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 Sneaky Leaky Sump Technique?

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edlithgow
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PostSubject: Sneaky Leaky Sump Technique?   Sat Sep 14, 2013 12:11 pm

Following my more-trouble-than-it-was-worth inconclusive investigation of metal in my oil, I now have a leaky sump gasket.

You have to take quite a lot of bits off to remove this sump, and its quite tricky getting them all lined up (in the right order, duh!) to go back again. I've done it, IIRC, 3 times now. I'm not getting any better at it and the novelty has definately worn off.

I'm thinking that next time, after I put the sump back, instead of putting the car back together and test driving it, I should just overfill the sump.

If it doesn't leak overnight with the oil level above part or all of the sump gasket (I havn't decided on the optimum level but I suppose you could gradually ramp it up, perhaps calibrating a dipstick specially) then it'd perhaps be ok in operation (with the oil level restored, natch) and putting it back together is less likely to be a waste of time.

Make sense?

Forseable but unforseen (by me) snags?
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Father Tiresias
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PostSubject: Re: Sneaky Leaky Sump Technique?   Sun Sep 15, 2013 12:14 am

Not likely to work I'm afraid. When an engine is running there is positive pressure in the crankcase and it is this that forces oil past gaskets and seals, not the oil simply splashing around in the sump. Now, if you overfillied it to a level above the crankshaft and then applied a pressure or about 30mBar constantly to it that may show up a leak.
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edlithgow
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PostSubject: Re: Sneaky Leaky Sump Technique?   Sun Sep 15, 2013 9:02 am

Do car sumps get pressurised? I've heard of pumping losses for single cylinder motorcycle engines but I suppose I assumed the piston movements more or less cancelled each other out in a 4 cyl engine.

Mines 3 though, come to think on't, so maybe there are pressure changes that the crankcase breather can't accomodate?

I suppose if I blocked off the breather I might be able to pressurise the engine. Bit of a faff though, and it'd be a steady rather than a pulsating pressure.
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Father Tiresias
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PostSubject: Re: Sneaky Leaky Sump Technique?   Sun Sep 15, 2013 7:19 pm

Yes they do. That's why there is a crank case ventilation system. Without it seals would blow all the time.
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edlithgow
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PostSubject: Re: Sneaky Leaky Sump Technique?   Mon Sep 16, 2013 2:14 am

Father Tiresias wrote:
Yes they do.  That's why there is a crank case ventilation system.  Without it seals would blow all the time.
Sorry, but that doesn't appear to make sense. If I reverse your argument, it seems to work better.

" No they don't (get pressurised). There is a crank case ventilation system to prevent it. If it did get pressurised, seals would blow all the time"

Presumably the dipstick would blow out too.

The existence of the crankcase ventilation system seens (to me) to imply that an external negative pressure (from the intake manifold) is applied to the crankcase. I suppose the net pressure will be that (with a minus sign) plus any pressure due to blowby and piston pumping, so it'll be hard to predict, variable, and I can't see me being able to measure it, though I could try and rig up a manometer.
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Father Tiresias
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PostSubject: Re: Sneaky Leaky Sump Technique?   Mon Sep 16, 2013 8:18 pm

Ok, let me put it like this.  I work in the engine research and development department of a large construction and agricultural machine manufacturer.  I spend my life working in a facility where we test and develop diesel engines and measure typically 200+ different parameters on an engine under test. Measuring crankcase pressure is an important parameter that we have to look at when engines are being developed or are under test.  Excessive pressure indicates a problem with pistons/rings and is likely to result in leaking crankshaft seals and high hydrocarbons in the emissions.  We also measure the quantity of blow-by which is a direct result of a pressurised crankcase. On a 4.4 litre four cylinder diesel any crankase pressure in excess of 30mBar is likely to be a problem.  Blow-by in excess of 80 l/min is a sign of high crankcase pressure due to a fault within the engine and will lead to leaking seals.  They do not generate a negative pressure.

When crankcase pressure is too high blowing out a dipstick is not uncommon.  It's usually followed by a stream of oil too!!  Many times I have seen oil splattered up the wall of one of our test cells due to the expulsion of the dip stick.  Most dipsticks have a section that is a tight fit around their collar to prevent this. It is not uncommon on any car engine to find that the dipstick is not pushed all the way down the tube when you go to check the oil. Crankcase pressure causes this.

