GREAT. ANOTHR FIRE in my GMC

B

Bob Dunahugh

New member
Sep 17, 2012
2,784
4
3
Heading back from Chicago. I started to hear an exhaust leak. maybe? Finall=
y pulled into a rest area to see if a band clamp had come loose. Everything=
looked solid, and clean. Then went about another 60 miles. As it seemed t=
o be increasing in sound. But not bad. Seams on the mufflers looked great.=
And the muffler was nice, and shiny. Back on the road again. About 50 mi=
les more. A trucker came along, and started flashing his lights. As he went=
by. He turned on his inside lights so I could see him pointing down. I was=
on I-80 in Port Byron Ill. I was just coming up on a lighted off ramp. Fo=
und that the aluminum on the bottom of the floor was solid. But got hot eno=
ugh to get the insulation to burn. Then the floor plywood. A police officer=
showed up. I got the smoldering material out with some water. The officer=
called the fire department just to check things out. Good idea. I pushed =
the aluminum down to get an air space between the insulation, and aluminum.=
That kept the heat away. So I could make it my last 90 miles home. =
I used band Clamps. So when I got home. It only took about 10 minutes to=
get a 9/16th wrench out, and remove the muffler. It seems that harmonics =
had built up in the top side of the muffler. And the muffler's two outer la=
yers simple started cracking. Then pieces of the double outer shell fell a=
way. So this pointed the exhaust directly at the floor. I have photos if an=
yone's interested. So don't make the assumption that I did. That the alumin=
um skin will protect your floor against fire. Or that a muffler that looks=
new is always fine. I'm adding a stainless steal heat shield above both mu=
fflers. Bob Dunahugh 78 Royale
 
Didn't GMC has shielding over the mufflers, stock? :) Is there an
aftermarket source or do you have to make them?

Sammy

GMCless in Charleston. SC

> Heading back from Chicago. I started to hear an exhaust leak. maybe?
> Finally pulled into a rest area to see if a band clamp had come loose.
> Everything looked solid, and clean. Then went about another 60 miles. As
> it seemed to be increasing in sound. But not bad. Seams on the mufflers
> looked great. And the muffler was nice, and shiny. Back on the road
> again. About 50 miles more. A trucker came along, and started flashing his
> lights. As he went by. He turned on his inside lights so I could see him
> pointing down. I was on I-80 in Port Byron Ill. I was just coming up on a
> lighted off ramp. Found that the aluminum on the bottom of the floor was
> solid. But got hot enough to get the insulation to burn. Then the floor
> plywood. A police officer showed up. I got the smoldering material out
> with some water. The officer called the fire department just to check
> things out. Good idea. I pushed the aluminum down to get an air space
> between the insulation, and aluminum. That kept the heat away. So I could
> make it my last 90 miles home.
>
> I used band Clamps. So when I got home. It only took about 10 minutes
> to get a 9/16th wrench out, and remove the muffler. It seems that
> harmonics had built up in the top side of the muffler. And the muffler's
> two outer layers simple started cracking. Then pieces of the double outer
> shell fell away. So this pointed the exhaust directly at the floor. I have
> photos if anyone's interested. So don't make the assumption that I did.
> That the aluminum skin will protect your floor against fire. Or that a
> muffler that looks new is always fine. I'm adding a stainless steal heat
> shield above both mufflers.
>
> Bob Dunahugh 78 Royale
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
73 and 74 (I think) did not have any kind of shielding. It's just 1" plywood above the mufflers. When I replaced my exhaust recently I put a couple of large sheet pans above the mufflers.
Glad the trucker was able to see you were smoking and you could put it out before things got too crazy.
Jared Kohl
Rappahannock County, VA
1973 Painted Desert "Onslow"

Didn't GMC has shielding over the mufflers, stock? :) Is there an
aftermarket source or do you have to make them?

