Hi Dylan - welcome to the forum, especially as you are a fellow 944S 16V driver!
I was interested to see that although you are in Windsor, you used Pie up here in Suffolk. I too have always had very good service from Pie, they are a very professional, knowledgeable and approachable bunch - especially Chris, and for me they are only a few miles away!
With regard to your 'tappet' problem, could it be a broken valve spring or a loose valve seat? For one tappet to go so bad as to be very noisy is unusual, they normally all degrade and get gummed up and noisy over time. However, given your 'backfire' event and subsequent cam-box noise and poor running, a valve problem might be the cause?
If it is a tappet, you do not have to take off the head to replace it (or all of them), but you do need to remove the cam cover, the cam chain and camshafts, along with the belt, etc. There is some information regarding tappet types in a recent thread on this forum where I provided some alternative part numbers - do not buy Porsche sourced ones unless you have deep pockets...
You mention that the revs are a little erratic, is the tacho simply following the engine (as it should), or bouncing around in an unrelated manner?
Also, o2 sensor and Maf...? I wasn't aware that any 944S had an o2 sensor over here in the UK, also unless you have had a conversion, you should be running an AFM (Air Flow Meter). The carbon track in the AFM can get worn and cause erratic idle and hesitant performance too. Also, vacuum leaks can do the same and there is quite a lot of rubber pipework to check.
The grounds for the instrument cluster should be checked and cleaned where they connect behind the cluster to the bulkhead, as well as the 'rugby' ball connectors on the back of the cluster circuit board, these are a well known poor connection & bouncing gauge problem area. If a search here doesn't come up with any info, go across to the USA Pelican Parts forum, where there is loads of info on cleaning grounds and the connectors, etc.
If all the gauges flickered off at the same time, it could be a bad connection from the harness to the cluster. These can be a problem as the contact on the cluster appears to be just solder which gets dirty over time with oxide build-up. Give those a gentle clean with the ink end of a pencil eraser - do not go too hard as they are delicate connections. A broken wire in the supply would also cause a failure, but unfortunately, there are several other places and components that could do this too!
Hope some of this info helps - good luck in your investigations. Post up a good photo of your 'S' too...
, but the owners can be very knowledgeable and helpful from that side of the pond. I am a member of Pelican Parts forum and never had any issues with the guys (and gals) on it - they are a pretty international bunch anyway.