Also bear in mind that some CCV system are open to air and are not recirculated back into the inlet.  If there were no pressure there would be no need for an open CCV system.
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edlithgow
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PostSubject: Re: Sneaky Leaky Sump Technique?   Wed Sep 18, 2013 9:53 am

You say above “We also measure the quantity of blow-by which is a direct result of a pressurised crankcase” Surely the relationship is mostly the other way around i.e. crankcase pressurisation is a result of blowby?

(Though apparently there may also be some positive feedback

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.” Vacuum applied at around twelve to fourteen inches of mercury (HG) will improve ring seal, allowing lower tension rings to be utilized.”)

That said, a lot of your post seems to relate to fault conditions, where excessive blowby (and/or vent blocking, presumably), causes positive pressurisation. AFAIK I don’t have such a fault condition, (though I can’t exclude it) and I’m unclear whether you are saying that positive pressurisation is normal.

I’m also unsure to what extent biggish, newish diesels are comparable to my ancient sub-litre, 3 cylinder normally aspirated and carburreted petrol engine, and I havn’t, so far, been able to establish what crankcase pressures might specifically be expected of it. The consensus seems to be that petrol engine crankcases are “normally”under slight negative pressure.

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“Measure the amount of vacuum in the crankcase. With the engine at normal operating temperature, block off the PCV breather tube or vent to the engine (usually the hose that runs from the air cleaner housing to the valve cover on the engine). Pull out the dipstick and connect a vacuum-pressure gauge to the dipstick tube. A typical PCV system should be pulling about 1 to 3 inches of vacuum in the crankcase at idle.” – (that’s presumably the lowest it goes, since the vacuum will tend to decrease as the throttle opens)

Crude test on a Volvo

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In high performance (especially turbo’d) engines, blowby, piston pumping and heating all increase and may cause crankcase pressurization, to the point that some racers fit vacuum pumps to their sumps. I suppose the higher compression of diesels might also result in more blowby.

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Blowby of course does not generate a negative pressure, but my crankcase vent system is connected to the inlet manifold, downstream of the air filter. In a carbureted, throttle controlled engine, this is at a (variable) negative pressure.

I suppose if the crankcase venting system had a pressure control valve, it could be set to normally maintain a positive pressure (at least relative to the inlet manifold) within the crankcase. AFAIK my crankcase venting system doesn’t do this, and I don’t see why it would, since positive crankcase pressure is (more or less) a bad thing .

Without a pressure control valve, assuming the ventilation isn’t restrictive, the crankcase will be at the pressure of whatever its vented to, either the inlet manifold or (in an “open” system), atmosphere.

I’ve only had two diesels (a Maestro and a Dodge truck, both Perkins). I didn’t have an engine manual for either, and didn’t work on them to any significant extent. IIRC my Dodge truck had an engine powered pump to power the brake servo, presumably because there was not enough engine vacuum to do it, which suggests significant negative inlet manifold pressures are not always the case with diesel engines.

However, according to Racor, who make crankcase ventilation systems (surprisingly, emission control of diesel crankcase vapours apparently wasn’t required in the US until 2007) at least some diesels may be at negative crankcase pressure during normal operation.

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“The third integrated feature of the CCV is the pressure regulation valve. It balances the pressure in the crankcase, protecting it from the high vacuum created by a dirty air filter and todays high mass flow turbocharged compressors. Our pressure regulation valves monitor crankcase pressure ensuring that it maintains a range of -4 to +4 inches of water ”

There are various graphs showing crankcase pressure against turbo restriction, inlet pressure, etc. They are mostly negative, and the implication seems to be that, without the Racor system, they would be more so.

“As the turbocharger pulls more air through the engine air filter, a vacuum is created. This vacuum is referred to (as) an air, or turbo “restriction”. This vacuum is translated to the crankcase via the closed crankcase ventilation system, unless a pressure regulator is present”

So positive crankcase pressure seems generally to be a fault, and I don’t currently have any reason to think I’ve got it.
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Father Tiresias
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PostSubject: Re: Sneaky Leaky Sump Technique?   Wed Sep 18, 2013 10:06 pm

You say above “We also measure the quantity of blow-by which is a direct result of a pressurised crankcase” Surely the relationship is mostly the other way around i.e. crankcase pressurisation is a result of blowby?

Badly phrased on my part.  It should be 'crankcase pressure is a direct result of blow-by'  As you say, the other way around.

A typical PCV system should be pulling about 1 to 3 inches of vacuum in the crankcase at idle.

At idle there can be a vacuum but as soon as the engine starts working the pressures within the combustion chamber increase to the point where some gas will get forced past the rings giving a positive pressure in the crankcase.