Sammy

GMCless in Charleston. SC

> Heading back from Chicago. I started to hear an exhaust leak. maybe?
> Finally pulled into a rest area to see if a band clamp had come loose.
> Everything looked solid, and clean. Then went about another 60 miles. As
> it seemed to be increasing in sound. But not bad. Seams on the mufflers
> looked great. And the muffler was nice, and shiny. Back on the road
> again. About 50 miles more. A trucker came along, and started flashing his
> lights. As he went by. He turned on his inside lights so I could see him
> pointing down. I was on I-80 in Port Byron Ill. I was just coming up on a
> lighted off ramp. Found that the aluminum on the bottom of the floor was
> solid. But got hot enough to get the insulation to burn. Then the floor
> plywood. A police officer showed up. I got the smoldering material out
> with some water. The officer called the fire department just to check
> things out. Good idea. I pushed the aluminum down to get an air space
> between the insulation, and aluminum. That kept the heat away. So I could
> make it my last 90 miles home.
>
> I used band Clamps. So when I got home. It only took about 10 minutes
> to get a 9/16th wrench out, and remove the muffler. It seems that
> harmonics had built up in the top side of the muffler. And the muffler's
> two outer layers simple started cracking. Then pieces of the double outer
> shell fell away. So this pointed the exhaust directly at the floor. I have
> photos if anyone's interested. So don't make the assumption that I did.
> That the aluminum skin will protect your floor against fire. Or that a
> muffler that looks new is always fine. I'm adding a stainless steal heat
> shield above both mufflers.
>
> Bob Dunahugh 78 Royale
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

>
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Could you by chance have had a partially plugged muffler or were these new ones?

Happy it wasn't any more serious than it was.

> 73 and 74 (I think) did not have any kind of shielding. It's just 1" plywood above the mufflers. When I replaced my exhaust recently I put a couple of large sheet pans above the mufflers.
> Glad the trucker was able to see you were smoking and you could put it out before things got too crazy.
> Jared Kohl
> Rappahannock County, VA
> 1973 Painted Desert "Onslow"
>
>

>
>
> Didn't GMC has shielding over the mufflers, stock? :) Is there an
> aftermarket source or do you have to make them?
>
> Sammy
>
> GMCless in Charleston. SC
>

>
>> Heading back from Chicago. I started to hear an exhaust leak. maybe?
>> Finally pulled into a rest area to see if a band clamp had come loose.
>> Everything looked solid, and clean. Then went about another 60 miles. As
>> it seemed to be increasing in sound. But not bad. Seams on the mufflers
>> looked great. And the muffler was nice, and shiny. Back on the road
>> again. About 50 miles more. A trucker came along, and started flashing his
>> lights. As he went by. He turned on his inside lights so I could see him
>> pointing down. I was on I-80 in Port Byron Ill. I was just coming up on a
>> lighted off ramp. Found that the aluminum on the bottom of the floor was
>> solid. But got hot enough to get the insulation to burn. Then the floor
>> plywood. A police officer showed up. I got the smoldering material out
>> with some water. The officer called the fire department just to check
>> things out. Good idea. I pushed the aluminum down to get an air space
>> between the insulation, and aluminum. That kept the heat away. So I could
>> make it my last 90 miles home.
>>
>> I used band Clamps. So when I got home. It only took about 10 minutes
>> to get a 9/16th wrench out, and remove the muffler. It seems that
>> harmonics had built up in the top side of the muffler. And the muffler's
>> two outer layers simple started cracking. Then pieces of the double outer
>> shell fell away. So this pointed the exhaust directly at the floor. I have
>> photos if anyone's interested. So don't make the assumption that I did.
>> That the aluminum skin will protect your floor against fire. Or that a
>> muffler that looks new is always fine. I'm adding a stainless steal heat
>> shield above both mufflers.
>>
>> Bob Dunahugh 78 Royale
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org

--
Ray Erspamer
78 Royale - "The Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMCRoyale
414-484-9431
Web Site: http://ray-lisa.page.tl/
 
Bob, I used a large oil drip pan from Walmart, mounted it to the plywood above the mufflers with 1" spacers, then filled the space with expanding
firestop foam (available at lowes)..I'm glad everything is ok.
--
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.

