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Old 3rd May 2019, 05:57 PM
Prefferle Prefferle is offline
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Default S8 D3 truly random misfire

Hi folks,
A truly useful and informative forum.

I have been plagued by a misfire issue that is truly random. One day it’s cylinder 1 and 9 having misfire counts between 9 and 96 with other eight cylinders on 0. Other times these 1 and 9 cylinder will both be at 0 with other cylinders 4 and 6 having misfires with the others all at 0 - very strange indeed!

I’ve since completed the following:
Replaced coil packs and plugs.
Replaced all connector shells on coil packs
Cleaned earthing connections on cylinder heads for coils.
Replaced MAF connector shells
Unplugged MAF’s with engine idling, resulting in erratic idling as it should be.
Carbon cleaned with walnut shell blasting.
Visual inspected inlet manifold for broken plastic flaps - ok.
VCDS scan resulting misfire codes only for engine.

It shouldn’t be compression as the misfire cylinders can change.

Can anyone advise of any other checks I can run?

Should I power off the battery and unplug ECU connectors and check/clean?
Is there an earth connection that can be corroded or something that can cause this?

Thanks guys,
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Last edited by Prefferle; 3rd May 2019 at 06:19 PM.
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Old 3rd May 2019, 08:32 PM
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steamship steamship is offline
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Is the battery in good health? Have you checked the vacuum lines for a possible leak? I don't know if VCDS can monitor fuel pressure, but what about a possible clogged fuel filter?
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Old 4th May 2019, 06:18 AM
Prefferle Prefferle is offline
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Quote:
Originally Posted by steamship View Post
Is the battery in good health? Have you checked the vacuum lines for a possible leak? I don't know if VCDS can monitor fuel pressure, but what about a possible clogged fuel filter?
Thanks steamship,
Battery good - less than a year old and no VCDS codes.
Vacuum lines mostly replaced - i’ll look into these again.
Fuel rail pressure checked on VCDS, so have not replaced fuel filter - I’ll replace.

one thing when I reset all codes with engine at idle - it sounds like the engine smooths out for a few seconds before pulling misfire codes again.

I willl report back with findings.

Thanks.
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Old 4th May 2019, 10:49 AM
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steamship steamship is offline
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Looks like you've covered everything that I can think of.

I just re-read your original post and noticed this line:

Should I power off the battery and unplug ECU connectors and check/clean?


I only recently discovered that the ECUs on the D3 are just lying exposed in the plenum chamber, and now wondering if that area might be the cause, as I'm thinking that since the symptoms are random in nature, maybe water has got into the ECUs, or at least into the connectors.

Since you replaced most of the vacuum lines, and they're probably worse to get at than on a 3.7 D2, I'd opt to investigate the ECU connector first, and at least rule them out before checking the vacuum lines.
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts

My DIY jobs - Replaced alarm horn, Replaced Coolant Temp sensor, Updated RNS-D firmware, Installed Reversing Camera and Digital TV, Dashcam Installation, Retrofitted Rear Blinds, Auto-Dimming Rear View Mirror, Chrome Boot Struts

Sold! 2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights

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Old 4th May 2019, 11:19 AM
Prefferle Prefferle is offline
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Thanks Steamship,

I’ll get straight on that. I’m also hoping to find and clean all earth connections I can find.

Regards,
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Old 4th May 2019, 05:45 PM
ainarssems ainarssems is offline
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A bit strange and I do not have any experience with D3 S8.

Considering this
Quote:
one thing when I reset all codes with engine at idle - it sounds like the engine smooths out for a few seconds before pulling misfire codes again.
I would say ECU detects a problem and cuts fuel to the problematic cylinders to save cats from unburned fuel going into them and damaging them and that results in more misfires. So I would monitor any odd behaviour after you reset and see if there are any problems/reports of misfire in that time to see if it can pinpoint offending cylinder, if that changes over time that it's got to be fuel supply or air metering issue.
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