Thread: Gasket replacement for Stock Manifold to 2.5" ZZP Downpipe

2002 Grand Prix GT
Stock manifold
2.5" ZZP downpipe

I have a pretty large exhaust leak at the back of the engine. It started small and now there is lots of fumes, and noise, and heat. I believe that the gasket that goes between the stock rear manifold and the ZZP 2.5" downpipe is bad.
First thought was that the rear exhaust manifold gasket was bad, but in other posts, it was revealed that those gaskets don't go bad often.

I can order this gasket from Intense: http://intense-racing.com/Merchant2/...e=3800_Exhaust

Now, what do I need to do to disconnect the downpipe from the manifold? What size are bolts? In order to remove the bolts, where is best place to reach them?
I am not finding much help on exhaust here when seaching

That is the right gasket but most autoparts stores should have them. The stock gasket fits the aftermarket DP. Make sure you isolate the leak first. The flex pipes go bad on these also. I cant remember the size of the 3 nuts, Maybe 14mm???

sea foam your engine, then watch for the smoke to come out where its leaking from, some of these cars crack the rear manifold.

no need to undo the down pipe at all. just the manifold off the head. take the two dog bones off so you can push and or pull on the engine to move it as needed.

if you want to replace the downpipe gasket, get the front of the car up in the air. you need I think a 13mm and long extensions. You can get to it easy from underneath.
Tip, spray with PB blaster a couple days before hand and run the engine for a couple minutes to get some heat into the fasteners. The nuts should come free pretty easily then.

You install the new gasket with the crush donut (raised ridge) facing the manifold. A stock metal gasket will push onto the manifold studs and stay in place.

Originally Posted by Scottydoggs

sea foam your engine, then watch for the smoke to come out where its leaking from, some of these cars crack the rear manifold.

no need to undo the down pipe at all. just the manifold off the head. take the two dog bones off so you can push and or pull on the engine to move it as needed.

The engine has no mods except for HV3. The exhaust is all stock except for the downpipe, which is a ZZP stainless. On other threads, since this car is not highly modded that it is unlikely it is the exhaust manifold.
I have a walker stainless gasket 31605 ready to replace the one at the rear manifold and downpipe, but know you got me wondering if that may not be where the leak is due to all the noise, fumes and heat (exhaust gas is getting inside car) I have Seafoam and I can get it put into the upper intake and check for smoke.

Originally Posted by mguzzo

if you want to replace the downpipe gasket, get the front of the car up in the air. you need I think a 13mm and long extensions. You can get to it easy from underneath.
Tip, spray with PB blaster a couple days before hand and run the engine for a couple minutes to get some heat into the fasteners. The nuts should come free pretty easily then.

You install the new gasket with the crush donut (raised ridge) facing the manifold. A stock metal gasket will push onto the manifold studs and stay in place.

I'm pretty sure the leak is at the gasket between the rear exhaust manifold and the downpipe. As for the new stainless gasket, I have Walker 31605, it has raised a raised ridge on both sides in the center 2.5" cutout. When I go to put this on, which way does it go? Or maybe it don't matter since both sides of the stainless gasket look the same. Also, I have read that sometimes it is hard to get a tight seal. Many have installed a new gasket and there still is exhaust leaking. Should I use some kind of RTV sealant on both sides?

Originally Posted by NotEnoughGPs

The engine has no mods except for HV3. The exhaust is all stock except for the downpipe, which is a ZZP stainless. On other threads, since this car is not highly modded that it is unlikely it is the exhaust manifold.
I have a walker stainless gasket 31605 ready to replace the one at the rear manifold and downpipe, but know you got me wondering if that may not be where the leak is due to all the noise, fumes and heat (exhaust gas is getting inside car) I have Seafoam and I can get it put into the upper intake and check for smoke.

the stock rear manny cracks stock, replacements also crack. or it could be the stock flex.

The stock metal gasket, some parts places call it the pipe flange gasket (manifold to exhaust). Take the dog bones off to give more room, might have to disconnect the air intake box if that's stock, and I found it easier to get off if I discount the dp from the cat back too. Torque it to the stock spec. To be honest I've always had trouble with my downpipes getting small leaks, 2 bolt flanges suck

Well, this morning, when the engine was cold, I started it up. I then reached down to the flange at the top of the downpipe, and determined that was where the exhaust leak was. I didn't have to use Seafoam. I was glad it was not the flex section of the downpipe leaking, or worse the exhaust manifold gasket leaking.
I had purchased a new stainless gasket and raised the car up, and got the 13mm socket on the bolt on the passenger side of the flange. Much to my surprise, it was loose, I could have turned it by hand!!!!!
Another BIG lesson learned here for me: don't put off checking the tightness of bolts or screws or whatever. I am trusting that someone else properly torqued that bolt on the downpipe, but they did not (that dp was installed at a shop, not by me) And to think that I just suffered with all the exhaust gases for such a long time as this leak at the downpipe began last fall!.
Again, lesson learned. I know I have read on here that others have experienced similar issues.
Well, both of those bolts on the downpipe flange are not tight, and was a dream to drive this car now. Seems like it has more power, will probably get better fuel mileage, and I don't need to drive with the windows open.
I now realize why that leak was getting worse! With all the vibration on the rear manifold and the dp, the bolt continued to back out and the leak was getting worse.
So, everyone learn from my mistakes and check, check, and check for tightness when there is an exhaust leak (applies to trans pan leaks, and oil pan, and etc)

loose bolts, thats a rare one, most of the time they are rusted on there.

Originally Posted by Scottydoggs

loose bolts, thats a rare one, most of the time they are rusted on there.

Yeah, I kept spraying "trick shot" penetrating oil on the bolts for 24 hours. I thought they looked rusty. I figured I would be braking out the drill, and had no idea how to get a drill bit in there.
So, I also was surprised! Never expected that.

You guys don't read its a ZZP downpipe not his stock downpipe

Ya but stock rusted bolts won't

Originally Posted by Swiggles

Ya but stock rusted bolts won't

I bought the new ZZP downpipe and new stainless gasket in 2008. A local exhaust shop installed the pipe. I am not sure which bolts the shop owner used on the flange: if the original bolts were rusted, I believe the guy who owns the shop would have installed new bolts. He would have never reused the original bolts. He surely would have torqued the now bolts. I took the car back to the shop last year to have to straight pipe welded in for the resonator. I bet in order to install the straight pipe, he loosened the 2 bolts on the flange of the downpipe, and he just snugged them back up and never torqued them. It was a few months later that I began noticing an exhaust leak. Since then, the leak worsened, and I figured with the high miles, it was the rear exhaust manifold that was cracked.
All the time it was the bolts getting loose on that damn flange.

After torquing the bolts on the downpipe flange, I disconnected the battery to get the PCM to relearn. I went on a trip on Sunday, it was like driving a new car!
The believe all the time the exhaust leak was there, the engine was running rich.
My fuel mileage seems to have increased some. On one 200 mile drive at highway speeds , the DIC was showing 34 mpg.