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Voltage drop causing rpm spikes on startup

Posted: Thu Apr 21, 2011 3:12 am
by usagoal
I made some progress on an issue I've been having during startup with my car. I've had rpm spikes during cranking, sometimes reaching 9000rpm. I've tried modifying the tach circuit with the help of other members, shielded wire, rerouting wire, etc...with no luck. I reviewed one of my datalogs and noticed it happens when the battery voltage drops to around 8-9volts. I put a battery charger on and tried again, this time there were no spikes at all. None. I tried this about 5 or 6 times and there was not a single spike. I also have the battery mounted in the trunk (1000 cranking amp battery with 0 gauge cables running up to the engine/starter. I'm debating whether I should try a larger cap or just move the battery back to the engine bay. There's nothing I can do about the voltage drop regarding the physical setup (battery, cables, etc). Any opinions? I can post a datalog when I get to work if anybody would like to see. Thanks.

Re: Voltage drop causing rpm spikes on startup

Posted: Thu Apr 21, 2011 4:15 am
by Matt Cramer
You might want to try more skip pulses; this can often fix an initial startup RPM spike.