MPG.. OUCH!

[Gump2773]

Filled up last night. Did the math and got 13.1 mpg on the tank. When I filled up, the pump stopped at 16.3 gallons. I kept pulling the nozzle out more and more, and by the time I was done I got 18.34 gallons in. Decided to run 2 containers of techron through to maybe clean some things up. Havent found any muffler shops that will check the cat for free, most wanna charge $100/hour labor to check it.

 

[MedicatedMike]

Wow, that CAT is a a PITA....at least for me! I sprayed the bolts a few times leading up to the first attempt, but no luck today.

Changed the oil to AMSOIL SIG SERIES with a flush and changed the o2 sensor in the manifold (with a AC Delco one of coarse). Go figure the thing I thought would be the hardest (the o2 sensor) wasn't THAT bad. :wootwoot:

The cat on other hand, especially the bolts at the manifold end, were just tight as can be and without a lift, we could not get any way to get a break bar in there to show it who's boss.

We Didn't bother trying to get the 2 bolts loose at muffler end, if we couldn't get the 3 at manifold end loose. Will give it one more shot in next few days and dare I say it....pay for the first labor on this TB.....


****Also, the box the cat came in had a sticker that said "Gaskets included" but I only have one "donut gasket" in my box. One end of the pipe is "flanged" though, so maybe it doesnt need one and the stickers plurality is a typo?? Has anyone bought the magnaflow before and remember if they received 1 gasket or 2?

 

[xx_gearhead_xx]

...... the air leaking IN to the exhaust stream will skew the O2 sensor readings to be lean. Lean= PCM richening the mix......

Hmmm, I think you meant out, since it's kinda hard to suck air in in the exhaust side of anything. You would lose the backpressure you would normally have with everything airtight.

 

[deekster_caddy]

Hmmm, I think you meant out, since it's kinda hard to suck air in in the exhaust side of anything. You would lose the backpressure you would normally have with everything airtight.

Remember that exhaust travels in pulse waves. To the pressure side of every wave is a negative pressure area behind the wave. So fresh air can be sucked in through an exhaust leak under light/mid throttle. Once you get to heavy throttle/high RPMs the exhaust no longer travels in readable pulse waves, but more of a stream. That's why the PCM can't use the O2 sensor at heavy throttle. The O2 sensor feeds a voltage output to the PCM. The PCM measures cross-counts, or how often the voltage produced by the O2 crosses 600mv. At light throttle, fresh air disrupts the waves and measurements, lowering cross counts and skewing the measurement towards lean. That's also why when reading the O2 sensor output from the PCM with a scan tool, it becomes a steady number at heavy throttle - there are no more pulses.

 

[xx_gearhead_xx]

That's an interesting point, though, with 6cy pumping through one manifold I imagine that there would be little time for that to happen. If there's less backpressure your allowing more air to be pulled thru the engine which would draw more fuel and air thru, giving you the crappy mileage. As far as lean codes, that would come from the engine being able to move more air and having to add more fuel to compensate. Atleast that's my take on it, now is it that the 02 sensor is so sensitive to changes that just a small amount of air would throw it off on idle/light throttle? I understand you have to have a header on an engine as it would run like crap if there wasn't something there to channel the exhaust even on a carb setup. I'm just trying to understand, I'm all about learning something new. I've just not heard of it explained like that.

 

[Sparky]

It isn't drawing more air through the engine with a crack in the manifold. It is only drawing a little bit of extra fresh air into the exhaust stream, enough to screw with the O2 sensor readings. It isn't a lot, but enough to make it think it is a tad on the lean side so it adds fuel to "compensate." What you get then is it actually running a bit rich, but the little bursts of fresh air getting sucked into the exhaust manifold dillutes the rich exhaust and makes the O2 sensor happy, and the computer happy. Result? Crappy mpg, possibly more carbon build up in the engine, and possibly earlier cat failure.

 

[deekster_caddy]

Basically, if there is an air leak anywhere after the MAF sensor and before the O2 sensor, you will get a lean reading from the O2 sensor. If the leak is pre-combustion, you will have a truly lean mixture. If it's post-combustion, you will have a false lean reading. Either way the PCM tries to correct with more fuel until it can't correct any further and throws a code.

 

[xx_gearhead_xx]

Def got the picture now, thanks for clearing that up.

 

[northcreek]

I have only had this vehicle about three months and have not taken any dedicated highway trips so most of my experience has been around town and rural areas.Today I filled up and found that I got just over 18 mpg and I have never got worst than 16mpg.
I think that the AFM system is the reason and 3:42 gearing helps too.The AFM system has a great reputation for reliability and is seamless in operation.
Also the mpg and actual gallons used which are displayed in the DIC were accurate to the tenth with my actual calculations,I find that pretty amazing since I always thought that those DIC readouts were just feel-good info....I'm just about ready to make the switch to syn. oil so maybe I can bump the mpg up a bit more..just sayin',Mike.

