R&R rear exhaust manifold 2004 Pontiac Grand Prix

Messages
27
Location
Pennsylvania
Thread starter
Not sure if anyone has a lot of experience with the GM 3800 and w-bodies... I’m pretty sure I’ve got a cracked rear manifold, and I’m working on a replacement. I’m in the process of a LIM gasket, so I’m thinking about doing it now. I know a lot of people like to drop the subframe a few inches and take it out the bottom, but I’m working in a stall with a pit, and I have nothing to support the motor with, so that’s not an option for me. I’d rather not go that path anyway... I don’t want to mess with that. Im trying to estimate how difficult it’ll be to do from the top while I have the blower and rear valve cover off, the dog bones out, and the motor strapped forward. I’m not that worried about the LIM gasket, it’s just this exhaust manifold I’m stressing over.
 
Messages
531
Location
Virginia
Hardest part is getting the heat shield off. Spark plugs boots are iffy. It’s a lot easier if you grab a flexible ratchet with the ujoint handle
 
Messages
27
Location
Pennsylvania
Thread starter
Last time I did the plugs I greased the boots... I remember last time was a nightmare getting those boots off. If that’s the hardest part, I’ll be thankful, lol. Once I have it disconnected from the downpipe, it should just just come straight up, I hope?
 
Messages
1,421
Location
MN
If you're already got all that taken apart, it should be pretty simple to take out the top. Just take the plugs OUT, or there's a good chance you'll break one off. Assuming you already thought of that, however.
 
Messages
531
Location
Virginia
Yep, takes a bit of maneuvering to pull out the mani. Removing the crossover and intake piping makes it much easier, but those have to come over to pull the EGR.
 
Messages
23,758
Location
MA, Mittelfranken.de
The rear manifold can be removed in the position you have the engine now, no need to lower the subframe. At this age the problem you may run into is rusted fasteners on the heat shields, pipes and manifold studs. Soak everything down with a good penetrating oil, Kroil, Liquid Wrench, Blaster all work okay and leave it overnight if you have the time. Being in the salt belt this is how I do it. After everything has been soaked use a impact gun with an extension and wobble on the exhaust to manifold nuts, light trigger so it rattles off, heat it if necessary, you are pulling the manifold so if they do break its not the end of the world. Remove the shield with 2 nuts at the crossover pipe and remove the crossover, it has bolts on the left side and studs on the right. Remove the O2 sensor and all brackets including the lifting bracket from the manifold. Remove the 3 head shield bolts, if they break its not a big deal you are replacing it anyway. Remove the EGR pipe to manifold bolt from underneath (one bolt). Not comes the fun part, removing the manifold nuts. You need to assess their condition before getting into it or you could end up pulling the head when they break. If they look a little rusty but a 6pt socket fits properly on them (not loose like its going to round) soak them good, let sit then spray again then put the socket and ratchet on the nut, tap the ratchet with a small dead blow hammer a few times. You don't need to beat the life out of it just hit it repeatedly to shock it and get some penetrant into the threads. Loosen them all then remove the manifold. If the socket feels like its going to round off stop and get a damaged head removal socket before going any further, proceed as you would with a normal socket but make sure the socket is absolutely square and all they one the nut. Worst case is you need to blow the nuts off with a torch then go after the studs once the manifold is out, its not as bad as it sounds just a PITA with torches and heating to get the studs out with grips but better than broken off flush studs on the head. Post back with questions if you have any.
 
Messages
747
Location
On the road Midwest
Thank you Trav; always enjoy your posts. My two cents worth is that Kroil is the best penetrating oil on the face of the planet. My profession is in heavy industry (power generation, oil refineries, chemical plants, etc.). NO maintenance departments in any of these places use "liquid wrench or PB blaster" or anything except Kroil. Spray down the 02 sensors as well when applying penetrating oil to the exhaust studs; and when you get them all out, replace them with stainless fasteners and use Nickel anti-seize on them.
 
Messages
23,758
Location
MA, Mittelfranken.de
Kroil is awesome, I buy it by the gallon and use a refillable air can! Unfortunately not everyone can get it without buying online and waiting for it. Liquid wrench seems to do a decent job, not as good IMO but it works given a bit more time.
 
Messages
27
Location
Pennsylvania
Thread starter
Alright, got it out and new one in from the top... plenty of space. On the torque specs, I wanna make sure. The torque thread on my main forum says 22-24ft-lbs, a lot of other sources say like 3 different specs? Like manifold bolt - 22ft-lbs, manifold stud (inner) - 22ft-lbs, then manifold stud (outer) - 89in-lbs? That’s a big spread between 22ftlbs to 89 inlbs... and in my car there’s only 1 nut on the manifold, the right-most fastener is a nut where the stud stays in the head. All the rest of them came out as studs with the nuts affixed to them. So what meaning does inner/outer have if the outermost one was a nut? Does that get 22ftlbs or 89inlbs? 89inlbs sounds like really low torque for this anyway...
 
Last edited:
Messages
23,758
Location
MA, Mittelfranken.de
All studs and nuts holding the manifold to the head are 22 ft.lb EGR bolt 21 ft.lb Heat shield 89 in.lbs Use all new studs and nuts and use never seize on the everything especially the studs that go in the head. Do not reuse any studs that came out with the nuts.
 
Messages
27
Location
Pennsylvania
Thread starter
Just curious, not doubting, what’s the purpose of not reusing the studs that came out with the nuts? I thought only one or two were supposed to be a nut and the rest were supposed to come out as studs. When I did the front years ago it was the same way, two nuts and four studs.
 
Messages
23,758
Location
MA, Mittelfranken.de
If they have permanent nuts then yes you can reuse them if they are not rounding off or rusted. IMO its good practice to replace them anyway if they are out as they can become strained when removing them if they are tight, they tend to fatigue with heat and age. I rather swap them out rather than have one break when tightening it (that's a genuine PITA) or break in service and warp/crack the manifold and have to do it all over again. Your call I am just telling you how I work and why. 2cents
 
Messages
8,299
Location
Texas
Originally Posted By: Trav
If they have permanent nuts then yes you can reuse them if they are not rounding off or rusted. IMO its good practice to replace them anyway if they are out as they can become strained when removing them if they are tight, they tend to fatigue with heat and age. I rather swap them out rather than have one break when tightening it (that's a genuine PITA) or break in service and warp/crack the manifold and have to do it all over again. Your call I am just telling you how I work and why. 2cents
I agree with you Trav. To go so far into a project and a stud breaks on you its game over - repeat.
 
Last edited:
Messages
27
Location
Pennsylvania
Thread starter
Hey guys just wanted to stop and update. Your advice was spot on, manifold came out the top fairly easy with the motor strapped forward. LIM gasket is done and all new lifters while I had everything apart, cleaned all the funk off the lim and supercharger, flushed out the cooling system. Everything started right up and it’s running like a champ now, except I still have a little ticking exhaust leak that’s infuriating me... though it’s a lot less than it was. I just don’t have the mental fortitude to take it back apart again frown Thanks for your responses, though, they were really detailed and helpful.
 
Last edited:
Top