1998 XJR with 200k miles. It was running well AFAIK, and a cooling system hose blew (infamous under-the-supercharger hose). Pulled off road as quickly as possible. It got hot. After towing it home I tested compression which was low/none in most cylinders . I'd just done a fair bit of work on it, timing chains/tensioners, front coil springs, shocks, etc, and I just got disgusted and parked it. Somewhat later: I finally decide to tear into it and figure out how bad the damage is. I got the cylinder heads off yesterday. One dropped valve seat, otherwise they seem intact. I'm taking them to a machine shop today to see what they think. At 200k they're undoubtedly overdue for a valve job or replacement.

With the heads off, I've been looking at the cylinders. All pistons are intact and nothing seems seized, so I guess it could be a lot worse. I've done some online research about Nikasil coatings and the cylinders look ok to me. No scratches, gouges, corrosion or obvious damage. I can still see the crosshatching on the cylinder walls, which I hope is a good sign. I'm not equipped to pull the engine for any further examination of the bottom end.

So my plan is to repair/replace the heads, put it all back together and see what happens. Is this a bad plan? Thanks for any comments or advice.

 

Little or no compression on multiple cylinders and only one dropped valve would make me leery. I had a similar failure and found glazing at the tops of the cylinders was the cause of low compression. Numbers were in the 30’s across the board.

 

When I had my 98’ XJR (now my daughters car with 271k miles) back in 15’, I had almost the same situation but it had 234k miles but no dropped seat and it was (and still is) the original engine with Nikasil. My personal opinion is that you’re on the right path. Get both heads surfaced, all new valve stem oil seals, grind the valves and seats, have the machine shop do the valve adjustment for you (you’ll thank me later), use the multilayer steel head gaskets with new head bolts that take a T60 Torx socket. The car runs like a scalded dog after the head job was done. Do not use the fiber head gaskets, make sure the block surface is clean and straight before installing the gaskets and heads. 4.2 liter engine mls head gaskets is what you want.