Gas mileage falling fast...

[sprucegoose]

I can't figure this one out. My 98 Tahoe 4 dr. 4Wd has always been about 12-13mpg city, 14-16mpg highway. All of the sudden about 2 months ago it started really dropping off fast. Hit an all-time low of 8.9mpg around Christmas pulling a double snowmobile trailer!

I had the truck loaded down pretty good, but never have I seen below 12mpg on the highway in this situation. I used auto 4wd occasionally on slippery roads as my tires are really bad right now, but that too isn't usually isn't a big factor. It is getting about 10mpg city and 12mpg highway right now.

It seems to be running smooth, with no evident loss in power, but it is almost like there is some kind of extra drag. I feel like I have to give it more pedal just to keep it at a cruising speed. It doesn't run rough, but the plugs have about 80K on them so I ordered a set of plugs and wires. Wires are off the original motor which I replaced at 106K so they are overdue.

Is it possible the injectors are plugged or something in the fuel system that is going? My feelings are there maybe is some sort of bearing or driveline thing going on, but I don't feel any vibration, hear any noise, or have a pull to one side. Any thoughts on what to check next here? Any help is much appreciated! : )

[planedude]

check your rear brake cylinders mine were siezed up causing the brakes to stay on slightly.

[sprucegoose]

Originally Posted by planedude

check your rear brake cylinders mine were siezed up causing the brakes to stay on slightly.

That's a good point, I have to get it up in the air to see if there is any friction there. I still have never had these rear drums off since I owned the truck (12/99 at 44K mi.)! I tried to get the drums off once, but couldn't get them off over the shoes. Didn't have the rubber plugs to fill the knock-outs in the back side of the back-up plates to get at the adjusters at the time.

I can't believe these are the original rear brakes, but they still seem to be working. They hold with the Ebrake anyway... Will put it up on the jackstands tomorrow, too cold out there right now! : )

[planedude]

I have a 96 chev 4x4 ext cab 5.7 and I'm not getting very good mileage either. I think my 265 aggressive tires are not helping plus this year is supposed to be the worst on fuel. Also have u dun a tuneup lately?

[sprucegoose]

Originally Posted by planedude

I have a 96 chev 4x4 ext cab 5.7 and I'm not getting very good mileage either. I think my 265 aggressive tires are not helping plus this year is supposed to be the worst on fuel. Also have u dun a tuneup lately?

I haven't really, like I said in my orginal post. I just ordered new wires and plugs from gmpartsdirect.com. Should have them on Tues I hope. Gotta pull my air filter, but it's only been in there a year or so...

I know they switch to winter blend reformulated gas around here now, but even when I am in an area where I can get "good" gas it hasn't improved much. I've had these 265/75/16 Michelins on mine since 2000. I don't think that affects mpg too much unless they aren't aired up enough. I run 42 psi. in mine, they are C load range tires.

[supcabman]

I Have A 97 2wd Tahoe And I Run 31x10.50 And I Run Them At 50 Psi I Went From A P255-70r15 And It Cut Mine From 16 To About 14 In The City And I Also Tow A Utility Tralier Loaded It Is About 3500 Lbs When I Tow I Only Get 10 But I Have Noticed It Seems To Be Getting Harder To Keep My Milage Up I Also Think It Has Something To Do With The Gas Right Now

[sprucegoose]

Well I lifted the wheels off the ground tonight and each one seems to be pretty normal, as far as drag. No brakes hanging up or obvious bearing failure. Also pulled the air filter and that looks like new. Better safe than sorry on that though, have had 2 filter boxes on cars I bought recently that were totally blocked with mouse nests!

I'm still thinking it is a combo of the winter blend fuel, and ignition parts needing replacement. I'm PO'd that gmpartsdirect had to special order my plugs and wires. If i knew that I probably wouldn't have gone through them...

Still will run some fuel/top end cleaner thru I think just in case I have some buildup in there that my injectors aren't functioning 100% Anyone have a preference on what brand to use there?

[planedude]

i'm gonna try a mass air flow soon. I will let u know if its any better

[sprucegoose]

Originally Posted by planedude

i'm gonna try a mass air flow soon. I will let u know if its any better

I would assume you would get a "service engine soon" light coming on or codes stored if that failed wouldn't you? What does a new one run?

[planedude]

Yeah fur sure if it failed but I'm wondering if its just not up to par. not sure how much they are i'm looking into different options , maybe ebay

[Thunderbolt]

How many miles are on the truck ?? It is a very good possibility your pre-cat O2 sensors are causing your problem. The pre-cat O2 sensor will make your mileage drop without ever setting the SES light or causing any noticable power loss.

[planedude]

thats exactly what I was thinking on the way home tonight, I have cahnged the left hand one but not the right. Will try.

[sprucegoose]

Originally Posted by Thunderbolt

How many miles are on the truck ?? It is a very good possibility your pre-cat O2 sensors are causing your problem. The pre-cat O2 sensor will make your mileage drop without ever setting the SES light or causing any noticable power loss.

Wow, almost forgot about this post. Wasn't getting the email updates from responses anymore, sorry!

You know, I never thought an O2 sensor could go bad without sending a code. This may be something worth trying, thanks!

I did FINALLY get around to doing that tune-up this weekend, and I discovered something that may have really helped also. I went to go change the plugs and wires and discovered this thing still has a dist. cap and rotor! I don't know why I thought this had just coil packs... must have been thinking of the Northstar in my Caddy. I bought a new AC Delco cap and rotor as well, and completed the job on Sat. Took a test cruise and it sure seemed to wake it up. Much more power on acceleration!

Have yet to do a test fill/run to find out what type of mileage I'm getting now, but I'll bet it improves a lot.

[sprucegoose]

Today's gas mileage... 15.3! Much better than what I was getting. Aprox. 2/3 of that was highway, and it was only 47 miles, so we'll see how it does in the long run. I think this was the bulk of the proiblem though. Why did I wait so long!!

[GiGsolydCustoms]

hey all, try this for better gas milage. i have a 97 dr sport i have 285 tires and run a bosch platnum +4 plug (real hot) though im having some fuel issues, i still get better than 15 mpg. as a matter of fact the last highway run, i got 20 mpg. keep the tach under 2000 rpm, and dont gun it at take off from a light or stop. i tried going it at 2500rpm and found i had a higher fuel consumption, bigtime. the tune up never hurt you either sprucegoose.
just a thought