69 350/350 intake manifold gasket

[Driftwood]

I'm leaking oil out of the rear of my intake manifold. I went online to AutoZone and came up with Felpro MS90314-2. Is this the gasket I want to install, or do I want a "high performance" gasket? Getting ready for this weekend's project...

Thanks
Andy

 

[Mpit69]

Are you sure its the intake and not the valve covers or oil pressure unit?

 

[tonyk72]

ditto

I had a leak back there too...passenger side. I used Permatex Black for the seal. Other's on this board have advised it's the best way to go...I've had no problems so far...

 

[Driftwood]

I think it's the manifold because the top of my transfer case is totally soaked in oil and the top (when open) of my hood is covered in oil after driving and it's leaking straight down the back of the engine. BTW, where is the oil pressure sensor on the engine? Thanks.

 

[Jack]

BTW, where is the oil pressure sensor on the engine? Thanks.

Andy:
69 has no OE op "sensor" ... it has a OE mechanical gage (68 thru 74 C3 have mechanical op gage) and its pressure line (small hose) connects at top rear of block ... just behind the intake manifold. Standing in front of car ... it is just to right of distributor. This oil pressure line and fitting is famous for leaking and will cover your hood w/ oil just as you described. I suggest replacing the line if there's any question of its age/viability. The OE line on my 71 was plastic ... yours is likely plastic also. If this is the culprit ... consider yourself very lucky that you've found it now ... it can easily cost you a motor if /when it lets go completely. Various sources/methods for op line repair ... to stop leak in mean time ... get a pipe plug (1/8" NPT) from hardware store ... remove the OE fitting from block & replace w/ the plug ... the gage won't work but it'll stop the leaking.
JACK:gap

 

[Driftwood]

Thanks a lot for the heads up. It definitely looks like the original plastic line-- kind of a scary setup. I'll plug it off and see if my oil leak clears up.

I'll post a new thread for the best fix.

 

[Driftwood]

What gunk for gasket?

I've got some Black RTV for the corners. What do people recommend to put on the intake manifold gaskets themselves-- just a tack to hold it in place, or some RTV, or that blue high temp Permatex stuff? I'll start tackling this tomorrow after work.

Andy

 

[Jack]

Andy:
If you're certain the oil leaks are not confined to op gage line ... then RTV in the corners should be all you need. BTW, we NEVER use the front & rear rubber pieces that come w/ intake gasket set ... we substitute a thick bead of RTV along center edges of block ... from one corner across to the other. Put a dab of RTV on intake bolts' threads too. We just freshened a circle track motor again last nite ... did intake as above ... practiced this afternoon ... no leaks. I like red or blue ... oil leaks more easily identified than with black.
JACK:gap

 

[Driftwood]

Great-- thanks for the tips. I capped off that oil pressure line tonight and changed the valve cover gaskets. All the existing gaskets I've changed so far (oil pan, valve covers) seem old and brittle, so I'm thinking I should change all the gaskets I can get my hands on.

I'm not 100% where the leak(s) is yet, but I'd like to do the intake gaskets because I want to paint the manifold anyway.

Once I stop all the leaks it should be much easier to keep everything clean and detect any future leaks.

Andy

 

[Jack]

Andy:
Your old OE intake probably has a stamped steel sub-pan attached to underside of intake with a few rivits. If so ... this pan is probably full of old, fully-cooked to coke oil residue. It'll pay you to pry out the rivits and remove pan ... take the intake & pan to auto machine shop and have it hot-tanked (vatted) ... so that all that residue is washed away. Those hard bits of residue will work their way out of pan & fall down onto lifter valley & cam ... and they're very abrasive & can clog passageways. If you remove rivits carefully & straight out ... they'll go back in & hold tight.
JACK:gap

 

[Driftwood]

OK I'll look at that when I remove the manifold and see if it should be cleaned. BTW- I'm thinking maybe I should replace the thermostat while I'm at it. What temperature do you suggest?

 

[Driftwood]

I removed the intake manifold today. Wow Jack was absolutely correct. That manifold pan is completely filled with hard, black gunk. I mean totally packed. Now I'm glad I'm replacing the gaskets and cleaning this pan out. Would a full manifold pan affect performance at all-- air flow, carb tuning, HP? Also where is that residue coming from, that would collect in an intake manifold?

Andy

 

[Jack]

I removed the intake manifold today. Wow Jack was absolutely correct. That manifold pan is completely filled with hard, black gunk. I mean totally packed. Now I'm glad I'm replacing the gaskets and cleaning this pan out. Would a full manifold pan affect performance at all-- air flow, carb tuning, HP? Also where is that residue coming from, that would collect in an intake manifold?Andy

Andy:
There may be a small degradation of performance from gunk ... not too much.

Each head has an exhaust crossover port located at its center ... and the intake has a runner that connects the two ... the intake runner directly connects to only heads’ exhaust x-over ports ... no other connection. That intake runner is used to heat the choke stove for an automatic choke and to help prevent carb icing. The runner gets very hot and the pan is there to help prevent heat transfer to oil. It is normal for the oil to coke/gunk in pan ... not good but normal. There’s usually a drain hole in bottom of pan but ... over the years ... it clogs up too.

Suggestion: Take a good center punch and lightly peen/stake just outside pan-rivit holes in intake ... all around holes’ circumferance ... will tighten holes up ... also use a dab of loctite on rivits.

Suggestion: You can block off both exhaust x-overs ... it will help cool the intake charge (good) but may cause a bit slower warmup ... and it is LIKELY your choke will need to be permanently wired/set open. We use thin sheet steel (about 18 gage) about 3/16” both wider and taller than exhaust x-over ports ... gives about 3/32” overlap at each edge ... just use red silicone to glue tabs directly to both heads’ x-over ports ... then put the intake gasket on ... the intake will torque down as normal & won’t leak. Some gasket sets come with block-off tabs ... they’re not as good as homemade ones. Most aftermarket hi-po heads do not have exhaust x-over ... same applies to intakes.
JACK:gap

 

[JohnZ]

I wouldn't advise trying to re-use the original rivets - once they're driven in the first time and removed, their retaining feature is worn off; you can order new ones at your dealer under GM #10229247. They come ten to a bag for about $5.00.

 

[Jack]

I wouldn't advise trying to re-use the original rivets - once they're driven in the first time and removed, their retaining feature is worn off; you can order new ones at your dealer under GM #10229247. They come ten to a bag for about $5.00.

Better safe than sorry ... didn't know GM sold them ... new rivits certainly better than used ... get new GM P/N 10229247 ... thanks JohnZ!
JACK:gap

 

[Stallion]

OK I'll look at that when I remove the manifold and see if it should be cleaned. BTW- I'm thinking maybe I should replace the thermostat while I'm at it. What temperature do you suggest?

The best to go with a C3 is 180 for the tstat. Good luck!!

 

[Robin7TFour]

drift hope i catch you in time. try using permatex copper. it designed for hi-heat applications. works great for me. also let the manifold sit overnight before starting ,this will give sealant time to cure ..

robin

 

[Driftwood]

Not too late

Haven't put it back together yet. Successfully removed pan and cleaned manifold and also painted it tonite. Will probably start putting everything back together tomorrow. Should I let the front and rear sealant set up first before inserting manifold? Or do I want to squish it down before the RTV or Permatex cures?

 

[JohnZ]

I've been using Permatex Ultra-Copper on intake ends for many years; I let it "skin" for 5-10 minutes before installing the manifold. If you let it sit first for several hours or longer, it won't bond to the block rail and the manifold surface.