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Oil in air intake

3K views 8 replies 4 participants last post by  kiwijules0505 
#1 ·
Hi,

I realise that the breather hose vents into the air intake before the throttle body, but is this amount of oil normal?

156 2.5 v6

Thanks


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#2 ·
It can depend on how car is driven as well as conditions. The oil is clean so that's good but it may be worth cleaning the oil separator (in the top cover , I think) to make sure there is not a lot of old oil deposits clogging it.

If it's all clean anyway, it could be just a bit or urban use that causes it.
 
#3 ·
Thanks Fruity. It's a track car, but due to lockdown it's just been started up occasionally for the last few months. Will have to take it for a blast sometime soon.

Will also check the separator - I didn't even know there was one...

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#4 ·
Actually, being a track car, where it is full throttle and high engine speed all the time, this may be the reason. An occasional clear out is one thing but asking maximum performance from it all the time could somewhat overwhelm the separator, I think. In other words, in normal use, I don't think there would be an issue but it may be a trait of simply being a track car.
 
#6 ·
If the engine is mostly used on track at WOT then this could be why there is oil being deposited into the atmosphere side of the the throttle plate and induction pipe.

There are two 'breather' tubes. Tube 'A' is located between the crankcase and the plenum chamber. Tube 'B' is located between the crankcase and the main induction pipe.

In 'normal' use, most of the time the throttle plate is only open to a fairly small degree. At small throttle openings there is a strong 'vacuum' in the plenum chamber, and blowby gasses vent from the crankcase directly into the plenum via tube A, while fresh air is simultaneously 'drawn' into the crankcase through tube B (from the induction pipe, this fresh air helps to better purge the crankcase of harmful blowby gasses).

However, at and approaching WOT the pressures on each side of the throttle body more or less equalise, with plenum chamber pressure rising to almost match the near atmospheric pressure that exists in the intake pipe.

At / approaching WOT the crankcase pressure rises substantially because more blowby gasses are produced, case pressure increasing significantly above the pressure in the induction tract (maximum blow-by would be likely to occur at peak torque I would expect).

With the increased crankcase pressure, as well as equalised pressures in the plenum and induction pipe, pressurised blowby gasses now escape from the crankcase not only throughjust the one breather tube, but via both of the breather tubes (i.e. A and B). Gas flow through tube A continues as before (though probably at a reduced volume due to the pressure rise in the plenum), but flow through tube B reverses. Blowby gasses now vent into the plenum chamber via tube A as well as into the intake pipe via tube B.

The vented blowby gas is quite likely to carry a significant quantity of oil with it, and to then deposit this oil not only into the plenum chamber but also into the induction pipe, which I think is probably why you are seeing oil there...

Oil is not a good thing for the engine to be breathing. It in effect becomes incorporated as part of the inducted 'fuel' charge and has the effect of lowering the octane of the inducted 'fuel' as a whole (i.e. petrol + oil mixed). How much of a real problem this might be is hard to say, possibly inconsequential, possibly not, depending...

For a track only car, it might be worth considering venting the crankcase into a catch tank, rather than allowing blowby gasses and oil to be inducted. Also keep in mind that blowby gasses have a reduced oxygen content, so if you can replace inducted crankcase gasses with fresh air then there should be at least some power pay off...

Regards,
John.
 
#7 ·
I used to track my 2.5 156 and got a little oil but not as much as that. The above is a good explanation thanks, does anyone understand the differences in the setup between the cf2 and cf3? The cf2 has 2 pipes T pieced coming out the rear bank to the intake pipe, and the oil seperator fitted that goes to the plenum. But on the CF3 the rear bank breather just goes to the plenum and there is a seperate breather via the front bank that goes to the intake pipe and apparant no oil seperator?
I considered fitting a catch tank to my car as im sure after I internally cleaned out the inside of the plenum chamber it improved performance. But ive been confused by the inlet and outlet setup needed for catch can to work properly when there is 2 sepearate pipes on my cf3...
 
#8 ·
"I used to track my 2.5 156 and got a little oil but not as much as that. "

That much oil in the induction tract does tend to suggest that blowby is probably a bit excessive. In other words the piston rings have probably seen better days.

Regards,
John.
 
#9 ·
Thanks for the info John. With 120k miles on the original, unopened engine and having done around 300 competition laps (around 600 miles) in the last 3 years, I won't be surprised if the rings are a bit tired.

I've always said I'll race with the original engine till it dies and then I'll upgrade to a 3.0 out of a 166, but the 2.5 just keeps going...

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