How to properly smoke test

[Wasatcher]

Hey guys,

I built a homemade smoke tester to check for vacuum leaks. My question here is when I'm using 1 MAYBE 2 psi to push this smoke through the motor is it necessary to remove the oil cap (like you'd do in a boost pressure test) to relieve pressure in the crankcase so you don't damage the seals?

Common sense tells me 1-2 psi isn't enough to hurt anything, but I removed the oil cap just to be safe because I'm paranoid. The filler neck is the only place I'm seeing smoke which is good news but I wonder if the the smoke is just taking the path of least resistance and not doing its job.

What do you say? Safe leave the cap on? I'm using a bike pump to push the smoke so I am in complete control of the flow of air.

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[whispering_eye]

I would say if 1-2 PSI of pressure blows a ring you had problems already. I run 700 PSI of nitrogen at work with the oil cap on/off. You shouldn't worry about rings with that amount of pressure. My bet is they are made to withstand 13-14 MPa (~1900 psi) of internal combustion pressure from a design standpoint. If you want to be extra super safe, run the test with the cap off. No harm done and peace of mind if you are worried.

[Wasatcher]

I'm talking about the seals in the crankcase, not the piston rings.

I'm filling the engine with smoke through the intake. I'm almost certain I'm just being paranoid here because if a seal can't withstand 1-2 psi then WTF is it gonna seal, but better safe than sorry which is why I'm asking. I even blocked off the airbox so no smoke can coat/contaminate the MAF sensor.

For example here it's advised to remove the oil cap. But this is when pressurizing with 14+ PSI, I shouldn't have to worry about damaging anything with a mere 1-2 PSI through the intake right?

[whispering_eye]

Ah sorry misunderstood which seals. I'd say you're fine. Generally speaking they're designed to withstand a few psi of vacuum due to negative cc pressure while at full throttle. Take the cap off and you won't need to worry. Not a big deal if you do it with or without the cap. You aren't testing the case seals anyways.

[Wasatcher]

Well I WANT to leave the cap on so the smoke stays inside the motor since a lot of it is leaking out through the filler neck.

You've helped fully convince me I'm splitting hairs here. If 1-2 psi could hurt something then the PCV system would do it

[viper_crazy]

Oil pressure is around 60-80 psi in the block during road operation and people have increased their boost to like 25 psi on some builds... How is 2 psi, or even a boost test to 10 psi going to be an issue with seals? Seems to me, your turbo would have taken care of that a long time ago.

[Wasatcher]

Originally Posted by viper_crazy

Oil pressure is around 60-80 psi in the block during road operation and people have increased their boost to like 25 psi on some builds... How is 2 psi, or even a boost test to 10 psi going to be an issue with seals? Seems to me, your turbo would have taken care of that a long time ago.

While I see your point, you're talking about the areas of the block pressurized by the oil pump, not crankcase ventilation. I think you'll find the vast majority of people will recommend you remove the oil cap when doing a boost test to allow excess crankcase pressure to escape without damaging seals.

I will agree I'm just being a paranoid worry wort. I've simply never pressurized the engine at all without removing the cap. But 1-2 PSI doesn't really count as pressurizing I suppose

[viper_crazy]

Originally Posted by Wasatcher

While I see your point, you're talking about the areas of the block pressurized by the oil pump, not crankcase ventilation.

Yes. Right! Dots connected... *click*

[Wasatcher]

Originally Posted by viper_crazy

Yes. Right! Dots connected... *click*

I'm not sure what that's supposed to mean or how it contributes at all.

[sebastian323]

Originally Posted by Wasatcher

While I see your point, you're talking about the areas of the block pressurized by the oil pump, not crankcase ventilation. I think you'll find the vast majority of people will recommend you remove the oil cap when doing a boost test to allow excess crankcase pressure to escape without damaging seals.

I will agree I'm just being a paranoid worry wort. I've simply never pressurized the engine at all without removing the cap. But 1-2 PSI doesn't really count as pressurizing I suppose

I suppose you mean that there are oil passages in the block that see pressurized oil, and that there are other passages in the block that also contain oil but the oil in there is not pressurized - such as the large drain passages that lead from the heads into the crankcase and the oil pan. The oil has lost pressure by the time it reaches these points as it has flowed through the various holes in the crankshaft, camshafts, AVCS, turbo etc. and lost pressure after encountering these restrictions. These are the passages that the oil seals are on.


I think even though a car may make 25 lbs of boost or more, the oil seals do not see this pressure through the crankcase ventilation system because when an engine is running the exhaust system is constantly relieving the built up pressure via the exhaust valves . However, when preforming a smoke test the engine is in a stationary position , where assuming no valve overlap, any cylinder that has the intake valve open ( and thereby exposed to the smoke test) dopes not have the exhaust valve open, so no smoke can escape via the exhaust system.

Perhaps one could set the blowoff valve really loose to have that be an emergency release point ?

Does anyone know at what PSI the oil seals blow ?


I left the oil cap off while smoke testing and all the steam came out of the cap. i slowly tightened the cap or placed my hand over where the cap goes to modulate the pressure to detect smaller sized leaks, as these were not very apparent when the cap was completely off. At one point the connection where I was injecting smoke popped off. I hop this thing came out from excess pressure and I did not blow an oil seal somewhere....