Massive decrease in fuel economy

[vonVeezelsnider]

In the last couple months the Malibu has become an absolute fuel hog. Went through the last tank of gas @ about 14-15MPG with about 70/30 City/Hwy. Less than 170 miles last time from full to the fuel light coming on. Absolutely absurd. For comparison in about 60/40 driving I'm pulling 25-27MPG consistently in the 07 Impala. Both have the 3.5L engine and 4 Speed so I don't get it. The last four or five tanks of gas have been progressively getting worse and worse. Can't be bad gas as all our cars are filed at the same Shell station. Injectors and MAF were serviced last summer and I've run thru some of the Techron stuff to no avail. There are no warning lights and the engine isn't missing, no fuel leaks under the car at any time... No change in tire pressure, driving habits etc...

Any idea what would cause fuel economy to almost go in half? I have to admit that hot on the heels of the strut mounts being replaced (first time) along with the Intermediate Steering Shaft (3rd time - on its fourth ISS) recently to make it stop clunking around, this is a bit of a pain in the behind.

If it means anything the car has 69000km (~43000mi) on it right now.

Edited October 19, 2011 by vonVeezelsnider

[67impss]

My guess would be a bad O2 sensor there may be up to 4 (2 upstream + 2 down stream of the Catalytic converter) that may or may not set a code. A good scan tool is invaluable at this point Best of luck also AllData.com is very helpful with trouble shooting flow charts if you attempt the repairs yourself.

[SAmadei]

I'd second the O2 sensor. The one closest to the engine is the most important in this aspect.

Edit: Even if the O2 was replaced "recently", some engines just don't get along with particular O2 sensors long term, so I would research the best O2 sensor brand to go with the 3.5.

Edited October 20, 2011 by SAmadei

[Drew Dowdell]

My guess would be a bad O2 sensor there may be up to 4 (2 upstream + 2 down stream of the Catalytic converter) that may or may not set a code. A good scan tool is invaluable at this point Best of luck also AllData.com is very helpful with trouble shooting flow charts if you attempt the repairs yourself.

this

[Drew Dowdell]

I'd second the O2 sensor. The one closest to the engine is the most important in this aspect.

Edit: Even if the O2 was replaced "recently", some engines just don't get along with particular O2 sensors long term, so I would research the best O2 sensor brand to go with the 3.5.

I've been reading that for some reason the Bosch O2 sensors don't get along with GM computers. Just find an AC Delco

compare it to the feedback rating of the Bosch

[ocnblu]

I wonder if any of this may have been the problem with my 2010 Colorado... no SES light, nothing, just wicked guzzling.

[David]

My neighbor saw this happen with his malibu and changed out all the O2 sensors and it worked great but then about 3 months after that he started getting bad milage again. Could not find anything and by chance the local neighborhood police officer who lives around the corner suggested putting a cam on his driveway. Found out the local youth have been hitting auto's that are in the dark and stealing gas since the prices are so high.

If the O2 sensors do not fix this and everything seems to run smoothly, I would suggest seeing if someone is stealing gas from your tank. Good luck with this.

[vonVeezelsnider]

If the O2 sensors do not fix this and everything seems to run smoothly, I would suggest seeing if someone is stealing gas from your tank. Good luck with this.

That would make sense and I've considered that as a possibility but honestly we park the Regal and my Impala out there right next to it and this hasn't been an issue on the other cars... Makes you wonder if they would steal gas from just one of three cars. My mom really only uses the thing to commute and she has secure underground parking where she works.

Edited October 21, 2011 by vonVeezelsnider

[SAmadei]

I'd second the O2 sensor. The one closest to the engine is the most important in this aspect.

Edit: Even if the O2 was replaced "recently", some engines just don't get along with particular O2 sensors long term, so I would research the best O2 sensor brand to go with the 3.5.

I've been reading that for some reason the Bosch O2 sensors don't get along with GM computers. Just find an AC Delco

compare it to the feedback rating of the Bosch

Yeah, Bosch is the brand I'm mostly referring to. But IIRC, they tend to run poorly as soon as you install them. Some other brands do the "install and 3 months later work like crap" routine. I've heard some bad things about Denso brand, as well, here and there.

I forget which, but I know Bosch work really well for one of the GM drivetrains... but I can't remember to save my life. Might be the 3800 series II. Of course, with that, you have to watch what plugs you use.

Details, details. ;-)

[vonVeezelsnider]

Bosch apparently does work well with the 3800, I've heard that too.

[vonVeezelsnider]

The Malibu continues to be an enigma and the story continues to develop.

Looks like this fuel-burning issue is more inexplicable than anyone thought. Oxygen Sensors not the problem, fuel economy still in 11-14 MPG range. Took car to Chevy dealer where it is right now and as of yet they are completely stumped as to what the problem is. No diagnostic codes, no missing, engine appears to be in perfect running order and condition. Fuel system service service was just done within 8000km, they actually called us to tell us that they have no idea what the problem is. I faxed them the spreadsheet I'm using for fuel economy tracking and hope to hear from them later today after they take another look.

