2.3L ('02-'11) Knock knock. Who's there?

[B2 Addict]

I've got an 03 Ranger, 2.3, automatic, 2wd that I'm absolutely sick of..

It has a "knock" that has developed under the hood, it only happens when the engine gets to operating temp at idle and I turn the headlights and/or put the transmission in gear with my foot on the brake. Otherwise it goes away...

When the "knock" starts, the Speedo needle will slowly fluctuate between 0 and 10 mph, sometimes going as high as 20 mph and back down to 0 all while the vehicle is not moving, doesn't matter if it's in gear or park.

I can't for the life of me figure our what is causing it nor can I isolate where it is coming from, even with a mechanics stethoscope. It does not sound like a rod knock and sounds as if it is coming more from the topside some freaking where. It sounds like if you were to slightly tap your knuckles on a desk. I've tried an oil change with a thicker viscosity oil and Motorcraft filter but it didn't change anything. Alternator tests fine, only code I have is an evap code.

I can try to explain it more clearly if my post is confusing.

 

[RonD]

Reads like Alternator is failing, and leaking AC voltage, speed signal is AC, and AC in general causes issues with the ALL DC voltage components in a vehicle
Google: AC Ripple Testing

You can't hear it when its cold?
Or only barely hear it?

You can remove fan belt from crank pulley and start the COLD engine, battery light will stay on
Listen closely
If no noise in super quiet engine bay(no fan) then shut off engine, you can only run cold engine 1 min with no water pump

Spin alternator manually and see what it feels like

 

[B2 Addict]

Thanks RonD, I'll give it a shot. It's pretty much nonexistent when the engine is cold. I can turn on headlights, blower motor, and put it in gear while cold and very rarely hear a very slight knock here and there.

The alternator voltage tested perfect but it did show a slight flutter that was undetectable in Voltage readings.

*After Googling AC ripple test, I think you may be onto something. The "flutter" I described looks to possibly be the ripple you referred to.. **Actually, I seemed to have accidentally left out the "flutter" description from the alternator in my original post, but ripple would have been the correct term. It makes more sense now and may very well be the answer to my issue. Kudos to you my friend !!

 

[B2 Addict]

Well, I'm back... The old alternator sounded like sandpaper when spinning the pulley and did show a slight ripple when tested so it didn't hurt to replace it.

With that said, the new alternator seems to have stopped the speedometer needle from moving but the knock is still there.. I'm at a loss.

 

[RonD]

So knock is from somewhere else

COLD engine
Pull off fan belt from crank
Start engine, can only run it 1 minute or so, but can repeat test as much as you want when engine is COLD again

Engine bay will be very quiet with no fan, see if you can locate the knock

 

[B2 Addict]

The only problem with that is the knock only shows up when everything gets up to operating temp, it doesn't do it when it's cold. Will it hurt anything to try that once it gets warmed up?

 

[RonD]

Yes, engine will overheat