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Old 01-05-2009, 11:33 PM   #1
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Wow. 95 ZR2 gets really poor mpg

So a friend of mine has a 1991 S-10, no frills, just 4WD longbed. He without the topper gets about 19-20 in town and upto 25mpg on the highway.

So I bought a 1995 GMC Sonoma Highrider (ZR2), I knew it would get slightly worse mileage since it sits higher and has a more powerful motor. But 11.96mpg?? That's just terrible if you ask me. Granted, I had just bought it, hit -20* lots of idling (pointless since high cold idle is about 1000rpms) and the fuel pump died, and I've been enjoying it's performance abilities.

But 12mpg?? Something isn't right here. I was hoping for at least the EPA of 15mpg. I didn't think I was driving any more spiritedly than the EPA test would assume.

The truck has 173,000 miles, and its mostly highway driving. Not a leak in the motor or drivetrain, well maintained by a local mechanic. It could use some of the deep stuff (bushings, wheel bearings, etc) but I it doesn't needs them currently. The doors and plastic panels rattle more than the suspension does.

Long winded shortened: 12mpg intown. Should be closer to 15mpg. Bought the truck for off-roading and towing, not mpg, but it doesn't need to get poor mileage either. Suggestions from anyone who's familiar?
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Old 01-06-2009, 03:41 AM   #2
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Those older compact pickups do great in their base models, but all the bells & whistles severely affect FE. The off-road packages usually come with all kinds of stuff.

Which engine is in your friend's truck? Which engine is in yours? How about transmissions?

The differential you've got is probably geared for towing and off-roading. That can make a huge difference. Do you have big or gnarly tires? They do too.
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Old 01-06-2009, 07:48 PM   #3
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if i remember correctly the 95 4wd s-10 have the 4.3L v6. most likely automatic as the manual with a v6 is rareish. the 4wd all have the same rearend (might be 3.73 but probably 4.11)

the 1st gen s-10's with 4wd im not sure, they could have put a 4wd with the 2.8L 4 banger but i kinda doubt it...

yea 12 doesnt seem right. id check the O2 sensors a they like to die when milleage goes up there. yea the fuel pump is one of those quirks the s-10's have. id check the plugs and wires(or change them anyways then you KNOW thier new) change all the fluids(unless he has records or somehting)

also since a mechanic owned it it means he knows what hes doing when he disables something etc. (btw dont rely on the SES light to tell you theres a problem, that thing only comes on if there's a major issue you can feel/see lol)

if you know him ask if hes done any performance things to it as i know there's lots of little things you can do to get extra oomph outa the truck while sacrificing mpg.(like put a performace PCM in it for one thing) but if theres nothing else done to the truck i wouldnt worry about it.

yea tires make a big difference (if thier non stock the spedo/odometer wont read right)

but if its -20*F i would just be happy it starts :P
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Old 01-06-2009, 08:42 PM   #4
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Remove your oil cap with the engine running. Do you feel air blowing out?
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Old 01-06-2009, 11:59 PM   #5
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All the 4WDs after 1989 had the 4.3, before that you could have had the 2.8 V6. I have the 4.3 V6, friend does as well. Difference: His is the TBI, mine is the CPI (central port injection, 1 injector, 6 nozzles). 165HP vs 195HP. small difference, although I read that the internal parts have changed quite a bit (balancing shafts, roller rockers, cam lobe profiles, etc).

We both have the Borg Warner 5 speed, I have the NP233 3 position electronic differential (no neutral) his is the NP231 4 position floor shift. No offense to others, but an auto truck is worth scrap to me. I need my manual when enjoying the offroad, the primary reason I bought the truck.

With the ZR2 package I get 31x10.5R15 tires, a huge jump over the stock 235/75R15 tires, 29" vs 31". They certainly weigh more, I can feel that.
I have the 3.73 rear gears. Stock gear for most the 4WDs. I don't know what my friend has, but I'd guess it's the same. It doesn't feel like his has the 3.43 or 4.11 gears. Granted the 2" tire difference may change that. We planned to swap tires just for giggles (see how it looks on his, see what awesome tires grip like on mine), so I'll get an idea then.
I have plugs and wires to swap out, just haven't had a chance yet, been too busy between when it first died (runs fine now) and holiday festivities. It starts fine at -25F, didn't even crank slow. There is a slight drop in rpm when cold if I touch the throttle, but it doesn't die. Even loading the motor with the clutch and tapping the throttle won't kill it. It goes away after about 30 seconds of running. I'm assuming its a cold injection problem, something a $271 part would fix.

I don't think the previous owner ever modified the truck beyond stock. He was too much a down to earth guy. And the windshield has the "oil change" date sticker on it. It was utility to him, not a toy. But I won't rule anything out.

The ABS is out, the light is on, and I CAN lock all 4 tires on ice. Not an issue, as I prefer no ABS. I like the predictability when offroading (and power braking ). I haven't looked into the reason yet, as it isn't stopping (bad joke?) the vehicle from running properly otherwise.

