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Poor mileage, no SES or MIL codes rough idle closed loop

 
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JimP
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PostPosted: Thu Dec 05, 2013 10:34 am    Post subject:  Poor mileage, no SES or MIL codes rough idle closed loop Reply with quote

The last couple of months I have noticed a steady decline in my fuel mileage and I have a rough idle once the car is warm and in closed loop. I have had the SES light come on once upon adding a magna flow catalytic converter but once I cleared the code it has not come back on.

When at operating temperature the car idles rough when stopped at a light, my highway fuel economy has dropped significantly. I initially suspected winter gas but I fill my wifes SUV and my car at the same station and her mileage is still very close to what it used to be (summer months). I don't let my car idle for long periods of time, it's well maintained, my commute is 80 % highway and I'm seeing less than 18mpg if not worse right now. It's 3.8 Litre 2003 Impala LS with the sport appearance package, I have 171,000 kilometers on it now.

Any suggestions where to look, I suspected the fuel pressure regulator, but it's fine, not leaking fuel into vac line, I cleaned the MAF sensor, the throttle body and IAC. I was thinking 02 sensors but no codes, my tool only scans and clears codes it does not run diagnsostics.

Thanks,

Jim
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ZSPOT
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PostPosted: Thu Dec 05, 2013 5:33 pm    Post subject:   Reply with quote

How often do you run fuel injector cleaner?

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JimP
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PostPosted: Thu Dec 05, 2013 5:40 pm    Post subject:   Reply with quote

I used injector cleaner a couple of tanks ago due to this issue, before that I throw in a can of seafoam now and then. What has me stumped is the rough idle when in closed loop after car is warmed up. I'm borrowing a snap on scanner tonight that runs diagnostics and real time data, hoping to look at fuel trim, I'm sure it's running pig rich but no signs of black smoke or smell in the exhaust. I hope to be able to see what the o2 sensors are doing. This things burns more gas than my Marauder that has 2 extra cylinders!
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ZSPOT
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PostPosted: Thu Dec 05, 2013 5:52 pm    Post subject:   Reply with quote

Only other thing I can think of off the top of my head, would be a leak at the vacuum harness to the FPR, or the EGR valve sticking.

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JimP
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PostPosted: Thu Dec 05, 2013 6:55 pm    Post subject:   Reply with quote

ZSPOT wrote:
Only other thing I can think of off the top of my head, would be a leak at the vacuum harness to the FPR, or the EGR valve sticking.


Will check those too, buddy with scanner should be here in 30 minutes. Thanks!
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98ss
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PostPosted: Thu Dec 05, 2013 9:40 pm    Post subject:   Reply with quote

I suffer from the same thing, but my mileage isn't as bad as yours. I suspect my 02 sensor is the culprit, that or as said above i have a vacuum leak somewhere. So it would be interesting to find out.
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JimP
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PostPosted: Thu Dec 05, 2013 9:50 pm    Post subject:   Reply with quote

Used the scanner, got the car hot, all the #'s look good \
A.F. 14.7:1
STFT was 0%
LTFT was between 3-5% went to zero when driving.
I have no misfire, no other codes and o2 sensors look good with precat going between .100-.900 mv, graph looks good, post cat voltage was between .700-.800 mv.

Going to fill it up again, I got 180 miles on this last tank and it's bone dry. If any one has any ideas let me know pls. Car had a very recent tune-up, it's driven 80% highway and not driven hard. It's making my Marauder look line an economy car mileage wise.
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98ss
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PostPosted: Thu Dec 05, 2013 10:19 pm    Post subject:   Reply with quote

JimP wrote:
Used the scanner, got the car hot, all the #'s look good \
A.F. 14.7:1
STFT was 0%
LTFT was between 3-5% went to zero when driving.
I have no misfire, no other codes and o2 sensors look good with precat going between .100-.900 mv, graph looks good, post cat voltage was between .700-.800 mv.

Going to fill it up again, I got 180 miles on this last tank and it's bone dry. If any one has any ideas let me know pls. Car had a very recent tune-up, it's driven 80% highway and not driven hard. It's making my Marauder look line an economy car mileage wise.


