Exhaust noise manifold cracks Dorman replacement

[Schurkey]

I searched the Trailblazer forum, got 22 hits none of which seemed to appropriately address this particular issue.

Trailblazer/Envoy/etc 4.2L exhaust manifold cracking is horribly common. Some have problems because the bolts that hold the manifold to the cylinder head have had the bolt heads broken off while the vehicle is being used (prior to putting a wrench on them). Even if the manifold isn't cracked, the gasket will leak if the bolts are broken.

I've heard but not confirmed that the last two years of Trailblazer/Envoy/etc. production have an extended warranty on the exhaust manifolds due to the common nature and high expense of replacing the manifold when the bolts break--$1000 to nearly $1500 has been quoted for non-warranty replacement of the exhaust manifold. This does not help those of us with older vehicles that don't have this extended warranty.


Multiple cracks on back side of manifold


Large crack near boss for O2 sensor

The exhaust manifold is almost completely covered in a two-layer stamped metal heat shield. My 2003 Trailblazer had an exhaust leak around the opening in the shield where the O2 sensor pokes through. The fault started out with a whistle noise (which I thought was a squealing belt at first) that soon progressed to an obvious exhaust leak that got worse and worse. It was typically loudest when first started, but quieted down as the manifold warmed up.

The aftermarket has provided a non-GM "solution" packaged as a "Kit" for 2002--2005 vehicles. This one-part-number (Dorman 674-777) kit includes a new iron manifold, manifold-to-head gasket, exhaust pipe donut, studs, and nuts, and a heat shield with studs and nuts. Everything you need except new manifold-to-head bolts (more on those, later) Sometimes these aftermarket parts are nicely improved, and solved what GM, Ford, Chrysler, etc. couldn't figure out. I'm not sure that's what happened in this case. While the "kit" concept is very nice, there are quality issues. Time will tell if this is a good choice.

The Dorman web page for the 2002--2005 part is here:
http://www.dormanproducts.com/itemde...SEName=674-777
A similar product for 2006-2007 vehicles is available under Dorman part number 674-990
http://www.dormanproducts.com/p-6727...origin=keyword
and for 2008--2009 vehicles, the part number is 674-869.
http://www.dormanproducts.com/p-5118...origin=keyword


I got my replacement manifold kit from Amazon for $123 shipped to my home. It is packaged poorly.



The new exhaust donut was damaged in transit. The existing donut looked fine, I re-used it after bending the little tiny spring tabs that keep it secure in the exhaust manifold while the pipe is getting bolted-up. The Dorman-supplied donut doesn't even have these spring tabs.



The Dorman manifold is very similar--but not identical--to the original GM item. In general, the casting is of lesser detail and quality, all the GM foundry marks are MIA as I'd expect; and the machining is rougher.

(Continued)

[Schurkey]

Note the poor casting and rough machining. (It looks like the Chinese "machined" it by kicking it across a concrete floor.) The casting flaw doesn't actually matter as long as it doesn't promote cracking. The rough machining is a disappointment. If I had been better prepared, and with more time, I'd have had this Dorman manifold planed for straightness and surface finish. This manifold was about .006 out-of-true. It's borderline for acceptably flat, and probably rougher-than-acceptable to boot.



Even internally, the casting and the machining is not up to GM standards. The internal vanes are not shaped as nicely as GM; and the machining for the donut is missing the chamfer. Neither is a deal-breaker.



Another source of cost-cutting is the heat shield. The original GM shield has the inner layer trapped by the outer layer by folding the outer layer around the whole circumference of the inner layer, as seen on the lower shield. The Dorman shield has just a few tabs on the outer layer folded over to secure the inner, as seen on the upper shield. I also had problems getting the Dorman shield to clear the casting projections on the cylinder head above the two rows of bolts. Nothing some prying and bending with pliers couldn't fix--but I shouldn't have to.


While I'm peeved that the new manifold is .006 out-of-true, the old one is many times worse. You can see daylight between the bottom of the straightedge and the manifold at both the #1 and #6 (pictured) runner. I suspect that these manifolds move around a lot due to heat expansion, and eventually they warp and crack from the temperature changes. This also explains why all but one of the bolt-holes are so huge--so the manifold can slide back and forth under the bolt heads as it heats and cools.



Of course, of the eleven bolts holding the manifold to the head, only nine came out without breaking. Perhaps I'm lucky--others have had more broken bolts than I did. The broken bolts had nuts welded to them, and then they turned out. You can also see the Lang re-threading tap I used to assure that the holes in the head still had usable threads. This tap isn't long enough to go to the bottom of the holes--but it would thread in readily, and didn't pull any metal out. The threaded holes were just fine.


