99 grand prix plenum replacment

[ricerider]

Just bought a 99 grand prix with 3.8l
I will be replacing both intake gaskets as well as the coolant tubes IF they havent already been done

My question is, is there way to tell if the plenum has been replaced with a dorman or other updated plenum without tearing the whole thing apart?

does the gm plenum say "gm" on it somewhere?

[Fivefingerdeathpunch]

It might say GM on the black plastic somewhere, I forget.

There is a date stamp on the plastic though, you might have to look up how to read it.

As for intake gaskets, look between the head and lower intake, if the gasket is black it's prolly factory, if silver then it's been changed to the upgraded aluminum style.

[BillBoost37]

Look at the top of the black plastic plenum. There are three things that look like one handed clocks. Read the numbers and look at the arrows. One will go 1-12 (that's months), another will go 1-31 (that's days) and the last one will have some years. You can tell the production date of the manifold that way.

If it has a 98 or 99 year, it's likely the original manifold.

[ricerider]

its not "if" but "when" with the original gaskets and plenum correct?

[BillBoost37]

No, not always. The original plenum, I've seen over 200K miles. The upper plenum with heat cycles will crack in the EGR pipe area, when that happens you want those coolant ports plugged. I've done lim's and plugged coolant ports, reusing stock LIM on many cars. W/o coolant in that area, the motor will not have an issue.

[ricerider]

I am planning a family trip cross country this summer and just need the car to be good to go. If i can stop the problem by plugging the ports instead of spending $100 on a new plenum im all for it!

Is there a write up on this forum somewhere on how to do it?

[BillBoost37]

Search on my user name and the word e a sy without the spaces. Look for modding a LIM.

I plugged a car, drove it 8+ hours straight to Erie PA, that car drove back to say hi (and help me with a roof) then back to Erie. That was about 5 years ago.. car is still running great.