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Discussion Starter · #1 ·
RC in sig started to miss recently, pulling the plug wires found that #5 and #7 cylinders were not firing.  Today, while the motor was running, I pulled the valve cover breather off to find a huge constant suction.  All I can figure is that I have a huge vacuum leak at the intake manifold gasket back at the #5 and #7 intake ports.  

360, edelbrock heads, stock 4bbl intake, Q-jet.  Has approx 30k miles on the motor since rebuilt.

Any other possible causes that I am overlooking?  Looks like a fun holiday weekend digging into the top end of motor!

Thanks,
Ty
 
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With the engine running spray propane or staring fuild near the vaccum leak and see where it goes, if it revs up at all you have found your leak.

it could be something as simple as the intake bolts are loose...

i had a vaccum leak on my dodge in my sig, and it was a square bore carb. with a quadrajet intake, and it was leaking at the sides of the carb gasket
 

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if you do not find a vacuum leak. you might want to check the cam for burnt lobes. i had a 318 with a burnt lobe on the LH bank. it had a dead miss and the brake peddle was hard as a rock. it sounded as if it had a vacuum leak. i drove it for 30K with the miss. before replacing the Eng.
good luck
 

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Discussion Starter · #5 ·
gen1dak said:
Usually when you hear a loud sucking noise, it turns out to be a loose or holed large vacuum line.
Not a loose vacuum line but rather, huge suction through the valve cover breather hole when I removed the breather with the motor running.
 

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BIGTRUCKNUT said:
Not a loose vacuum line but rather, huge suction through the valve cover breather hole when I removed the breather with the motor running.
that would be the PCV valve being bad and not the intake gasket
 

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Discussion Starter · #7 ·
DODGEBOYS said:
that would be the PCV valve being bad and not the intake gasket
I'll check the PCV before digging deeper. Thanks.
 

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l,d plug it off and then see what happens
 

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Discussion Starter · #9 ·
Nope, not the PCV valve.  Pulling the intake this weekend. :(
 

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Ty,

Let me know if you need some help this weekend. I'd be open to a wrenching session and should be off Friday and Monday for the long weekend.

Alex
 

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Discussion Starter · #11 ·
driveramsII said:
Ty,

Let me know if you need some help this weekend. I'd be open to a wrenching session and should be off Friday and Monday for the long weekend.

Alex
Alex,
I pulled the top end off the motor tonight, found that the intake manifold gaskets were jacked up - squished out or shifted at install. Putting new head gaskets on while I have it apart, currently have the .026" MP composite gaskets that have been a problem and are not being produced any longer - replace them before they sh!t the bed.
Will be working on it Saturday morning/mid day, pick up the gasket set at 0900. I'm at 4791 Darby Ct, Va Beach if you aren't doing anything. Stop by. Just got a new bender too but need to make a stand for it.
How's your stuff coming along?
Ty
 

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I had the same problem. With my last rebuild, I used the Fel-Pro intake gaskets that were intended to block the exhaust crossover that was recommended by folks here. After a few months, it started to lose power and I noticed oil fouling on my plugs. The gaskets shifted all over between the runners, sucking air from the outside and oil mist from the inside.
 

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Discussion Starter · #13 ·
KThaxton said:
I had the same problem. With my last rebuild, I used the Fel-Pro intake gaskets that were intended to block the exhaust crossover that was recommended by folks here. After a few months, it started to lose power and I noticed oil fouling on my plugs. The gaskets shifted all over between the runners, sucking air from the outside and oil mist from the inside.
Exactly! Did you switch to a different gasket? I have Edelbrock heads now that don't have a crossover port anyway. I'm going to glue the gaskets down better and be careful with manifold bolt torque.
 

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Yes, I switched to the OEM style Fel-Pros, I think the others were marketed as "high performance". And just an FYI for future readers, these gaskets are not adequate on their own, to block the crossover, mine blew right through. You should use the ones Mopar makes that have stainless block off plates.
With that said, I now understand the system better, and realize since I drive in a cold climate in winter, I'm better off with the crossover functioning.
 

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Hey man, sorry I just saw your reply a little while ago. Its been a crazy weekend with family and parties and such. My truck is going pretty well. Got 99% of the wiring redone and its running great. Gonna re-add it to my insurance and run to DMV this week hopefully if I can get off early one day. Give me a holler if you ever find yourself needing a hand or anything. Still have my number?
 

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Discussion Starter · #16 ·
Finished the complete tear-down of the top end of the 360 to include pulling the RPM heads, removing all the valves/springs for a thorough cleaning, replacing gaskets, reinstall and tune.  Once timed and the carb tuned Wednesday the thing is running great. 

In all, it looked like the intake manifold gasket squished/shifted on 4 intake ports and was drawing oil in to the cylinders from the lifter valley.  Also fixed a bad oil leak down the back of the motor that was coming from the back side of the intake manifold.  Idles pretty good now although it should get even better next week after I pull and discard all the old, rotted emissions vacuum plumbing.  I was getting a consistent 10 mpg around town and work commute lately with the leaks.  Maybe it will jump back up to the 12s again. 
 
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