Thread: Intermittant misfire...

Hi,

I have a 1990 RRC Vogue 3.9L V8. After I replaced the old alternator (52 amp) with a 108 amp Bosch unit I have been having an intermittant misfire ever since.

It usually happens after 10 mins of driving. If I remove the plug to the alternator it still misfires, however, if I remove the plug from the air flow meter the misfiring dissapears.

I have installed new fuel filter, plugs, leads, rotor, dizzy cap, ignition module and coil twice, it's quickly adding up.

Has anybody out there in V8 land experienced such a problem?


Much appreciated,
CSK.

If you do a search you may find the old posts on this,.

The main reason for this to occur is stray EMI from the Bosch alternator into the wires which cross from the distributor to the coil, most notably the signal wire from dizzy to coil.

The fix is either to replace the wires with shielded coax, with the outer earthed at both ends, or to reroute the wires well away from the alternator eg along the LH head to the firewall and around.

This is an old and well known problem and fix.
BTW, probably because the ECU went into Limp home when the MAF was disconnected that the misfire disappeared, or maybe the EMI is transmitted along that pathway to teh ECU.
Shield teh wires and the problem should disappear.
Regards Philip A