Colonial Travelers
 
Bob nice presentation at MCACN. Glad you caught it in time. Are these the NAPA 18425 or the Walker sound fx 18425 presumably the same part retagged?
I was about to go with these but now in question. It usually takes a backfire to do the "exploding cigar" but you think the metal resonant "oil
canned" till it fatigued? Also you stated theses are dual skin mufflers but in the picture they don't look like it.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First
 
Bob, whoever said lightning doesn't strike twice in the same place didn't
own a GMC. We are glad that the damage wasn't worse. I belive Dan Gregg had
something similar happen to his coach. Do you have blocked cross overs and
dual mufflers? There really needs to be a good deal of space between the
exhaust system and floors. Many use stand offs and sheet metal shields
under there.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

Bob nice presentation at MCACN. Glad you caught it in time. Are these the
NAPA 18425 or the Walker sound fx 18425 presumably the same part retagged?
I was about to go with these but now in question. It usually takes a
backfire to do the "exploding cigar" but you think the metal resonant "oil
canned" till it fatigued? Also you stated theses are dual skin mufflers
but in the picture they don't look like it.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First

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Unsubscribe or Change List Options:
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The problem with the aluminum that's there from the factory. It's up in con=
tact with the insulation. That caused the insulation to burn. Then the plyw=
ood. Thus there's no heat protection. I need to install a heat shield with =
an air gap. Bob Dunahugh ________________________________ From: =
Bob Dunahugh Sent: Tuesday, November 22, 2016 11:5=
4 PM To: gmclist Subject: GREAT. ANOTHR FIRE in my GMC=
Heading back from Chicago. I started to hear an exhaust leak. mayb=
e? Finally pulled into a rest area to see if a band clamp had come loose. E=
verything looked solid, and clean. Then went about another 60 miles. As it=
seemed to be increasing in sound. But not bad. Seams on the mufflers look=
ed great. And the muffler was nice, and shiny. Back on the road again. Ab=
out 50 miles more. A trucker came along, and started flashing his lights. A=
s he went by. He turned on his inside lights so I could see him pointing do=
wn. I was on I-80 in Port Byron Ill. I was just coming up on a lighted off =
ramp. Found that the aluminum on the bottom of the floor was solid. But go=
t hot enough to get the insulation to burn. Then the floor plywood. A polic=
e officer showed up. I got the smoldering material out with some water. Th=
e officer called the fire department just to check things out. Good idea. =
I pushed the aluminum down to get an air space between the insulation, and =
aluminum. That kept the heat away. So I could make it my last 90 miles home=
. I used band Clamps. So when I got home. It only took about 10 m=
inutes to get a 9/16th wrench out, and remove the muffler. It seems that h=
armonics had built up in the top side of the muffler. And the muffler's two=
outer layers simple started cracking. Then pieces of the double outer she=
ll fell away. So this pointed the exhaust directly at the floor. I have pho=
tos if anyone's interested. So don't make the assumption that I did. That t=
he aluminum skin will protect your floor against fire. Or that a muffler t=
hat looks new is always fine. I'm adding a stainless steal heat shield abov=
e both mufflers. Bob Dunahugh 78 Royale
 
My first walker sound effects after installing HEI and "forgetting" olds counterclockwise dizzy rotation...single wall for sure...boy did my ears
ring...neighbors were concerned as well :roll:

http://www.gmcmhphotos.com/photos/data/6323/medium/image103.jpg

--
Sean and Stephanie
73 Ex-CanyonLands 26' #317 "Oliver"
Hubler 1-Ton, Quad-Bags, Rear Disc, Reaction Arms, P.Huber TBs, 3.70:1 LSD Honda 6500 inverter gen.

Colonial Travelers
 
I had those Walker dual mufflers. One of them burned a hole in the top of it the size of a softball. I drove it home from Minneapolis (about 400
miles) that way. I do have the GM aluminum floor and received no heat damage to the floor in 400 miles. I did drive with ear plugs as the noise was
unbearable.

I replaced the whole system with a single rear muffler with a heat shield on it.

I worked on Dan Greg's coach in Jim K's shop after he burned a hole in the floor above one muffler. We found a piece of scrap aluminum in Jim K's
junk pile and covered the bottom of the floor above the mufflers. I also got him some mortar mix at the hardware store to fill in the hole from
above. I never knew how or if he finished off fixing the hole.

