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Cooperative Motor Works

Vintage RV & Motorhome restorations - "The GMC Coop"

You are here: Home / Cracked Intake

Cracked Intake

June 15, 2017

GMC Motorhome, Look, this intake thing is real!

 

Mornin, look…….. I had some updating to do on a couple of projects around here but this came up yesterday that I think will make all of you pause who have not addressed this as yet.

Now after I say what it is don’t blow this off and get some coffee…… you now cannot say you didn’t know about this!  Here it is, class….. this as tough love or whatever but you must see this:

Our 455 and for that matter as I have seen 403 Olds motors used in a GMC motorhome or a Tornado have an affliction.  Naming this nemesis I call it a “cracked intake”.  Yeah, a pretty special name but in the case of our Olds “concave” design intake it refers to a crack you will find on about every intake I see.  Between the large secondary carb ports is a thin wall of metal separating the 2 big holes.  Look hard and you will probably see something like this

 

 

 

OLYMPUS DIGITAL CAMERA
Showing up better being magna-fluxed, this crack is caused by the hot exhaust gasses bending under the carburetor on the concave designed intake.  Take a look at the intake on your motor…..  If you can see the burnt, rusty exhaust path from the left to the right head….. there you are!
 This heat causes hot spots in the intake metal casting causing the metal to crack at the weakest spot.  When this crack reaches the floor inside the intake  Mafnaflux-2 the exhaust gasses below contaminate the air/fuel mix going to the combustion chambers …. and YOU are now on the side of the road!  Yeah, yeah, yeah we’ve all heard this…. let me show you what we ran into yesterday!
So our project is to bring a coach back to the road.  It had been a loved, long owned GMC willed to his daughter who wants to show her family what it’s like to be brought up camping in a GMC Motorhome.  A Low mile 1973 coach, is good overall shape that drove in sounding and looking proud.
Explaining the issue of the crack in the intake possibly causing issues in the future the direction was to pull the intake, block off the exhaust crossovers then install the new FiTech fuel injection system.  Needless to say we would pull the intake and do this anyway but with the FiTech system I feel this is a must.
OK, pulling off the carb, this is what we found  Marlin intake 2   Yep, you can see the crack…. lemme show it to ya before cleaning off the surface!  Marlin intake 1 Look at THAT buildup of carbon and it’s been bubbling out of the crack for a LONG TIME!
Could this be a sign of what’s to come?  Now remember this coach was running fine, drove in and from the owners report drove pretty good!  RIGHT?………. pulling up the intake, look at the bottom of the intake itself  Marlin intake 6  THAT my friend is a huge clump carbon from cooked oil because of this concave intake manifold design!
Oh, but wait… there’s more!  Take a look at the box of Keeblers we have on the tin “turkey tray” gasket that was trying to keep the oil from being cooked!  Marlin intake 5  One small clump of that stuff in your oil passage and the motor is done.  This woman’s Dad is still in this motorhome and has been holding it together for her!  This motor is giving out it’s heart to stay alive!
No…. there’s still more!  Pulling up the tin gasket look at what was hiding under there…….  Marlin intake 4  2… countm… 2 huge carbon bricks!  The entire middle of this motor had been filled with cooked oil carbon!!!!!! And if any small piece of any of that were to flow through the oil passages the outcome would not have been good!
AND THE MOTOR RAN PRETTY GOOD!!!!!!!!!!  No foolin, we were all first impressed on it’s compression and all and thought…. maybe we should let this intake slide, it looks and runs pretty good….. low miles…. are we wasting time and money?
NO… we weren’t and it would be time and money well spent if you are still running around with an intake that looks like this  Paul-cracked-intake-3Note the rusty burn color across the middle……. brother you have now been forewarned!  And like it looks to be the new fad in making a final statement….. I’m throwing the microphone to the ground…………….
Spread the word and we’ll see ya tomorrow……  Jim working on motor.

 

 

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