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Discussion Starter #1
After replacing my fuel spider, plugs, fuel filter, MAF, fan clutch and many other parts, I'm still getting pretty terrible mileage from my 2001 RWD. Not sure what could be causing it. I'm not getting any codes, and the truck seems to run fine. I was able to get 317 miles per tank every time with my old S10. Seemed consistent no matter how I drove. This truck, however, was a mess when I got it. Pretty much everything was trashed. I'm wondering if there's something I missed. Pretty much the only stock part of the fuel system left is the pump, and it tested as holding pressure. Fires the truck up on the first shot every time, so if it's going bad it would surprise me.

What are some other things that could cause bad mileage, without tripping the check engine light?
 

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Been there Done it
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The fact it's an S10 with a 4.3 enters in also. They don't get the kinda of mileage a Honda with a 2.nothing does.
 

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B4U Task Force Admin
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If you don't drive it like you stole it, and the tune is in order, 4.3's will get respectable milage for what they are.
 

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Discussion Starter #6
What is bad mileage ? What drive train rear end? What kind of driving? Winter- summer?
To me, any mileage that's worse than what I was getting previously, since it's a lot of the same equipment swapped onto the new truck. I want to be able to get about 300 miles in before the low fuel light comes on. Right now I'm getting closer to 200 miles per tank. I'll have to watch it more closely over the next week to see what it averages with its new MAF.

I don't know much about the rear end. It's a RWD truck, with drum brakes on the back. It looks like the rear end from the old truck, but I can't be sure if they're the same model.

Driving conditions are summer and fairly dry. I live in the Southern part of the US. I do a lot of highway driving, and a fair amount of idling in traffic. None of this ever effected the old truck.
 

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B4U Task Force Admin
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I did a 268 mile round trip yesterday in the wife's Blazer2. I just filled and calculated...19mpg. Frfeeway speeds were between 65-75. I won't complain about that.
 

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To me, any mileage that's worse than what I was getting previously, since it's a lot of the same equipment swapped onto the new truck. I want to be able to get about 300 miles in before the low fuel light comes on. Right now I'm getting closer to 200 miles per tank. I'll have to watch it more closely over the next week to see what it averages with its new MAF.



I don't know much about the rear end. It's a RWD truck, with drum brakes on the back. It looks like the rear end from the old truck, but I can't be sure if they're the same model.



Driving conditions are summer and fairly dry. I live in the Southern part of the US. I do a lot of highway driving, and a fair amount of idling in traffic. None of this ever effected the old truck.
Look in your glove box and get the rpo code for the gears. What size tires?

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Mr Goodwrench's Evil Twin
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Do you have the ability to view the trucks fuel trim numbers in the live data stream? If something is causing poor fuel economy, there is a good chance it would be reflected in the fuel trim numbers.

You could check the coolant temp sensor as well, these will generally cause a vehicle to run excessively rich when they go bad and often times won't set any codes. Often times they will give a reading of -44 degrees when they go bad, but not always. After the truck has been sitting overnight, look at the coolant temp on the scanner (don't start the truck first), it should show very close to what the ambient air temperature is.

What about a fuel leak? Have you checked for leaks, particularly around the filler neck and all the connections in the fuel lines? What about those drum brakes... are they dragging? That would reduce economy as well.

All that said, it would help to know what the actual calculated fuel economy is. Fill the tank full, till the pump stops itself and do not overfill the tank. Reset trip odometer when you fill up. then drive until its almost empty, fill the tank full again. Take the number of miles driven (as seen on trip odometer) and divide that by the number of gallons you put in on the second fill up. It would be much easier to say there is or isn't a problem if we knew the trucks actual calculated fuel economy.
 

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Someone may correct me if I am wrong but wasn't there 2 different capacity tanks for S-trucks over the years? Maybe this truck you have now has the smaller tank compared to your last one.
 

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You can check tank capacities at the GM Heritage site. For the most part, they ranged from 18.5 to 19 gallons.
 