His coach did not have the GM installed aluminum floor.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
This is good to know. GM uses dual wall construction for a reason. I don't think I will go with these single wall units. $32 you get what you pay
for. Cinnabar has a stock replacement for double the price. I will see if they are application specific dual wall or just a reseller of the Walker.
If single wall, I will go to my muffler manufacturer source and see if I can get something custom made as I have in the past. A genuine GM assembly
line muffler, even if in bad condition would be of great help if anyone has one for external dimmensions and internal baffeling design type.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First
 
These mufflers had about 3000 miles on them. And were double wall. Weren't =
cheep mufflers. The left one is fine. Made in USA. Bob Dunahugh _=
_______________________________ From: Bob Dunahugh =
Sent: Wednesday, November 23, 2016 9:58 AM To: gmclist=
Subject: RE: GREAT. ANOTHR FIRE in my GMC The problem with the a=
luminum that's there from the factory. It's up in contact with the insulati=
on. That caused the insulation to burn. Then the plywood. Thus there's no h=
eat protection. I need to install a heat shield with an air gap. Bob Dunah=
ugh ________________________________ From: Bob Dunahugh Sent: Tuesday, November 22, 2016 11:54 PM To: gmclist=
t.gmcnet.org Subject: GREAT. ANOTHR FIRE in my GMC Heading back f=
rom Chicago. I started to hear an exhaust leak. maybe? Finally pulled into =
a rest area to see if a band clamp had come loose. Everything looked solid,=
and clean. Then went about another 60 miles. As it seemed to be increasin=
g in sound. But not bad. Seams on the mufflers looked great. And the muff=
ler was nice, and shiny. Back on the road again. About 50 miles more. A tr=
ucker came along, and started flashing his lights. As he went by. He turned=
on his inside lights so I could see him pointing down. I was on I-80 in Po=
rt Byron Ill. I was just coming up on a lighted off ramp. Found that the a=
luminum on the bottom of the floor was solid. But got hot enough to get the=
insulation to burn. Then the floor plywood. A police officer showed up. I =
got the smoldering material out with some water. The officer called the fi=
re department just to check things out. Good idea. I pushed the aluminum d=
own to get an air space between the insulation, and aluminum. That kept the=
heat away. So I could make it my last 90 miles home. I used band =
Clamps. So when I got home. It only took about 10 minutes to get a 9/16th =
wrench out, and remove the muffler. It seems that harmonics had built up i=
n the top side of the muffler. And the muffler's two outer layers simple st=
arted cracking. Then pieces of the double outer shell fell away. So this p=
ointed the exhaust directly at the floor. I have photos if anyone's interes=
ted. So don't make the assumption that I did. That the aluminum skin will =
protect your floor against fire. Or that a muffler that looks new is always=
fine. I'm adding a stainless steal heat shield above both mufflers. B=
ob Dunahugh 78 Royale
 
Bob,

I'd like to update the Fire Prevention document we created that's on the GMCES website and would like to make recommendations on how
to prevent what occurred in your. I've copied and pasted some parts of your emails below and have questions.

1) I started to hear an exhaust leak. maybe? Finally pulled into a rest area to see if a band clamp had come loose. Everything
looked solid, and clean.

What did you hear when you were checking the exhaust?

2) It seems that harmonics had built up in the top side of the muffler.

This is not clear to me, are you saying that sound waves caused the double skins of your muffler to vibrate and wear a hole through
them?

3) The problem with the aluminum that's there from the factory. It's up in contact with the insulation. That caused the insulation
to burn. Then the plywood. Thus there's no heat protection. I need to install a heat shield with an air gap.

How close were / are your mufflers to the aluminum sheet?

4) Do you have any photos of the failed muffler?

5) What brand was the muffler?

6) Where did you buy it?

Regards,
Rob M.
The Pedantic Mechanic
USAussie - Downunder
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Bob Dunahugh
Sent: Thursday, November 24, 2016 5:34 AM
To: gmclist
Subject: Re: [GMCnet] GREAT. ANOTHR FIRE in my GMC

These mufflers had about 3000 miles on them. And were double wall. Weren't cheep mufflers. The left one is fine. Made in USA. Bob
Dunahugh

________________________________
From: Bob Dunahugh
Sent: Wednesday, November 23, 2016 9:58 AM
To: gmclist
Subject: RE: GREAT. ANOTHR FIRE in my GMC

The problem with the aluminum that's there from the factory. It's up in contact with the insulation. That caused the insulation to
burn. Then the plywood. Thus there's no heat protection. I need to install a heat shield with an air gap. Bob Dunahugh

________________________________
From: Bob Dunahugh
Sent: Tuesday, November 22, 2016 11:54 PM
To: gmclist
Subject: GREAT. ANOTHR FIRE in my GMC

