For every 50,000 km. The spark plug should be replaced. To replace the spark plug for a 1999 4Runner is relatively easy provided that you know the procedure. Before you start popping up the hood, you should have the following item handy:
- 6 new spark plugs. According to the Toyota's owner manual, double ground spark plug should be used. But in the NGK website mentioned a single ground is ok. That's all depends on your preference. I would recommend you to follow the Toyota's instruction.
- 3/8" wrench, 3/8" Torque wrench. The regular wrench is mainly for you to loosen the spark plug and other screws. Do not use the torque wrench for any loosen job as torque wrench is only designed for tightening. TEST YOUR TORQUE WRENCH BEFORE TIGHTEN ANY PLUG.
- 10" wrench extension is a must or you can't even get access to the spark plug. 10" is a good length for the job but longer is better.
- Spark plug socket is not the same as a regular socket. It is deeper, and it has a rubber boot inside of the socket, which is used to hold the spark plug.
- 10mm and 12mm sockets, Philips screw.
- Dielectric, Anti-seize, Penetration oil.
REMEMBER POWER TOOLS IS USEFUL BUT NOT FOR THIS JOB.

1. Popup the hood, you should be able to see the engine compartment like this. It is a Toyota 1999 4Runner 5VZ-FE 3.4L 6 cylinder engine.

2. Start removing the spark plug from the passenger side. Remove the hoses, clips and screws from the following circled locations. You can remove all hold air intake including the air filter but I don't recommend it because you will need to disconnect the MAF sensor. if you damage the MAF, it will be an expensive repairing. So don't touch that part. Also don't touch the MAF sensor. it is too easy to be broken.
3. Press the rubber tube and push it toward to the right to remove the air intake.




4. After the air intake is removed, you should be able access to 3 left side spark plugs. Just make sure do not get any dirt into the engine air intake.
5. Press the tab and remove the connector, also remove all 3 screws to get access to the spark plugs.
6. Now you can see the spark plug. Use the air compressor to blow all dirt out of the hole. If it is not clean enough, use a wire brush to clean it and spray it again.

7. Now you can remove the spark plug. Just make sure to use a regular wrench to remove the plug. Never use torque wrench to remove anything.
8. This is a stock spark plug from Toyota. As you can see, it is a twin ground spark plug from Denso, The model number is K16TR11. The old one is pretty dark but the new one should be in sparkling silver.
Before you install the new plug, put a little bit of anti-seize on the thread. Too much anti-seize is not necessary and not recommended.
9. This time you have to use a torque wrench to tighten the plug. First, gently drop the plug in the hole. and just use the extension and spark plug socket without the wrench. Hand tighten the plug lightly. Do not force the turn hardly. if you damage the thread, you will need to paid for an expensive repairing job. Hand tighten until you cannot turn the plug anymore. then connect the extension bar to the torque wrench at 18nm. Tighten it until you hear the click sound.
10. Before you reconnect the cable, put some dielectric first. It will make a better contact for the plug and cable. And it will make you a bit easier to remove the cable when the next time you have to replace the plug.
Repeat the step 5 to 10 for all 3 spark plugs. Then you can reinstall the air intake system. Make sure to reconnect all hoses. And secure all cable and hoses.
11. To remove the driver side is not too hard except the last one plug against the firewall. But still not too hard provided that you have a pier.
First, remove the cable holder for the spark plug cable. then remove the diagnostic connector by press the tab at the back.
12. Remove this nut. Make sure do not drop the screw or it is too hard to find it again.
13. Remove the cable from the holder. And remove the holder. And remove the screws from the circled locations. If these screw is rusted, spray some penetrating oil. Then wait for a couple of minutes and try again.
14. Now you can pull the cable to get access the first plug. remove the circled cable then you can get access to the second cable. if it is too hard to be remove. use a pier.
DO NOT PULL THE CABLE. You must pull the boot directly.
Repeat the same procedure to clean the hole, remove the replace the spark plug.
15. Pull the bar to the left then you can get access to the final plug.
When all plugs are replaced. Reverse the procedure to reconnect all nuts, screws, cable.
DOUBLE CHECK ALL INSTALLATION BEFORE YOU START THE ENGINE.
My work has been done perfectly. Now the engine is running a lot quietly and smoother. Don't know the mileage improvement yet but I would say it will be a improvement for sure.
Any question. Please leave the comment.