02 silverado fuel mileage went down ALOT!!

I have a 2002 Silverado extended cab short box 4x4 with the 5.3L engine. I bought it in 2004 with 36000 miles and got around 19 mpg highway and 14 city. Now with 100000 and flowmaster cat back exhaust and brand new cat's from gm I get 10 mpg city and 13.9 highway. I also replaced the plugs about 1000 miles ago with ac delco rapid fire platnums and the fuel filter earlier this year (15000 miles). The truck still runs fine no loss of power or drivabilty but the gas mileage just sucks. Any ideas on how to get it better???? I am thinking of getting a cold air intake kit but I am not sure if that is a "cure" or a fix to my problem or if it is even a problem due to its age and mileage.

Have you checked for any codes?? Did the mileage get worse before or after the new exhaust?? Parts stores usually check codes for free. Post any codes.

Well there is no ses light on but I will check for them, I know that some don't light the ses light. The fuel mileage was real bad when the cats plugged up but it back to where it is now after the exhaust work.

Well there is no ses light on but I will check for them, I know that some don't light the ses light. The fuel mileage was real bad when the cats plugged up but it back to where it is now after the exhaust work.
I suspect the new exhaust system has much less back pressure, and this is the problem, as this will effect the pcm control on mixture and timing. I get 19 hwy and about 14-15MPG around town I still have stock exhaust. this is a stupid question but, the o2 sensors were replaced when the cat's were replaced ? I hope so............

I lost a lot of MPG when I had the plugs replaced...

No I did not replace the o2 sensors.

No I did not replace the o2 sensors.
the o2 sensors should be replaced every 100,000mi and whenever the cat's are plugged with carbon this is what I have read many times before. I have never had this happen to me cat's plugged but with the overly rich mixture which i am sure you have i would do this o2 replacement. the up stream o2 sensors control the pcm the rear sensors monitor cat performance.if you can get the o2 sensors checked with scan for a reasonable cost that would be also recomended before replacing the o2.

My work has a snap on scan too that should monitor the o2 readings, I will check that out, in the next few days. Thanks for the replys.

Just a few options...

The O2 sensors are a big possibility, also, the spark plugs may not be burning the fuel correctly. Verify the specs on the plugs to see if they match factory. Also, the exhaust will tear down your mileage, but should increase HP/Torque. It shouldn't affect it as much as you said, but it could change 1-2 mpg. You should also buy a few cans of Seafoam and run it thru the gas tank. This will be sure to clean the injectors. The intake might also need a seafoam fogging to eliminate the carbon buildup. Make sure you also use Top Tier gas. Do a search for gas stations providing such gas.

Thanks for the info Scott, I will try the seafoam and see if that makes a difference. I did check for codes, and did not come up with any. I also only run BP fuels, 89 with ethanol, and have been trying the 87 non ethanol to see if that makes a difference. I used to run the 92 ultimate fuel but that is a little expensive right now.

Just make sure you change your fuel filter after running the seafoam in your gas tank...it will pull all kinds of crap out of it and clog up the filter.

Well after 200 miles of highway driving and 75 miles in town it now is getting 10.7 mpg. I am putting it on the snap on modis to look at the o2 sensors data.

Look at IAC position and injector PW at idle while you're there.

Carbon buildup can cause both to go up (sending fuel mileage down). If they do, the PW stays high even after you step on the gas. IAC generally will be between 20 and 40 counts, PW less than 3 mS. Seafoam probably won't get it all, especially around the throttle body. You may need an induction flush.

Ok here is the main points of the data I got from the scan, engine running for approx 3 min
o2 b1 s1 ranging from 60 to 800 o2 b2 s1 ranging from 60-800
o2 b1 s2 around 789 o2 b2 s2 around 127 then after 2 more min went up to 780's. I did notice o2 b2 s1 not changing as fast as b1 s1 (bank 1 sensor 1) iac 83 inj pw 3.9 both banks

then after the engine running for 11+ min

o2 b1 s1 ranging from 60-800 o2 b2 s1 ranging from 60-800 not as fast
o2 b1 s2 around 742 o2 b2 s2 around 126

inj pw 3.2 both banks and iac around 40. From that data I am believing that bank 2 sensor 1 is in need of replacement, and in which case I will replace both primary sensors. Any other ideas or tips??

What about the catalytic converter? That too can drive the mileage into the bucket.

To really look at the injectors, you need to look at the waveform with an oscilloscope. With 100k, they are probably in need of replacement, so I would replace them anyway. I don't remember the exact frequency, but the waveform won't even show up on a 2.5mS scope. Looking at the datastream will tell you to replace one if the voltage is 450mV and doesn't move, but being slow on a scan tool doesn't necessarily mean it/they are bad.

If O2 sensors dont get the mileage back to where it was, you may want to get an induction flush done. Your numbers were right at the top of the range, so you could easily get an increase.

The cats were replaced 200 miles ago, passenger side wes plugged. Now what is an induction flush?? Is that something I can do at home or something the local dealer would have to do??

because your cats were bad i new that at 100,000 mi the o2 sensors were off. now after replacing the o2 sensors and you confirm your hopefully improved mileage , clean your maf sensor with CRC maf cleaner.

If you haven't resolved your milage problem you might check your fuel regulator . It has a vacuum line on it that will suck fuel into the engine if the regulater leaks. It does not leak externally, only internally. Pull the vacuum hose off and see if it is wet inside. It is located on drivers side of engine in middle of intake manifold. about 50-75 bucks. Good luck

That was actually the first thing I checked, everything was dry. Thanks.

I suspect the new exhaust system has much less back pressure, and this is the problem, as this will effect the pcm control on mixture and timing. I get 19 hwy and about 14-15MPG around town I still have stock exhaust. this is a stupid question but, the o2 sensors were replaced when the cat's were replaced ? I hope so............ I got my 02 z71 with 21k on it, I got 15 mpg. I bought a programmer, a cold air kit, cat back exhaust, and a throttle body insert. then I got 17 mpg. I now have over 128k on it, now with 3" straight pipes...still adverage 17 mpg:wtf:

I got my 02 z71 with 21k on it, I got 15 mpg. I bought a programmer, a cold air kit, cat back exhaust, and a throttle body insert. then I got 17 mpg. I now have over 128k on it, now with 3" straight pipes...still adverage 17 mpg:wtf:

with this mod that you have done on 02 sil 5.3l what octane of gas do you use to get this mpg ? what pcm programmer did you use?

Back to the fuel regulator--Did you check it right after driving the truck or after it sat for awhile? Sometimes they only show gas in vacuum line right after driving. Good luck!

with this mod that you have done on 02 sil 5.3l what octane of gas do you use to get this mpg ? what pcm programmer did you use? I almost always run 87 octane, I used a hypertech programmer set to use 87. I had 2 1/2" duals, 3" duals, and now a 3.5" single stainless exhaust setup that belongs on a diesel. (it was free) I'm running a cat but no muffler. the 2.5" and 3" had a muffler...the exhaust being my only mod that changes, I thought it was strange that you wrote the lack of backpressure caused the fuel mileage to drop, when mine has not.

After it sat for a while and while running, all dry. I just got my o2 sensors and I am going to install them tomarrow and see what happens.

Well I just replaced the o2 sensors today. The passenger side seized and stripped the 02 bung out. I got lucky and was able to retap the bung and now they are in, I will let you all know if this helps out the fuel mileage.

I will let you all know if this helps out the fuel mileage. Did it?