If they can't find anything I have a couple other shops I am familiar with whose honesty I trust who maybe could give a second opinion. With an entire tank burning through in less than 170 miles of mixed driving I have a feeling this issue is going to be either expensive to fix or expensive to live with.

Edited November 21, 2011 by vonVeezelsnider

[z28luvr01]

Does the car otherwise run and drive normally?

bad fuel filter (assuming the car has an out-of-tank filter)?

clogged air filter?

low tire pressures?

fuel leak somewhere between the tank and the engine?

[vonVeezelsnider]

Does the car otherwise run and drive normally?

bad fuel filter (assuming the car has an out-of-tank filter)?

clogged air filter?

low tire pressures?

fuel leak somewhere between the tank and the engine?

-Engine is running perfectly, and it's not at all down on power or behaving in an odd way.

-Fuel filter is of the in-tank variety

-I changed air filter this summer and the old one really wasn't that dirty but I changed it anyway

-Tire pressures this whole time were in the 30-33psi range which is at or above Manufacturer specs. I check all tire pressures bi-weekly

-Fuel leak ruled out as well by the techs at the dealer

-Car is not out of alignment either we determined.

head ---> desk

Edited November 21, 2011 by vonVeezelsnider

[Camino LS6]

Is there ever a strong odor of fuel?

[Camino LS6]

... or maybe the "secure parking" isn't so secure.

[Robert Hall]

... or maybe the "secure parking" isn't so secure.

Maybe try a locking gas cap..

[vonVeezelsnider]

No odour of fuel at all, trying out a locking gas cap for a tank and see where that gets me methinks would be the next logical step from here.

[vonVeezelsnider]

Locking gas cap is on, tank is brimmed. We shall see what happens. They said that they scanned fuel trim data and found it off spec but not too far off spec. There was a PCM calibration update that had been out for the car for a while pertaining to hot-crank issues so they installed that while they were at it. They said that I should monitor the problem for a couple tanks to see if it persists. I guess it really says something when the dealership service department can't find something to do about the problem.

[Camino LS6]

It will be interesting to learn the results.

[vonVeezelsnider]

Locking gas cap experiment complete. Noticeable difference in Fuel Economy from before = None.

#$@!

Now I don't know what step two is.

[Camino LS6]

Bummer!

Back to the dealer?

[vonVeezelsnider]

Maybe a different dealer as the guys I usually deal with came up empty. I was looking at the printout from the PCM reflashing the technicians left in the car for me at my request and noticed that the last time the PCM was re-flashed (for rough idle in December 2010), they appear to have installed software for a different variant of the LX9 engine that has a slightly different PCM (perhaps the U-Body variant of the LX9?). This was corrected as well when they flashed it with the right software (for sure) this last time. I was thinking to myself that maybe this was a contributing factor as well but seemingly it is not. Hopefully this is will be gotten to the bottom of sooner rather than later and not with a whole bunch of continued diagnosis fees etc...

[Camino LS6]

Believe me, I feel your pain.

[vonVeezelsnider]

Well.. As quickly as this issue arose it seems to have almost completely rectified itself. Returned 21.5MPG on the last tank with the same driving mix, up from 12.4MPG on the same tank from two weeks ago (when the weather was actually twenty degrees warmer than it was during this tank worth of driving). I have no idea what caused the problem or why it went away so suddenly, but I'll take it.

Of course, this really is six of one and half a dozen of the other as a new problem has just arrived on the horizon. I was driving it around on Friday and parked it at the mall and went into a store for five minutes, came out and it wouldn't start again. Plenty of electical power, just kept cranking and cranking with no inclination to turn over. Knowing that on occasion the 3.5L Epsilon cars develop a 'mild' case of not wanting to start when the engine is warm, I left it for 30 minutes and went back to start it again, this time it would crank and crank and eventually *try* to turn over, engine power crept up to just under 500RPM and the whole thing starts shaking quite acutely. I gave it a little throttle to see I could trick it into turning over but that didn't work and it just deicded to stall out. Left it for another hour and came back, more of the same but eventually, with a little throttle input and a couple tries it roared to life. I tried to replicate the problem this weekend by driving on a circut and returning home, replicating the circumstances and it won't repeat itself. Strange.

[Drew Dowdell]

Water in fuel?

[vonVeezelsnider]

I don't think so... Wouldn't it run crappy after it finally got going again?

Once I got it to turn over finally, it ran like it was supposed to do normally and continues to do so on the same gas that was in the tank when it happened. Usually buy gas from PetroCan where there is gasline antifreeze built into the mixture during the winter months.

[Robert Hall]

Ghost in the machine.

[67impss]

Von after rereading from the start I'm thinking you might have the scan tool plugged in while driving and look at the info on the O2 as far as lean/rich codes also the outside air temp sender could be faulty. Anyways would need a flow chart as this is just fishing w/o a scan tool & flow chart.

Good luck & keep us informed