puff of blue smoke at higher rpm decel

[Barry_R]

Compression tests and leakdown tests will not show up anything for oil control issues. Oil might even make those numbers look better. Compression sealing is top ring. Oil control is second ring and oil ring.

Want to verify intake sealing? Pull the rockers and block off all vacuum openings. Make a carb block off plate for the carb from scraps (or one of those engine lifting deals). Add or open up an 1/8NPT opening somewhere and attach your radiator pressure tester. Pump up 20psi and look/listen/soapy water check. Be ready to be surprised at places you'd never expect to leak - carb mounting studs etc.

[Belgian1979]

to reply to comments :

Compression is as good as when I checked it after the engine had been run in. So from that side I would say the rings should be ok.
Oily plugs tend to vary, sometimes for instance a lot of oil on number four, then next run nothing etc.
Yes, the valve bowl area seems to shine from oil, however I cannot be sure that it's not gas either. The injectors have a tendency to load up the walls as well.

When assembling the intake manifold I had the impression that the seal on the valley side was a little larger, but not a lot. Tried several thicker seals, but they created other issues. To be sure they were sealed i used blue silicone around the intake ports. When using a bore scope I can still see traces of the silicone.

I use a catch can, but I'm not impressed with the quality. The hose from the can to the engine had oil in it. So assumed that it was drawing in oil through there.

Previous runs with high rpm bursts and subsequent decels and an oil level check afterwards showed that I lost about 200-250 ml during that run. This is however a little difficult to assess as I'm also using an accusump which does create some variability in oil level depending on the oil pressure when you shut down the engine. Anyway using the same way to measure it seemed to be about 1/4-1/5 of a quart. I then disconnected the PCV and connected it to my airboxes to test it out. Last drives I did it stayed constant, however I had less chance on bursts to high rpm as well. Nonetheless no noticable change in oil level. But I payed special attention to the exhaust smoke color this time. Still blue smoke when on decel.

The tricky part is that the pcv feeds to a vacuum chamber underneath the ITB's. So if it drew oil through the ITB's it may have ended up in the vacuum chamber and the engine is now trying to suck out what remained in there after moving the pcv line to the air boxes.

Intake vacuum at idle is about 9-10" and when under decel close to 25"

[BrazilianZ28Camaro]

Take a look into the header primaries, should be easy to find what cylinders are burning oil. From there, inspect valve seals and guides, then check the intake port of those cylinders.

If nothing is wrong there, I'd inspect rings.

What oil are you using?

[ptuomov]

Vacuum log still having some residual oil in it from the previous breather setup is a good theory.

Is there any chance that some (but not necessarily all) of the smoke is just from the engine running rich under deceleration?

[geraldtson]

Just experienced similar problem on 3 year old trouble free SBC 350 crate motor. Oiling number 8 cylinder plug. Leakdown was good,compression good,so pulled springs and guides felt good. Replaced with new PC type valve seals while i was there. Immediately problem still there? Pulled intake, all intake ports on right bank looked a little oily especially number eight. Intake gasket had small cut (pinch) hole at bottom of gasket and was sucking oil. Replaced with felpro bluestripes and problem solved. Its might be worth following same order?

[Newold1]

Belgian 1979, I remember pics of your crass ram ITB intake system. The vacuum chamber you have on that system could be collecting amounts of oil from the crankcase breather system and with the lower vacuums of acceleration running and idling it's possible that the lower vacuum is not pulling that accumulation of oil into the intake runners much and which runners get the oil moves around a bit depending on where the oil level is closest to inlet openings that feed vacuum to the chamber. At high vacuum readings such as decel the vacuum is strong enough to pull that oil in the vacuum chamber up into the intake port and down to that cylinder to be burned during the decel. It might be the case where the oil vapor at high crankcase pressures is enough that its being collected in the vacuum chamber until there is a strong decel vacuum suction present.

Can you rig a crankcase pressure sensor with lets say a 0-30psi range readable and does your ECM have a pressure readout capability like fuel pressure, etc.? This would show if you've got excessive crankcase pressures and its pretty easy to determine where that is coming from. I know on some engines that use ITB and a vacuum chamber for things like IAC, PCV, and power brake vacuum feed that they sometimes put a small metered hole at the lowest point of that chamber that allow the oil buildup to drip back into the crankcase which in your case would be the lifter valley. Just some thoughts.

[Belgian1979]

I have some time today after work. I'm going to try and suck out the oil out of the vacuum chamber. Won't be easy though.

As for oil I'm running a 5w30 synthetic. HV pump with high pressure spring. 60 psi while running. 40 psi at idle.

Measuring crankcase pressure us possible, but i have non suiteble ports connected to the crank case.

[Belgian1979]

I was unsucessful at trying to suck out oil if it would be in there. I cannot bend the tube so that it reaches the floor of the chamber.

Maybe a question : someone here know of a valve that I can use in the pcv line that I can actuate electrically as a shut off valve ?

[Promodwrench]

Every time that I've encountered a motor with leaking valve seals it always smoked on startup after sitting (especially after sitting overnight) on some of them (but not all) the smoke would lessen after warm up.

[Belgian1979]

Every time that I've encountered a motor with leaking valve seals it always smoked on startup after sitting (especially after sitting overnight) on some of them (but not all) the smoke would lessen after warm up.

I haven't noticed this.

[engineguyBill]

Generally speaking, if the rings have failed there will be oil smoke upon acceleration - as well as deceleration. Spark plugs will also show signs of oil being burned.

Smoke upon deceleration is typically an intake valve guide/seal problem. As vacuum is developed within the intake tract, oil is pulled through the valve stem/guide into the combustion chamber therefore producing blue smoke. This will also be evident on the spark plugs.

Worn exhaust guides/seals will not show up as smoke, nor will the oil be evident on the spark plugs. Bad exhaust guide/seal will result in oil being drawn into the exhaust system and may even shoe up as wet exhaust pipes or even oil dripping from the exhaust.

[Belgian1979]

Hi Bill,

I agree, this is the general idea.
I've driven her some more today and this time I stayed well below the rpm's at which I usually see the smoke coming on decel (<3500-4000). It was roughly pulling something like 15" of vacuum on these drives and under decel. Did see no blue smoke in the rear view mirror. Plugs seem a lot dryer as well.

Now if I could just get that oil out of that vacuum chamber if it's there.

[Newold1]

Could you give us a sketch of a front view cutaway of your vacuum chamber as it is configured into your ITB intake manifold. Maybe someone here could give you an idea of how to extract any oil there without removing the manifold. Based on your lower rpm decel with 15" of vacuum I am starting to lean more towards a ring seal issue. Do you know the exact set of rings, first, second and oil ring types and tensions, etc.? What were your ring gap dimensions and do your pistons have an accumulator groove in the land under the first compression ring?

[Belgian1979]

What happened ? A lot of replies have suddenly vanished

[modok]

Compression tests and leakdown tests will not show up anything for oil control issues. Oil might even make those numbers look better. Compression sealing is top ring. Oil control is second ring and oil ring.

Well said. I think you do not mean to argue but every time you say " compression is good so rings are good" It just pisses us off.
If you are trying to piss me off, it's working.
If not, then ok.
I don't know if it IS or isn't the rings. Don't know. it could be the computer, a leak, guides, ect, but if you are asking us to DOUBLE CHECK your thought process, then of course we will all tell you, you haven't rules out the rings.
Don't want to hear it....then, that's your problem. Just saying how it is.