I have been chasing this issue for almost a year now. I wrote about it in another thread but I'll transfer all the relevant info to this one:

Codes: P0300, P0302, P0304, P0305, P0306
(Random misfire + cylinders 2, 4, 5, 6)

What I've replaced:

• Coil pack
• Spark plugs (twice - first with NGK, then with Bosch)
• Spark plug wires
• Both valve cover gaskets (I wasn't getting any oil on the plugs, but I figured why not)
• Both upstream o2 sensors
• Fuel filter
• Air filter
• Coolant temp sensor
• Intake air sensor
• Crankshaft position sensor
• ECU

As well as:

• Cleaning the MAF (with MAF cleaner of course + a second time a few months later when the issue returned)
• Putting a can of Techron in and putting a can of Redline in (a ~month apart when the issue returned)
• Replacing most of the vacuum tubes on top of the throttle body
• Swapped injectors from left bank to right and right to left
• Swapped cam position sensors

Shop tested:

• Fuel pressure
• Smoke test for leaks
• Compression test

I'm truly near the end of my line with this. My last guesses could be a clogged cat, or a coil pack. I don't think it's the coil pack because I get the exact same cylinder misfires with both the original and the new. Could it also be an EGR valve? Fuel pump? Could I have not tightened the plugs enough? Something REALLY stupid like that?

Is there a way to test cat clogged-ness? I would have figured that would be a downstream o2 issue.

I also have been getting gas a all different places so I don't think it's that. I only run premium and have occasionally found places that are ethanol free... but I still get the misfire.

1999 2.8 A4
All stock
163k


ANY suggestion is welcome, thanks!

 

Don't know how you'd check a cat other than visually inspecting the core and/or swapping it for catless or known-good cat.

Looks to me like the only other thing you haven't tried is moving injectors around. Maybe move injector 2 to cylinder 1, and injector 4 to cylinder 3 or something like that, to see if the misfire follows.

 

What's it like trying to get the fuel rail out of there on the V6? All I can find is walkthroughs for 1.8s and last time I assumed something on a similar model would be just as easy it was the fuel filter.

 

On my '96 at least, the fuel rail was so simple that I was impressed with Audi on that one. Four screws, and the rail with injectors lifted right up, rubber hoses stayed attached. Seeing as #5 and #6 don't have their shared coil cylinders affected, you might suspect injectors, but not four of them, and not intermittent. I've had one injector clog on mine in 260,000 miles, and when it quit spraying, it stayed like that. I wouldn't suspect fuel pump, EGR valve, or plug tightening. I would suspect a bad connection somewhere between the ECU and coils. If the misfires are worse if the throttle is opened, then perhaps a bad ground or +12V power to the coils.

 

Ill definitely take out the fuel rail and move some injectors around - hopefully it's super easy but I'm concerned it may not be.

Its odd but the misfire comes a few seconds to minute AFTER opening up the throttle quite a bit. Like I'll accelerate to pass somebody and it'll be fine during but after I get back into the lane and slow down to non-passing speed THEN the CEL will come on.

I tried to find where the ground is but couldn't. I figured theres someplace that's not the battery? Also if you say my power to the coils are bad, should I just look into the battery itself as the issue? It's a costco brand which I think is fairly middle-of-the-line, but I've never put my voltmeter on it - it always starts the car just fine.

 

I'm not all that familiar with the 2.8, but based on that description I wonder if the CTS (coolant temperature sensor) or IAT (intake air temperature) sensor could be at fault.
These can be tested for sure with VAG-COM / VCDS, and maybe also generic OBDII readers.

Also didn't read any mention of you looking for/fixing vacuum leaks.

 

Yes I did check for vacuum leaks and found none but I'll admit I'm not positive how to check "well" i.e. for really small leaks - which would probably be the type to only cause issues at high speeds.

Unfortunately I only have a generic obdII sensor so I can't do a whole ton of testing. I haven't found anybody with one in my area (seattle) without having to go to a shop and pay a ton just to borrow it.

 

Purchased

Screw it I'm desperate. You could tell me the rear control arm causes this and I'd give it a shot. Also, it take longer to get a real vagcom and test (and probably cost more) than it would to just buy these new parts and replace them.
ECS would salivate if they read this thread.

 

Hope they help! You've already tried so much.

 

What are you using to pull codes, and was that everything that came up? You also said the CEL comes on after returning to cruise, but there is a delay from when misfires start and when the CEL turns on. Under what conditions do you feel the misfires? Any feeling of loosing power?