What else should I clean when replacing intake plenum?

[SmoothTS]

Thanks to Ranger, AJ, and Submariner, I have learned that my 2000 Seville STS has a cracked plenum between the Throttle Body and intake manifold. I will be fixing this sometime within the next two weeks. My car has 115,000 miles and I changed the plugs and boots about 500 miles ago. The new plugs didn't help my recent reduction in fuel economy (drop of about 3 MPG over the past 6000 miles). I checked my Fuel Pressure Regulator and it seems to be fine. So, I was thinking I could also clean the TB and the EGR valve and maybe inspect the MAF sensor at the same time I am replacing the plenum. What do you think? Is there anything else I should hceck out while I'm working under the hood. Aside from oil, my new plugs, air filter, tires, and battery, nothing else has been changed on this car.

[submariner409]

You'll never have a better access to the backside of the throttlebody bore than during a plenum replacement: Lay in some TB cleaner and have at it. Make sure the PCV supply and suction plastic piping and rubber couplers are in good condition. Double check the starter electrical connections. Spend the extra bucks and replace the injector O rings (intake manifold "sockets"). Make sure you torque the intake back down properly: it doesn't take much.....89 inch/pounds = 7.5 ft/lb, and, although it isn't usually necessary I personally would use new (and they would be the new style) manifold gaskets, too.

BEFORE you pull the intake manifold itself, use a shop vac and smaller hose to get the lion's share of the accumulated sand and grit from around the manifold runner flange areas. 115,000 is a LOT of crap over the hood......and an amazing percentage of that winds up sitting on top of the engine.

[Ranger]

Definitely do the TB & EGR.

[quantum]

I see alot of "cracked plenium" threads, what are the idicators that a plenium is cracked, besides crappy mileage? Just curious if there was a list of symptoms...

[SmoothTS]

Originally Posted by submariner409

Spend the extra bucks and replace the injector O rings (intake manifold "sockets").

Could you please explain what you mean by "intake manifold 'sockets'"? This has me totally confused.

I assume you're suggesting injector o-rings as they can leak with age. I don't have to remove the fuel rail to remove the intake manifold, do I? This is a 2000 N*. I am thinking about sending my injectors to Cruzin to have them tested and cleaned sometime in the future. Should I just wait on the o-rings until then?

[Ranger]

Originally Posted by quantum

I see alot of "cracked plenium" threads, what are the idicators that a plenium is cracked, besides crappy mileage? Just curious if there was a list of symptoms...

High idle, rough idle, surging, fuel trim DTC's.

[submariner409]

The injectors are a press fit into the molded "sockets" (holes, sleeves, cavities) in the intake manifold, use O-rings for the lower seal and the entire fuel rail/injector assembly is held down by studs and nuts. If you're going to do an injector cleaning later, I see no reason to change the O-rings now if the injectors aren't loose in the manifold. The FSM directs you to remove the injector/fuel rail assembly prior to removing the intake manifold, and you'll want to do some cleaning, so another 5 nuts shouldn't be much more work.

[SmoothTS]

My plenum, intake gaskets, and injector o-rings are here (thank you Lindsay Cadillac!!) so I'm going to tackle this tomorrow morning! Will I need the GM fuel line connector tool to get the fuel rail out of the way? Do you have any final suggestions or hints that might be able to help me? I have already read the TB cleaning threads so I think I have that part understood. I'm going to shopvac and clean as much of the engine as possible too. I think I will leave the EGR alone as others have said the EGR has a pretty good error code system and will let me know if it's having problems. Then again, maybe I'll get the urge and stop by my local stealer to buy an EGR gasket and clean it anyway! Thank you again for the help. I'm always open your to advice! Wish me luck!