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Oil breather / rocker breather

Richnd1974

Enthusiast
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Bournemouth
I noticed that when I remove the oil fill cap I get a wosh of fumes, it seems to be backing up, see video, so replaces the pipe that runs from the t piece to the bottom of the exhaust manifold as it had a little red connector piece in that was seriously hampering air flow. So questions, is that bit meant to be there and the pressure I am seeing is this correct. I have also checked the pipe from the rocker cover and its chuffing well so not blocked
 
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hi - in my van with the 1108cc engine the breather pipe goes full-bore from carb top to rocker cover, and this pipe tees off down to the manifold via a restrictor (1mm or so in diameter). This is how it is supposed to be; hope this helps.

Lovely engine bay by the way !
 
Aha thanks same as mine then, so the little red bits meant to be there then, which way around does the restrictor go, towards the exhaust manifold? The smallest opening towards the manifold or t piece
 
In mine, the restrictor is not a valve (but check it by blowing into a cup of water each way!) so orientation doesn't matter.
 
Mines wider one end then narrows but agree shouldn't matter... Will re install later, the pressure I get when I remove the oil cap do you get something similar?
 
There is a little chuffing but not visible; when I loosely hold a piece of A4 paper just under the gear-change rod with the oil cap off, the paper just about hovers in place at tickover. There is obviously a net flow out of the cap, but nothing I am worried about. To put this in context, the engine is 102,000 miles old, and after a compression-test has an average dry cylinder pressure of 158psi (I think that theoretical new values were about 166psi, so I am happy with my engine condition).
The 'paper test' may be worth trying (maybe I will patent it) to compare net outflow of gasses.
 
Compression tester ordered, any ideas of what psi readings I should get on a 956cc engine?
 
The following may be all a load of boll*cks but I think that the maximum psi on a 'dry' compression test depends only on the compression ratio (and health) of the engine.
I think that the 956cc (Sierra, or Cléon) engine would probably use a 8.3:1 compression ratio (1986-91) - or possibly 8.5:1, 9.25:1, or 9.5:1, though these higher ratios could be non-R4, later-built engines. Lower compression ratios were usually set to enable lower-octane ('lower quality' - as in the olden days) fuels to be used successfully.

The rule-of-thumb I read of and used for my 'as new' pressure calculation was: pressure=compression ratio x 20psi. If that is correct, then the engine's 'as new' pressures for the 8.3:1 956cc engine would be 166psi (or up to 190psi for the highest compression ratios), but your readings should be lower than that, at least due to wear. Wear or damage in the pistons/piston-rings/bores/valves will lead to a lower pressure from a compression test.

Without further knowledgable input, I would look to achieve 150-160psi for a healthy engine, with readings reasonably consistent between cylinders (say 10-12psi max difference). Particularly low readings in one or two cylinders often points to damage inside.
A 'wet' compression test (with a squirt of oil in the cylinders to further seal the bores) would produce slightly higher readings, but can usefully pinpoint leaking bores if the pressure jumps significantly.

Any professional mechanic doing your compression test should know what a 'good' result will be for your engine; I may be a bit off but hopefully not a mile away!
 
Something I've learned about Renault engines that are driven very, very hard - as if a pretend sports car - is that they can quickly give symptoms of burned valves, which can also contribute to a lower or varied compression cylinder to cylinder. However, a 10 minute simple valve adjustment fixes it immediately! Brilliant motors....they seem to save themselves from drivers like me :)
So if your compression numbers are unsatisfactory, try the test that I use [whenever it seems to run less than perfect] - with engine idling, pull off each spark plug wire one by one. If on each one the sound changes dramatically, and evenly, then all is probably ok. When one or two seem not to change the rpm or pitch much...those valves are the ones I know need a bit of tweaking.
You real mechanics are respectfully invited to contradict or agree...as I don't pretend to be a mechanic [only a pretend race driver] and mostly I learn by trial and error, and error, and error.
Michael Stechow
 
I'm aiming to do a compression test next week as tool has just arrived, I shall report back, thanks for all the comments
 
Something I've learned about Renault engines that are driven very, very hard - as if a pretend sports car - is that they can quickly give symptoms of burned valves, which can also contribute to a lower or varied compression cylinder to cylinder. However, a 10 minute simple valve adjustment fixes it immediately!

I have just reset my tappets (valve clearances) which has improved the engine noticeably! A few valves were 2 thou (inches) out - one exhaust was 2 thou closed, another couple of valves a thou or two too open. Amazingly easier to fine tune the mixture and tickover now :) Set at Inlet: 6 thou; Exhaust 8 thou.
 
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