Here's a really fine self-inflicted means of generating the dreaded misfire codes in the OBD. On my new-to-me '94 855 Turbo with 120Kmi, one of the first things I noticed was the top engine torque mount rubber bushing was shot. I chose the process of replacing it the hard way, (which is to remove the entire bracket in order to press out the bushing). Astutely noticing that the vacuum lines were badly cracked, I properly replaced them with new hoses carefully noting the routing. Upon reassembly, reconnecting the various sensor wires, the last one done was the two-prong connector for the crank position sensor (not the three prong cam position sensor) and plugged it into its mate and leave it perched atop the vacuum lines that snake all over the place in there. (Lots of skinned knuckles along the way---just not the same kind of operating room that I'm used to from my 240 days.)
Upon startup, all seemed well & had a nice drive off down the road. A bit of spirited throttle mashing to watch the tach and turbo boost work their magic brought on a curious thing.....the tach ran up to 4500, then upon backing off, dropped to zero. Engine running fine, driving down the road, but the tach reading zero. More leadfoot work to get a good downshift and presto...tach jumps to 4500, then drops to zero. CEL lights up at this point. Hmmm.....not good.
Off to Advance Auto....OBDII reader shows misfires on random cylinders, misfires on 1,2,3,4,5, etc. etc. Got them to reset it and 1.5 miles down the road, it's back on. OK, time to inspect the wires and pull the plugs. Wires are new from P.O. and look just fine. Cap & rotor just a bit dusty but no ignition misfire tracks. Cleaned the hell out of it anyway. Plugs 1,2&3 look good enough to pass for new--I mean perfect. Number 4 is carbon coated completely black--not wet, just black powdery carbon totally over it. Number 5 not bad, just not perfect like 1,2,3.
Hmmmm.....maybe a stuck injector on 4 is causing a misfire?? OK, pull the injector rail--not for the faint of heart--thought I had bled off all the pressure at the schrader valve, but you'd be surprised at how much remains! Not that I know what I'm looking at, but decide to swap 4&5 just for diagonistic purposes, same with the plugs so maybe the problem will move to #5 and I'll know what's up.
Button it all up again, learn how to clear the codes from the little underhood readout station (bless you Volvo on the '94's) and off down the road again. (BTW I'm leaving out all the searching for an injector at numerous part shops and pick 'n pull.....another story.) Dang! CEL back on 1.5 miles down the road.
OK, time for some old school ATS. I really had the feeling this was self inflicted and was somehow associated with the engine mount work. Pull out the airbox again, peering under the mass of hoses/wires and what to my wondering eyes does appear but the lead for the crank position sensor is routed on top of the vacuum lines, positioning it perfectly parallel to and right along the #4 plug wire for about 2-3 inches. YES! Induced signals from plug wire into the very sensitive crank pickup were driving the computer crazy. Moved it back to intended position and all's well with no CEL after 300 miles. Engine must have been in limp mode and only firing #4 now and then during the whole episode.
Moral of the story? Look at the fun I had?? Just be careful to put it back where the Viking Gods built it or you'll be sorry!