Sorry it wouldn't let me edit, but I looked at your mileage and its about 100k. I would have to guess lower intake gaskets.
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JimP
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PostPosted: Fri Dec 06, 2013 11:45 am    Post subject:   Reply with quote

I sprayed carb cleaner around the vac lines as well as lower intake, the idle wasn't affected / no change. I can hear a hiss while the car is running and a whoosh when when I turn it off. It has me stumped right now, the fuel trims looked good so if there is a vac leak I'd like to be able to find it. I'll dig into a bit further and get back if I find anything. Let's hope it's not the lower intake, bad timing for me busy time of year.
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PushrodPower
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PostPosted: Fri Dec 06, 2013 12:37 pm    Post subject:   Reply with quote

A longshot, but have you checked the TB gasket? I just recently had this problem on Impala. Idled like crap etc. But was also associated with a CEL.

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How much miles are on those seats? How much pounds of ass were applied to the seat during every trip? Lol

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PostPosted: Fri Dec 06, 2013 4:11 pm    Post subject:  Re: Poor mileage, no SES or MIL codes rough idle closed loop Reply with quote

JimP wrote:
The last couple of months I have noticed a steady decline in my fuel mileage and I have a rough idle once the car is warm and in closed loop. I have had the SES light come on once upon adding a magna flow catalytic converter but once I cleared the code it has not come back on.

When at operating temperature the car idles rough when stopped at a light, my highway fuel economy has dropped significantly. I initially suspected winter gas but I fill my wifes SUV and my car at the same station and her mileage is still very close to what it used to be (summer months). I don't let my car idle for long periods of time, it's well maintained, my commute is 80 % highway and I'm seeing less than 18mpg if not worse right now. It's 3.8 Litre 2003 Impala LS with the sport appearance package, I have 171,000 kilometers on it now.

Any suggestions where to look, I suspected the fuel pressure regulator, but it's fine, not leaking fuel into vac line, I cleaned the MAF sensor, the throttle body and IAC. I was thinking 02 sensors but no codes, my tool only scans and clears codes it does not run diagnsostics.

Thanks,

Jim
mine did the exact same thing after winter blend was switched over from summer fuel 15.9mpg =14.90 1L/100 km for two straight months

MPG has recovered though took over 2 months though, to get back to normal 32mpg 100% highway only driving 32 = 7.351 L/100 km



with summer fuel I can get as high as 38MPG = 6.19L/100 km under ideal conditions minimum traffic, flat grade exactly 44mph or exactly 78mph is the ideal speed/MPG ratio for this car.. I found these two spots will give the absolute best miles per gallon..


but the ECM has modification to shift early into 4th



I was on the I15 several days ago 100MPH barley touching 2200 rpm
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JimP
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PostPosted: Sat Dec 07, 2013 3:18 pm    Post subject:   Reply with quote

For now I guess I have to chalk this down to the winter gas. I can't find any vacuum leaks, diagnostics show that everything is working as it should, STFT, LTFT, O2's my buddy even put in on his emissions machine and it passed with flying colors.

My last tank was 15.2 L/100km, I'm usually at around 11.2 combined or mid 7's on the highway. Still you guys have me wondering about the lower intake manifold gasket but there do not appear to be any signs diagnostic wise to indicate there is anything wrong. I'll keep you posted.
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PostPosted: Sun Dec 08, 2013 1:07 am    Post subject:   Reply with quote

JimP wrote:
For now I guess I have to chalk this down to the winter gas. I can't find any vacuum leaks, diagnostics show that everything is working as it should, STFT, LTFT, O2's my buddy even put in on his emissions machine and it passed with flying colors.

My last tank was 15.2 L/100km, I'm usually at around 11.2 combined or mid 7's on the highway. Still you guys have me wondering about the lower intake manifold gasket but there do not appear to be any signs diagnostic wise to indicate there is anything wrong. I'll keep you posted.
you can try disconnecting the battery to reset the ECM computer. it may reset your radio stations and time also....
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