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12/6/2019 7:27:02 PM
Posted: 2/23/2011 12:48:18 PM EST
Keeping in mind that we've got 10% ETOH fuel and it's well below freezing up here, my '01 Impala keeps getting worse in MPG.
3.4 liter engine, ~135K with most of that being highway mileage.
Two years ago (~25k since) I installed Bosch platinum plugs, Belden wires, NAPA fuel filter. Air filter is good, mobil 1 in the crankcase.

five years ago the GM dealer replaced the intake gaskets under warranty (they didn't ask, just did it)

For quite a while it would set a #3 cylinder mis-fire code and I'd run a bottle of injector cleaner through it and it would go away....finally I pulled the plenum and changed the wrong injector, but haven't had the frequency of setting that same code. Changed the T-stat while it was exposed.

The car always seems to run well, I've no reason to believe the exhaust is plugged in spite of it being the original system.

Used to get ~36mpg loaded & hauling ass down the interstate, then went to ~32, 30, 29, now 25mpg. if that.

I'm not a GM guy, and really hate working on cars (I prefer tactical vehicles & trucks) The scan tool isn't showing anything....I'm thinking about pulling the intake, changing all the injector o-rings since the mi-fire problem changed after pulling the injector rails (occasionally we smell gasoline, but never see any leaking fuel AND there's word that alcohol fuel is causing problems with seals & rings) there's been talk of cracks & leaks with the intake on these engines, but this one is all aluminum, never seems to looses coolant....Maybe I should have used A/C plugs: NAPA Prolink shows Iridium plugs as OEM, but even the AC plugs called out are platinum.
I'd sure appreciate any and all advise: with 87 oct. fuel at $3.41 and a 55 mile round trip everyday it's really adding up.
Link Posted: 2/23/2011 1:51:10 PM EST
Alot of things can affect fuel economy. Engine, trans, exhaust, tires, even a bad wheel bearing can increase rotational inertia.

Of all the things in your OP; the smell of gas you mentioned really bothers me. This should not be the case, and if you can smell it you have a leak or problem somewhere. I'd concentrate my efforts on finding & fixing this issue, then seeing what that does for the fuel economy.
Link Posted: 2/23/2011 1:59:17 PM EST
Fuel economy always goes down when winter weather rolls in. Sometimes 5%, sometimes 25%. It all depends on circumstances.

If your car isn't kicking any codes and is running properly, I wouldn't dig too deep.

You didn't mention your tires- cold weather tends to screw with tire pressure, too. I've seen midwestern weather shifts drop my tires by damn near 10psi.

You also might consider where you get your gas. I no longer buy gas from AAFES because their lowest-bidder fuel performed poorly. Keep track of where you buy your gas and how your car handles it.

Good Luck.
Link Posted: 2/23/2011 2:06:23 PM EST
Without diagnosing the car personally, it's hard to say exactly what would fix it.

I would definitely find the source of the gas smell first, leaking gas is wasted no matter what.

Additionally, I would clean the MAF, run 2 cans of GM top engine cleaner through the engine, and then change the primary oxygen sensors. While the sensors are out, I would run an exhaust back pressure test on both banks.

All these things are maintenance work, and can help fuel mileage tremendously, and the back pressure test might uncover a restricted catalytic converter or muffler.

The Bosch platinum plugs are about the last thing I would put in any car, and could be contributing to the misfire situation, but if you aren't feeling it skip, that would be a lower priority for me. I use the original plugs in everything as a general rule. That goes double for calibrated driveability parts, like TPS, MAF, BARO, IAC, ECT, and O2 sensors.