One bolt broke leaving about 1/4"--3/8" of stud sticking out of the casting. I turned a 5/16 nut onto those threads, welded the nut to the stud, and it turned out with some effort. The other bolt actually screwed out just as far, proving it wasn't seized to the head--but after turning out about 3/8", it then broke flush with the casting--in the rearmost bolt hole, so it was moderately difficult to get at! I had to add weld material to the stud so I had something to hang the nut on. I welded almost a dozen 5/16 nuts to that stud, and they all broke off. I welded a 3/8 nut to it, and it held. Again, the bolt turned right out, but took a lot of torque to do so.






Original bolts are grade 9.8. I bought replacement bolts of a superior strength. The new bolts are probably as Chinese as the manifold, so there's no guarantee that they were made to spec. My local hardware store didn't have eleven of either kind, so I took all nine of the regular bolts, and added two flange-heads to make eleven. One potential problem if this ever has to come apart, is that the regular bolts use a 13mm socket, while the flange-head bolts use a 12mm socket. I hope I remember that a hundred-thousand miles from now! Each 35mm-long replacement bolt was just slightly longer than the originals, so I added a hardened ("Grade 8") 5/16 flat washer of sufficient thickness to make up the difference. These bolts would need a washer anyway, due to the oversized (by design) holes in the manifold. Only one hole in the manifold is "normal" size--all the rest are huge. The new bolts were all installed with Permatex copper-based anti-seize. GM specifies a thread-locking compound--which is why these bolts turn so hard, all the way out. they're chemically locked in place. No amount of penetrating oil will make a difference, the oil can't get past the chemical thread locker to do any good. The thread-locker will prevent the bolts from corroding, but the anti-seize I used instead should make removal easier since the bolts won't be locked in place. [MUCH, MUCH LATER EDIT] DO NOT USE ANTI-SEIZE ON THESE BOLTS. THEY COME LOOSE IN SERVICE, WHICH IS WHY GM WANTS THREADLOCKER[/MUCH, MUCH LATER EDIT] I torqued the manifold-to-head bolts to the GM spec of 15 ft/lbs. GM says to make three passes, each at 15 ft/lbs. I made a first pass at about 10 ft/lbs, and then three at 15. Before I put the heat-shield on, I ran the engine for ten minutes, let it cool and then made a final pass at 15 ft/lbs. Only a few of the bolts turned, and none turned very much.


The Dorman kit comes with exhaust pipe studs and steel nuts. I simply refuse to install steel nuts on exhaust flanges, so I bought two packages of studs and brass nuts just to get three metric brass nuts. Brass won't corrode to steel, so removal later is all but assured.

Other manifold replacement notes:

I really believe that the biggest reason the manifold bolt heads sometimes have popped off before even putting a wrench to them, is because the manifold moves around a lot due to expansion and contraction. As the manifold expands, it puts sideways pressure on the bolts, and I don't think the original bolts have acceptable fatigue strength.

All of my bolts still had the heads on them, and in fact even the ones that broke, didn't break until I'd turned them out about 1/4" or more. I think that if I'd used a swivel-socket so that no side-pressure went on the bolt from the excess force needed to turn them, that they might not have broken at all. SUPPORT YOUR RATCHET HEAD, don't let the ratchet or wrench push the bolt sideways as you turn them out.

Be sure to install the manifold gasket properly. The manifold has one hole that is just the size of the bolt. The gasket has one hole that is similarly small. Put the small hole in the gasket so it's aligned with the small hole in the manifold.

There's an air conditioning pipe-and-hose that goes across the engine. I held it up and somewhat out-of-the-way with a 4x4 chunk of lumber under the pipe, and on top of the engine plenum. It won't go much higher due to the way it's bolted to the air conditioning accumulator.

I removed the air cleaner and the windshield washer fluid reservoir. Easy enough to do, and it opens up a lot of area under the hood.

I used anti seize when I re-installed all the threaded hardware on this project, including the heat-shield studs and nuts, and the studs and nuts that secure the dipstick tubes. The heat-shield hardware seemed very weak. I did NOT torque the heat shield studs and nuts to the GM spec, because they got "rubbery" at about half the torque GM wanted. More cost-cutting by Dorman.

