2012 GMC Terrain 3.0L SLE VIN 5 (Flex) 200k miles
I assume that I really need/have to remove the intake manifold when replacing the (rear) spark plugs. Which I would assume means replacing the gasket(s). Would that mean replacing both the upper & lower gaskets? Throttle body gasket as well while doing this job to clean it? Using iridium plugs AC Delco 41-109. First time for me to work on this vehicle so appreciate any insight.

Thank you.

I saw a post on the TerrainForum.net that mentioned you can get 4 plugs out w/o removing the manifold and another one almost out but the last one requires the removal. Sometimes you can get by w/o replacing gaskets but I think it's good practice to do that anyway.

200,000 on a set of plugs is pretty darn good. I did my Caddy 3.6 at about 80,000 and they didn't really look too bad but I figured with my mileage accumulation pattern, and chassis rust, that was likely halfway to the scrappage time for the car.

Check out this video on youtube
https://www.youtube.com/watch?v=nGONpPQl1P4

Doesn't look terribly difficult. you do have to remove the intake manifold, but it looks like you only have to replace the lower gasket, and that is only if you really want to. I would do it though, Since the gasket is only like $20.

Yes, get new gaskets for the intake manifold, as well as valve cover gaskets. You might as well do that while you're in there. Replace every gasket you have to remove!

You should use NGK's new ruthenium spark pugs. Part #91276. They will last even longer than iridium, so you will NEVER have to pull the intake again!

Thank you all for the input. I plan on doing it next weekend. BTW, it's my sister's vehicle so work will occur away from home.

You just need the Upper Plenum Gasket. This is a easy straight forward job. Plenum bolts call for 17 ft lbs.

Oil was in the spark plug wells. I assume that means a valve cover gasket kit is required with new spark plug tube seals?