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Twin cam crankcase pressure low, VC pressure high

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eschneider

eschneider

Jedi Warrior
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Got a weird one.

Customer is finishing the cosmetic resto of a Lotus-powered Morgan (recently purchased, basically an "unknown" engine). After we re-gasketed and re-sealed and painted the engine, it is now spewing oil.

The oil was 3-4 quarts overfilled. Fixed that, but now there is very little pressure coming from the draft tube (vent from the right rear of the cylinder head), and very high pressure when the oil filler cap is removed.

we know the engine has loose rings (consensus is that this is the result of a race-spec engine rebuild from the 80's) - but this should have caused high crankcase pressures, and we have the reverse problem.

the car does smoke heavily when started. <edit> Loose valve guides wouldn't be exposed to combustion pressure, though - the valves are closed at that point. I don't think it's a cracked head or head gasket - compression and leakdown were low but equal on all cylinders.

why would there be such a differnce between crankcase and valve cover pressure? Aren't the two essentially the same given that the timing chain passage connects the top and bottom end of the motor?

There is also a profuse amount of oil under the valve cover with then engine running - is that normal?

edit - I did check for obstructions in the crank vent from the fuel pump cavity to the outlet at the head - compressed air blew though without apparent restriction.
 
The easy stuff first... There *should* be a lot of oil in the troughs. The cam and "buckets" ride in it. That's normal.

The crankcase pressure is surely the rings, the oil smoke on startup is oil weeping down past the guides after it has been shut down, sitting on (likely) the exhaust valve heads. Likely what killed the rings!

Is this a Weber head or a Stromberg?
 
DrEntropy said:
The easy stuff first... There *should* be a lot of oil in the troughs. The cam and "buckets" ride in it. That's normal.
Click to expand...

OK. What about a pressurized filler cap?

DrEntropy said:
The crankcase pressure is surely the rings
Click to expand...

I have LOW crankcrase pressure - I can't feel any pressure at all coming out of the breather tube. Shouldn't loose rings be HIGH?

DrEntropy said:
Is this a Weber head or a Stromberg?
Click to expand...

Weber 45 DCOE's. This motor was supposedly "built to race spec" in the 80's. Customer is confident the loose rings are "race spec" - I'm not as convinced.
 

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eschneider said:
...Fixed that, but now there is very little pressure coming from the draft tube (vent from the right rear of the cylinder head), and very high pressure when the oil filler cap is removed.

...
Click to expand...

I'm working from a not always reliable memory here but the only "tube" I remember at the right rear of the head is the rubber tube that serves to drain the oil from the head back into the crankcase. This drain was just about adequate when everything was working well and they're easy to mess up when the head is installed. I don't remember any vent in that location, at least with the stock setup.
 
There's a vent there, it runs to the air box, John. :wink:
<span style="font-style: italic">
EDIT: Shoulda said: "on standard engines"... An 8"~9" steel tube, mild kink about 2" from one end, shouldered on both ends to allow rubber seals to be smushed into place when the back side of the airbox is bolter to the carbs. </span>
 
Many hipo TC's also employ an additional breather on the front face of the cam cover.

I have this on mine, it certainly reduces crankcase pressure/ oil leaks.
 
After consulting with Doc Entropy, I decided the pressure in the valve cover was "pulses" rather than pressure. To test the idea, I put a rubber glove over the oil filter cap - it "fluttered" more than "blew up"

here's a quick video, just for giggles. Just playing with the camera, really.

https://s62.photobucket.com/albums/h110/schne92650/?action=view&current=doorleyvent.mp4

So the final thought is:

1) the oil leak was primarily caused by overfilling

2) the very strong pulses in the valve cover - dunno if that's normal, and it seems like it would stress the valve cover gasket.

3) I'm still concerned there is NOTHING coming from the crank vent tube, but it might make sense that the pulses are baffled/muffled by the chamber in the head.

Even if it's 5 years from now, if anyone reading this has a similar experience or further insight, I'd love to know.

edit: after we witnessed oil bubbling around the exhaust ports (through the new gasket) the customer is now resolved that the motor needs to be "refreshed" in the near future. Some better photos of the front vent would help - sounds like this is an improvement that should be added when the motor is apart.
 
It gets th' pressure from crankcase to bottle simply for settling.

LP's mill is a result of years of experience fettin' with 'em. The photo of the front vent above is what makes for no backpressure or crankcase pressure.

BTW: the vented cam cover fitting can be either home-made or acquired thru Dave Bean... Go for doin' it yourself, Eric. It was well documented ~someplace~ I can't recall just now (HELP me MB). Fairly well accepted in twink circles as a good solution.


Thanks for the pix, LP.... And dont' be such a stranger here!!!!

:laugh:
 
Was just wondering as the oil filler seems to be blocked off in the picture, hense the need for another vent source? Excellent looking motor!
 
Thanks.

It is actually an aftermarket oil cap setup. A sleeve with internal threads is epoxied inside the oil filler opening. A screw in cap with o ring eliminates the leaking problems that go with the stock cap. Looks better too. :smile:
 
The vented cam cover referred to by Doc above, was documented as a cure for oil leaks in TC Europas.
Read it Here.
 
Lotus West and L.O.U. carried that as well.

Funny/interesting the T/C Europa guys show the alternator PULLEY on the exhaust cam and site ITs seal as a leak point. :wink:
 
The document in the link I sent shows the alternator pulley on the inlet cam, where it should be.
Here's my engine . You can see the breather pipe coming from the front of the cam box.
 
I wuz takin' th' wizz, Roger. :wink:

The cam mounted pulley arrangement was/is introducing another opportunity for the thing to leak. As if there weren't enuff already. :devilgrin:


I did miss-speak with 'exhaust' as opposed to intake. my bad. :shocked:
 
A little bit of oil leaking from the engine, gearbox, final drive, etc., stops some of the chassis rusting away.
 
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