In high performance (especially turbo’d) engines, blowby, piston pumping and heating all increase and may cause crankcase pressurization, to the point that some racers fit vacuum pumps to their sumps. I suppose the higher compression of diesels might also result in more blowby.

Diesels have a much higher compression than a turbocharged four cylinder.  Combustion chamber pressures can get as high as 170 Bar when turbo'd.  Same thing happens with petrol engines, stick a turbo on it and combustion chamber pressures increase forcing more gas past the rings and pressurising the crankcase.

Blowby of course does not generate a negative pressure, but my crankcase vent system is connected to the inlet manifold, downstream of the air filter. In a carbureted, throttle controlled engine, this is at a (variable) negative pressure.

What about the effects of pressure in the combustion chamber when the engine is under power?

That said, a lot of your post seems to relate to fault conditions, where excessive blowby (and/or vent blocking, presumably), causes positive pressurisation.

No, the point I was making was that (on diesels) a crankcase pressure of up to 80 mBar is not unusual.

“The third integrated feature of the CCV is the pressure regulation valve. It balances the pressure in the crankcase, protecting it from the high vacuum created by a dirty air filter and todays high mass flow turbocharged compressors. Our pressure regulation valves monitor crankcase pressure ensuring that it maintains a range of -4 to +4 inches of water ”

In other words the suction effect of the turbocharger can draw presssure from the crankcase via the CCV system, not that the crankcase is normally a vacuum.  As they say, a side effect of high mass flow turbochargers. This would be normal on a 'low blow' system. Once combustion chamber pressures rise, again there will be blowby and a pressurised crankcase.
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edlithgow
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PostSubject: Re: Sneaky Leaky Sump Technique?   Thu Sep 19, 2013 9:01 am

Thanks. That's clearer.

This is interesting (to me) because I hadn't really thought about it before.

Quite apart from the implications for my half-assed leak testing technique, it seems possible that assessing (if not actually measuring) crankcase pressure could be a relatively simple test of engine health, though you might need experience with a particular engine to interpret it.

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I found some specs for some Detroit Diesel engines which seem to max out (as in, reach the limit of acceptability) at 1” of water at 28000 rpm.

They suggest that “A slight pressure in the crankcase is desirable to prevent the entrance of dust.”

This suggests (though it doesn’t actually say) that slightly positive pressures are “normal”.

This is an apparent contrast to the figures shown in the Racor link, which shows negative pressures over most of the operating range, going positive at high outputs. Presumably diesels, and perhaps engines in general, vary.

Its quite hard to get figures for “normal” petrol-engined cars, especially bangers.

This thread goes into “How to test the BMW E39 pressure-controlled crankcase ventilation system (CCV)?” in rather obsessive detail

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That’s of course a rather different car and system to mine, but 4WIW

“A properly functioning pressure control valve is designed to maintain a slight vacuum (approximately 10 - 15 mbar ) in the crankcase which assures reliable crankcase venting during all engine operating conditions.”

This at least seems to establish the general principle. (which is the opposite of that stated in the Detroit Diesel link)

A few pages down, actual readings are reported for an M62tu engine

“Eng Temp-----RPM---------Vacuum in inches [of water]
Cold-----------1300--------4.64" (at startup, Sec air pump running)
54°C------------600--------3.61" (Sec air pump just stopped)
70°C------------550--------3.58"
80°C------------500--------3.37"
93°C------------500--------3.32"
102°C-----------500--------3.29"
108°C-----------500--------3.28"

If the engine was held (1500-4000 RPM) the vacuum was 2.92"
The RPM didn't matter.”

I don’t believe my system has any control valve, the crankcase is simply connected to the air cleaner via a baffle.

AFAIK the pressure in the air cleaner will always be negative with the engine running, so I should have negative pressure in the crankcase IF the breather pipe (and baffle) are clear (and wide enough), unless I’ve got a lot of blowby.

This guy

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reports on a 1971 Triumph 2.5 litre saloon (nice shape, IIRC) which is arguably a more comparable level of technology.

He was getting - 6psi (i.e below atmospheric) at idle, and -1 to 2 psi at 80 mph. Above that no reading, but he apparently couldn’t read positive pressures on his gauge, so it’s a fair bet it went + flat out.

He was testing a second DIY breather that he’d added, so perhaps a stock system would go positive earlier. OTOH, he did suggest that Triumph used a one-size fits all system which was under-capacity for the 2.5L.

Maybe mine is more adequate. I’ll probably have to measure it to find out.
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