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7.4 L intake manifolds keep rusting

F

Floatation Device

Contributing Member
My intake manifolds rust of both sides of the carb mid center of the manifolds on both engines. They are stock. What can be done to stop the rust? I have 4 bbl weber carbs.
 
What is you concern? Somr performance issue or mearly cosmetic? If performance what issue are you concerned about? If cosmetic, pull them, have them prepped and powder coated. I think I've seen aluminum intakes for marine engines as well?
 
As noted above, this is likely to just be a cosmetic issue. Not uncommon in high humidity environments where its cool at night and dew forms on exterior surfaces. In the engine compartment, this dew still forms, but instead of burningoff in the sun, condenses and sits in the hollows of the intake manifold. The other possibility is that rain water is getting in thru a seam above the engines in the engine box/deck hatch. I have the same issue and have toyed with the idea of painting the rust spots with a rust conversion solution and then painting with black BBQ rustoleum. Not done this yet... far down the list.
 
There is nothing you can do to stop the rust. That part of the intake gets as hot as the exhaust and it will burn off any coating you put on. You can try some BBQ paint.
 
Powder coating cures at around 400 degrees. If your intake is getting that hot you have other,bigger problems. Some powder coatings will withstand 1000 degrees once cured.
I'm sure BBQ paint won't do that. It is much cheaper and easier though.
 
Sorry to say. What ever the exhaust gas temp is coming out of the cylinders is how hot that part if the intake can get. The exhaust passes through the intake under the carb. If your boat is over prop-ed and the engine is over loaded at cruise RPM the exhaust gases can reach 900-1000 degrees.
 
I haven't owned a 3.0 since about 2000 so forgive this if it's a stupid question. Are the intake and exhaust manifolds seperate pieces of steel or two different functions in a large combined piece of steel?
 
[QUOTEDocksidematineservices;487636]Sorry to say. What ever the exhaust gas temp is coming out of the cylinders is how hot that part if the intake can get. The exhaust passes through the intake under the carb. If your boat is over prop-ed and the engine is over loaded at cruise RPM the exhaust gases can reach 900-1000 degrees.[/QUOTE]

I'm having some trouble understanding this. I don't understand how the intake, any part of it really, can exceed 500 degrees when the autoignition point of gasoline is like 530/540 degrees?
I always thought thst you wanted the temp of the air/fuel mixture to be as cool as possible until it entered the cylinder. What you're describing seems to be super heating it?
 
We are discussing the rust on an intake manifold of a 7.4ltr not a 3.0 ltr.


There is an exhaust cross over port on the intake manifold that runs directly under the carb.
 
Docksidemarineservices said:
We are discussing the rust on an intake manifold of a 7.4ltr not a 3.0 ltr.


There is an exhaust cross over port on the intake manifold that runs directly under the carb.
Click to expand...

Sorry, don't know why I had the 3.0 on my mind.
Still don't understand how the intake temp could exceed the autoignition point of the air/fuel mixture.

Another choice for the op could be a ceramic coating. Powder coating could still be used as long as the normal operating temp of the piece does not exceed the curing temp of the powder, usually 250 or 400 degrees depending on the powder. There are some high temp powders that claim to be ok up to 1000 degrees but results seem to be iffy. An IR temp gun could easily tell you what temp the manifold is actually reaching to help with your decision. Having said all that, if it were mine, I'd treat the rust, clean and prep the part and use BBQ grill paint.
 
Yes, I've been reading up on the design. It seems it's some screwey GM scheme that heats a coil for the carburetor choke. My reading also showed that, as I brought up, preheating the air/fuel mixture by heating the intake like that is a concern. If the engine is a newer, injected, engine that passage can be blocked off with restricters in the intake gaskets. IMO, it's a jacked up, over engineered design with more disadvantages than advantages.
 
No disadvantages for a carbed engine. Block off that port on a carbed engine and see how it runs. It is necessary. Injected engines are ecm/pcm controlled. A carb is just a controlled fuel leak.
 
I see disadvantages even on a carbed engine. Cold, dense fuel/air is best for engine performance. Heating the intake manifold is counter productive and a crappy design if its only purpose is to activate the choke. A carb with an electric choke is an effective, and far simpler method. GM definitely threw KISS out the window when they came upbwith that system.
I never said to block the passage with stock carb. I know its necessary for that jacked-up system to open the choke.
 
This is NOT some GM "jacked up" or "screwy" scheme. Running some exhaust gas under the intake manifold is a solution used for well over 60 years (that I know of) by virtually every V8 manufacturer, ever. It is NOT part of the "choke heater" scheme although the plumbing for many choke heaters is in the same area on many engines. The over under stacking of the intake and exhaust manifolds on many inline engines accomplishes the same thing. On many automotive setups, this flow under the carbs was "controlled" by a thermostatic valve on one of the manifolds called a "heat riser" valve. Back in the day, this often rusted and stuck to "amusing" results. I've seen this setup on MOPAR, FORD, Studebaker and GM engines... ( a happily misspent youth!).
 
Thanks for all the feedback. The engines are fine and are prop'ed properly. The rust is cosmetic. I am very peticular about the appearance of my engine and compartment. I have some rust in othere areas like the genset that I just replaced all the sheet metal. Took other advice and replaced the genset pan with a glass pan. Never heard of BBQ paint but will try. Thanks again.
 
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