Will a plugged intake manifold cause blow-by?

[Bohemian]

SBC 350 4 Barrel, GM Cast Iron Spread Bore Intake Manifold.

I seem to remember back in the day that if that exhaust crossover in these old cast iron spread bore SBC Intake Manifold's was plugged up with carbon it could cause blowby...

I also seem to remember having to knock the 4 rivits out of that steel pan on the bottom of the intake in order to cleanup all the carbon there because it causes a carburetor heat soak condition that causes hard starts after engine is warmed up, but engine otherwise starts first try when cold.

Any of this sound familiar to any of you all?

thanks in advance.

 

[mechted]

I also seem to remember having to knock the 4 rivits out of that steel pan on the bottom of the intake in order to cleanup all the carbon

Yep. When I tore down my motor I found exactly this. And the exhaust crossover/EGR piping was completely plugged. At the time I figured that's why it failed a smog check. Pull that manifold out in favor of a Edelbrock when I rebuild it, but it was going back into a non-smog vehicle so I didnt have to worry about

 

[tRustyK5]

I dunno about causing blowby, but the heat soaked hard start is a classic 'extra' you get with exhaust crossover through the intake.

Ever wonder why most intake gasket sets come with block off plates?

 

[diesel4me]

I've seen a few intakes that had a lot of sludge baked into hard carbon under that tin shield ,and one had a crack in the intake under all the grunge,that would open up after the engine warmed up,which created a vacuum leak and it was sucking oil from the valley into the intake and burning it..engine always ran good after a cold start,but once it warmed up it had a bog or hesitation on acceleration,after the crack opened up more I assume...that went away after an aluminum intake was installed..

 

[wasted wages]

Exhaust x over manifolds are a big killer of fresh rebuilds too..

Owner gets everything hot tanked, the big glob of crud gets partially broken down and the builder doesn't either know/care to pull the little pan.

Fresh oil washes out all the broken down sludge and grime into the new rebuild....

I've seen some that literally needed an air chisel and sandblasting to get that baked on crap off of the manifold.

 

[diesel4me]

We had to use a hammer & a flat chisel to get most of the carbon off that intake that was cracked,it was like coal,with slimy greasy oil underneath...the exhaust passages in the intake get hotter than the exhaust pipes probably since its the first place they pass by on their way to the exhaust manifolds..

The engine ran much better after we replaced the intake--it no longer wanted to stall when warmed up in drive at idle,sometimes it would idle lower & lower until it stalled at a stop sign or red light,and have a bad flat spot when you tried taking off from a dead stop..it was sucking in oil from the valley and probably exhaust fumes from the passages in the intake for the heat riser..the center cylinders spark plugs would have a ton of brown crust on them too,while the others looked normal..

We decided to pull off the intake after hooking a vacuum gauge up,we could see it drop down a lot once the engine warmed up and it would flutter down to near 5" at idle just before stalling--when cold it had a fairly steady 18"...when opening up the mixture screws seemed to help,we figured it had a vacuum leak,but everything we could think of externally was perfect,no cracked hoses,everything was new,fittings tight,vacuum modulator OK,etc..spraying carb cleaner on everything didn't change the idle speed...so we figured it had to be "internal"..