and still is a lot of times, if mileage went bad the o2 sensor was telling the engine to run rich.If you have the original ones, might be due for new ones just as preventive maintenance.
My mileage went bad for a couple of weeks and me and the guy next door think someone was taking a gallon or 3 out of our tanks at night. He had the same problem. Mine went down to about 12-13 for about 3-4 weeks, then back up to normal 15. The past couple of weeks my engine seems like it finally broke in. At 180k+ my acceleration is better and mileage went up to 16. It's fun to drive again.
98 C1500 Silverado extended cab 5.7 auto 2wd Fort Worth, TX.


 
CDAUSA 
Needs to Get Out More Member
Posts: 12906

Loc: Texas Panhandle
Reg: 12-31-01
03-15-07 10:53 AM - Post#1120356    
    In response to new96er

I would check fuel pressure first, then, if you have been buying fuel at the same place, change to a fuel you know comes from a different refinery----if there is such a thing.

Are you using oxygenated fuel?

 
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-15-07 07:17 PM - Post#1120719    
    In response to CDAUSA

I've been getting fuel at different places, so I'm not sure about that. Some has ethanol, up to 10%. I'l probly check plug wires tomorrow after work, and look at cap and rotor.
Thanks so far....
'96 Silverado ext. cab. 5.7l


 
CDAUSA 
Needs to Get Out More Member
Posts: 12906

Loc: Texas Panhandle
Reg: 12-31-01
03-15-07 07:24 PM - Post#1120735    
    In response to new96er

I have no doubt I could run 100K on the AC plugs, regap and run another 50K, check to make sure the plats are still there and go another 50K and never lose a HP or MPG---------but, that is not the case running oxygenated fuels. You sure might check to see, especially if it has been phased in without you knowing. 2-3 mpg drop is common with it, and a waaaay shortened plug life from what has been said here.

As long as the wires are cared for, never been greasy/oily or hurt, they will last 120-140K without problems, but will be the first to go.

I changed my Vortec cap and rotor at 245K, just because I felt guilty, from their condition at that time I could still be running them without failure but is good to check them.

 
jbknight311 
Frequent Contributor
Posts: 1011
jbknight311
Loc: Fort Worth, TX
Reg: 11-27-06
03-15-07 09:03 PM - Post#1120837    
    In response to CDAUSA

While I know you are right, and I have changed a lot of things over the years that didn't need it, I just like working on it at times and still am stuck in the 70's with plug and tune up stuff. At least I don't have a little tiny fuel filter in the carb inlet to worry about any more and I still get a kick out of thinking about that when I come across the half dozen I have stashed in old boxes of crap I can't throw away.

I have noticed a difference in platinum plugs over the years. the AC's are the best I found. I tried Bosch a few years back in an Olds I had and they really did not act right at highway speeds. I pulled them out after about 5k miles.
98 C1500 Silverado extended cab 5.7 auto 2wd Fort Worth, TX.


 
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-16-07 06:09 AM - Post#1120990    
    In response to jbknight311

I checked for codes last night, none. There was 1 icon flashing and the manual says it represents the catalaytic chamber (??) or something like that. It went on to explain how when cold it sends more fuel until the O2 sensors warm up then the computer goes into closed loop etc. If the icon is flashing it means the check or test has not been completed. SO after driving to work this AM, I checked it again, still flashing. So I think I have to focus on those sensors. I think the converter is not plugged, as I can accelerate hard for as long as I want without any problem.
'96 Silverado ext. cab. 5.7l


 
jbknight311 
Frequent Contributor
Posts: 1011
jbknight311
Loc: Fort Worth, TX
Reg: 11-27-06
03-17-07 07:17 PM - Post#1122035    
    In response to new96er


I took a trip to Austin this weekend, a lot cleaner then the scumbags on Greenville Ave in Dallas celebrating St. Pats day. Bottles, cans and litter all over the place here, Austin was nice and respectful. Anyway, my mileage was real close to 17mpg on the highway. I have no idea how, but it seems like the motor is just breaking in and running good. 184k miles and a standard 15 to 15.5 on the highway and now it is running better and burning less.
I wonder if it has anything to do with the seasonal gas I read about a couple of years ago?
98 C1500 Silverado extended cab 5.7 auto 2wd Fort Worth, TX.


