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  #16 (permalink)  
Old 04-04-2004, 03:57 PM
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Knock, Vacuum leaks [Assuming A Fresh Engine]

Hi,

Doc here,

Does this Car have An Air Injection Reaction System, And If So is It hooked up? Not withstanding that, Is it hooked up Correctly?

A Faulty Diverter Valve Can Do Exactly what your describing if the valve is blown apart inside. It can pump air in the wrong direction.

The Noise you will hear, (Knock) will come from the pump area (front of engine, also, you may or may not hear it at the exhaust manifolds. )

If your getting bubbles at gasket areas, This indicates Pressure, not Vacuum.

To test this theory, put a vacuum gauge at any manifold port you can access. Should read between 17 and 25 lbs. Open the Oil cap and observe the Vacuum gauge. It Should not Drop or Dip. While you have it hooked up, give it a slight rev, the gauge should not flutter. It's OK to dip, then rise quickly once on rev.

Again, A Faulty A.I.R can cause this.

Does the engine Run and Idle normally otherwise? (cap on ect..) No stumble, or any other signs of misfire?

Couple of things to check, Remove the PCV valve, Hold it next to your ear and shake it, Does it Rattle? If not get a new one.

While you have it out, Spray some Carb Cleaner into the bottom end until it runs out the top, shake it around and do it again, you'll see some funky Stuff come out of it!

Another thing to try, is with the engine running, Spray Carb cleaner around where the Intake manifold mates to the Heads. The Engine should Not speed up. If it does, You have a bad Gasket, or the wrong Manifold installed, or a wrapped Surface.

Make sure the Air Cleaner is on while doing this, as overspray can cause the engine speed to increase, giving a false indication.

Leaky Head gaskets Can cause Crankcase over pressure. Did you Re~Torque the Heads after Break~in? If not, Try it, If the gaskets are not too damaged, or Teflon Type They May reseal. Usually, this type of leak is accompanied by a misfire, water in the oil or vise~versa, but it's not inconceivable that a leak so small could be the culprit, slowly building pressure on a "tightly" vented Crankcase.

Do A compression test. It may or May not show as really out of spec if it's just a real tiny leak but if 7 cyls ARE pretty much identical and one is low by several pounds, you may want to rethink the head gaskets.

There are some good start points anyway.

I suspect (If you have one) The AIR injection System.

However, I would eliminate all the above also.

Let me Know How it turns out!

Regards,
Doc

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Old 04-04-2004, 04:35 PM
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hey doc,the engine is stock 350 sbc rebuilt with around 40,000 on it,no egr,oxygen sensor or any emmisions control except a pvc valve,just gas and fire.very simplistic motor about an 80 model engine.no high performance parts either other than holley street avenger carb all stock.did a compression test on it all cylinders are 160 all the way around,heads are great and were recently serviced to ensure no future problems.

does anyone else have a sbc 350 with a solid oil cap or does everyone have a vented oil cap?

ok knockng sound is coming from the timing chain cover when i put the cap on,could the cover actually be sucked against the chain or gears?getting worried now!

Last edited by 73chevyhelp; 04-04-2004 at 04:35 PM.
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Old 04-04-2004, 06:03 PM
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The oil cap is usually solid, providing there is a breather at the rear of the pass side vc.
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Old 04-04-2004, 06:19 PM
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I HAD THE SAME PROBLEM ON A 350 . IT WAS THE BAFFLES
RUBBING THE ROCKERS . YOU COULDN T SEE ANY RUBS
I TOOK THE BAFFLES OOT AND IT QUIT
HOPE THIS IS SOME HELP
BILLY R.
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Old 04-04-2004, 06:31 PM
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Jmark's right. If you have an unrestricted crancase filter, as you should have, then you should have next-to no noticable suction when you remove the non-vented oil fill cap.

It sounds to me like you have full curb-idle vaccume in your crancase with no place to go, once the non-vented filler cap is in place. It's VERY possible, it's pulling something around enough to cause the knock you're hearing. If this is the case, I wouldn't run it long without venting the crakcase properly... Too much crancase vaccum or 'pressure' raises hell with oil seals and can turn 'em inside out..

