Another bad mileage Tb

[hawkeye405]

Hi guys,

I know there's a bunch of bad mileage topics but I'm having a difficult time finding one that gives you the steps to take or things to do in order to locate what causes the bad mileage and where to start to fix it.

I have a 2003 TB 4.2L I6 that we purchased new in 2002. We did all the maintenance as required to date. Four months ago, my wife purchased a new CUV so I sold my Monte Carlo and began driving the Tb. It had just over 85,000 miles and runs fine but gets a whopping 11 mpg with average city / hwy driving. I even replaced the oil with Royal Purple, installed new iridium plugs and ignition coils. I also replaced the air filter with a K&N filter but I'll be installing the Green filter when it arrives. My wife never bothered to check the gas mileage at all.

So where do I start?

 

[littleblazer]

My steps were:
1. Check thermostat if bad check cat too after its replaced.
2. Clean cpas, check for oil in the connector
3. Make sure the O2 sensor is working properly.

 

[Daniel644]

My steps were:
1. Check thermostat if bad check cat too after its replaced.
2. Clean cpas, check for oil in the connector
3. Make sure the O2 sensor is working properly.

don't forget cleaning the throttle body.

 

[djthumper]

Did you change the transfer case fluid? That is every 50,000.

Check your MAP sensor and clean it if it needs it.

 

[Mooseman]

If you feel like reading 9 (yes, 9!) pages where one member is trying (and still trying) to figure out his mileage woes, head on over to this thread:
http://gmtnation.com/forums/topic/13500-only-getting-160-miles-on-25gal-tank/

Pretty much all of our collective knowledge on mileage issues is in there and despite that, still having issues.

DJThumper: The MAP is basically a vacuum sensor and not cleanable AFAIK. Now the MAF is cleanable with MAF cleaner but he doesn't have one for that year.

 

[djthumper]

The MAP has a sensor that many times can be cleaned, though it is a long shot. Which is why I mention it, if it is built up. I know his year did not come with a MAF...

At this point I am thinking that you need to get a diagnostic tool to take readings from the system. When you put in new plugs did you use AC Delco plugs? With only 85K on the clock, I probably would not have replaced the coil packs.

 

[hawkeye405]

Thanks for the response guys. I cleaned the throttle body when I replaced the plugs and coils. What is the cpas and how do I test the o2 sensor? I changed the rear differential fluid but not the front. Ill do that today.

Two of the coil packs were bad so I replaced all 6. I have a scan tool but not sure which one. Ill check that later.

 

[littleblazer]

If it can read live data, that is how you would check the o2 sensor.

 

[No Tyme]

+1 on the MAF sensor. I use an air compressor and blow out my air filter 1 time at oil change and always shoot the MAF sensor with CRC MAF Cleaner... On the next oil change I replace air filter, and still shoot the MAF... I like to keep my machine clean!
Still on the fence on the "green" filters...

 

[hawkeye405]

So after doing more reading of the thread mooseman posted, I did some work on my truck. I replaced the fan clutch and the thermostat, put in new coolant, changed the oil and poured in Royal Purple 5w-30 and replaced the cpac. I power washed the engine bay and started the truck. It now runs like new, smooth and quiet. I'll start keeping track of the mileage after the next fill up.

 

[C-ya]

Did you change the transfer case fluid? That is every 50,000.

I changed the rear differential fluid but not the front. Ill do that today.

Don't forget the transfer case. Every 50K miles. GM Auto Trak II.

What is the gear ratio in the rear end? The 3.42 trucks seem to get good mileage while the 3.73 and 4.12 (?) trucks get worse and worser, respectively. 11 mpg does seem pretty low. I have the 5.3 in mine with 3.73 gears and I can maintain about 13 with basically city driving.

The 4.2 engine is pretty picky on plugs. AC Delco is what it likes. With your scanner, look at the front and rear O2 sensors and make sure they are switching like they are supposed to. If not, and one is lazy, that could cause excess fuel consumption.

 

[DocBrown]

MAP sensor is really cheap and takes less than 10 minutes to change. Don't bother cleaning it, just get a new one.

Check the actual operating temperature. You can do this with the Torque Pro app and a an OBD II Bluetooth reader. If it's low, you need a new thermostat. And if you do, there is a high probability that your CAT is clogged. Very common between 85-120k miles with the I6.

 

[IndianaEXT]

And if you do, there is a high probability that your CAT is clogged. Very common between 85-120k miles with the I6.

New member here. Before I go starting my own thread with my own woes, I have seen this mentioned many times (clogged cat). Is this something that can be done for the weekend warrior in a garage to unclog or is it recommended to take to a shop? My truck currently has around 140k on it and I can guarantee it's all original equipment.

 

[DocBrown]

New member here. Before I go starting my own thread with my own woes, I have seen this mentioned many times (clogged cat). Is this something that can be done for the weekend warrior in a garage to unclog or is it recommended to take to a shop? My truck currently has around 140k on it and I can guarantee it's all original equipment.

You can't unclog it. The insides melt. It has to be replaced. Judging from your name you have an EXT. Replacing that one is harder than a SWB TB. It's a 4 hour job if you have a torch to heat the bolts. If not, well it's probably all day. Trouble is that it's really expensive to have it done. It's a catch 22. You can do a generic CAT only replacement but I've only negative things about that. Best to get the OEM replacement.

 

[hawkeye405]

I had mine cut out and a new one welded in at my local muffler shop. $235 complete. My truck runs great and you can actually feel the exhaust coming out at the tail pipe.