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Discussion Starter #12
Thanks so much for the responses, lots of good info. I always suspected my tank was about 19 gallons in the old truck. The new one appears to be identical.

Do you have the ability to view the trucks fuel trim numbers in the live data stream? If something is causing poor fuel economy, there is a good chance it would be reflected in the fuel trim numbers.
I've been meaning to get an adapter so I can plug my laptop in. Otherwise I'll have to take it to a shop. That's a last resort.

You could check the coolant temp sensor as well, these will generally cause a vehicle to run excessively rich when they go bad and often times won't set any codes.
I didn't even know it had one, so this is good info. I might try swapping the CTS from my spare engine that I saved from the old truck. It had no problems as of the last time I drove it. I've been meaning to change the coolant anyway.

What about a fuel leak? Have you checked for leaks, particularly around the filler neck and all the connections in the fuel lines? What about those drum brakes... are they dragging? That would reduce economy as well.
I already put new drums on it. The old ones were pretty bad. And I'm fairly sure it doesn't leak fuel. There's no gas smell, at least. No drips or anything, and I've spent a fair amount of time under it cleaning off all the mud from the previous decades of this truck's life.
 

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Figure out your real fuel mileage, rather than just saying X amount of miles before the light comes on.


1. Go to the gas station and fill up your tank until the pump clicks off. Don't "top it off". Reset your trip meter.
2. When you're low on fuel, go back to the gas station (preferably the same one) and fill up again the same as step 1.
3. Look how many miles are on your trip meter. Divide it by the number of gallons the pump is showing. That is your fuel mileage.
4. Report back to this forum and post your results so we have an idea of what you're talking about.
 

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Discussion Starter #14
Alright, I've refilled the tank today. I put 14.90 gallons in, and the trip was at 167 miles when I reset. We'll see how it does when I fill up again. When I filled up today, the needle was well under the quarter tank mark but not entirely on E. Close enough to make me nervous. I realize now that all those years when I was driving until the fuel light came on, I was probably nearly bone dry in the tank. Very dangerous to do in my city, I don't know if I have the nerve to drive like that again. I live in a place where there's a good chance I can die if my vehicle breaks down. I'm also not entirely sure this new truck has a working low fuel light. But could it really have had three to four gallons left in it with the needle as low as it was?

I guess we'll see after this next tank burns through.
 

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I know my fuel light comes on when there's about 8-10 Liters left (aprox. 2-2.5 gallons) I have a bad habit of running it past the E line...... I've gotten good at guessing how much gas will fill my tank based on how far I drove with the light on lol. Not that I would ever recommend it but you might be surprised how far you can go with the low fuel light on.
 

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Discussion Starter #16
Ok, the results are in. I was able to drive the truck 233 miles before filling up. The low fuel light came on, and the truck was sputtering as I pulled into the station. I was only able to put in 14.825 gallons, which confuses me. Shouldn't the tank be bigger? Because I filled up with roughly this much before. So I guess my tank is only 15 gallons or so? I know the truck was just about on fumes today.

It seems like replacing the CTS did improve economy. I also replaced the EGR valve. Are there any more things that would rob economy and performance? The truck drives normal on surface streets, but seems to lag somewhat in the top end. It's like there's a flat spot in the acceleration above 70 mph. I might try swapping the throttle position sensor and cleaning the throttle body out.
 

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Discussion Starter #18
You did, but clearly it's not taking 19 gallons. There's no way that tank had more than a gallon in it when I filled up today. There's no certainly way I still had 4.17 gallons in there. If that gas station had been even a few hundred feet down the block I wouldn't have made it.
 

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Discussion Starter #19
I might have solved the mystery of the missing economy and acceleration, since the truck gave a p0420 this evening after a long drive. I can't decide if I want to go ahead and put a new cat in there, or first try the trick I saw on Youtube of putting a gallon of paint thinner in the tank. Though I'm fairly sure it would be a waste of time, as that trick appears to be for cats that are dirty, not plugged up as I suspect mine to be.
 
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