Heading back from Chicago. I started to hear an exhaust leak. maybe? Finally pulled into a rest area to see if a band clamp had come
loose. Everything looked solid, and clean. Then went about another 60 miles. As it seemed to be increasing in sound. But not bad.
Seams on the mufflers looked great. And the muffler was nice, and shiny. Back on the road again. About 50 miles more. A trucker
came along, and started flashing his lights. As he went by. He turned on his inside lights so I could see him pointing down. I was
on I-80 in Port Byron Ill. I was just coming up on a lighted off ramp. Found that the aluminum on the bottom of the floor was
solid. But got hot enough to get the insulation to burn. Then the floor plywood. A police officer showed up. I got the smoldering
material out with some water. The officer called the fire department just to check things out. Good idea. I pushed the aluminum
down to get an air space between the insulation, and aluminum. That kept the
heat away. So I could make it my last 90 miles home.

I used band Clamps. So when I got home. It only took about 10 minutes to get a 9/16th wrench out, and remove the muffler. It
seems that harmonics had built up in the top side of the muffler. And the muffler's two outer layers simple started cracking. Then
pieces of the double outer shell fell away. So this pointed the exhaust directly at the floor. I have photos if anyone's interested.
So don't make the assumption that I did. That the aluminum skin will protect your floor against fire. Or that a muffler that looks
new is always fine. I'm adding a stainless steal heat shield above both mufflers.

Bob Dunahugh 78 Royale
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
> The problem with the aluminum that's there from the factory. It's up in contact with the insulation. That caused the insulation to burn. Then the plywood. Thus there's no heat protection. I need to install a heat shield with an air gap. Bob Dunahugh

My '73 had no insulation. I was under there for some other reason an
noticed charcoal above the right muffler.

I used cookie sheets above the mufflers. About a 1/2" air gap between
bottom of the "shield" and plywood floor.

http://www.gmcmhphotos.com/photos/muffler-heat-shields/p25883-heat-damage-from-hot-mufflers.html

I have since replace the mufflers, too.

Kelvin
'73 23' in Eugene, OR.
 
When Bob mentioned "harmonics", I'm sure he is referring to resonance of the exhaust pulse that vibrated the top of the muffler shell until it cracked
and then fragmented. I have seen this many times on a multitude of vehicles, all with aftermarket mufflers. It happened to me on a 6 cylinder vehicle
with the 18 inch muffler less than a year after it was replaced. Got it replaced with a 20 inch long muffler and it lasted until I replaced the entire
exhaust system. Most of the other ones I replaced on customer's vehicles due to cracking of the shell were the 18 inch "turbo" mufflers, and for some
reason I have not figured out, they almost always cracked on the top.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.
 
If you want a good source for a cover to protect the floor, go to Walmart in the Auto section near the oil and buy a drip pan. They are approximately
2' by 3' an have a 3/4" or lip so all the way around. Mount them by the out side lip to the floor and you will have a 3/4" or so air gap. You could
also add one extra screw in the center with some stainless washers as an extra stand off if you are worried about it rattling. Those pans are around
$11.00 at Walmart.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Perhaps add a few vents along the leading and trailing edges to let air flow through the gap as well......

Pete

> If you want a good source for a cover to protect the floor, go to Walmart in the Auto section near the oil and buy a drip pan. They are
> approximately 2' by 3' an have a 3/4" or lip so all the way around. Mount them by the out side lip to the floor and you will have a 3/4" or so air
> gap. You could also add one extra screw in the center with some stainless washers as an extra stand off if you are worried about it rattling.
> Those pans are around $11.00 at Walmart.

--
Cary, NC

No Coach yet but decided it will be Definitely wet side bath, 77 or 78, 403, hopefully with good original Imron paint.
 
> Perhaps add a few vents along the leading and trailing edges to let air flow through the gap as well......
>
> Pete
>
>

> > If you want a good source for a cover to protect the floor, go to Walmart in the Auto section near the oil and buy a drip pan. They are
> > approximately 2' by 3' an have a 3/4" or lip so all the way around. Mount them by the out side lip to the floor and you will have a 3/4" or so
> > air gap. You could also add one extra screw in the center with some stainless washers as an extra stand off if you are worried about it rattling.
> > Those pans are around $11.00 at Walmart.

That is a good idea.

It would not take much to add them prior to installing the sheet(s) on the coach. They could be as simple as drilled holes or cut slots.

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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