UK: Diesel Misfire cylinder 5?
 

2010 A8 4.2TDI CDSB engine

I’ve had engine issues for months if not years now, long story short I’ve found the source of the sluggish engine behaviour, a misfire, there’s no fault codes at all and other than the turbo speed sensor on turbo 2 showing that it’s running slower than turbo 1 there are no other tell tale signs of anything being wrong (other than the huge delay in takeoff and lack of boost)

back to the long story short…

did an injector leak test and all 8 returned the exact same amount over a 1 minute period.

I then did a compression test using the glow plug holes. All came back good.

as soon as I removed cylinder 5 glow plug it was covered in carbon build up, I assume this is the misfiring cylinder.

in VCDS I have an injector deviation on this cylinder of +0.49 which when doing the research doesn’t suggest that there’s anything immediately wrong but to keep an eye on it in the future?

I have ordered a new injector and will see if that makes any difference.

what I’d like to know is…

1. The results from the injector leak test shows good returns, what’s the likely hood the injector is bad?

2. Is it essential that the new injector is coded straight away or can it be done a few days later?

3. what else could cause the misfire?

I thought that as long as compression was good and fuel and air is good then wham bam everything’s good?


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Cylinder 5 glow plug compared to new one.


Thanks in advance for any feedback / advice.

Dave.

Leak down test cyl 5 and see what you get.

thanks for the suggestion.

Doesn’t a leak down test offer some insight as to why a low compression cylinder is leaking though?

All 4 tested cylinders have virtually the same reading suggesting that there is no loss of compression?

A cylinder that passes compression can still fail leak down and cause issues while actually running.

.
Just spitballing here (as I have no TDI experience), but perhaps the glow plug connection or wiring has a fault. :confused:
.

Wiring is a possible but most common is a bad injector.

fitted a new injector but same running issues unfortunately.

Yesterday i removed and inspected the intake and swirl flaps, WOW!!

I’ve seen pictures posted in forums showing various states of carbon buildup and Dan honestly say mine are as bad as any I’ve seen elsewhere. There were a couple of large chunks approx 20-30mm in diameter that had dislodged when removing the intake manifold that were as moist covering the valve inlet holes albeit on cylinders 6 and 7 so not immediately contributing to the misfire on cylinder 5. The internal diameter of the intake had been reduced by almost 50% where the EGR gasses join the intake internally which were almost fully blocked.
I cleaned it all up and refitted. Had a very large amount of carbon bagged up when I’d finished, will post pictures below.

results were that it has increased In response but the misfire is still there. On occasion if I rev the engine slowly whilst passing between 1500 and 2000 RPM I can feel and hear the misfire, this seems to disappear after 2000RPM but must still be there as the turbo never manages to spin at full speed. Logs yesterday stated max speed of 166,000RPM verses 227,000RPM on turbo 1.
Max speed being in the region of 232,000RPM.

I am going remove the intake again today and try the clean out the intake ports on cylinder 5 as my ch as I can, I’m thinking of scraping out any build up and then sucking the debris out with a hoover.

Stay tuned…

https://cimg2.ibsrv.net/gimg/www.aud...9b33bab00.jpeg
https://cimg3.ibsrv.net/gimg/www.aud...a68ca8d45.jpeg

I don’t have the equipment to perform a leak down test, the car is booked in for diagnostics on Wednesday next week so hopefully they might be able to perform it.

So I took the intake off today and investigated the intake ports in the cylinder head, couldn't even see any of the stem down the square ports, completely covered in carbo build up, can't see the valve head but there must only be 25% air passing in to the cylinder through that valve, the valve down the round port are more visible but still have carbon deposits. looks like I've found the issue..

Will update as an when I get it sorted...

Thanks for any advice you guys have given me..

Just an update...

Had a compression test performed by a local VAG specialist, cylinder 5 250PSI, cylinder 6 360PSI, all rest 400, they couldn't perform a leak-down test as they don't have the tools..

It's booked in at another place further away for the leak-down test next week.

Cylinder 5 and 6 were the worst by far with the build up of carbon in the ports, both valves in cylinder 5 could not be seen as they were covered in carbon and one valve in cylinder 6 was almost covered.

Any ideas of a possible fix without pulling the motor and splitting the heads??

I have had a Terraclean performed which did nothing (well, maybe made it run a little smoother) also thrown bottle after bottler of fuel treatments in too and no improvement.

The intake ports have been walnut blasted by myself and are now clear, my next attempt at fixing is going to be opening the valves on cylinder 5 and going to town with a soft but firm brush soaked in carbon cleaner to try and make sure there's nothing stopping the valves from seating correctly, can't hurt right??