Was gonna call this “Got Crack” but I felt that a bit too much. Yesterday I did a test drive in a coach preparing it to go home. The coach has been running great pulling it around the shop…..keeping it plugged in and all.
As I pointed out before we really want to shake a coach down before it leave if we can so yesterday, I fired that mother up for a drive. Let off the brake…mash the gas…got 1 to2 shift…3rd kicked in and things are going well….for @ 2 miles and the motor lost power and shut down. CAPCON…we have an issue!!
Dropping into neutral the motor fired right back up…put her into gear and it died again. Hmmmmm….flashers on I pulled into a sidestreet for a little more investigation. The thing fired right back up, dropped into gear mashed the gas and goose egg……motor dies.
I let the coach sit for a couple of minutes, she fired right up..put it in drive, idle was way off so keeping my foot in it I started the struggle to get back to the shop. As long as I pumped the gas…..making the accelerator pump squirt….I could make my way at 10mph or so. The motor would die….. I’d fire it up and pump along a little more til she died again.
What is this? It’s not fuel related…I had fuel squirting…it’s not spark , she fired right back up every time…………. It’s vacuum related, putting a vacuum gauge I had @ 12″ of vacuum at idle, putting the coach into gear we only had a rough needle touching 5″. Your motor running…shifter in park with your dash AC off there should be @ 18″ of vacuum and shifting into drive vacuum should only drop 2-4″ with a smooth needle action.
This puppy has a pretty big constant vacuum leak…….Checking the intake I looked for open vacuum lines, maybe a loose base gasket on the carb…. spray WD40 on the intake seal listening for any idle change………looking to where the vacuum was escaping…..nothing. What is it?
You have probably heard of this before….an intake crack but do you understand why and what causes ths in our motorhomes. For those that know all about this go for some more coffee…but wait, maybe you could learn something so stick around.
Not only is our GMC a unique motorhome, they also sport a very unique intake manifold. Unlike this aftermarket Edelbrock, he carb. is actually sitting lower than the heads (note the downturn passages) bending hot air from side to side inside the intake heats up the center of the intake manifold big time. You can see a rust mark from side to side on the middle of your intake. This one is rusted and pitted from the heat
Whe metal gets hot it expands, when one section of a cast metal part gets hot while another area is cooler the metal could crack and that’s pretty much what happens on that this wall between the 2 large secondary parts. Pull off your carb..look carefully and you may see this That crack continues and once it reaches the floor inside it will allowhot exhaust gasses to mix with your intake vacuum. When your intake vacuum dips below 5″….in other words when you mash on the gas….when you go below 5″ your motor will simply shut off. No fuel is being socked into the cylinders.
EVERY intake used in a GMC Motorhome…even a 403 intake is susceptible to this….it had to be an error from GM…if they would have made this again they would have changed it…but it took many years for this issue to show up….they probably didn’t know about this. GM was proud to announce the GMC Motorhome was built for as much as a 15 year life span….yup and what year is it now? These are things we are finding out as time marches over our dreams.
Every GMC intake manifold can and probably does have this problem and you may not know it. You WILL when your crack gets long enough! So, what to do? Do we throw that baby out and get some new bath water?
Over the years there have been fixes….an aluminum intake copy without this crossover was made. Hard to find them and being a small production parts sometimes it may not fit perfect. Time being the money sucking pig of the world…….. taking the time to fit the intake is not a good choice for us.
After cleaning the intake up…having it dipped then checked for cracks…if the only one is the one we know of….no other cracks……. Installing this intake with a fiber (Mr. Gasket 404 for the 455 motor) and a pair of stainless steel blockoff plates like these We make our own now…..far less cost…… the exhaust crossover will be blocked off and even if the intake swells up from heat exhaust gasses will be stopped by the plates and your problem will be solved.
By this time, this project is pretty much a must. You may be running along fine…this crack forms and you will not be fine….and this will sooner or later happen to EVERY 455 and 403 motor. It’s not if but when!
Cost for this project is 1 guys work for a day…say 6-8 hours labor…….a Mr. Gaskets 404 gasket for the 455, a tube of Permatex Ultra-grey RTV sealant, a pair of the SS blockoff plates ……. I change the 180 degree thermostat any time I open the cooling system (any brand but a Robertshaw…ask me why). That’s it….@ $100 in parts, a few cusswords oh and @ $120 for a machine shop to clean and magnaflux the intake.
When you purchase one of these coaches to have and hold….I would be sure this project is taken care of right away. If you have a rusty burn mark through the middle of your intake…..Bob’s your Uncle!
So we pulled off Ben’s intake yesterday, it’s at the machinist getting cleaned up so maybe tomorrow we’ll put it all back together
I hope this helps you understand if you had not already this really is not an elective project but one everyone needs to have done. Yes…I mean everyone!
It’s after 8am…I’m milking the clock…need to get out there and work….see ya tomorrow