I dislike SES or SOL-as I call them-lights. Some reading has told me I have a 95.5, which may or may not be ODBII. I haven't looked for the reader port yet, but that'll seal that question if I find it.

I'll check the air/oil thing right now (I love my job for reasons like this).
Okay, put my hand over it, didn't feel a breeze. Covered it with my palm, felt more like it was sucking my hand in. Hmm. From what I know, the crankcase should have positive vacuum, hence for the blowby valve. I think I'll check that out tomorrow, see if its working or not. I may just pull the EGR as well and clean it out, it might be junked up and fouling a little.

So, bypass (real name eludes me at this hour) valve, EGR, plugs, wires, anything else anyone could think of? I know few will read this till tomorrow, possibly after I attempt any of this. But hey! There's always the next day!
Thanks for any and all input guys and gals.
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Old 01-07-2009, 05:39 AM   #6
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When you swap tires temporarily, don't forget to calculate the circumference difference into any speed, distance, or FE numbers.
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Old 01-07-2009, 07:09 AM   #7
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Well, if you didn't feel air blowing out your engine is alright mechanically. Is your SES light on? Whether it be OBD 1 or 2 the SES usually means the engine is running on a default fuel map. That could cause bad mileage but mpg numbers in the 11s is pretty low, that's gotta be a combination of things.
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Old 01-14-2009, 08:24 PM   #8
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if you're having a little fun and even one wheel bearing is starting to loosen up, 12 mpg is perfectly feasible.

91 s10 is going to have either the 2.8 v6 POS or the 2.5 L4 gutless econo-engine. either can have manual or auto trans and either can have 4wd. your friend probably has the 2.5 L4 with the MPG hes getting. mine gets 26 mpg mixed driving, 29 hwy (75 mph highway) without overdrive.

the 4.3 can get decent mileage but it needs 2wd, manual, and a very light foot. 4wd saps mpgs, auto saps mps, off-road packages drop the diff ratio which kills mpgs, off-raod packages also raise the ground clearance forcing you to punch a bigger hole in the air. a friend of mine has a 4.3 in a full size with 2wd and 5M tranny getting 18+ mpg mixed driving. it's a big engine and it'll drink big if you give it the slightest chance.

I work in a shop that does suspension and I can tell you that your wheel bearings are either bad and need to be replaced or they aren't. you'll start to lose mpg as soon as you get any play. same goes for the rest of the parts.
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Old 01-14-2009, 09:18 PM   #9
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How accurate are the numbers your friend is giving you for the older truck?
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Old 01-14-2009, 11:12 PM   #10
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Quote:
Originally Posted by kamesama980 View Post
91 s10 is going to have either the 2.8 v6 POS or the 2.5 L4 gutless econo-engine. either can have manual or auto trans and either can have 4wd. your friend probably has the 2.5 L4 with the MPG hes getting. mine gets 26 mpg mixed driving, 29 hwy (75 mph highway) without overdrive.
Incorrect. GM introduced the 4.3 in 1989. Before that it was the 2.8 with 4WD. 4WD only came with the 4.3 after that, all the way through 2003. He does have the 4.3.

Quote:
Originally Posted by kamesama980 View Post
the 4.3 can get decent mileage but it needs 2wd, manual, and a very light foot. 4wd saps mpgs, auto saps mps, off-road packages drop the diff ratio which kills mpgs, off-raod packages also raise the ground clearance forcing you to punch a bigger hole in the air.
Very true. Mine is not an auto. The diff ratio is 3.73, not the 4.10. Even the non-ZR2 and 2WD came with the option of 3.73. I completely understand the concept of wind resistance and the variables related. The EPA reduced mileage from my friend's setup to mine, 20/24 to 15/20. I accept that, but I don't accept 12 vs 15mpg. I want that 3mpg back. I have not had enough miles to test whether a heavy foot and fast shifting or a light foot and fast shifting (ie low rpms) produces the better.

Quote:
Originally Posted by kamesama980 View Post
I work in a shop that does suspension and I can tell you that your wheel bearings are either bad and need to be replaced or they aren't. you'll start to lose mpg as soon as you get any play. same goes for the rest of the parts.
Suspension wise the only problem I have, noise it makes, issue it gives, is one missing bump stop for the front left. All points lubed, all rubber without tears. The A-arms may be soft in the rubber, but aren't rotted or damaged beyond age.


You have a completely different setup than I. Other than both made under GM, we don't even have a single thing in common. Hard to draw conclusions from your experience other than basic mechanical workings of vehicles.


Quote:
Originally Posted by DarbyWalters View Post
How accurate are the numbers your friend is giving you for the older truck?
Very accurate. This friend isn't an acquaintance, I was roommates with him for 2 years, and known him and his truck for 4 years. I've helped him many times fix things with his truck. I'm very familiar with its running gear, options, and setup.
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