New Member with Poor Gas Mileage

[emawoman]

New member who's already learned a lot just by finding this forum.

I bought a used 2006 Vibe base from a GM dealer a few months ago. Around 95,000 miles now and seems to run good. However, gas mileage is poor, less-than-expected 23 mpg, no matter city or highway, no matter gentle or more aggressive driving.

We replaced air filter, put Techron fuel additive in two times, cleaned MAF sensor, using the excellent instructions we found on this forum. Bought new spark plugs and PCV valve but haven’t put them in yet. Plenty of power and no CEL, so we don’t think O2 sensors or catalytic converter would be the cause.

I searched for reasons for the poor mpg and found a thread about “jack rabbit starts and poor mpg of 19-21” and think mine may have an ECM computer problem. The Vibe was squealing the front tires when we would drive off from a stop. Since they were more worn than the back, I imagine the previous owner had had the same problem for some time. We had the tires rotated so the better ones are on the front.

I looked at the ECM recall threads. Ours is a 89661-02K41. Hard to read those small numbers, but with a mirror, I think mine is 8746, so the chart said it was “good” and not eligible for the replacement under the recall.

I still think the lousy mpg is due to some defect in the computer. Is there any way I can hack the existing computer to fix it to get better mpg? I’ve looked online for used and new replacement ECMs. I’m wary of a used one (from eBay) unless I could get one from an owner who swears he got at least 30 mpg. I’ve also read that swap outs may need programming. Does a dealer have to do it, or can I do it myself? My husband thought of RockAuto as a place that might carry new ECMs. Not sure which ACDelco model to pick there—Vibe is AT, FWD, base w ABS. ACDELCO Part # 19205364 MODULE,Powertrain Control w/ACTIVE BRAKE CONTROL(JL4) or 4-SPD AUTO TRANS(MU4); Base Model. If we have ABS, do we need w/Active brake control? There’s only one choice for the Matrix there: STANDARD MOTOR PRODUCTS Part # EM11375 Reman, which oddly enough is my exact same 89661-02K41, says w/o Traction control.

Can someone advise me on what to do? Has anyone replaced ECM like mine that was not covered in the recall? And where do I complain; maybe there should be an extended recall to cover mine, too.

We like our Vibe, but we would like to get better gas mileage. Thanks.

[GregC]

Odds are its not the ECU. Change the plugs and PCV now and see what that gets you. Then clean the throttle body use good gas no ethanol that got me 4mpg more.

[Chiadog]

Raise the vehicle and check for a dragging brake/brakes. Then run an engine compression test to determine the condition of the engine. 23mpg in a Vibe sounds like a BIG problem.

[emawoman]

We plan to do what GregC recommends, but I have my doubts that that will get us an additional 7 mpg, which is what I thought I would be getting when I bought the car.

As for the dragging brakes, it doesn't seem that the car would be so peppy-moves just fine, coasts easily and requires very little pressure on the gas pedal. As I said, giving it just a light press on the gas when moving from a stop squeals or spins the tires. So it seems to me the computer is giving too much gas at that point. Engine is quiet and we have lots of power, so I doubt engine compression is bad.

What I would like to know is if anyone has replaced ECM for bad mpg and gotten good gas mileage afterwards. Also, did you just plug in a new computer or did it have to be programmed? Can my existing ECM be reprogrammed, and if so, can I do it? I'm not afraid to tackle any computer; I replace hard drives and add RAM to mine all the time.

[joatmon]

Did the over sensitive throttle recently develop or was it there the day you bought the car? The 06's had electronic throttle, drive by wire. I have never driven one but apparently it gives a different feel than traditional mechanical throttle. I don't know if the electronic throttle control is a separately programmed module of if it is done by the main ECM

Not sure if the ECMs in these cars can be reflashed, recalls seem to involve replacement rather than reprogram. If it can be reprogrammed, it would probably take more effort and money getting the proper interfaces, software and flashing computer than it would take to get a different ECM. Probably, but don't know for sure

Do you see the same sensitivity on the throttle while at speed? If you're rolling down the highway, do the RPMs change radically with slight changes in gas pedal position.

[GregC]

I got mine a 03 150,000 miles 12-12 first tank 22.7 my last tank 27.8. Do check for brake drag, the gas pedal by wire does feel different.

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[Salsa Guy]

If the Front AF (O2) sensor hasn't been changed that could be a start. Because it could cause the engine to run rich but the PCM would never know it. Also when changing that reset the ECU (disconnect the negative side of the battery for 10 min).

The PCM/ECU should be replaced in your car. I would think the GM dealership would have checked that out before they sold it. That would be the first thing to check before spending another dime.

[blindman10]

I doubt you will see 7 mpg with the new plugs and pvc, but if it's anything like mine when I got it, those items along with an all round tune up freed up a hand full of mpgs. The ECU reset and a clean throttle body should help too.

I may have missed the reasons for the ECU recall but did have mine replaced a few years ago as it was included in the defective units. Was this behavior you are seeing included in the recall? I'm thinking that if your ECU p/n doesn't fit in the recall then you are looking in the wrong place and certainly won't get it done for free.

[vibenvy]

Welcome to GenVibe! That certainly sucks to hear about the poor mileage you're getting with your Vibe. I hope you get the issue/s figured out and resolved ASAP. Post some pictures of your Vibe when you get a chance !

[emawoman]

I'm reviving this thread because, what do you know, I just got a recall notice from GM about the ECM. Took it in today and the service guy said they need to reflash the computer for this recall.

We have been getting 23 mpg overall, rarely as much as 26. We still are of the opinion the ECM is at fault for the poor mpg. We'll see if this reprogramming is of any help. Does anyone else have any experience with this recall?

And a general comment about computers. A regular PC can run, although not optimally, if it has some bad software. If there is some really evil malware, it may be nearly toast and you have to erase and reformat it completely. But some malware or bad software might just slow things down and you can still use the PC, even successfully clean it and get it back to speed. So, with a car computer, I wouldn't take anything for granted. I'd expect the ECM to be subject to the same problems PCs can have.

[Salsa Guy]

Very Doubtful. Again I'd suggest looking into a new Front O2 Sensor. It maybe running rich and could foul the CAT, then you'll have 2 issues