Lost Fuel Economy - Possible O2 Or Cat?

[truss64130]

For the past month, I've noticed a rather significant drop in mileage in my 02 sierra RCLB 4.8. Usually I am around 14-15 MPG using winter grade gas, but the last three tanks have yielded just a hair above 11. I changed gas stations once thinking it was bad gas, but there was no change. I have had the truck for just over 2 years and am currently at 79300 miles.

 

Ever since I had the truck, there would be a slight rattle in 4th gear around going about 40 - 45 MPH. I also notice an exhaust odor, and even a sulfur smell a few times, when the truck is on the highway going up hill (under a load).

 

When I tapped on the Catalytic converter the other day, I did hear some rattling, although I'm not sure if it is the heat shield. Every video I have seen on the net seems to show the cat rattling all of the time, not just at one RPM.

 

Everything I am reading seems to point to either O2 sensors or a dieing Catalytic Converter. Since I am barely within the 8 year / 80,000 mile cat warranty, and if it / they are indeed the culprit, I wouldn't mind a free exchange. What kind of advice would you recommend?

 

-Todd

[Jsdirt]

Rotten egg smell & a rattle point to the cat. I would check that out. If that's covered, bring it in - they'd probably replace that without a hassle, unless it was damaged externally in some way (like off-roading).

 

A cat in pieces will definitely hurt mileage.

[truss64130]

Just got off the phone with my local dealer. they pretty much said that unless it throws a code (which it has not) they cannot tell if it is the cat. From my reading, they should be able to do some sort of test to see if if the cat is on its way out, right? Maybe I need to find someone in the area that has access to a scanner. I've been considering buying AutoEnginuity or similar to see things like O2 sensors and whatnot, but I can't afford it this month...

[Nytemare]

For the past month, I've noticed a rather significant drop in mileage in my 02 sierra RCLB 4.8. Usually I am around 14-15 MPG using winter grade gas, but the last three tanks have yielded just a hair above 11. I changed gas stations once thinking it was bad gas, but there was no change. I have had the truck for just over 2 years and am currently at 79300 miles.

 

Ever since I had the truck, there would be a slight rattle in 4th gear around going about 40 - 45 MPH. I also notice an exhaust odor, and even a sulfur smell a few times, when the truck is on the highway going up hill (under a load).

 

When I tapped on the Catalytic converter the other day, I did hear some rattling, although I'm not sure if it is the heat shield. Every video I have seen on the net seems to show the cat rattling all of the time, not just at one RPM.

 

Everything I am reading seems to point to either O2 sensors or a dieing Catalytic Converter. Since I am barely within the 8 year / 80,000 mile cat warranty, and if it / they are indeed the culprit, I wouldn't mind a free exchange. What kind of advice would you recommend?

 

-Todd

Its winter time...you are going to notice a decreased mileage due to the addition of more ethanol in order to prevent water from being in the fuel.

Unless you have a code...its probably neither. You can have the performance of an O2 checked, it could be on the edge of failing and causing a loss of fuel economy, but most of your issues are likely stemming from "winter gas".

[truss64130]

I appreciate the reply about winter gas. Do they change the formulation from year to year? I have only dropped to 11 once before. It was hunting season in the winter and I was using four wheel drive pretty much the entire time through the mountains. I only ask this as I have been through 2 other winters before without suffering this much. The truck usually runs 16-17 MPG in the summer and 14-15 in the winter.

[Nytemare]

I appreciate the reply about winter gas. Do they change the formulation from year to year? I have only dropped to 11 once before. It was hunting season in the winter and I was using four wheel drive pretty much the entire time through the mountains. I only ask this as I have been through 2 other winters before without suffering this much. The truck usually runs 16-17 MPG in the summer and 14-15 in the winter.

I have heard from a few people now that they are using way more ethanol in this years fuel...any truth to it...I have no idea.

 

You can have your O2 sensors checked for activity and performance on using the tech II scan tool....but it sounds like someone at the place you called doesn't want to do the job. An O2 on the edge of failing may compound the effect of the winter fuel too....

 

Also, have you cleaned your TB and intake lately?...Is it time to change plugs and wires?...Have you run some injector cleaner through?

My guess is that its tune up time...and you will also need to find a place that knows how to use a tech II if you want to get the activity of the O2 checked.

