1998 Jimmy, fuel mileage sucks!

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otis24

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Oct 30, 2005
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I purchased a 1998 GMC Jimmy (4.3 V6, MFI) about a month ago. It was a one-owner vehicle. The previous owner changed oil about every 3k miles. I just replaced the plugs, wires, rotor and cap. I am getting about 10 MPG or less in town. I drive about three miles to work, back home for lunch, back to work, and them home for a total of twelve miles. The engine is not throwing any codes. My previous vehicle, a 1994 GMC Sonoma, with 4.3 V6 TBI was getting about 15 MPG in town. The engine on my Jimmy seems to lope when it is cold.
Should I expect better MPG? What is a possible fix (O2 sensor, EGR valve)?
 
I think the problem is that these trucks were never stellar in the area of fuel economy to begin with and the fact that your drive so little (12 miles) in the day doesn't allow the engine to fully warm up, or it just warms up, this combined with frequent shut-downs and start-ups in between doesn't help either. You are always driving when it may be running a richer mixture.

These trucks are built solidly and also contribute to the excess fuel usage IMO.

Try taking it for a good long run on the highway and see what kind of results you get, I bet you it would be much better than what you are seeing now.
 
Do independent repair shops typically have any type of computer they can hook up to the Jimmy (or any other domestic vehicle), to monitor what's actually going on with the engine? I'd at least check with a few shops and see what diagnostic capabilities they have, before I started blindly swapping parts.
 
I have a 1996 Jimmy and it is not great on gas but your truck seems to be lower than normal. I had a major impact when mine was worse on gas recently. ended up changing the TPS. I also have to agree with the post of getting it on the computer to see what part is out of spec. the big issue with this motor for me has been the ERG valve as it tends to get little pieces of carbon stuck in the small pintle hole. also did you try and clean up the throttle body at all ? I took mine off and cleaned off the carbon on the underside of it. been a solid truck and mine will get 20 mpg on the highway if you drive it in the sweet spot. how many miles are on the truck ?
 
My son had a 96 Jimmy, it ran great. The MPG in stop and go short hops were around 13-15, nice highway runs close to 20. Extended idling and long warm up times really wasted gas in his. I'd suggest a good highway run to check MPG and work from there.
 
Originally Posted By: demarpaint
My son had a 96 Jimmy, it ran great. The MPG in stop and go short hops were around 13-15, nice highway runs close to 20. Extended idling and long warm up times really wasted gas in his. I'd suggest a good highway run to check MPG and work from there.


So if the mileage was 13-15 your 10mpg isn't too far off, and I really think this is because the car only goes 12 miles worth of short trips and is constantly running rich because the engine never gets to fully warm up.

Take it for a good run on the highway and see what the mileage is. A good way to tell is to fill up your tank, run it on the highway and then fill up again and see how much was used versus the mileage...
 
I guess in a round about way yes. They weren't the greatest for MPG's. I would also imagine since the OP lives in South Dakota its pretty cold, and that engine is running rich for a while, and then is shut off. Now if he idles it to warm it up, yes 10 MPG could very well be. I'd take it on a long highway run and see how she does before I started tossing parts at it. I'd also have a look at the air cleaner, and clean up the TB and EGR valve for starters, that's cheap enough, and see where that leads.
 
I am heading out to Sioux Falls today (120 miles one way). Think I'll take the Jimmy. Also, will dump 1 bottle of Seafoam in the tank. This should give me a better idea how it does on gas. I haven't had this vehicle on a highway trip yet.
 
I wouldn't dump in the Seafoam until the way back. I would fill it up drive one way and fill it up again to see how much fuel was consumed... Then put the Seafoam in for the trip back... Then you have an accurate picture of your engines consumption with/with-out Seafoam!

Have a good trip!
cheers3.gif
 
Run some Auto-RX if you are concerned about fuel efficiency. It will maximize ring seal and even up/restore any lost compression.

Also will tighten up all the rubber seals as well. Good luck on your new to you Jimmy.
 
You could also run a 5w20 oil in it in the winter to allow it to flow more freely because your engine is cold all the time...
 
ARX will do good for this engine... They run a bit rich IMO and some carbon buildup is unavoidable. So ARX is a good choice.
 
If it's running a bit off, could be the infamous lower intake manifold gaskets leaking. Is the thermostat good? Is it getting to just below 210 on the gauge?
 
I agree with Stevie C.
It can actually get below freezing in SD, and city driving is murder on gas mileage.
But...
O2 sensors can get lazy and not throw a code - the other parts/sensors generally will if they are bad.
Maybe a plugged exhaust or cat converter.
 
Given that you've only had the truck for a month, your location, and the kind of driving you are doing, I'm not surprised at the MPG you are getting! For reference, the closest climate data I got with a quick search (Mitchell, SD) shows the average temp for December to be 15.7, and January to date as 16.9, with the average lows being 5.2 and 6.1, respectively.

City driving in cold kills MPG. My truck, a 99 F150, which gets 15 MPG on a year around average, has been getting about 10 MPG recently. My Jeep Cherokee, which averages 17 MPG, has been getting about 13 MPG the last month and a half.

I'd not worry too much about it if the thermostat is good, and wait for warmer weather. If the MPG doesn't come up accordingly, then I can see a problem.
 
Another thing that can help is plugging your engines block heater in when your engine isn't in use so that it's warmer and uses less fuel when you start-up and go.
 
Originally Posted By: MNgopher
Given that you've only had the truck for a month, your location, and the kind of driving you are doing, I'm not surprised at the MPG you are getting! For reference, the closest climate data I got with a quick search (Mitchell, SD) shows the average temp for December to be 15.7, and January to date as 16.9, with the average lows being 5.2 and 6.1, respectively.

City driving in cold kills MPG. My truck, a 99 F150, which gets 15 MPG on a year around average, has been getting about 10 MPG recently. My Jeep Cherokee, which averages 17 MPG, has been getting about 13 MPG the last month and a half.

I'd not worry too much about it if the thermostat is good, and wait for warmer weather. If the MPG doesn't come up accordingly, then I can see a problem.


Looks like we're on the same page.
 
I'm just 70 miles west of Mitchell. And yes, it has been very, very cold. My trip to Sioux Falls got me 17.3 MPG highway. Speed was mostly 65 MPH. I'll start plugging in the block heater and see if that helps.
 
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