Good luck, keep us posted.
Link Posted: 2/23/2011 4:08:12 PM EST
Thanks for the replies;
Tires are Goodyear installed last Feb. with proper inflation.
New wheel bearings all around six months age.
The gas smell has been bothering me, but it's faint and so far I haven't found anything wet.
Checked the PCV valve the little jumper hose from the plastic tube into the control valve is whipped, but it sucks closed when I cover the valve, which also closes with a snap when covered.
Drives me nuts: I can start it stone cold and it's idle just as smooth & nice, when warm it still seems to runs nice 99% of the time.
I've heard of catalitic converters becoming plugged, there was even an extended recall (of course I missed it)...I'll have to find a shop that can test it for back pressure. (A long time ago I learned about testing converters, but I don't remember much except drilling a hole ahead and behind the converter....I could do that, and MIG over it, but what to use for a gauge?)
O2 sensors will cause the engine to run rich, but I would have thought they would throw a code?
I'm used to decreases during the Winter months, but never this bad.
AAFES....what a joke they've become.
Sometime it's a tough call between throwing parts or paying $90. an hour for labor, plus list price for parts.
Link Posted: 2/24/2011 2:57:05 PM EST
1st thing I would do is replace the O2 sensor. A sensor that's not working properly won't usually throw a code. O2 sensors are usually shot at 100k miles. Check to see how many O2 sensors you have though. Some new vehicles have as many as 4. My wife's 2003 Highlander has 3 (V6 engine). Don't worry too much about the after cat sensor. It's just there to monitor your cat converter. One of the first signs of a bad O2 sensor is a drop in gas mileage.

Cat converters can plug up over time. Any decent muffler shop should be able to check it. Probably run you $30 or so. Takes about 30 mins to do...max.

Go with the exact plugs recommended for your vehicle. Bosch makes good stuff but their platinum plugs won't work right in some vehicles.
Link Posted: 2/24/2011 3:02:35 PM EST
Just out of curiosity, did you start using higher octane gas when your mileage started to drop off? If so, try going back to the lower octane gas.

Since we're nearing the end of winter I'd be willing to bet you'll see an improvement in your mileage starting even if you do nothing - particularly if most of your driving is short trips where the engine ends up spending most of its running time below its most efficient temperature.
Link Posted: 2/25/2011 4:21:43 AM EST
Run some regular..non-alcoholic gas..and some fuel conditioner. Change air filter and check tire inflation

The real gasoline will help...and I've noticed my GM vehicles will drop off on fuel economy if I don't run a tank of fuel with conditioner in it every once in a while.

This winter..I've been using Lucas fuel conditioner..the stuff for diesel and gasoline...the stuff definately improved mileage..and contains no alcohol to dilute already diluted fuel.

I'm fortunate to live in a location where I can buy plain old gasoline...and that's what we run..unless we're forced to refuel out of town.

We have a new 2010 vehicle..and I've been keeping track of the mileage. Fuel economy drops of considerably with gasohol..as much as 5 mpg..but usually in the 2-3 mpg...just depends on how much good fuel is in the car..and how shitty the E-10 gasohol we occasionaly pump in it is.

Something to consider..I think the new vehicles rated mileage is calculated on E10 fuel...or new V6 Challenger is rated like 16-26 mpg...with gasoline that is all gasoline..can average quite a bit better than that...like 20-29 city/highway mileage.
Link Posted: 2/25/2011 11:12:05 AM EST
[Last Edit: 2/25/2011 11:18:16 AM EST by Keekleberrys]
Link Posted: 2/25/2011 3:16:01 PM EST
for 2011...
http://www.chevrolet.com/owners/chevy-manuals/
"Use regular unleaded gasoline with a posted octane rating of 87 or higher."

Your engine doesn't have a high enough compression ratio to require (or benefit from) premium. You'll end up dumping unburnt fuel out the tailpipe.


The problem is exacerbated by ethanol. Ethanol increases octane and decreases BTUs (energy) per gallon. Some people see the ethanol sticker and use a higher octane fuel to 'make up for the ethanol'. Worst of both worlds......you're already getting less energy per gallon with the ethanol, choose to pay more $/gal for a higher than recommended octane rating, and then dump unburned fuel out the tail pipe.
Link Posted: 3/16/2011 5:33:54 AM EST
Here's a "never would have guesed that tips"
There is a fresh air tube that runs from the intake boot to the firewall side valve cover; the rubber grommet was whipped and has been for some time. I replaced that part and mileage jumped 5mpg! Keep in mind this is NOT the PCV valve side, so I thought it was just ventilation air for the crankcase.
While we're at it, the U shaped vaccuum line at the throttle body that connects to the PCV valve is spongy/soft, but doesnt' seem to be leaking.....it and the PCV valve are goin to be replaced.

Also: couple weeks ago I changed outr the Bosch Platinum plugs with ~25K on them for AC platinuim (OEM) and picked up 2~3mpg.
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