 
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-22-07 06:36 PM - Post#1126192    
    In response to jbknight311

cecked the O2 sensors on the w'end they were good. took it in to the dealer today. they said it gave codes P0420 and 0430. cats are plugged. GM price over $900 plus install. got it done down the street for about $600. ouch
'96 Silverado ext. cab. 5.7l


 
CDAUSA 
Needs to Get Out More Member
Posts: 12906

Loc: Texas Panhandle
Reg: 12-31-01
03-22-07 06:55 PM - Post#1126216    
    In response to new96er

So you replaced them? Is there a chance you saved the cats or can retrieve them, I would like a pic of these cats............plugged.

Were you able to find out if you use oxygenated fuel?

Are these sensors behind the cats originals??

 
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-22-07 08:02 PM - Post#1126279    
    In response to CDAUSA

I got the old ones. The sensors are front and back of the cats. I'm not sure what you mean by oxygenated fuel though.
'96 Silverado ext. cab. 5.7l


 
CDAUSA 
Needs to Get Out More Member
Posts: 12906

Loc: Texas Panhandle
Reg: 12-31-01
03-22-07 08:27 PM - Post#1126302    
    In response to new96er

Looking at Canadas requirements, from what I have read they have settled on 10% ethanol as being the most required, so it should have little to no affect.

This is an example to the extreme, but did you smell any fuel in the ECM enclosure?

If and when you have time, I would really-really like a look at a stopped up honeycomb cat. Those being suspect, are checked with a method of backpressure where you drill a hole in the exhaust pipe ahead of the cat, put a fitting in and take the motor up to 2,000---should not read more than 2.7# of pressure.

 
Fullsize92Blazer 
Senior Member
Posts: 1250
Fullsize92Blazer
Age: 26
Loc: Phx, AZ
Reg: 10-12-04
03-22-07 08:46 PM - Post#1126312    
    In response to CDAUSA

The 96 up have a MAF and a MAP sensor dont they? Couldn't you just check with a scanner to read excess backpressure without having to drill holes into things?
'08 Yamaha PW50
'08 Polaris Ranger RZR 800
'95 S10 Blazer
'91 Toyota 4Runner
'86 3/4 ton Suburban





 
new96er 
Member
Posts: 54

Loc: Saskatoon, Sask. Canada
Reg: 04-29-03
03-23-07 06:38 AM - Post#1126468    
    In response to Fullsize92Blazer

I gotta assume they did check it with a scan tool. I was specific about that when I left it there 'cause my code reader did not pick up codes. I didn't see any drilled holes, so it must of been the MAF or MAP sensor. The charge was only $48 for a scan......
'96 Silverado ext. cab. 5.7l


 
CDAUSA 
Needs to Get Out More Member
Posts: 12906

Loc: Texas Panhandle
Reg: 12-31-01
03-23-07 07:58 AM - Post#1126522    
    In response to Fullsize92Blazer

  • Fullsize92Blazer Said:
The 96 up have a MAF and a MAP sensor dont they? Couldn't you just check with a scanner to read excess backpressure without having to drill holes into things?



"Drilling holes into things" is likely the best way to get a correct reading, and certainly the best route for a DIY guy, or only route for most.

I doubt I would trust readings from the MAP sensor for this purpose, certainly not the MAF.


 
JeremyB 
Senior Member
Posts: 696
JeremyB
Reg: 10-31-01
03-23-07 03:34 PM - Post#1126767    
    In response to CDAUSA

I was thinking they pulled out O2 sensors and put in pressure guages to check the differential pressure at the cat

 
rc 
Member
Posts: 4

Reg: 03-24-07
03-25-07 06:19 AM - Post#1127734    
    In response to new96er

if your new cats didn't fix it, clean your maf, you would be shocked how little filth it takes screw with them, (and not give up a code). and if your maf isn't sending the right signals, the o2's signal dosn't mean too much.
the map is more related to the tps, these also could cause poor mpg.
remember new parts can still be junk parts, (especialy from a discount auto parts store - lifetime warrenty or not)

Edited by rc on 03-25-07 06:25 AM. Reason for edit: No reason given.

 
jbknight311 
Frequent Contributor
Posts: 1011
jbknight311
Loc: Fort Worth, TX
Reg: 11-27-06
03-25-07 07:48 AM - Post#1127779    
    In response to rc

  • rc Said:
new parts can still be junk parts



Ain't that the truth
98 C1500 Silverado extended cab 5.7 auto 2wd Fort Worth, TX.


 
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