Short Story: The Dorman exhaust manifold kit has certain issues; poor machining being the most important. I have no idea how long the Dorman manifold may last. Maybe it's better than the GM manifold, maybe it's worse. On the other hand, considering how common this problem is; and how many of us have broken bolts in addition to cracked manifolds, even GM screwed the pooch--I can't recommend spending double or triple (or more) for Genuine GM parts in this situation since we know the GM failure rate is sky-high. The Dorman part is supposedly "Lifetime Warranty"; but that's for the parts--not the labor. When bolts break, the labor is the lion's share of the expense and headache.

I had forgotten how wonderfully quiet this vehicle is when the exhaust is properly sealed-up!

[j cAT]

On the mounting of the manifold to the engine I would use 1/8inch thick stainless steel washers under the manifold bolts. smooth the surface that the washer will sit on . this will allow the manifold to move slightly. also do a 50% torque on the bolts run engine then torque to slightly below the torque spec like 5 FT LBS. dry torque !

[DeltaP]

The Dorman hardeware kit, packaged separately, comes with the washers attached to the bolts,(steel). A nice days work there!

[Schurkey]

Originally Posted by DeltaP

The Dorman hardeware kit, packaged separately, comes with the washers attached to the bolts,(steel).

These guys:
http://www.amazon.com/Dorman-03413B-...s=03413+Dorman

The larger photo shows them to be Grade 10.9, similar to the ones I bought. Less expensive, too. I'd have bought them, if I'd known they existed before I replaced my manifold.

Unlike the GM bolts, these appear to have no threadlocking compound.

[andrew321]

I just ordered a dorman replacement for my 97 dodge dokata 2.5 original had two cracks in it for awhile. gonna find out how she cmpairs to the old one soon

[Schurkey]

Almost exactly five years later. I'm not certain of the miles--I don't think I recorded the mileage when I replace the manifold in '14; but I suppose about 80K since then. If I can confirm the mileage I'll add it to a later post.

I've got excess exhaust noise and not all of it is coming from the exhaust pipe joint ahead of the muffler. Yup, I think the manifold is cracked again. And the heat shield is a mess.

I've been in contact with Dorman, and with Amazon. Dorman says these manifold kits are "Lifetime Warranty". Supposedly, there's a warranty-replacement manifold kit on it's way to me. I'm not seeing it on my Amazon orders page, though.

I have "30 days" to remove and replace the manifold. Amazon expects me to ship the old one back to them; and they'll provide a prepaid address label to handle the return.

I guess I'm about to find out how well that anti-seize works. I can do this...but I really didn't want to. At least I know to have the "new" manifold planed by a machinist before installation.

New O2 sensor ordered as well. That's on me, I'm paying for it. I can't see going to this much work and not stuffing a $40 sensor in the thing while I'm in there. I may order the Dorman hardware kit, too. Haven't decided for sure.

[Blue Bowtie]

I remember this well-detailed thread, and wondered if/when you would get to experience the benefits of your careful assembly. It seems that you will. Hopefully the anti-seize does its job.

[j cAT]

the exh manifold should be with slotted holes so the metal can move on cold/hot.. do the proper torque procedure. also I do the torque 3x .. low med then the spec torque.. I start engine and run it with the low torque so the manifold is not gonna bind up .. then when at normal temp , continue with the torque spec..

[Schurkey]

Originally Posted by Blue Bowtie

I remember this well-detailed thread, and wondered if/when you would get to experience the benefits of your careful assembly. It seems that you will. Hopefully the anti-seize does its job.

Yes, anti-seize prevented broken bolts (but I also used a swivel socket to ensure no side-force on the bolts as I removed them.) Some were a little bit stiff as they turned out, but not nearly as bad as the original bolts.

Originally Posted by j cAT

the exh manifold should be with slotted holes so the metal can move on cold/hot.

All but one bolt hole are either slotted or oversized. the one standard-sized hole in the manifold needs to be matched to the smaller hole in the gasket. That's also the reason for the three-layer steel gasket between manifold and head--so the layers can slide around as the manifold expands.

Originally Posted by j cAT

do the proper torque procedure. also I do the torque 3x .. low med then the spec torque.. I start engine and run it with the low torque so the manifold is not gonna bind up .. then when at normal temp , continue with the torque spec.

More-or-less what I did both times.

I also ran the engine before installing the heat shield, so that I could go around the bolts while it was hot. My service manual says "18 ft/lbs" in one place, and "15 ft/lbs" in another. I went with 15.

I should probably pop the heat shield off again and go over the bolts now that it's got a few hundred miles on it.