 

[RayVoy]

Mike, those are good numbers, I think you may be giving the AFM (know as DOD on your truck) a little more credit than it deserves

Around town driving (any town in NY state) would not too often see the truck in 4 cyl mode very often.

Probably good maintenance, good gas and a GOOD driver share more credit.

 

[gmcman]

most of my experience has been around town and rural areas. Today I filled up and found that I got just over 18 mpg and I have never got worst than 16mpg.

Yeah.....those are very decent numbers. If I averaged 18 in the city all the time I would need to check to make sure someone didn't put 60 PSI in my tires.....

 

[northcreek]

Mike, those are good numbers, I think you may be giving the AFM (know as DOD on your truck) a little more credit than it deserves

Around town driving (any town in NY state) would not too often see the truck in 4 cyl mode very often.

Probably good maintenance, good gas and a GOOD driver share more credit.

Ray, My truck is AFM,DOD was the term used on GM's earlier offerings.I agree that AFM would not be engaged at low speeds but, on rural roads at 45-50 mph I can barely perceive a slight change in the exhaust note,and the tach tells me that it's not shifting into(out of) OD.
So I would guess that in my case AFT is in use 30-40% of the time wether I'm aware of it or not.I may do that AFM indicator modification at some point since it would be nice to know if I'm driving a 4 or an 8 ....Mike.

 

[RayVoy]

Ray, My truck is AFM,DOD was the term used on GM's earlier offerings

Sorry Mike, I thought '08 was the change year.

I may do that AFM indicator modification at some point since it would be nice to know if I'm driving a 4 or an 8 ....Mike.

What is the mod?
My '05 Envoy was the I6, so I didn't have it, my '09 has it, and the DIC tells me when it's in AFM, I never see it "around town", maybe my right foot is a little heavy.

 

[northcreek]

Sorry Mike, I thought '08 was the change year.

What is the mod?
My '05 Envoy was the I6, so I didn't have it, my '09 has it, and the DIC tells me when it's in AFM, I never see it "around town", maybe my right foot is a little heavy.

I wish I had that feature,my DIC doesn't tell me dic but, this is the mod that might fix that > Active Fuel Management / DoD Indicator - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

 

[RayVoy]

Dan is still active on the os, maybe ask him how it worked out over a period of time.

I haven't taken the time to look up the rest of the circuit that Dan tapped into (I sold my manuals after I traded the Envoy), but I don't feel good about the resistor he has inserted in series with the injector (if indeed, that is what he did).

 

[northcreek]

I don't feel good about the resistor he has inserted in series with the injector (if indeed, that is what he did).

Looking at the schematic again I see what you mean about the series resistor . Also I'm not comfortable with destroying the integrity of the injector harness .
I would be more apt to go with the vacuum gauge solution as the one member suggests...quick,simple and no real risk.

 

[Gump2773]

Just a small update. I still have not had the cat changed out but I ran 3 bottles of techron through and did my test this morning. Filled up and did the math, 14.9 mpg all city. Thats a large step up for 9.5 mpg in the start. I would really like to see 16 but I know thats pushing it.
Also, I wish my 02 had the fancy computer dash stuff. All I have is a trip odometer.

 

[joe mama]

I wish my 02 had the fancy computer dash stuff. All I have is a trip odometer.

Problem solved, plus you'll have a scan tool to read codes:
Amazon.com: ScanGaugeE Compact Multifunction Trip Computer with Customizable Real-Time Fuel Economy Digital Gauges: Automotive
I just got one, and I really like it.

 

[RayVoy]

Filled up and did the math, 14.9 mpg all city. Thats a large step up for 9.5 mpg in the start. I would really like to see 16 but I know thats pushing it.

Keep in mind, we may be still getting "winter gas" from the pumps; and, winter, or summer, the regular gas has been deluded with alcohol, both will reduce the mpg.

 

[Sparky]

I went on a (relatively) long trip recently that half of it was at 60mph and half at 65 (with a smattering of in town at the beginning and end). Got 21.5 mpg that tank. Best I've gotten in a long while, but I haven't gone on a highway trip like that for a long time either.

My best tank was 23 only a couple months after I got it, I'm not sure how that happened but I wish it would happen more often

 

[tcjack]

I have an 04 TB with 98K miles and auto tran and the 4.2 I6.

I just had a tuneup, new plugs and fuel filter. I am getting close to 21 on full highway at 60-63 mph. Have not clocked just riding around town, but I have noticed that when first starting up my average will drop fast. Think that first 2-3 minutes driving around town really sux.