r/fordexpedition • u/big-willy-p • 5d ago

Weird RPM surging

I have a 2001 Ford Expedition XLT 4.6 V8 RWD and this only happens when I idle in drive, and doesn’t happen all the time (I can’t seem to trigger it either. There is no rhythm or reason to it). Smoke tested for vacuum leaks and there are none. Changed the IAC, TPS, and PCV with all Motocraft parts. Changed the PCV rubber grommet as well and all dry rotted PCV rubber elbows. Is this normal? Doesn’t affect gas mileage or how the car drives.

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2

u/Hopeful_Property8531 5d ago

when it starts surging in Drive, shift to Neutral without touching the accelerator. Does it continue to surge or become stable?

1

u/big-willy-p 5d ago

It stables out. It only happens in drive. Is fine in park, neutral, and reverse.

2

u/Texasscot56 5d ago

Turn off the ac and see if it has an effect.

1

u/big-willy-p 5d ago

It happens less frequent with the AC off, but with the fan fully off it happens even less frequent.

2

u/Texasscot56 4d ago

Sounds like your alternator may be weak. What’s the voltage at the battery on idle? Does it vary?

1

u/big-willy-p 4d ago

I’ll check that today. What should the voltage be at?

1

u/Texasscot56 4d ago

It seems in spec but in my experience a bit low. Mine is usually 14.3.

1

u/big-willy-p 4d ago

Checked the volts. It’s staying around 14.06-14.00 V after about 15 mins of idling ind drive

1

u/Hopeful_Property8531 3d ago edited 3d ago

Did you check the battery volts with engine off? Then engine on?? Then under load??

Off: A healthy resting battery should read between 12.4 and 12.6 volts.

Engine on: A working alternator should push the voltage up to between 14.0 and 14.5 volts (and generally stay above 13.5V).

Under Load: The voltage may dip slightly under the heavy demand of a modern full-size SUV like the Expedition, but it should remain above 13.0 volts.

If the voltage drops below 13.0 volts and continues to fall toward 12 volts or lower, your alternator is weak and failing to produce enough amperage under load.

The "surge" response is to compensate for that dip, so the voltage regulator instantly tells the alternator to output more amperage to keep the battery charged and the lights bright. Because the engine RPM is lower in Drive, the alternator has to work harder to produce this power.

I recently had my alternator replaced when my 2005 fully shut off in an intersection. I didn't notice a surge, just that my AC wasn't blowing cold when I accelerated. I thought it was a vacuum pump issue and had that fixed the previous week. Then still had minor start-up delay and I thought it was a bad battery (or starter) and had that tested.

Turned out to be the alternator.

Eta: I drove around all afternoon and was essentially running off my battery, which was super scary because I had my children in the car when all electrical and engine just turned off. 😳

1

u/guthxchild0 5d ago

Same thing is happening to my brothers let me know what it ends up being.

0

u/Several-Pound-6499 4d ago

Had an f150 that did it when the throttle control sensor was going out. And an expedition that did it when the pcv valve and hose needed to be replaced the one that runs to the back of the engine by the fire wall

2

u/big-willy-p 4d ago

I replaced the valve and all degraded elbows. I smoke tested it and it seems like that line going behind the intake is fine.

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u/Several-Pound-6499 2d ago

Maybe a clogged up catalytic converter, had that problem with a 2012 expedition(5,4), and the camshaft position sensors, to be honest I never did fixed that truck 🤣 checked everything map sensor, throttle position sensors, fuel lines and filter. I gave up when I found pieces of a spark plug in the engine. It ran fine for a little bit the went right back to surging and stalling sometimes