NEED HELP Recommended Exhaust Manifold Replacement?

[MrShelley]

Well, the Trailblazer project ('08 4.2L LT) has been mothballed for the last couple of years and as soon as I pull it out of the shop and drive it a few days, the exhaust manifold cracks. Pulled the old one off today (what a chore) and as expected, it's nearly split in half. So, the question is...what do I replace it with?

GM #12637060 (order online)
Dorman #674869 (in stock at local O-Reilly)
Something else? A header is tempting, but cost prohibitive.

Longevity is most important. I've saved up the money to drop 4.56s and a locker in the thing so I can use some of that money, but I don't want to be buying a new exhaust manifold in a year.

CURRENT SETUP
2008 Trailblazer LT 4WD
MarkMCs 3" body lift
MarkMCs 3" suspension lift
MarkMCs 1.5" wheel adapters
Helo 835 wheels (17x8)
Massive Speed Systems Adjustable Panhard Bar
Bilstein 24-103336 Front Shocks
Bilstein BE5-6248-H5 5100 Series Rear Shocks
Flipped Front A-Arms
BF Goodrich All-Terrain TA KO2s (285/70/17)
MDB Fabrication Front Bumper (HardTrailz old bumper)
MDB Fabrication Armor/Skidplates
KC HiLites LZR Lighting

Pending:
4.56 Gearing
Powertrax Locker
Custom Roof Rack
Fold-Down Fullsize Spare Carrier
Winch

 

[6716]

Nice rig!

I almost feel like any one here will do you just as well as any other. There's no electronics or moving parts. You might have just got lucky with one that cracked early, I don't see a ton of complaints about the exhaust manifold.

 

[flyboy2610]

Go with GM. I don't trust Dorkman for anything but door panel fasteners.

 

[Mooseman]

Well it's the GM version that cracked, like all of them. Dorman says that theirs has reinforcements where the cracking was prone. I think all the aftermarket ones say that but not the GM one. I'd get the Dorman. Their non-electrical parts are usually pretty solid.

 

[xavierny25]

Dorman FTW on this part anyway. I'm on my third exhaust manifold, first was of course the original 2nd was some crappie of the shelf unit that I had a shop put in while I was out of town and the wife had the truck. Needless to say it didn't last long maybe about 2yrs. I installed a dorman and could tell when compared side by side that the dorman is way beefier and better all around.
The pos the shop put it. Unfortunately I couldn't locate a pic of the dorman I put in.
There lower and upper control arms are good hard parts as well.

 

[MrShelley]

Well it's the GM version that cracked, like all of them. Dorman says that theirs has reinforcements where the cracking was prone. I think all the aftermarket ones say that but not the GM one. I'd get the Dorman. Their non-electrical parts are usually pretty solid.

Kinda the way I'm leaning. Plus, I can have it installed in the truck by tomorrow. It also comes with new flange nuts, flange gasket, and a shiny new heat shield that will save me a few bucks. The GM part doesn't. 🙄

 

[MrShelley]

Dorman FTW on this part anyway. I'm on my third exhaust manifold, first was of course the original 2nd was some crappie of the shelf unit that I had a shop put in while I was out of town and the wife had the truck. Needless to say it didn't last long maybe about 2yrs. I installed a dorman and could tell when compared side by side that the dorman is way beefier and better all around.View attachment 102007
The pos the shop put it. Unfortunately I couldn't locate a pic of the dorman I put in.
There lower and upper control arms are good hard parts as well.

You sure that ain't a picture of the one lying on my shop floor? 🤣

 

[xavierny25]

You sure that ain't a picture of the one lying on my shop floor? 🤣

Lol, I think they all crack in about the same spot. Dorman is definitely the way to go. I'd grab and replace the manifold bolts while your at it. BTW nice rig!

 

[MrShelley]

Already got the new bolts, I was PRAYING that the one broken bolt I could see from looking under the fenderwell was the only problem. As always, it couldn't be easy and as soon as I pulled the heat shield I knew broken bolts were only part if the problem. Oh well, still love the truck, and can actually work on it if (when) something breaks. Thanks for the compliment, I get more looks with it than my family gets with their Jeeps.

 

[budwich]

I had mine replaced by GM just inside the warranty boundary (10yr, xxx miles). It lasted less than two years. They said "that's the way it is some times"... right. The dorman has made it past year two so I am hoping it will go a bit longer..... BUT, because of the virus, I haven't done any "heavy towing" which causes significant heating / torquing issues which likely pushes the design to the "brink". good luck... when gm did the replacement, it took three days as they had to deal with broken bolts (three if I recall). I broke the front one during my attempt... it wasn't fun.

 

[Mooseman]

IIRC, there is Loctite on the OEM bolts so they may benefit from some heat prior to attempting to remove them. You may even notice that some may already be broken.

When I was replacing the engine in my '02, the manifold was cracked on both the old and the used replacement engines and several bolts were broken. Although it was on the engine stand, I heated each broken bolt in the head with oxy torches and was able to turn them using the tip of a small screwdriver until I could grab it with pliers. I had 5 or 6 of them.

 

[MrShelley]

IIRC, there is Loctite on the OEM bolts so they may benefit from some heat prior to attempting to remove them. You may even notice that some may already be broken.

Only had one bolt that had snapped the head off. Got it out pretty easily with vise-grips. I say "pretty easily" because it HAD to be the bolt closest to the firewall. I think the body lift made it a bit easier, but it was still pretty close quarters.

 

[MrShelley]

Installed the replacement Dorman manifold. Flanges are definitely thicker than OEM, but areas where the cracks happened look the same to the naked eye. Crossing my fingers and hoping for the best. Running quieter than ever, so this issue may have developed so slowly than my ears grew accustomed to it. Still, not bad for a truck with only 95K miles.

Replaced the engine coolant sensor at the rear of the engine (Stant 74192) which is located right next to the exhaust port with the broken bolt. The wires looked kinda "crispy" and I figured it was easiest to replace while everything was off that side of the engine. Started her up and immediately threw a P0128 code, so looks like I'll be replacing a thermostat next. What are the odds? May put that off until I pull the front diff for the gear change though.

 

[Mooseman]

Don't wait too long. You can cook your cat while running low temps like that.

 

[MrShelley]

Don't wait too long. You can cook your cat while running low temps like that.

The fun never ends!

 

[Schurkey]

1. Expect to haul any Dorman exhaust manifold to the machine shop for planing of the gasket surface before installing. Every Dorman manifold I've ever seen was warped coming out of the box. Some have a rough gasket surface, too. They should be FLAT and SMOOTH.

2. The Dorman heat-shield for the Trailblazer is a piece of crap.

3. Inspect any Dorman manifold carefully for casting flaws. Here's an iron booger from the inside of a "427 Corvette" manifold. Dorman QA is non-existent. I've heard that the manifolds made in Viet Nam are actually worse than the manifolds made in Communist China.


Having said that...the Dorman manifold for the Trailblazer Atlas six-popper seems to be more-durable than the Genuine GM piece.

Bolt-breakage on the Atlas exhaust manifold bolts can be minimized by using a swivel-socket so there is no side-loading of the bolt during removal.