This was my mother's car and she was the sole owner. It's still stock with exception to the tires. It has 43K miles on it. Driving home on the highway, outside temp was about 85 and the engine temp was hovering around 200 degrees. Cruise set at 70mph. All of a sudden, the tach dropped to zero and KABOOM! Holy crap, I thought I blew up the engine. Looked in my rearview mirror and a large plume of black smoke can trailing out of the tail pipe. Car is dead and we coasted to the side of the road. Opened the hood to see if could see anything. Everything looked fine, no smoke, no fuel smell, no burn marks anywhere. Got back in the car and tries to start it. It fired back up, no funny sounds, idling as usual. Slowly pulled forward and crawled at about 25mph, then, it happened again, tach dropped to zero and she backfired. Not as loud this time, since I took my foot off the gas as soon as I saw the tach drop. Pulled over thinking I might find a loose wire or connector. Jiggled some harnesses, got back in the car and she started right up again. Slowly made back up to cruising speed and continued for about another 8 miles and then the bottom fell out of the tach and once again, pow! We managed to limp the rest of the way home which was about anouther eight miles. Idled it back into the barn as if nothing ever happened. Reading some of the other threads on possibilities, I didn't find a 'like' situation since no one mentioned their tachs dropped to zero before the backfiring. Any suggestions or ideas would be greatly appreciated.

 

Having followed a thread about tach filters, if the wire from the distributor to the tach filter shorts momentarily, it will kill the ignition, and may eventually kill the ignition permanently. If it is temporary, the ignition can restart and ignite the unburned fuel in the exhaust system. This may be what is happening.

To test, disconnect the tach wire from the distributor and see if it runs ok (tach will be inoperative). If it works ok, disconnect the tach wire from the tach filter and connect it directly to the distributor's tach terminal. If it works ok now, your tach filter probably has a short, or its wires do.

Read this thread:
http://forums.corvetteforum.com/show...529&highlight=

 

That's a good start. If connecting the tach directly kills it, the tach board is bad. If you leave the tach disconnected, and it still does it, the Ignition module in the distributor is going south.

 

Thanks guys. I can't tell you how horrified I was when this thing went off. I'll look up that thread and start testing from there. It sounds manageable at this point.