Engine speed and tachometer jumping

[rcfanatic]

Yesterday I was driving for about 2 minutes when my car ('86 325 with Motronic 1.3) began to surge and buck, and the tach was jumping from 0 to 4k and everywhere in between. I found a place to pull over. The car idled almost all right but jumps around a little. When revving it up, it sputters and struggles to maintain rpm and the tach jumps wildly all over the place. I replaced my battery and alternator 3 days ago, so I checked them first. The battery was at 12.4V with engine off, and 13.8 with it on. Battery terminals were tight. I unhooked the ground and let it sit for a moment to see if anything might reset. Started the car again and there were no changes. So I checked most of the connections under the hood that go to sensors as well as fuses. They all seemed OK. When my girlfriend came to get me five minutes later, I tried starting the car and it ran fine to get me home. So I've been reading on the forum and can't find a problem exactly like this, where both the tach and engine show sporadic behavior. Here are some of the culprits to similar problems:

SI board
Coil/Distributor
Injectors
Spark plugs
FPR
CPS
Battery terminals
TPS
DME plug
Coolant temp sensor
Main/Fuel relays
AFM

The only ones that seems like they would effect both engine and tach are the CPS and coil/distributor. Should I start here in replacing parts? Any input is appreciated.


Thanks,
Adam

[jlevie]

That set of symptoms could be:

Intermittent power to the DME
Flaky timing reference sensor
Flaky DME