2008 4cyl G6, groaing between 1900-2500RPM, poor accel, poor mileage, help! - Pontiac G6 Forum
 
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post #1 of 10 (permalink) Old 05-12-2012, 10:00 PM Thread Starter
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Question 2008 4cyl G6, groaing between 1900-2500RPM, poor accel, poor mileage, help!

I'm not an idiot, but I know enough to know I need a mechanic to do a lot of stuff. I'll try to be complete here, but feel free to ask me dumb questions. I won't be offended.

A few months ago we had some accel issues, and took it to a mechanic. Great guy, came well recommended. Plugged in the code scanner for free, found it was "random misfire" and replaced the spark plugs. I do IT work, and their notebook needed a little help, so it was a labor swap. The car ran great after that but started to seem to have a similar but not the same issue with acceleration. This was in late March.

The issue now is that if I gun it, the RPMs will eventually jump up there and it'll pick up while before that wouldn't help (bad plug and all), but between about 1900 and 2500 it's sluggish, and you can hear the engine groaning. The fuel economy is shit now, while if I have it up at high way speeds it'll go back up to 25-30, yet at street speeds it's under 20, between 14 and 19. I had a car-full of people thursday night, and there was barely enough power to move at all in that RPM range.

The kicker is, there are no diagnostic codes being tossed out, they put the code reader on, nada. The mechanics were a bit confused by that too. One suggestion was to try a trans fluid/filter change, but they admitted that might not fix this issue. By the maintenance schedule it could use one soon, but I'd like to hold off for a few months if I can on that, and focus on this issue.

The car is an 08, 4cyl (and previously peppy as heck for a 4cyl), 60k miles, my girlfriend is the only owner, oil change every single time it needs one by the oil life meter. They'd check the air filter and while at the most recent oil change the dealer said the air filter was still good, it was closing in on replacement time, this was in February. I replaced it in March before we diagnosed the spark plug problem thinking maybe the loss of power was the filter being clogged, (but it was the plugs then).

My amateur thoughts were maybe the MAF is malfunctioning in a way that the computer isn't seeing as a failure mode yet? Or maybe with the bad plug in there for a month, fuel-rich exhaust has fouled the catalytic converter? I've seen people with similar sounding problems put in a tank of good gas and a can of Seafoam and it's helped, but I'd rather hear someone say I should try that before I try random things.

So, what info did I leave out that might help, and I appreciate any feedback anyone might have.

Last edited by burnte; 05-12-2012 at 10:05 PM.
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post #2 of 10 (permalink) Old 05-13-2012, 01:58 PM
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It almost sounds like the unburned fuel probably melted or damaged part of your cat. You can usually run an exhaust back pressure test. You remove the o2 sensor and put in a pressure gauge specific for the job. There should be a spec on that. Unless they simply don't do that with newer cars, I'm not sure.

Do you have any foul smells coming from the exhaust, like rotten eggs or anything of that nature?
Does it seem like it's low on power all the time? Or after you've been driving for a few miles and it's warmed up?

I don't know if you have 1 or 2 cats. I have an '09 2.4 and I have two cats, hopefully you only have one, be cheaper to fix if that is indeed the issue.


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post #3 of 10 (permalink) Old 05-13-2012, 11:06 PM Thread Starter
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If it's the catalytic converter then it's under warranty until 100k miles or 5 years (part of the power train warranty). Doesn't matter how long it's driven, it's weak in that RPM range, and a little weak below it, there is no warm up time or anything. It only picks up to relatively normal power above 2500/2600 RPM. I'm not sure about the smells, I'll try to take a whiff tomorrow.
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post #4 of 10 (permalink) Old 05-14-2012, 05:41 AM
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The cat should actually be covered by the 8 yr 8k emissions warranty. I'm sure a dealer can diagnose and fix it all under warranty
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post #5 of 10 (permalink) Old 05-14-2012, 02:58 PM
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i was wondering if im also having cat issues in my 07 3.5L v6. I have terrible acceleration lag where i will push to gun it from a stop and it litterally will turtle for two-three seconds...then all of a sudden jump full throttle into gear...still acceleration isnt what it used to be. I also noticed that there are times where im riding at a 20 roll and try to launch and i get about the same thing...shitty acceleration.

