dvs_dave

Yikes, how did that happen? Someone forced some wrong size bolts in, cracking it?

Either way, a vacuum leak like that will cause the rough running you have so you’ll need to sort that first, then see how you get on with the throttle body/pedal position errors if they’re still present.

Lost Cosmonaut

I've seen it with several used S8s that have been through the ringer, seems relatively common actually? Most of the used upper manifolds I've seen on eBay seem to have cracks around the TB mounting tabs. This particular engine has been through a rebuild and I'm guessing someone just cranked the heck out of the screws when installing everything. I didn't want to have to get a new upper manifold part but looks like my hand got forced. I wouldn't be surprised if this has been the cause of most of my issues, especially since they've been progressively getting worse since I bought the thing.

At least I bought it low enough that throwing some money at it is worthwhile.

Lost Cosmonaut

OK so here's what I've done since last month:

New Y plenum (manufactured 2023 according to the date code!)
New gaskets
New throttle body bolts (old ones were stretched and bent and stripped on one side)
New bolts for the Y plenum (somehow managed to snap one putting it in)

Inspected the inside of the manifold and it looked good, the previous owner put in the upgraded blue flaps and they look great and have been working so far.

Put everything together, ran a throttle body adaptation on both sides. So far everything has been running great. Still has a little bit of a strange engine noise from 1500-3000 that makes it sound like a V8 more than the smooth V10 sound. Ran through the last tank of suspected bad gas and put in a new tank from a reputable place and immediately it started running better, the 1500-3000rpm roughness is still there but is better. Also all my CELs went away and no new ones have popped up other than the recurring one for the air pump which I know is unplugged.

Next step will be putting in the new fuel filter I have and then doing an oil change. If the new fuel filter doesn't help my next step will be plugs and coils.

But so far I've been driving it all over again and it's been going good!