Gas Mileage Drop

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Fastbird

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This is odd........We installed a Magnaflow Cat-Back on the 00 LS8 back in September. Since then........our gas mileage has suffered instead of taken a slight rise.

Before, the little display would calculate about 410-425 miles on a full tank. Now, it's down in the 360-385 range. Not a huge drop, but a drop none the less.

I know some of you are thinking "More power = can't keep your foot out of it." But I put it through a highway ONLY tank and it still only managed 365 miles on the tank. And that was almost bone dry. And, I was being VERY easy on the gas.

I'm thinking one of a couple of things: A -- I cracked a pipe whilst wiggling the old pipes outta there. Or B -- just a strange coincidence that the O2's took a dump at the same time. I am not getting any audible sign of a cracked pipe, so I'm thinking O2......

Getting a pretty strong fuel smell when we park the car and get out which I've been attributing to a DTC being thrown for a "Canister Evap System small leak detected" issue after the dealership replaced the fuel pumps earlier this year.

I'm scanning the car later tonight when I take a drive and am going to see what that does. I'm also probably going to wipe the computer to a clean slate (pull the PCM fuse) to reset the block learns and see if that helps.

Just posting this to see if anyone else had a fuel mileage drop after a cat-back install.
 
I usally only get about 265-285 and now I added the intake and got about 320-345 and I drove a tank on highway and got more like 450ish !!!!! And I kinda speed alittle avg speed was about 80 or so.....
 
No, it's not the pipes. This gas smell has been around since the new pumps went in. I have a hunch that the stealership tweaked an Evap line.

Here's the scan information that I found interesting:

TPS: 18% at idle. ZERO throttle. I'm thinking that should be 0% at idle because that should be a closed TPS condition.

My rear O2's were both staying up high. Left side was 750-850 and right side was consistently 840-860 at idle and would move from about 700-900 when driving. The front O2's were cycling very nicely, and the block learns were adding 3.6% on the left side and 1.4% on the right side. WELL within reason. My question is, is it normal for the rear O2's on OBDII cars to not cycle much at all???

Before we put the exhaust on the car, it was bone stock and consistently about 385-400 miles per tank. Now, with the current tank, it's only going to get not even 350. That's enough of a drop, especially with a more free flowing exhaust, to cause me concern.
 
Fastbird...I can't see anyway the rear 02 sensors can cause a problem. They are there for one reason...to calculate the effiency of the cats. They don't contribute to the fuel map.

I think it's possible the fuel smell you have and the evap problem could be a contributing factor. That's worth investigating.

It's also possible that the exhaust system moved your torque band up enough to cause the fall in mileage. That wouldn't surprise me one little bit.

And lastly, this seems to be happening at about the same time the weather is turning colder. Every car I have ever owned in my life get's worse mileage in colder weather than the summer months. May just be a coincidence.
 
TPS: 18% at idle. ZERO throttle. I'm thinking that should be 0% at idle because that should be a closed TPS condition
Click to expand...
0% is what it is when the car is off if you broke your throttle cable the car will stall becasue it "chokes out" It would be like plugging a straw and nothing will get in AIR....and I cant help you with the o2 I dont know what is normall for this car....Nice computer though HIGH TECH....Maybe becasue of the back pressure that you no long have is causing this....Did you unplug the battery after installing and running the car I would try this and let the computer reset its self.....
 
The gas mileage thing has been going on since Sept when it was still summer heat. I used my buddies scanner last night to clear the EVAP code (P442), and if nothing improves today then I'm going to wipe out the computer and see what happens. I was going to get under the car today and do some digging, but wouldn't you know it.............it finally got cold out there.

The TPS thing, unless OBDII works completely different that OBDI, the TPS should read 0% throttle at idle because it's on a biased scale, I.E. when you initially start the car, that current closed throttle position becomes 0%. IN other words, when you start the car, say the TPS is being given .50 mV......this would be logged in the computer as 0% TPS or closed throttle. Then the next time you start the car let's say the TPS was showing .30 mV......now that would be the 0% or closed throttle.

The reason the car would not choke out at 0% TPS is because of the IAC......an air bypass system in the manifold controlled by a valve to regulate the idle of the motor.

2001LS8Sport said:
It's also possible that the exhaust system moved your torque band up enough to cause the fall in mileage. That wouldn't surprise me one little bit.
Click to expand...

That thought hit me initially too as it does feel a little sluggish down low now, but the reason I'm bringing all this up is because no one else seems to have seen such a dramatic drop in economy after a simple cat-back install.

Thanks for the help everyone, I'll be sure to keep you all posted on what I find.
 
Believe it or not, it could also be a clogged fuel filter. My mileage tanked. I couldn't figure out what was wrong. Changed my fuel filter (I was really remiss, it had been ~75K miles) and both my occasional pinging and low mpg issues were resolved.
 
LS4me said:
Believe it or not, it could also be a clogged fuel filter. My mileage tanked. I couldn't figure out what was wrong. Changed my fuel filter (I was really remiss, it had been ~75K miles) and both my occasional pinging and low mpg issues were resolved.
Click to expand...

Really?!?! I may have to take a look at that then. As far as I know the fuel filter hasn't been changed yet.

.........any pointers for the process????
 
410-425!

Fastbird said:
Before, the little display would calculate about 410-425 miles on a full tank. Now, it's down in the 360-385 range.
Click to expand...


My 02 is %100 stock, and after a fill up my range is always around 370. My mpg is usually around 21.


How did you get 410-425?
 
LS4me said:
Believe it or not, it could also be a clogged fuel filter. My mileage tanked. I couldn't figure out what was wrong. Changed my fuel filter (I was really remiss, it had been ~75K miles) and both my occasional pinging and low mpg issues were resolved.
Click to expand...

That is an excellent point! I've seen more than one car's mileage improve with a new filter.

The filter is located behind the left front inner fender. Remove the tire (easier if you do) and pull the plastic rivets that hold the rear portion of the inner fender. remove the liner and there it is in the same area as the cruise transducer. Easy swap from there.
 
LikeNew02 said:
My 02 is %100 stock, and after a fill up my range is always around 370. My mpg is usually around 21.


How did you get 410-425?
Click to expand...

I dunno. The car was bone stock and consistently averaged around 22-23 MPG with our normal driving. It's well below that now. We just filled up yesterday and the range was showing 360 after fill up. Usually the range would start at about 385 and after 15-20 miles would have bumped up to about 410-425 range.
 
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