What's the best way to change the rear three plugs on a `99 Regal? From the top or from below?

Is this a LS or GS? I do mine from the top on either model.

LS, do you need to remove the cowl or anything?

Just remove the plastic engine cover where it says Series II 3800. Also, although not necessary it would help to undo the front 02 sensor electrical connector from the bracket and remove the bracket. Just two 13mm nuts hold it on to the exhaust manifold. Then it will be easier to get your hands in there to remove number 2 and 4 plugs. Hardest part will be getting the spark plug ignition wire boots and metal shields off the plugs. It takes patience. I would highly recommend replacing the ignition wires in the process.

Thanks for the tips. Any recommendations on what plugs to or not to use?

You're welcome.

If you have the original AC Delco double platinum plugs - 41-921 they will be hard to come by and discontinued. The recommended AC Delco plug is now the 41-101 Iridium which is made by NGK for GM. Gap .060".

Other suitable plugs are the Autolite double platinum APP 606 or NGK TR55IX Iridium. Gap .060". Try hard not to use any Bosch spark plugs but it is your choice. Good luck.

This past weekend I changed the spark plugs on my 98 Regal LS, so here are my thoughts:

As BNaylor said, removing the spark plug wire boots is a real pain. I had to use a little shot of WD40 on the exposed bit of the plug which got drawn up into the boot as I worked at pulling it off. Eventually they pop off. Without the WD40, I doubt that I could have removed the boots. When I replaced the boots, I gave them a little shot of silicone lubricant so they will be easier to remove when I can afford to replace the wires. I also intend to pop them off and on from time to time when I am under the hood. Removing the wire boots is the bulk of the job.

Regarding access to the backside plugs, I got to the one under the alternator from the side, under the alternator. The other two I was able to access from above. On the back, top of the engine, there is a nasty plastic hook thingie which holds some wires. I was able to twist and pop out of its metal bracket. It will bite if you don't remove it. This is probably the O2 sensor bracket that BNaylor mentioned. Coming from above, the metal bracket did not get in my way. I then put some padding on the dogbones and climbed up on the engine, put my knees on the padding and I was able to easily access the two rightmost backside plugs.

The O2 sensor is kinda in the way when removing the last plug. Fortunately, I bought a spark plug socket drive from Autozone ($5.95) which worked perfectly for that spark plug (I thought that I had wasted my money until I got the that last plug). I have read that removing the O2 sensor is difficult.

Good Luck

When I replaced the boots, I gave them a little shot of silicone lubricant so they will be easier to remove when I can afford to replace the wires. I also intend to pop them off and on from time to time when I am under the hood. Removing the wire boots is the bulk of the job.

The O2 sensor is kinda in the way when removing the last plug. Fortunately, I bought a spark plug socket drive from Autozone ($5.95) which worked perfectly for that spark plug (I thought that I had wasted my money until I got the that last plug). I have read that removing the O2 sensor is difficult.

Good Luck

That depends on the pre-CAT 02 sensor. Weather and how much corrosion you get, etc. I had good luck with PBBlaster on the bung threads and using a universal 02 sensor socket which has the cutout for the wiring. Engine/exhaust should be warm. Some people use BFFI when replacing plugs and you may damage the 02 sensor so it doesn't hurt to remove it first but it is up to you.

On the silicone grease which is a good suggestion important part is to make sure it is dielectric in nature. When you get the AC Delco replacement wires which is the Premium Silicone 7mm wires they will come with pre-applied dielectric silicone grease on both ends.

Some people go through the extreme of removing the dogbone engine mounts and pushing engine forward to get some extra clearance which is very minimal unless you remove the exhaust down pipe or tranny motor mount but that is not really necessary. Plus you may get some weird annoying knocking sounds after completion unless you get the engine back in the original spot.

Are double platinium plugs OEM or is that what you recommend?

The OE Delco plugs were the 41-921 which is double platinum. Good for possibly up to 100K miles. I use Autolite APP606 (double platinum) in my wife's Regal LS which are a good alternative spark plug. I use NGK plugs in my Regal GS.

The current recommended Delco brand replacement is the 41-101 iridium which are 100K mile plugs but made by NGK for GM.

Really your choice but they all work fine. I've had bad luck with the Bosch brand though.