A Short Story

So a friend asked me to look at a 3.0 L Taurus at his shop. The car originally came in for a P0420 code and they put an anti-fouler on the rear o2 sensor. Whatever.

During the road test the car developed a high idle and idle surge. They replaced the Idle Air Control Valve twice hoping to fix the issue. It didn't. So the upper intake manifold and gasket were replaced along with some vacuum hoses. Still the same symptoms.

I get called to look at it. Hook up the scan tool observe high positive fuel trims and the idle complaint. No codes, they were probably reset. Maybe it was a false positive, maybe not. Reset the Keep Alive Memory and go for a test drive.

The Idle seems to settle around 700 rpm except that it jumps about 50 rpm every second. Fuel trims on the other hand are still high. They go normal around 3000 rpm, spraying around the lower intake shows a vacuum leak on the lower intake gasket. Decide to smoke test the intake, smoke is pouring out of the valve covers and lower intake area.

So I recommend they do a gasket job first and get back to me if the car is still acting up.

Around a week later I get the call asking me to look at the car again. Not only did they replace the lower intake manifold gasket. They also replaced the upper intake manifold again for whatever reason.

Again I hook up the scan tool. Fuel trims look OK in the single digits, idle is stable around 700 rpm except for that slight increase every second. Interesting. Disconnecting the IACV confirms base idle is OK and the jump disappears. So plug it back in and try commanding a percentage. Seems like it's working fine, idle is stable no jumping.

Hmm, weird... I start looking at other PIDs and notice ignition voltage is a little low. Around 13.5v +- .1v, I've seen worse but it's not great either. try revving the engine to 2000 RPM. It doesn't seem to change.

So, it's time to look at what's going on in the charging system. The GENFDC PID is going to 50% duty cycle every second and correlates to the idle jump... Disconnect the 3 wire connector at the alternator. GENFDC PID goes to a 100% and the idle smooths out. Ignition voltage stays at 13.5v.

So surely the alternator is working in default mode? Trace the wiring back to the PCM and everything looks OK... Plug the 3 wire connector back in. GENFDC goes back to doing what it was doing before cycling to 50% every second.

  • Backprobing pin 1 the GENMON side of things looks normal, switching from 0-10v.
  • Backprobing pin 2 shows what you would expect considering what I was seeing on the GENCOM PID.
  • Backprobing pin 3 shows ignition voltage with no real voltage drop.


At this point I'm leaning towards replacing the alternator, and I tell my friend as much. But a it bugs me.

If the wiring to the PCM is OK (I have no reason at this moment to think that the PCM is the cause) and the alternator is in default charging mode. Why would it not be able to respond to changes in RPM and load? Maybe the regulator is stuck in default mode? It's a possibility and testing thus far is making me lean towards this.

I believe that the system sees a low ignition voltage and is attempting to raise it by commanding the 50% DC. Idle is then jumping accordingly.

Is there anything else I should be looking at?

Other than the oddities with the charging system and idle almost everything else seemed fine...

One thing I wasn't sure of is that the DPFE sensor stayed at around 1.01v, I thought this should be around .5v. I didn't have any access to a known good reading at idle. I unplugged the sensor just to check and the 5v reference and low ref is OK. The DPFE tubes were cold and the EGR solenoid was closed. So I figured I could rule this out.

  • You have done all you can do at this point, try an alternator.
    – Moab
    May 6, 2016 at 22:13

1 Answer 1


Got a call back from my friend, they replaced the alternator and confirmed a good idle on the road test. Charging voltage is back in the 14+v range and the idle jump is gone.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .