How to Change the Spark Plugs on your 3.7L Mustang

3.7L Mustang Spark Plug Firing OrderAt 11K miles on the car I was getting a random misfire code. P0300. I checked around and could not find an NKG plug so I purchased a set of Motorcraft SP-520 3.7L V6 Mustang Spark Plugs from Ford Dealer. They are platinum and run about $10 each.

I pulled out my SCT tuner and plugged it into the ODBII port and it gave me the P0300 code. A P0300 diagnostic code is a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0303 code would tell you cylinder number 3 is misfiring. Unfortunately, a P0300 doesn’t tell you specifically which cylinder(s) is misfiring, nor why. So my first plan of attack was to change the spark plugs.

The problem on my car seems to be related to Ford TSB 21474

This is a nasty ECU bug that can come into play with certain repairs or modifications. This effects both 3.7L and 5.0 Mustangs.

 Symptoms of a P0300 may include:

  • The engine may be harder to start.
  • The engine may stumble or hesitate.
A code P0300 may be due to:

  • Faulty spark plugs or wires
  • Faulty coil (pack)
  • Faulty oxygen sensor(s)
  • Faulty fuel injector(s)
  • Burned exhaust valve
  • Faulty catalytic converter(s)
  • Stuck/blocked EGR valve / passages
  • Faulty camshaft position sensor
  • Defective computer


If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Start with spark plugs, and coils first. In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. In some cases the problems could be fuel injectors.

Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Motorcarft SP-520 3.7L V6 Mustang Spark Plugs Here are the Motorcraft SP-520 3.7L V6 Mustang Spark Plugs in the box. Alternate part number CYFS-12F-5.

Motorcraft SP-520 3.7L V6 Mustang Spark Plug Gap

 These were gapped over .45 from the factory. Nick the tuner at ProCharger recommended I gap them from .35 to .38 (because of the blower)
Pull your engine cover if you have one. Also your strut tower bar if you have one of those also. Start with pulling the bolt holding the coil down.
 Then push the red tab up and pull the coil electrical connector off.
3.7L Mustang Coil  My first coil left the boot inside stuck on the plug. I tore it up trying to get it off. This was a $3 part at the ford dealer.
3.7L Mustang Spark Plug Get your 5/8 plug puller out and pull them gently. Also remember when you stick them back in thread them by hand. If you mess of the soft aluminum threads then game over.
3.7L Mustang Spark Plug Anti Seize Don’t forget a dab of anti-seize on each plug.
3.7L Mustang Spark Plug boot lube Also to keep your coil boots from sticking then you can apply a little bit of boot lubrication. Hopefully this will keep them from sticking next time you take them off.
3.7L Mustang Intake Manifold Removal No for the Passenger side. You will need to remove the intake manifold.  There are 7 bolts on the top.
3.7L Mustang Intake Manifold Removal 1 bolt for the PCV valve hose.
3.7L Mustang Intake Manifold Removal Electrical connector on top at the EGR valve.
Use your u-shaped puller to get this wire loom harness connector on the throttle body. There is another one on the other side of the throttle body.
I pulled this vacuum line at this location near the firewall.
 Then the EGR valve hose has a clip on top you pull out.
Now you can pull the manifold off the car.

The cylinder firing order for the 3.7L Mustang is 1-2-3 on the passenger side and 4-5-6 on the drivers side. The firing order is 1-4-2-5-3-6.

Then you have full access to the right bank.
3.7L Mustang Intake Manifold Now is the perfect time to clean off the intake manifold.
3.7L Mustang Intake Manifold Now reverse the procedure to put it all back together. Here is a top shot, you can see the 7 top bolt holes.