# Misfire codes on one side only - advice needed



## DeDe (Jan 30, 2009)

Hey guys!
I need some advice form my V6 ATQ Passat GLX (wagon)
I posted this one another forum with not much luck.
Here is my problem:
A day or two ago my car started to misfire. But not the normal, ulgy one from a dead spark plug. First I just noticed the MIL light flashing. 
Very interesting is the misfire on only one side (driver side, 4, 5, 6 cyl)
The pedal had a pretty good response, it was accelarating ok.
If you would hear it you would not say it is a misfire. But VagCom is counting them. 
I know how a real misfire is so I started with the usual suspects:
Checked the sparks plugs: good. Dry, no deposits. I changed them anyway on that side. No improvement.
Replaced the whole set of sparks wires complete with the coil modules. Same thing. The defect shows up shortly after start, once it passes 1500. Not at idle. Accelerating doesn't get it any better. 
I can only slightly feel a hint of hesitation. Something is going on but I can't pinpoint it. Obviously it is not major but I am afraid of doing more damage.
So I went further: checked the vacuum hoses and I found them in good shape(not perfect but alright).
My next step was the MAF sensor. Expensive piece. I got it from my mechanic friend. Put it on, and had the same codes. MIL flashing on, same defect, misfiring for the whole side.
I wonder what can cause such a small offset, that I get those vibrations, loss in power is not major, sparks plugs are good.
I am out of ideas. 
I wonder if anybody had similar defect and what was the cause. 
I have a Vag Com and any ideas on what to check further are welcome.
Thanks in advance!


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## O_o (Nov 12, 2009)

*FV-QR*

What's the compression like?


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## DeDe (Jan 30, 2009)

*Re: FV-QR ()*

Not measured yet. Will do though


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## tryin2vw (Dec 30, 2006)

*Re: Misfire codes on one side only - advice needed (DeDe)*

When was the timing belt replaced? It sounds as if the idler roller has lost a bearing, causing the left head to be out of time. This is a worst case scenario, but one that has been seen before.
Get a valve cover gasket and remove the valve cover on the left head. Rotate the engine by hand to TDC wheremark on the harmonic dampener aligns with the mark on the timing belt cover. The keyways on the cams should align with the arrows on the bearing caps, on both heads. If they do not, then a roller may be bad. If they align, then the cam position sensor (back near the firewall) is probably bad.


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## O_o (Nov 12, 2009)

*Re: Misfire codes on one side only - advice needed (tryin2vw)*


_Quote, originally posted by *tryin2vw* »_It sounds as if the idler roller has lost a bearing, causing the left head to be out of time.

... resulting in noticeably lower compression.

_Quote, originally posted by *tryin2vw* »_If they align, then the cam position sensor (back near the firewall) is probably bad.

... which would throw a code, if scanned.
You may be right, but I see no need to remove the valve cover... yet.


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## DeDe (Jan 30, 2009)

*Re: Misfire codes on one side only - advice needed ()*

Makes sense.
I will check the compression. 
I don't know when/if the timing belt has been done. It is on the to do list for me and I think it just made the top of the list....
The sensor for cam position on that side is towards the front and it throws no code. (I scanned and scanned.... after every move I made.)
The idler roller isn't the worse case, I thought about the cam chain adjuster going bad (loosing it's plastic piece) which is much more expensive than the TB kit.
Again, the misfiring is barely noticeable, the loss in power is minimal, you wouldn't notice it so easy if the flashing check engine light wouldn't be on. The misalignment is tiny so I hoped for a "lighter" defect.
Thanks for advices
Any other ideas are welcome.


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## tryin2vw (Dec 30, 2006)

*Re: Misfire codes on one side only - advice needed (DeDe)*

Cylinders 4-5-6 are on the driver side of the car. The cam position sensor for this bank is on the back of the head, on the firewall end of the head. The connector in front is the cam chain tensioner.
For what it is worth, TDC is measured on cylinder 3.
Also try removing the top timing belt covers. View the two-hole ears on the front of the cam sprockets with the engine at TDC. The ears should be absolutely horizontal, pointing at the other exhaust cam. Also, on the bottom side of the right (passenger side) cam sprocket, try to determine if the timing belt is taut or if it is loose. Taut means really taut, as in no play, banjo tight. 
Loose=bad


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## DeDe (Jan 30, 2009)

*Re: Misfire codes on one side only - advice needed (tryin2vw)*

I couldn't get the compression tool to measure. Yet.
But I checked the belt at TDC on the passenger side between the cam sproket and the tension roller and it seems taut to me. There was a little play but one has to see more than one to judge correctly. Or at least one tight and one loose to make the difference.
I will get back with more details.
Thanks!


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## DeDe (Jan 30, 2009)

*Re: Misfire codes on one side only - advice needed (DeDe)*

Just an update:
Compression is fine, 190 for all 6 cyls
But in the meantime we found out something new that might explain the misfiring codes on the second bank:
The Cat on the right side (cyl 1,2,3 !) is clogged, giving a lot of backpressure. I only had once a "insufficient flow" code that I somehow missed while erasing them but kept it saved.
Oxigen sensors all look like they are working, voltages fluctuating between 0.085 V to 0.750V
It is interesting how the system feels it, gets fooled by it and somehow makes the other side to misfire. I don't even think it is a real misfire...
I will get back with details.


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## O_o (Nov 12, 2009)

I don't think the ECU adjusts the mixture separately for each bank. I'm guessing it settles on a fuel rate appropriate for the average airflow of the two banks, making it rich on the clogged side, and lean on the other, causing the misfires.


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## Peter Badore (Jun 17, 2002)

*Re: ()*

On the Audi 2.8l V6 12V version AAH engine I thought that I read that each bank had separate
A/F control. With one O2 sensor per bank, the fuel/air ratios can be monitored more accurately than the VW VR6 2.8l engine which has only one O2 sensor for all 6 cylinders. And so why wouldn't the 2.8l V6 30V AHA code engine continue with that feature? Have you seen any VW or Audi technical literature that describes the 30V engine O2 sensor operation? I would like to know.


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## O_o (Nov 12, 2009)

*Re: (Peter Badore)*


_Quote, originally posted by *Peter Badore* »_Have you seen any VW or Audi technical literature that describes the 30V engine O2 sensor operation?

No. I'm just guessing. If, as you say, they had separate fuel control for each bank on the 12V, I don't see why they would stop doing that with the 30V, unless there were problems with it.


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## DeDe (Jan 30, 2009)

*Re:*

Update:
I changed the clogged cat (bank1).
I cannot see any codes (yet).
It improved the milage right away, from 10.5 to 9 or less l/100km (from 22 to 26 mpg for our friends in the US







)
And I can feel more power. 
Because it was a gradual clogging I did not noticed the loss of power but I could definitely feel the increase now.
Regarding the fuel mixture, I don't know much about how that is done, the only thing I have noticed before exchanging the cat was that the oxygen sensor adaptation values were negative on one side and positive on the other, indicating a richer mixture on one side and a leaner one on the other. 
I haven't checked the measuring blocks now to see how that afected the O2 values. Yet.


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## Slimjimmn (Apr 8, 2006)

*Re: Re: (DeDe)*

usual clogging is caused by carbon buildup from poor fuel and low temp thermostat. When I get those codes, I erase them and drive at high rpms to cook the cats clean







no codes


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## O_o (Nov 12, 2009)

*Re: Re: (DeDe)*


_Quote, originally posted by *DeDe* »_[T]he oxygen sensor adaptation values were negative on one side and positive on the other.

Well, there you have it, then: separate fuel adjustment for each bank. Only one MAF sensor though...


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