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Ford Flex Spark Plugs Replacement Guide
How to change the spark plugs in a 1st generation 2009 to 2019 Ford Flex SUV with the Duratec 35 3.5L V6 engine.

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2019 Flex 3.5L V6 Engine
Front Three Spark Plugs
Intake Manifold
This automotive maintenance tutorial was specifically written to assist owners of the first generation (2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018 & 2019) Ford Flex crossover SUV in checking or changing the spark plugs in the Duratec 35 3.5 liter V6 engine.

Owners of other Ford or Lincoln vehicle such as the Explorer, Fiesta, Focus, Fusion, C-Max, Mustang, Taurus, EcoSport, Escape, Edge, Expedition, Transit Connect, F-150, Ranger, MKZ, MKC, Navigator, MKX and Continental may also find these DIY instructions to be helpful. The procedure should be the same or very similar for the EcoBoost 3.5L V6 engine.

The OEM iridium tipped spark plugs in this 2019 Flex were part number Motorcraft CYFS 12F-5 (also known as CYFS-12F-5 or CYFS12F5).

A few other compatible replacement spark plugs with their part numbers include the following: ACDelco 41-988, Motorcraft SP-520 and NGK (6509) LTR6IX-11.

The tools and other items needed to complete this procedure include a 8mm socket with a 1/4" drive ratchet, a 5/8" spark plug socket, an extension bar, a 3/8" drive ratchet and a tube of dielectric grease.


Rear Ignition Coils
Front Left Ignition Coil
Slide Out Lock Tab
Since I was just checking the condition of the OEM spark plugs on this 2019 Flex at 30,000 miles, this guide will only cover replacing the front three spark plugs. The plugs are iridium tipped and should last up to 100K miles according to the service interval specification in the owner's manual.

To access the rear bank of three spark plugs, you will need to remove the upper intake manifold which is held in place by six 8mm bolts. This will also involve detaching the air intake hose and a few electrical connectors.

If you have access to compressed air or a wet/dry shop vacuum, clean off the top of the engine to help reduce the risk of having debris fall down into the cylinders when you are replacing the plugs.

I also recommend only checking or changing one spark plug at a time to further reduce the chance of having something fall down into the well.

The first step is slide out the white plastic locking tab on the electrical connector.

Push In Release Button
Disconnect Power Plug
Loosen Counterclockwise
Then push in the release tab before sliding the electrical connector straight off the ignition coil housing.

Loosen the single bolt that secures the ignition coil to the top of the engine by turning it in the counterclockwise direction with an 8mm socket and a 1/4" drive ratchet.

Remove Ignition Coil Bolt
Pull Out Ignition Coil
Ignition Coil Removed
Spin out the bolt the rest of the way by hand and set it aside in a safe place.

Rotate the ignition coil back and forth a few times to make sure the rubber boot at the bottom isn't stuck or "frozen" to the tip of the old spark plug.

Lift the ignition coil straight out of the spark plug well and set it aside in a safe place.

If your OBD2 / OBDII code scanner has indicated DTC (diagnostic trouble codes) that indicate a problem with the ignition coils such as P0351, P0352 or P0353, P0354 or P0355 you may need to replace a faulty coil.

A few compatible replacement ignition coil part numbers include the following: Motorcraft DG-520, Delphi GN10237 or Spectra C-720.

Spark Plug In Well
Spark Plug Socket
Loosen Counterclockwise
Attach the 5/8" spark plug socket to the long extension bar and a 3/8" drive ratchet.

Lower the socket down into the well and secure it to the top of the old spark plug.

Carefully loosen the old spark plug by turning it in the counterclockwise direction.

Try to avoid using excessive force. If you have trouble loosening the old plug, try spraying a small amount of penetrating oil down into the well and wait about five to ten minutes for it to work its way into the threads.


Spin Out Old Spark Plug
Remove Old Spark Plug
Inspect Old Spark Plug
Once the old spark plug is loose, detach the ratchet from the extension bar.

Spin out the old spark plug the rest of the way by hand.

Lift the old spark plug out of the well and detach it from the socket.

Inspect both ends of the old spark plug.

If the electrode tip appears to be covered in black soot or oil residue, the engine might be burning oil and should be inspected by a professional mechanic.

If the electrode tip appears to be white or ashy, the engine might have repeatedly been overheated.

Lower In New Plug
Tighten Clockwise
Apply Dielectric Grease
I recommend buying the original OEM Motorcraft CYFS 12F-5 iridium tipped spark plugs.

If you have a gap gauge, check the gap on the new spark plugs to make sure they match the manufacturer's specification.

Push a new spark plug into the spark plug socket.

The socket should have a rubber insert or a strong magnet to securely hold the plug in place.

Carefully lower the new spark plug down into the well.

Spin in the spark plug a few turns by hand in the clockwise direction to prevent it from becoming cross threaded.

Tighten the plug in the clockwise direction with the 3/8" drive ratchet to just past hand tight.

If you are re-installing the old plugs after checking them, tighten them to just a small fraction of a turn past finger tight.

If you are installing brand new plugs, tighten them until you feel the new crush washer collapse.

Avoid over tightening the new spark plugs to prevent from stripping the aluminum threads.

Some mechanics prefer to use anti-seize lubricant grease on the threads, but most spark plug manufacturers do not recommend using anti-seize since it can easily lead to over tightening.

Double check that the spark plugs are tight before moving on to the next steps.

Apply some dielectric grease to the opening in the rubber dust boot at the bottom of the ignition coil.

The dielectric grease will help seal out any moisture, dust or debris and ensure a reliable electrical connection.

Push In Ignition Coil
Spin In Silver Bolt
Tighten Clockwise
Lower the ignition coil back down into the well and push it on to the top of the new spark plug.

Rotate the ignition coil back and forth a few times to help spread the dielectric grease.

Line up the bolt hole in the ignition coil assembly with the corresponding hole in the top of the engine.

Spin in the silver metal bolt a few turns by hand in the clockwise direction to prevent it from becoming cross threaded.

Tighten the bolt in the clockwise direction with the 8mm socket and a 1/4" drive ratchet until it is snug.

Avoid over tightening the bolt to prevent from cracking the plastic ignition coil housing.

Push In Power Plug
Slide In Locking Tab
Spark Plugs Replaced
Slide the electrical connector straight back into the socket until it "clicks" securely into place.

Then push in the white plastic lock tab to secure the power plug.

Start the engine and listen closely for any strange sounds that might indicate a problem such as a loose spark plug or a disconnected electrical connector.

Be sure to record the spark plug change in your vehicle's service records.

Please check out all of my Ford Flex DIY Repair & Maintenance Guides.

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