I have a 2001 Ford Focus that has some funky problems at idle under load. A nice high-load scenario is by abusing the windows motors (rolling them up while windows are up), which causes the RPM to tank and the IAC kicks in after 200ms or so to keep the engine from stalling. ford_focus_load

What's weird is on another vehicle (2001 Saab) I see the voltage go down, but the engine RPM doesn't change at all! I've tried keeping my foot on the gas slightly to account for the difference in idle RPM, but still the RPM tanks.

I recently replaced the battery to see if this would help, but no avail.

Why wouldn't these intermittent electrical loads go through the battery?

I will look into the harmonic balancer. It's a mazda engine: Rough idle only under load

Other issues with car:

Update January 25 2018

The titles for the chart are:

  • RPM = RPM
  • ENGLOAD = Engine Load
  • IAC = Idle Air Controller %
  • VPWR = I think voltage at battery, will confirm
  • GENVDSD = Generator voltage desired. This tanks too, which doesn't make sense...
  • S11 = Upstream O2 sensor
  • S12 = Downstream O2 sensor

Tested resistance between battery negative and far end of engine block (< 0.1 ohm) and negative of fuel pump relay (3 ohms), both of which are less than 5 ohms. However I will check voltage next and look around for the ECM.

Update January 27 2018

Not sure what genmon means exactly, but it definitely moves. Also the gencmd (commanded duty cycle of generator from pcm) looks like it takes off too. VPWR is the voltage at the PCM, so I think that's ok. enter image description here

  • What's your idle rpm set to? It could be a voltage regulator I suppose.
    – GdD
    Jan 25 '18 at 8:44
  • 2
    I'd suggest it's just how Ford/Mazda implemented how their charging system works. On my cars, I've seen a visible drop in light output from dash/headlights when I do something like that, but not a dip in RPMs. Jan 25 '18 at 15:17
  • 2
    Try ministering the genmon and genmon pids to see what they do. I’ve seen bad voltage regulators do funky stuff on fords.
    – Ben
    Jan 25 '18 at 16:29
  • Monitor what the battery voltage is when you do this - a bad battery could be dropping the ECM voltage into the "scamble brain" area with unpredictable results. I've seen small four-cylinders that start and run fine, but a high electrical load drops the ECM voltage to 10 or 11 volts with really weird effects - and the alternator can't help much at idle.
    – SteveRacer
    Jan 26 '18 at 4:55
  • I'm not sure of all the parameters on your graph, so maybe that's not the issue. Still, I'd like to see ECM voltage as close as you can get to that terminal. Also, does the ECM have an alternator "idle bump" function?
    – SteveRacer
    Jan 26 '18 at 5:01

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