Mover's dime's worth

Last edited by PrimeMover; 04-04-2004 at 06:37 PM.
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Old 04-04-2004, 06:46 PM
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how do i vent the crankcase?
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Old 04-04-2004, 08:05 PM
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Generally, the crancase is vented from a hole in the opposite valve cover from the PCV valve's location. Usually the PCV is on the passenger side and the crancase is vented from the driver's side cover. In most stock applications where non-vented filler caps are used, a rubber hose is run from the valve cover up to the air cleaner's outer shell. A small 'crancase filter element' connected to this tube and mounted inside the AC's outer shell, outside the engine air filter element, helps balance vaccume and helps to make sure that the fresh air being pulled in by the "Positive Crankcase Ventilation" or PCV valve, is clean.

If you don't really dig the looks of another hose going up to the air cleaner, the crancase can be vented directly at the cover by using a vented, filtered filler cap. Moroso and Edlebrock both make them and they are readily available but without going all the way to the air cleaner with the breather, sometimes the PCV valve has a tendancy to suck a little oil back into the motor. If you vent this way, sometimes it better to modify a bit and put the PCV valve in the intake manifold valley area where there is less oil flying around.

Hope this helps

Last edited by PrimeMover; 04-04-2004 at 08:27 PM.
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Old 04-04-2004, 10:05 PM
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had same prob on 70 6.00 breather ended nightmare but started a new one.brand new 395 sanderson ceramic jet coated headers and rare M/T valve covers now have oil on them,breather did not fit snug enough a little electrical tape fixed problem.
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Old 04-04-2004, 11:15 PM
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i have never had a line to the air breather on the engine,i have a non stock air filter.im guessing to buy another oil cap with the breather holes in it,since no one has never had this problem before im going to call everyone i know to figure the problem out,thanks for the info guys even though all it did was confuse me.
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Old 04-04-2004, 11:33 PM
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i pulled the oil cap off and the knock got half as loud and now my carb is popping when i rev it whats going on?
everyone seems to be forgetting the popping... my first guess is that you have a broken ring, when your crankcase pressure is elevated (stuck pcv valve installed) you are forcing oil into a cylinder causing a knock and pop through the carb, once you lower the crankcase pressure (by opening up to gods free nature) the problem goes away

try and pull the plugs, if one is black and oily you are in deep **** and yoi gave a piston with a broken ring (and then some) a rebore and 8 new slugs should cure it right up...
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Old 04-05-2004, 07:17 AM
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Like has been said, you have to have a vent, ether in the valve cover or the oil fill cap. The pcv has to have air, or it will suck the valve cover gaskets out (in).

Ether use a vented cap, or replace the pcv with a vented cap. One in each valve cover.

Troy
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Old 04-06-2004, 01:25 PM
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the vented cap worked on the crankcase pressure and the fact that i had a bad plug fixed the problem.2 bad plugs on 2 cars what a day lol.
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Old 04-06-2004, 02:00 PM
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Glad its all better.

A few years back, there was an older Sun scope that came through the auction I go to. It was mint and I sure kick myself for passing on the bidding. A quick 2 minutes on the scope ( real ocilosicope) can spot most problems in no time. Oh well.
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Old 04-06-2004, 10:00 PM
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Cool.

PRIMO!

Dirty plugs can be caused by a lot of things - I wouldn't tear my motor up just yet. When I first got the five-three running, that slippery 350 fouled two plugs right out of the gate. After everything lapped in, my plugs are running a light grey now and I've got 3000 miles on the latest set of fishin' sinkers.

Happy cruisin'

Last edited by PrimeMover; 04-06-2004 at 10:00 PM.
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Old 04-07-2004, 10:55 AM
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Ok, so let me get this straight, I am supposed to have a breather on BOTH valve covers??? The stock valve covers had an oil fill and a breather on the same side so that's what I replaced them with when I went chrom Edelbrock.

Both the breather and the oil fill are on the same side. Am I risking damage on the other side????
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