Suspect leaking intake manifold
Moderator: Team
Suspect leaking intake manifold
I have a 348 ci Chevrolet engine with a custom made sheet metal intake. I have a problem with light smoke out the exhaust. The engine is a fresh rebuild. I would like suggestions on the best method to check for oil leakage by the intake causing the smoking issue with the intake still installed. thanks
Re: Suspect leaking intake manifold
I see something on this not too long ago bout how some use smoke. Another used a homemade carb block-off plate with and a nipple, then(with rockers backed off) used a vac pump to see if it held psi(low like normal engine vac psi).
Channel About My diy Projects & Reviews https://www.youtube.com/c/BOOTdiy
I know as much as I can learn and try to keep an open mind to anything!
If I didn't overthink stuff I wouldn't be on speedtalk!
I know as much as I can learn and try to keep an open mind to anything!
If I didn't overthink stuff I wouldn't be on speedtalk!
-
mag2555
- Guru
- Posts: 4608
- Joined: Sun Sep 18, 2011 11:31 am
- Location: Heading for a bang up with Andromeda as we all are.
Re: Suspect leaking intake manifold
I would do a leak down test first and confirm that the rings have all seated in on that bank of cylinders.
You can cut a man's tongue from his mouth, but that does not mean he’s a liar, it just shows that you fear the truth he might speak about you!
Re: Suspect leaking intake manifold
If your engine appears to be ingesting oil from the underside of intake ports in the lifter valley it is a pretty easy look see by just removing the carburetor and snaking a flex inspection scope down into each port. If you have all or a few sucking oil into the intake ports you will see the wet oil coating the inside of the intake ports near the head to manifold interface. If the heads were ever milled a significant amount and the intake was not milled a correct corresponding amount leaks like this would be very likely. If this is the case here its still a fairly easy thing to correct the angles of the intake to head surface and stop the "Suck In"!
The Older I Get, The Dumber I Get 
-
user-23911
Re: Suspect leaking intake manifold
Seal it off at the carb and pump it up with an air compressor.
You'll hear it hissing where it leaks.
If it's a sheetmetal manifold , it'll have plenty of pinhole leaks in it.
You'll hear it hissing where it leaks.
If it's a sheetmetal manifold , it'll have plenty of pinhole leaks in it.
Re: Suspect leaking intake manifold
Thanks for the replies, let me provide some addition info about the sheet metal intake. It is a tuned multi/port efi.
Re: Suspect leaking intake manifold
A few questions:
1. Have you done a leak down test on the cylinders?
2. Were the cylinder heads ever milled? , Especially an angle milling?
3. With the carb off can you see oil coating or oil residue coating the inside of the runners?
4. Are your valve covers or valley connected by a hose to a fitting at the base of the carb or into a port on the manifold? If there is such a hose, does it have a PCV valve inline on the hose or valve cover/valley?
5. Did you check the manifold fitment before installing?
I do this by using a 1/8" wedge at each end of the china rail at each end of the block, (no intake gasket in place) and then I use an inspection light and look at the gap between the head intake face and the intake manifold face itself and see if the two surfaces are parallel on the same angles. If they appear close and the bolt holes line up pretty well with just the intake gasket in place then usually the intake gasket will seal and the junction of the two surfaces will seal. If those angles are off and not parallel then most likely the heads have been milled enough to require the manifold faces to be machined on the same angle.
6. Since its a fabricated manifold meaning a welded construction, the intake faces on the manifold could have distorted from the heat of welding and they are not either straight and the faces may not now be on the same angle as the cylinder head faces. Put a good straight edge down the length of the faces and see if they are nice and flat and use a measurement and make sure the angle they are at is the same as the corresponding angle of the head intake face.
I suspect from what you are asking that obviously the oil is coming into the cylinders from one of four ways:
Either its in the manifold and either entering from the valley from bad sealing of the intake to head surfaces, or through a vent hose or PCV connection.
The oil is coming past the rings.
The oil is coming in from bad, worn valve guides or valve guides with bad or no seals.
Don't ASSUME anything when looking to find a problem like this, just look and check possibilities until you find the culprit.
1. Have you done a leak down test on the cylinders?
2. Were the cylinder heads ever milled? , Especially an angle milling?
3. With the carb off can you see oil coating or oil residue coating the inside of the runners?
4. Are your valve covers or valley connected by a hose to a fitting at the base of the carb or into a port on the manifold? If there is such a hose, does it have a PCV valve inline on the hose or valve cover/valley?
5. Did you check the manifold fitment before installing?
I do this by using a 1/8" wedge at each end of the china rail at each end of the block, (no intake gasket in place) and then I use an inspection light and look at the gap between the head intake face and the intake manifold face itself and see if the two surfaces are parallel on the same angles. If they appear close and the bolt holes line up pretty well with just the intake gasket in place then usually the intake gasket will seal and the junction of the two surfaces will seal. If those angles are off and not parallel then most likely the heads have been milled enough to require the manifold faces to be machined on the same angle.
6. Since its a fabricated manifold meaning a welded construction, the intake faces on the manifold could have distorted from the heat of welding and they are not either straight and the faces may not now be on the same angle as the cylinder head faces. Put a good straight edge down the length of the faces and see if they are nice and flat and use a measurement and make sure the angle they are at is the same as the corresponding angle of the head intake face.
I suspect from what you are asking that obviously the oil is coming into the cylinders from one of four ways:
Either its in the manifold and either entering from the valley from bad sealing of the intake to head surfaces, or through a vent hose or PCV connection.
The oil is coming past the rings.
The oil is coming in from bad, worn valve guides or valve guides with bad or no seals.
Don't ASSUME anything when looking to find a problem like this, just look and check possibilities until you find the culprit.
The Older I Get, The Dumber I Get
Re: Suspect leaking intake manifold
Remove the PCV if there is one, or through the oil fill, Introduce Acetylene or Propane in the crankcase, watch the AF on a WBO2, or better yet a 4 gas analyzer. If the intake is sucking air from the crankcase the A/F will change, become more rich, with the additional gaseous fuel.
-
rebelrouser
- Guru
- Posts: 1946
- Joined: Tue Sep 15, 2015 2:25 pm
- Location:
Re: Suspect leaking intake manifold
If the intake is pulling oil by a vacuum leak, the quickest way is to remove the PCV valve, block the other Valve cover breather, and take a strip of paper and hold it so the paper covers the grommet where the PCV used to be, rev the engine up and let come down quick. If it has no vacuum leak the paper strip will be pushed off the hole in the valve cover by blow by, if the crankcase is under vacuum from a leak it will suck the paper down. Doge LA magnum engines used to have a plate on the bottom of the intake and the bolts would work loose causing oil consumption due to vacuum leak, found lots of them with the paper trick.