fn_42

did they take the valve covers off and verify both cams?

As I found out, it's easy to screw that up: https://www.audiworld.com/forums/a8-...2871695/page2/

there's 15 chain links between cams on bank 2 and 16 on bank 1

I never saw misfire codes from this though

silverd2

Well first, something about your mechanic and maybe his "buddy":
Can't imagine why he would replace the tensioners for a camshaft position sensor code...tensioners throw their own unique code.

Anyway, if the chains were not put back in correctly (common mistake with many "mechanics" who don't bother to read the fine print) it could cause unexplained misfires on one bank only. Each side has a different number of links between exhaust and intake cams...15 links on driver's side (bank 2...5,6,7,8)...and 16 links on bank 1 (pass.). I've seen even those who "swore" they went by Bentley and match the links on both side..misfire and/or timing codes later.

Unless you find an electrical problem causing this, I'd have a mechanic properly check the outer belt timing first...and if that's correct. maybe pop the valve cover on the driver's side and check the chain timing.

All just a theory, but the chains had to be removed and replaced (tricky job anyway) to do the work he said he did...and the problem didn't happen till after.

Cheers

silverd2

I was still typing...with one finger

Gregg in the 603

Wow, Fn 42 and SilverD2, that is EXCELLENT input! I'm being optimistic here and really hope we're onto something! That makes pretty good sense, let's hope we're right! I was tasked with obtaining the parts. Since I could survive without the car for a while (I had a spare summer car), I told my mechanic to get to it when he was slow. So he let me source the parts. Do you think I ordered the wrong camshaft timing chain? I went totally by his recommendation, but he did not tell me the camshaft chain was bank-specific. Do you know the part numbers for the correct chains? Because I only ordered one for this job. Do I have the wrong chain on the cams???!!! Man, that would be amazing if this solves it!!!
As a side note, the camshaft tensioners made a horrible clackity-clack sound on startup and it was a good thing we replaced them. One of the shoes was totally worn down to nothing.
Again, fantastic input! Keep it coming!

tozoM8

It could be the chain tensioner. If it was replaced the chain timing could be off.the chain is same on both side, but the timing is 15-16 links since one side rotates the opposite direction from the other.

Gregg in the 603

Oh, okay then. So we're talking about links between cams. I see now. It's not a chain issue, so much as a timing issue. Wow, that's gonna be tough to confirm. The tensioners I know are bank specific and we sourced them directly from Audi, so I know they're correct... I guess will have to confirm with a third party?

silverd2

Right, not incorrect parts...possibly incorrect timing when installed. The drawing below shows bank 2 (drivers) timing and correct link count (15)....one of the trickiest parts of reassembly, getting that right.

The link below is a home job pulling and replacing bank one (pass) and getting the count right at 16 >>>

quattroworld.com Forums: Replacing cam chain tensioner and/or pads on bank #1 of 40V motor (shorter cut)...

Gregg in the 603

Awesome, thanks for all the great insight. I will take for action. The links and pictures should prove valuable. I'll update all when I get to it hopefully soon. Thanks again.
Gregg

Gregg in the 603

Update: Still have not been able to check the camshaft chain timing link count. I brought the above picture to my mechanic and he was convinced that they (him and his Audi tech friend) triple-checked the link counts and all timing concerns. Timing is spot on. I threw everything at him and he said that he checked all possible options, resulting in his conclusion that it must be the head. However, it was not misfiring before I brought it to him for camshaft tensioner/chain changeout. So why is it misfiring all on one bank now?!!!
He told me that the S8 head design is a little different than most heads in that the guides are not bored into a cast head but rather held together by an arm that holds all the valves. Is this true? If so, his theory is that upon warmup, the arm warps (twists) just slightly, leading to the exhaust valves being just slightly off, causing a misfire. Thoughts? Does anyone have a picture of the heads so I can try to visualize the problem?
I'm going to bring it to one more independent foreign motor shop for diagnosis and hopefully bad head confirmation. If it is a bad head, does anyone know where I can find one a little cheaper than Audi? Is used an option? I really hope so. Thanks for the help so far

silverd2

That doesn't sound right at all. The valve guides are permanently driven into the head at the factory, just like any alloy head.

In the A8/S8 40V engine, you have roller rockers, instead of cams sitting directly over the valves like the previous 32V engines. The multiple sets of rockers (with hydraulic spacers in the rockers) are on a removable pivot shaft that rides in journals in the head. There's nothing to warp or change shapes in there.

The valve cover would have to be pulled to confirm proper chain count/cam timing.