Epic2112

So, my car developed a new problem today. I was driving, and all of a sudden my tach drops to below zero (where it sits when the car is off). I'm in gear and coasting, and as I'm watching the tach it starts bouncing around, between 0 and maybe 5k RPM. I gave the car a little gas, and it sort of stumbled a bit, back fired pretty significantly, and then went back to normal. I drove all the way home after that without incident. Later, the car started up right away correctly, and drove around the block a few times with out an issue.

So, what are the possible explanations that I should have a look at? Here's my list so far:

1- speed and reference sensor wiring
2- wiring between speed and reference sensor connectors and DME connector
3- DME relay (I don't know how I'd test this, I'd have to wait for it to fail completely and plug in my spare relay to see if it's fixed)
4- DME

Can I rule out one of the sensors because the tach was going crazy, or do they both control the tach? Is there anything I should add to my list?

924RACR

I would absolutely say it s a likely sensor problem; on my 82 931, which is also a crank sensor ignition, I have an occasionally dodgy sensor connection at the ECU. When it craps out, which is only momentary, I get an instant tach drop with the big backfire (at 3000rpm/75mph on the highway, in particular, really makes people back way off!)...

Epic2112

Ok cool, that's one of the things I wasn't sure about. So my fuel is likely just igniting in the exhaust because the ignition cuts out when the sensor cuts out?