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1974 Cadillac RWD 472 exhaust diagnose problem question

Started by Scot Minesinger, August 23, 2014, 10:37:26 AM

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Scot Minesinger

Starting a new column, I posted the 1974 EGR question, and based on input it seems more likely that there is a problem with exhaust.

After a backfire at 70mph on highway due to coil failure (coasted to the side of the road safely-no worries) about a year ago the car has lack of power.  The car starts, idles and performs perfect under low loads.  However when you need some power it is not there.  Vacuum systems are good and there is no knocking.  I was thinking it was EGR (before the owner told me it started happening after backfire), but after reading posts and shop manual, the EGR may be fine. 

The car is quiet.  Theory is that the backfire damaged the muffler internally making it more restrictive and robbing power.  There was a post on a similar issue in Auto Restorer about a 1966 Thunderbird that ran hot and lacked power at speed (know one could diagnose it over the internet??!!-everyone focused on water pump and etc.) and it turned out one of the mufflers was clogged with rusted debris, yet the car ran quiet.  Once the muffler was replaced it was good to go. 

My question is how can I tell if the muffler is clogged or restrictive?

Thanks,

Scot
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

"Cadillac Kid" Greg Surfas 15364

Scott,
That depends upon your hearing and familiarity with what the exhaust should sound like. Of course the easiest way is to remove the muffler and see if the performance increases, or short of that listen carefully at the tail pipe for what would be "whistling" at the tailpipe exit when the engine speed is brought up to say 3500 rpm.  Of course if you knew what the back-pressure was supposed to be (and perhaps some one has that information for the forum), you could tap into the head pipe with a test port and see what pressures develop at the same engine speed.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Scot Minesinger

Greg,

I was worried that this would be the answer.  Hate to remove an original muffler on a 40 year old car that is already quiet, you know what a can of worms that is going to be.  If someone has an idea that is more decisive please advise. 

Thanks,

Scot
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

"Cadillac Kid" Greg Surfas 15364

Scott, you should be able to tell by the sound.  Is the mufflers shape distorted?
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

TJ Hopland

How about getting one of yours with a similar engine close by and then starting them both and comparing the sound and maybe temps with an IR gun?   What feels like putting a gloved hand over the tail pipe?

I suppose you could pull the EGR off, rig up a plug for the intake side and see what kind of pressure you get there?
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Scot Minesinger

I will have a similar car without this problem near by (mine has dual exhausts) and can check this out to see if any distinctive differences. 

Greg, what will the sound differences be?

Thanks for helping me out with your experiences on this obscure issue.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

TJ Hopland

If they are blocked they tend to be much and have a more muffled sound to them.  You also won't tend to feel 'pulses' at the tail pipe.   With a V8 and single exhaust its kinda hard to feel the pulses so that isn't the easiest test but may yield some info if you have the option to do an A B comparison.     
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Alan Harris CLC#1513

An engine with a restricted exhaust will have low vacuum at the intake manifold. Try testing it with a vacuum gauge.

Scot Minesinger

Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

TJ Hopland

StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Jon S

Scot -

Curious . . . Do you know if the backfire was exhaust or carburetor?
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

Jon S

The symptons almost sound like a clogged fuel filter.
Jon

1958 Cadillac Sedan De Ville
1973 Lincoln Continental Coupe
1981 Corvette
2004 Mustang GT

TJ Hopland

Running with a screwed up spark would usually be a backfire in the exhaust because of the build up of unburnt fuel.  With lack of spark things still flow and operate fairly normally in the intake side of things.    Carb backfires are usually a timing issue.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Scot Minesinger

the backfire was a one time thing when ignition coil failed.  It has not backfired ever again.  I will check timing
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty

The Tassie Devil(le)

I once identified a muffler problem where the engine would stop on occasions, then restart without problems till the next time it happened.

What happened was that the insides of the muffler had rusted out, and a piece of internal metal would occasionally become dislodged and block the outlet, causing the engine to stall.   Once off, the piece would fall out of the way, and the engine would start right up as if there was nothing wrong.

Any backfire will cause problems, and if you are saying that the 40 year old muffler looks good.   Internally it could, and most probably is a mess.

No 40 year old standard exhaust system is going to be rust-free, unless it hasn't been used from new.   Corrosion is always there with exhausts, and the less a vehicle is used, the more chance of corrosion.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

R Schroeder

As I and Tim stated in your other post, use a vacuum gage to find out if you have a restriction.

Video on how to do it.

https://www.youtube.com/watch?v=d-jp1IIJVVk

TJ Hopland

Interesting video.    I didn't know that without a load the vacuum should remain fairly steady.
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

R Schroeder

I have used a vacuum gage on my cars for 50 years. It will tell you just about anything , if you know how to use it.
I have one presently mounted on my engine. It is there 24/7.
If you have a tool box, you should have one in it.
Some help falls to deaf ears...........ha
Roy

Scot Minesinger

Yes, often a factory vacuum gauge is installed and called a "a fuel economy gauge", at least it was on my 1985 Chevrolet Caprice.

I though it was important to have it on my 100% removable return to factory stock (for shows) gauge cluster in my 1970 Cadillac. 
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty