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DTC's, Problems & Troubleshooting Getting an engine related DTC? Check here for assistance.

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Old 09-28-2010, 11:21 PM
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Poor MPG/Slight Surge @ low RPM

Hey guys. My 06 Grand Prix is getting only 21 MPG in 60/40 hiway/city driving and seems to have a slight surge at low RPMs (<1500). Seems like the average MPG for that kind of driving should be more like 25-27. The problem has never set the MIL.

Anyways, I feel like the symptoms would point towards a lazy O2 sensor. It may be good enough yet to not throw a code but bad enough to affect mileage. How can I tell if the O2 sensor is bad on my Actron scan tool? Otherwise if I replace just the upstream O2 sensor purely for maintenance reasons what are the odds it may help this problem?
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Old 09-29-2010, 12:54 AM
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Look at EGR, sounds like a clogged or partially restricted EGR flow, it will cause surge. Generally surging is from a lean condition.

Yes, and O2 could do it, but likely an 06 would trigger codes on a bad O2.

look for O2 cross counts, that is what you want to see.

Quote:
Originally Posted by 06_base_GRpr View Post
Hey guys. My 06 Grand Prix is getting only 21 MPG in 60/40 hiway/city driving and seems to have a slight surge at low RPMs (<1500). Seems like the average MPG for that kind of driving should be more like 25-27. The problem has never set the MIL.

Anyways, I feel like the symptoms would point towards a lazy O2 sensor. It may be good enough yet to not throw a code but bad enough to affect mileage. How can I tell if the O2 sensor is bad on my Actron scan tool? Otherwise if I replace just the upstream O2 sensor purely for maintenance reasons what are the odds it may help this problem?
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Old 09-29-2010, 02:56 AM
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Check your vacuum lines especially the elbows and tees the rubber gets brittle and cracks, I had to replace all mine 3 years ago. A small leak on the bypass valve, fuel pressure and pressure sensor line will cause poor gas mileage and not set a code. Vacuum leaks on the evaporator circuit will set a code.
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Old 09-29-2010, 10:20 AM
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Quote:
Originally Posted by n0dih View Post
Look at EGR, sounds like a clogged or partially restricted EGR flow, it will cause surge. Generally surging is from a lean condition.

Yes, and O2 could do it, but likely an 06 would trigger codes on a bad O2.

look for O2 cross counts, that is what you want to see.

When I say surge, I am more picky than most- the problem is a seat of the pants feel. It isnt something you can see in the tach but you can just feel something aint right...

As for the cross counts, does that mean how fast/often the voltage changes? I was watching the outputs btw the 2 O2 sensors last night and noticed the post-cat sensor was MUCH slower in changes than the pre-cat and voltage was steadily higher (post-cat)...
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Old 09-29-2010, 12:11 PM
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dying MAF will do it too.
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Old 09-29-2010, 12:26 PM
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Quote:
Originally Posted by 06_base_GRpr View Post
When I say surge, I am more picky than most- the problem is a seat of the pants feel. It isnt something you can see in the tach but you can just feel something aint right...

As for the cross counts, does that mean how fast/often the voltage changes? I was watching the outputs btw the 2 O2 sensors last night and noticed the post-cat sensor was MUCH slower in changes than the pre-cat and voltage was steadily higher (post-cat)...
Does the RPM change on the Data-log when this is occurring?
Does the TCC slip speed change on the Data-Log when this is occurring?
Does the TPS change on the Data-Log when this occurs?

If the RPM stays the same it might not be a surge.
If the RPM stays the same , but the TCC slip changes -- then you need to look at the TCC and determine what the TCM if driving it. Is it 100% locked or is the TCM ramping it up and down or or the TCC slipping the the TCM holding a steady %?
If the TPS is going up and down and the APP is steady -- You may have a Throttle Body failing and that is one of the issues of a bad T-body. This can give poor MPG, but it doesn't change That Much
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Old 09-29-2010, 10:55 PM
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Better way to describe it

Quote:
Originally Posted by Caleditor View Post
Does the RPM change on the Data-log when this is occurring?
Does the TCC slip speed change on the Data-Log when this is occurring?
Does the TPS change on the Data-Log when this occurs?

If the RPM stays the same it might not be a surge.
If the RPM stays the same , but the TCC slip changes -- then you need to look at the TCC and determine what the TCM if driving it. Is it 100&#37; locked or is the TCM ramping it up and down or or the TCC slipping the the TCM holding a steady &#37;?
If the TPS is going up and down and the APP is steady -- You may have a Throttle Body failing and that is one of the issues of a bad T-body. This can give poor MPG, but it doesn't change That Much

Ok so I've thought of a better way to describe the problem...It's more of a "BOG" than "SURGE". To take y'all back a few years, what happens reminds me of a bog from a carbureted car when the primaries were too rich then the secondaries opened up and it took off like crazy...That's how my car reacts when I accelerate from the on-ramp onto the highway- bog then downshift and go. It's a slight bog and relatively soft but very noticeable to me. It is also difficult to maintain highway speed consistently (~75mph). Plus the crappy idle/low end and gas mileage...

Here are a few measures I've taken for maintenance that haven't helped (keep in mind I've only owned the car a couple months):
  • Autolite Double Platinum plugs
  • full can of seafoam in gas
  • 8 oz of Lucas fuel inj cleaner in gas (separate from seafoam tank)
  • cleaned MAF with contact cleaner
  • new Air filter

Hey Caleditor-
I am really new to operating a scan tool (got it Monday) and don't know the abbreviations or even what proper parameters are for any given sensor value. I am thinking about picking up a Helms set for my car. They would explain diagnostics better for me right? I will watch the RPMs on the scan tool 2morrow on my way to work.

Last edited by 06_base_GRpr; 09-29-2010 at 11:02 PM.
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Old 09-29-2010, 11:04 PM
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APP = Accelerator Pedal Position Sensor
RPM = Engine speed
TCC = Torque Converter Clutch
TPS = Throttle Position Sensor
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Old 09-29-2010, 11:14 PM
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Quote:
Originally Posted by n0dih View Post
Look at EGR, sounds like a clogged or partially restricted EGR flow, it will cause surge. Generally surging is from a lean condition.

Yes, and O2 could do it, but likely an 06 would trigger codes on a bad O2.

look for O2 cross counts, that is what you want to see.

I watched the O2 sensors after I got to work this morning and the post-cat sensor voltage was relatively steady and low...~.18 volts It should be fluctuating a lot right? The pre-cat one was cross-counting like crazy! Voltage was much more normal on it too. Is the post catalyst sensor usually slower to change/ cross-count due to lesser heat?
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