Crankshaft positioning sensor? Not throwing any codes for it though. Any ideas?

The tach often has an issue where the decal slips down, which causes the post to bind on the decal.

The idle issue may be the idle air control valve, easy on the 2nd Gen, a pain on the 1st Gen. A common failure point for stalling or uneven idle.

Calvin Louie
1989 Porsche 928 S4 - Linen Metallic / Linen

Decal? The needle doesn't move. I've checked the iac valve, but im thinking that whatever the reason the tach isn't working for is it.

The tach face is simply a big decal. You have to remove the cluster and plastic cover, and slide it up a bit. Maybe apply some adhesive to keep it from sliding back down.

Calvin Louie
1989 Porsche 928 S4 - Linen Metallic / Linen

yes this happened in my old '94. The slipping of the face-plate decal kept the tach needle from moving...definately check this

I'll admit, it slipped down. Tach works like new now, but its still got a high idle, but when i wrap it out to 5000 rpms it goes down to like 1000 rpms. Otherwise, it is at 2000 to 2500. I've still got a 542 code coming up.

where was your needle stuck? mine has been stuck at about 1500 for a very long time; can anyone direct me to a link for step-by-step cluster access/removal?
1996 Mark VIII ; metallic beige charcoal

About at the 1000 rpm range. I did it myself, with if I can remember, was a 7 millimeter socket. First take off the headlight switch, by pulling the switch from the shaft it is on. then the black plastic shroud should be able to just pull out from there, which will allow access to the 7 mm screws. There are also screws going up in front of the cluster on the black trim piece. It helps to have the steering wheel facing as down as possible when doing this. Then pull the black trim piece out, which requires angling it top facing down I believe, and it comes out. then you have access to the cluster screws which you take the ones on the outermost edge that is holding it into the dash, and it will come out enough to disconnect the electrical harness in back, and then it is free. then you get to get some torx bits and take the clear lens off and have access to the tach sticker. It sounds like a lot but I did this in about an hour, I also cleaned my clear lens with window cleaner, and also the clear lens on the black trim piece. A razorblade helps when taking the tach sticker off, as you're lifting up VERY gently, remember, this thing CAN bend, stick the razorblade under it and let it do most of the work of releasing it from the glue it is held by. I just lined the holes on the tach sticker with the holes on the backing plate it sticks to and It looks and works like new. These Tach stickers only move a couple of mm's so it looks like it's in the right spot but it isn't. I could tell mine wasn't because my left blinker arrow didn't line up with the hole in front of it on the cluster. Hope this helps.

took care of it today, awesome instructions, THANKS- couldn't have done it without...used hot glue to reset the tach faceplate; also changed all the bulbs; pulling that plastic cluster out of the dash was tight; thought for sure it was gonna crack!
1996 Mark VIII; metallic beige charcoal

Hot glue is probably not the best choice for this particular job. It gets pretty hot in the car sitting in the sun, especially on top of the dash, so I'd be prepared to see some running glue or the face shifting again when it gets hot out.

Calvin Louie
1989 Porsche 928 S4 - Linen Metallic / Linen

yea, I had a feeling that was not the best choice...I did the job over the weekend at my girlfriend's house- not in my garage....just used what was convenient and available...also didn't want to use something that might ultimately damage the outer coating from behind