I did order the Dorman bolt set; the bolts are a touch shorter than what I used before. I think the hardware-store bolts bought the last time were a little longer than the originals.


For the record, the newest Dorman manifold is somewhat different from the previous Dorman manifold. The cast-in numbers and letters, and the Dorman trademark are in different positions compared to previously. I don't know if this represents an entirely new mold, or merely touch-up and repairs to the previous mold. For all I know, Dorman is now contracting with an entirely different company in China to cast the manifold. The heat-shield is noticeably different, too. They're down to only four tabs holding the layers together. The heat shield I took off was pretty-well totalled; big sections of the outer layer gone, and the thing was totally rusted below the O2 sensor.



I had to use Amazon "Live Chat" twice; once to assure them that I'm the same guy that bought the thing five years ago (address change and credit-card number change); and the second time to straighten-out the return of the old parts. I'm still not sure that Amazon has this right, so they may be charging me for the warranty replacement on the 15th; in which case I'll have to deal with them some more. Some companies e-mail a transcript of their "chat" sessions, but not Amazon. I told the person that I was taking screenshots of the chat session, but they closed the chat window before I was done. At least Amazon has records of the chat sessions, even if mine are incomplete.

[Schurkey]

Yet Another Update.

The first Dorman manifold was replaced with the second Dorman manifold, as detailed above.

The second Dorman manifold was replaced last weekend with the THIRD Dorman manifold. This one was not provided under warranty, as it may have been my fault that the exhaust leaked. Note that the second manifold wasn't cracked, just leaking at the gasket and the donut.

1. It is a mistake to use anti-seize on the manifold-to-cylinder head bolts. I have learned my lesson on that. Upon removing the heat shield, I had two bolts that were barely finger-tight, and a third--the very front manifold bolt--mostly unthreaded, almost ready to fall out. Huge exhaust leak at the #1 runner. The third manifold went back on with fresh Dorman bolts, coated in "Blue 242 medium-strength thread locker"; and the same 15 ft/lbs with multiple torque-wrench passes as before.

2. I have been having repeat problems with the donut gasket failing. Some of this may be due to the somewhat poor-fitting aftermarket exhaust pipe and catalyst I installed. The pipe is sold as a direct-fit, but in fact the bends on the pipe are not quite correct, leading to some misalignment. The bigger problem may be that I was not torquing the three nuts on the exhaust manifold outlet studs. I've been concerned that I'd bend the flange permanently attached to the aftermarket pipe/catalyst. This time I put the nuts at the service-manual specified torque of 38 ft/lbs. That's tighter than I have been going, and hopefully solves the loose-nuts-burned-donut problem.

3. I had my favorite machine shop plane the manifold at the gasket surface, AFTER they loaded it into their parts-cleaning oven for a few heat cycles to settle the casting. Dorman must plane these manifolds while still warm from casting; by the time the consumer takes them out of the box they're badly warped, and the original Dorman machining is rough, not smooth like it should be.

4. I refused to use the craptastic Dorman heat shield. The Dorman heat shield is a piece of junk. I bought a Genuine GM heat shield from the local Stealership. Test-fit the shield to the manifold, assure you have tool-clearance around the O2 sensor. I had to tweak the heat shield to get my O2 sensor socket to fit properly.

My '03 Trailblazer is back together, wonderfully quiet. Given the added expense of the machine shop labor, and the GM heat shield, I probably doubled the cost of the manifold kit. But my labor is free other than "shop supplies" and the electricity to run the air compressor to power the air ratchet. And beer. So I come out alright.

[j cAT]

[quote=Schurkey;7236675]Yet Another Update.

The first Dorman manifold was replaced with the second Dorman manifold, as detailed above.

The second Dorman manifold was replaced last weekend with the THIRD Dorman manifold. This one was not provided under warranty, as it may have been my fault that the exhaust leaked. Note that the second manifold wasn't cracked, just leaking at the gasket and the donut.

[ QUOTE ]

The manifold is the bolt holes . the portal hole of the bolts .. the various of the larger port holes .. grinding it ...manifold is the blue med locker ..
now the china is the crap parts ..

[Schurkey]

My machinist tells me that the Dorman exhaust manifolds made in Viet Nam are actually worse than the ones made in Communist China.

They must have twelve-year-olds pouring the molten iron into molds made by fourteen year olds, 'cause otherwise I don't know how they would screw 'em up worse than the Chinese.

But--I have to admit--GM didn't do such a fabulous job on the Atlas 6-popper exhaust manifolds, 'cause they failed so universally that Dorman had enough of a business case for knocking them off.