I’m a diyer and have always changed the plugs on my own Corolla myself. It’s an older one, early 90’s, so the procedure is bound to be a little different for a 2003. For me it’s a simple enough job, takes about 20 minutes to change out all four. It’s common sense to secure at least a Haynes or Chilton’s maintenance and repair manual for the car and read through the procedure before starting.
Here’s what you want to avoid at all costs.
- breaking off a plug while removing it. if you can’t get a plug out easily using the ideas here, don’t force the issue. stop and take the car to a shop.
- cross threading a plug while installing it. if you aren’t sure, stop and take the car to a shop.
- dropping anything into the spark plug hole. clean the hole before removing the old plug, then use common sense.
- overtightening the plug, or not tightening it enough when installing it
- using the wrong plug part number
First off, I buy the NGK replacement plugs, making sure I’m getting the same part number as is recommended in the owner’s manual. Buy your plugs at a dealership if you have any question about whether the part number is correct. Then I visually inspect each plug for any signs of cracked metal or porcelain, and that they are gapped to spec. Then I take the plugs out to the car, set up a work bench, and pop the hood. Next I remove the plug wires from the plugs. My wires are all of different length so it is clear which plug goes on which wire. To be sure you can number the wires with a marker before removing them. The plugs are down inside a deep hole, so you need an appropriately sized 3/8 inch drive spark plug socket and ratchet to get them out. Spark plug socket are taller than normal sockets usually. Use the type of spark plug socket that has a rubber insulator inside, helps prevent damaging the spark plug. Test the fit of the socket on one of the new spark plugs so you know how it feels when it seats correctly.
It’s a good idea to remove any debris in the holes before you remove the plugs. I just us a shop vacuum cleaner and make my own adapter to fit the hole. Compressed air would work too. To remove a plug, put an extension on the socket (but not ratchet) and hold on to the extensions while you place the socket over the plug to where it is seated. Then install the ratchet and while supporting the extension with your other hand, loosen and remove the plug. The plug may not come out with the socket and be left in the hole, so be prepared with a set of long forceps you can poke down the hole to grab the plug to pull it out. Sometimes the socket will stay down the hole too, and the only thing that comes out is the extension. You can tape the socket to the extension with some duct tape to prevent that.
As each plug comes out, number it with a marker. Take the time to inspect the tips of old plugs, comparing them to photographs you can find on the internet of what used plug tips should look like. Especially take note if one of the plugs looks much different from the others. They should all look more or less the same.
To install a plug, double check it is undamaged and the gap is correct. I apply a very thin layer of moly lube on the threads, but not everyone here agrees on that point. Helps making removal easier the next time. Put the plug in the socket (it will stay there if the socket has that rubber insulator) and attach the extension to the socket. Now carefully place the plug as vertical as possible into the hole. Thread it in by hand, holding only the end of the extension. If it grabs at all, lift it up and try again. It should start to twist into the hole needing with very little force at first. This is done to prevent cross-threading the plug. You should be able to twist it in 1 to 1 1/2 turns by hand using only the extension, after which you’ll need the ratchet. The final job is to tighten the plug to the correct torque spec. Make sure you know what this is for you make/model/year/engine. The safest approach is to use a torque wrench. If you don’t have one, you can estimate the torque using a bathroom scale. Say you want 15 foot pounds. Press on the scale to feel what 15 pounds feels like, then press on the ratchet handle one foot away from the socket with that same force. When all the plugs are in, use a flashlight to make sure the rubber insulator from the socket isn’t still stuck on top of one of them, and that there’s nothing in the hole other than the spark plug.
I always double check the plugs aren’t loose, remain at the right torque, after a week of use.