[truss64130]

Nytemare,

 

Thanks for the suggestions. I did the throttle body early fall, along with plugs and wires. I did use the NGK TR5 copper plugs that others have recommended. Also, before I changed the plugs, I ran 2 tanks of gas with seafoam + through the brake booster vaccuum line. The fuel filter is on my to do list, as it has almost been 20000 miles since that was changed. (also, replaced the fuel pump at that time with OEM)

 

I gotta get my hand on at least a basic scanner that can read real time O2 sensor readings.

[Nytemare]

Nytemare,

 

Thanks for the suggestions. I did the throttle body early fall, along with plugs and wires. I did use the NGK TR5 copper plugs that others have recommended. Also, before I changed the plugs, I ran 2 tanks of gas with seafoam + through the brake booster vaccuum line. The fuel filter is on my to do list, as it has almost been 20000 miles since that was changed. (also, replaced the fuel pump at that time with OEM)

 

I gotta get my hand on at least a basic scanner that can read real time O2 sensor readings.

Basic scanner wont give you much more than code reading ability.

you need a tech II or snap-on/Mac...etc equivalent scan tool to read realtime data.

 

Since you have done all the standard PM on your truck...it has to be sensor related...cats getting plugged or just that the ethanol content in the fuel is way too high.

[Jsdirt]

If your state government is predominately Democrat-run, you can bet on high ethanol content. Here in MA, my scan tool picked up 19.2% ethanol in the fuel, when ALL pumps are clearly labeled 10% content. No surprise there. This state INVENTED corruption!

 

I'm all for helping out farmers, but this is a bit ridiculous ...

 

Ethanol brings you poor mileage & low power ... but it gives the treehuggers that warm, fuzzy feeling ...

[truss64130]

Basic scanner wont give you much more than code reading ability.

you need a tech II or snap-on/Mac...etc equivalent scan tool to read realtime data.

 

Since you have done all the standard PM on your truck...it has to be sensor related...cats getting plugged or just that the ethanol content in the fuel is way too high.

 

By basic, I didn't mean too basic. I have an actron that will read codes and I/M Monitors already. I was thinking of getting a laptop based product such as AutoEnginuity. Even without the enhanced GM package, I should then be able to read live O2 data, or am I on the wrong path?

 

Thanks for the suggestions.

[Stevens03]

If you've not done it yet, clean your MAF sensor too.

[jhm]

Just got off the phone with my local dealer. they pretty much said that unless it throws a code (which it has not) they cannot tell if it is the cat. From my reading, they should be able to do some sort of test to see if if the cat is on its way out, right? Maybe I need to find someone in the area that has access to a scanner. I've been considering buying AutoEnginuity or similar to see things like O2 sensors and whatnot, but I can't afford it this month...

Yea they should be able to do a vacum test. They can also test for lazy O2 sensors. You may have to pay for it but if it turns out to be a plugged cat then you get it replaced free. By the way, any chance you have a bad spark plug or wire? That could drop your mileage too. Check this out:

http://autos.yahoo.com/maintain/repairqa/b.../ques083_0.html

[truss64130]

That link was a good read. I'll look into a vacuum test. I'll check the plugs and wires, but they were replaced not too long ago.

[redvett]

We the USA have to get away from our dependence on foreign oil and the Republican run goverment reluctantly supported going to ethanol fuels. Ethanol bring more power and cleaner burning engines.

 

 

If your state government is predominately Democrat-run, you can bet on high ethanol content. Here in MA, my scan tool picked up 19.2% ethanol in the fuel, when ALL pumps are clearly labeled 10% content. No surprise there. This state INVENTED corruption!

 

I'm all for helping out farmers, but this is a bit ridiculous ...

 

Ethanol brings you poor mileage & low power ... but it gives the treehuggers that warm, fuzzy feeling ...

[jhm]

We the USA have to get away from our dependence on foreign oil and the Republican run goverment reluctantly supported going to ethanol fuels. Ethanol bring more power and cleaner burning engines.

 

 

If your state government is predominately Democrat-run, you can bet on high ethanol content. Here in MA, my scan tool picked up 19.2% ethanol in the fuel, when ALL pumps are clearly labeled 10% content. No surprise there. This state INVENTED corruption!

 

I'm all for helping out farmers, but this is a bit ridiculous ...

 

Ethanol brings you poor mileage & low power ... but it gives the treehuggers that warm, fuzzy feeling ...

 

Ethanol over 10% can cause big problems for engines. It also drys out seals and harms plastic parts which is why you cant leave it in seasonal equipment. My small engine dealer told me to use 89 or better octane since the ethanol lowers the octane rating of the gas, if you put in 87 you are not burning 87. Shelf life of ethanol is about a month so if you have it in gasoline you better use it up in that time or get gunk.