Fuel economy is shit any more.

My gas cap acts funny here and there...its months in between but now and then my mssg will ready "Check Gas Cap" and when i pull over and release alot of pressure comes out.

Lastly my car runs VERY HOT underneath. If i stand next to it within a minute or two my shins are on fire.
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post #6 of 10 (permalink) Old 05-14-2012, 03:08 PM
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Thedrizzle - It could be a cat... If it's getting that hot from under the car it seems like it could be it. As for the pressure in the gas tank that sounds like it may be an evaporative emissions thing. Now I'm not sure but maybe it's possible that the excess back pressure from a bad cat is causing the engine to have trouble pulling a vacuum?

Poor MPG is a strong indication. Are you also under warranty? How many miles do you have?
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post #7 of 10 (permalink) Old 05-14-2012, 10:10 PM Thread Starter
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So, yes, it is the 80k/8year emissions warranty, my bad. However, you, Crusing6 sir, were 100% correct. It's the cat. Today the car FINALLY decided to throw a code and my mechanic friend read the code, catalytic system. I told him about your theory midday, and he said that was actually a brilliant insight and highly likely, and an hour later it threw the code confirming it. You have my gratitude, sir. I'm calling the dealership tomorrow. Than you so much.
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post #8 of 10 (permalink) Old 05-16-2012, 02:23 PM
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Quote:
Originally Posted by thedrizzle52 View Post

My gas cap acts funny here and there...its months in between but now and then my mssg will ready "Check Gas Cap" and when i pull over and release alot of pressure comes out.
The fuel system regularly pressurizes itself to test for an fuel vapor leak. Yours is failing that test occasionally...i.e.-not holding pressure for a specified amount of time. It does not affect drive-ability in any way.



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post #9 of 10 (permalink) Old 05-16-2012, 02:54 PM
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Quote:
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The fuel system regularly pressurizes itself to test for an fuel vapor leak. Yours is failing that test occasionally...i.e.-not holding pressure for a specified amount of time. It does not affect drive-ability in any way.
Greenman, the only noticable change is it starts dumping fuel rapidly. This is what prompted me to pull over immediately and check it rather then check it once i arrived at my destination. When i get the check gas cap message ill sometimes also get the flashing Check engine light on the dash. It will flash for a few moments and go away completely.

but during the time that i recieve both indicators i drop massive ammounts of fuel.

as for the CAT issues ive wondered for a while now if it may have been dented in or crushed at one point causing the back up and the heat issues.

Im no mechanic but the only thing i can chalk up the lag in acceleration to feels like my intake is clogged or possibly my fuel line.

The car is at 95K and i think its about ready for a new fuel filter....but i wouldnt think this would be the culprit
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post #10 of 10 (permalink) Old 05-16-2012, 03:34 PM
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Quote:
Originally Posted by thedrizzle52 View Post
Greenman, the only noticable change is it starts dumping fuel rapidly.
What exactly do you mean by that? Is the fuel gauge dropping rapidly or is the mpg dropping rapidly?


Quote:
This is what prompted me to pull over immediately and check it rather then check it once i arrived at my destination. When i get the check gas cap message ill sometimes also get the flashing Check engine light on the dash. It will flash for a few moments and go away completely.
A flashing CEL indicates a high rate of engine misfire that could damage your cat/s. Unburned fuel is going straight into the cat/s rapidly heating them up & melting the catalyst substrate. Your high-rate misfire is occurring intermittently. There's a chicken & egg thing going on. High rate misfire will destroy the cat & a clogged cat can cause a high rate misfire. It does sound like your cat/s is/are toast. I'd recommend a backpressure test on both banks. Cats are covered for 8yr/80K.


Quote:
The car is at 95K and i think its about ready for a new fuel filter....but i wouldnt think this would be the culprit
The fuel filter is part of the fuel pump assy & is generally good for the life of the car.



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