So I spent the weekend trying to replace the o2 sensors on my 2008 H3. Downstream came out just fine but the o2 sensor in the manifold would not budge an inch. Tried many rounds of PB Blaster, torching it, o2 sensor sockets, locking pliers and a pipe etc. but nothing. Got too carried away and ended up rounding off the sensor head. So basically it seems I have two options: Take it to a shop to see if they can get it out or replace the entire manifold.

Might seem like a bit of a stretch to replace the entire unit because of an o2 sensor, but both my cats have been burned for years according to a GM dealer service report in 2017. I've had a P0300 and P0420 for that entire time as well but really no noticeable performance issues other than a pretty rough startup on occasion. Just trying to weigh the options of paying someone hundreds of dollars (potentially) to remove a stuck o2 sensor in a manifold with a bad cat or biting the bullet and replacing everything.

Anyone have any experience with a non-OEM manifold and cat? Seen some on Rockauto that were reasonably priced but I'm really looking for a balance between quality and price. Not sure if that's possible but would appreciate any advice or personal experiences with replacing the manifold. Honestly I can't convince myself that dropping $1,000+ on a GM part is the way to go.

 

If its striped try some liquid wrench or croil penetrating oil. Whack it and put vice grips or pipe wrench...

 

The best way to get it out will be a Venom induction gun. It'll just take a bit and that baby will super heat the metal.

 

Heat works wonders where there was none before. Stuck exhaust fasteners or components like an O2 sensor get hot and cool over and over and so heat doesn't do much to break them free on occasion.

There is not much room in there to get a big set of vise grips or pipe wrench involved.

If I were to replace my exhaust manifold, I would do a MagnaFlow OEM replacement and get something back in performance for all the $$ and busted knuckles.

 

Thanks Doc. Had a feeling from your numerous older posts on this topic that you would recommend going the MF route. Really thought about it but just couldn't spend the $$ on the Magnaflow OEM. Was hesitant, but ended up just going with the Walker manifold. Should be here tomorrow so I'll give it a go this weekend. Unfortunately, I can already see two manifold bolts that are broken. The one closest to the front (still see a lot of the stud sticking out) and the one farthest back by the firewall (cannot see the stud - lucky me). I've been pulling the heat shield down almost every day and hitting them with PB Blaster and will do it one more time tonight. Same with the lower bolts/studs at the flange. Hoping for no drilling/helicoil use but I'm afraid that back bolt is going to be a problem. Any last minute advice would be appreciated!

One last question in case anyone sees this in the next 24 hours...I am about to pick up the Dorman HELP! #3411 M8x1.25.56mm Manifold Hardware Kit for the install on my 08 3.7L. Does the 56mm length matter that much? I've really seen a ton of variation in total length when searching for the proper hardware. Some went 30mm, others went 45mm (and said it was too short), etc. I obviously don't want to go with something too short but I just haven't seen a solid answer on the proper hardware to bolt the manifold to the head other than the M8x1.25 size

Front manifold bolt

Rear manifold bolt by firewall

 

Those exhaust manifold bolts break or crack and break when trying to remove often. Almost more common than not.

Most of the time you can drill into the bolt that broke off flush and then use an extractor bit to remove it. Go slow, be cautious, you will get them out. Only on rare occasion do you have to drill it all out and go with a helicoil type replacement and that is usually because somebody did not have a good extractor or thought the drill out would be faster.

I cannot remember the length of the Bolts and do not have time right now to dig that out.

Good luck.

 

56mm length studs should be ok(includes 6mm hex height for install). I installed 45mm length (12.9) studs, 10mm height flange nuts(10.9) Exhaust Manifold Stud Upgrade
OE bolts stud length 30.7mm:


Max depth in the head 28mm:

 

Yeah @hummerz I searched for what you did for a while but I had trouble finding something similar. VERY interesting development tonight though. Got started on the removal process and the header bolts were unbelievably easy to get out. I think I got each one out that wasn't broken using only one hand on the ratchet. Was really surprised to see that these were bolts instead of the studs/nuts. Can someone confirm that the OEM setup is studs/nuts and not bolts? Posted the bolt images below. All appear to be 9.8 M8s and roughly 33mm long (not including the bolt head). Should I still go with the DORMAN studs/nuts with the new unit or go get some bolts?

Anyways, I added a few bolts back and moved down to the flange thinking they would also be a piece of cake and what do ya know, they won't budge. I'm also having trouble with the space down there as well as the cat pipe and resonator pipe shifting about 3 inches every time I pull down on the ratchet to loosen the bolts. Is it necessary to remove anything to gain some more clearance? And is the pipe movement normal? I hit it with a lot of PB Blaster again and will get a torch to it tomorrow so hopefully that will help.

 

OEM 9.8 BOLTS to head, STUDS to exhaust.

 

You only need 45mm thread length to properly secure the manifold to head.