Background:
Early stock 18V engine on a 1971 BGT. Twin HS4 carbs with no smog equipment. Front tappet cover vents to a y-tube which plugs into each of the carbs (providing a vacuum). Alloy rocker cover with a vented cap. Car runs fine, oil pressure 50-60psi at all times. The tappet cover filter is clean and flowing (it was only recently off to replace the gasket). Oil consumption is about 1 quart per 500 miles, compression ~160psi on all cylinders.
Issue:
The vented cap 'blows', and there's a small smear of oil on the underside of the bonnet above where the rocker cover cap vents. I've read that a good test of the pressure setup is to place a card over the rocker cover opening with the engine running - the card should stick there due to vacuum, rather than vibrate off.
Question:
Is the vent in the rocker cap there to suck or blow (or both)? If it's there to suck i.e. a negative pressure under the rocker cover, why might mine seem to be blowing excessively?
Positive or Negative Pressure?
The MG Experience ~ MGB & GT Forum ~ Archives
MG MGB and MGB GT Tech Talk
MGB & GT Forum: Positive or Negative Pressure?
http://www.mgexperience.net/phorum/read.php?1,1547271
Join the discussion, post your photos, or ask your own questions. Membership is FREE!
18V engines should have non-vented caps. The early 18G-GH caps were vented, but not the 18V.
18V engines should have non-vented caps. The early 18G-GH caps were vented, but not the 18V."
The 18V's didn't have vented caps because they vented through a tube from the stock valve cover to the charcoal canister. If you are using a non-vented alloy valve cover, you need to use a vented cap to let air into the engine.
To understand how the crankcase ventilation works on 18GK, 18V engines (and I think 18GH as well) you need to study the evaporative loss system - it is an integrated part of the crankcase ventilation.
Unfortunately, the documentation is tricky to find - it was in the emissions control manual and not part of the standard manual. I have it, but don't have time to find it right now.
Basically, the gas tank is sealed. Fumes from the gas tank (perhaps due to expansion from sitting in the sun on a hot day) are piped to a carbon canister. The carbon canister is plumbed to a fitting on the rocker cover. Note that there is a restrictor in the rocker cover fitting to prevent too much flow.
The crankcase is also sealed (via a non-vented cap). A fitting on the carburetor applies a mild depression or vacuum to the crankcase via a tube into the tappet (side) cover. This sucks not only the blow-by from the engine but also the fumes from carbon canister.
If you need better or more accurate details let me know and I will type in the data from the emissions manual.
I'm sorry to say that with that oil consumption and pressure from the rocker cover cap, I suspect you have ring/bore problems.
Mr. Twist has a good video on the subject
http://www.youtube.com/user/Universitymotorsltd#p/u/40/eyfRz3QZGsU
Watched the video, but nothing about the vented/unvented caps in this one....Hopefully I'll find more info in this.
Thanks
Edward
The vented cap isn't hurting anything, but the excessive pressure and oil consumption indicate that your cylinders are, indeed, worn to the point that oil is bypassing the rings and is being burned in the combustion chamber. RAY
Blow -BY as someone mentioned..
500 miles/qt is excessive consumption.
Do a bleed down check to verify..
With the alloy valve cover, you should use a vented filler cap on non USA cars that were not fitted with the charcoal canister.
Have you checked the hoses that run from the tappet cover to the y-piece and to the carbs for clogging?
Has your engine been bored and honed recently and needs to have the rings bed in still?
Curtis
When the engine is cold, have someone watch the tail pipe at first startup....If a puff of blue smoke comes out, you could have leaking valve seals...I'm not sure how much oil
this will use, but it will definitly affect oil consumption...
If, car is not smoking (bluish smoke), when hot, and your plugs are not gummed with oil residue, then you may not be in that bad a shape...
My advice is to check your plugs for oil residue first (black and gummy)...That's the easy way to know if you are burning the oil..Then , have someone with a good eye, follow you to see if your car smokes (blue smoke) during upshifts (accelerating)...If it is, you will need an overhaul/ring job.
The vented cap isn't hurting anything, but the excessive pressure and oil consumption indicate that your cylinders are, indeed, worn to the point that oil is bypassing the rings and is being burned in the combustion chamber. RAY
"
I usually agree with Ray - but not in this case.
On my car, if I remove the non-vented cap, the idle actually rises. The car feels it as a vacuum leak. Now, I admit, the hole in the vented cap is small, so it may not be as profound as removing the cap - but it will still contribute some leakage.
Of course, if your blow-by is so great that you have a pressure, then Ray's point would be correct. But until you do a leakdown test and establish that your rings are leaking, your pressure problems may be due to bad or incorrect emissions plumbing.
With the alloy valve cover, you should use a vented filler cap on non USA cars that were not fitted with the charcoal canister."
Yes - that's what I have - not charcoal canisters for us Kiwi's!
[URL=http://img442.imageshack.us/i/coverroq.jpg/]
[/URL]Yes: when I suck at the carb ends of the y-hose, there's vacuum at the vent cap. Interestingly, there's a small amount of 'mayonnaise' in the lumen of the carb hose pipe where the y-hose attaches.
Has your engine been bored and honed recently and needs to have the rings bed in still?"
No, the lower end hasn't been touched in the ~40k miles I've driven the car.
When the engine is cold, have someone watch the tail pipe at first startup....If a puff of blue smoke comes out, you could have leaking valve seals...I'm not sure how much oil
this will use, but it will definitly affect oil consumption...
If, car is not smoking (bluish smoke), when hot, and your plugs are not gummed with oil residue, then you may not be in that bad a shape...
"
Pleasingly, the car doesn't smoke noticeably on either startup or when hot / accelerating / decelerating. Drives very nicely, in fact.
Here's one of the plugs - the rest are the same:
[URL=http://img840.imageshack.us/i/plugr.jpg/]
[/URL]No carbon canister on foreign 18V engines? Who'd have thought? The main purpose of the 18V seemed to be emission controls.
Okay - so your gas tank cap must also be vented - true?
The position of the tube that taps into the carbs does not cause much of a vacuum (certainly not manifold vacuum). If you are sure the hoses have no kinks then likely you just have too much blow-by. But why don't you confirm that with a leakdown test?
The oil filler cap on the photo is definetely not a true vented cap, which you need if you do not have the charcoal canister.
Milan
Some interesting reading here:
http://www.british-cars.net/mgb-technical-bbs/valve-cover-venting-2008042623312325919.htm
Without the filter vented cap, I guess I would be sucking some crud into the engine - were it not for fact it seems to be an erroneously positive pressure system at the moment (yes, therefore blow-by seems to be a distinct possibility).
I'm pretty sure I don't have a vented gas/fuel cap. So many MGB variations!
I'll take her to the local cylinder-head shop next week for a leak down check. That might also tell me whether the mayonnaise is due to condensation or something more fundamental.
Before ordering a new vented oil filler cap, make sure that you have the right rocker cover. Looking again at the photo I fear that yours will not accept the right cap unless it has two tabs inside the filler neck. If you have the tabs on the outside, the cap will not fit and you'll need to replace the rocker cover for the correct one (whether steel or alloy).
Milan
It does have the internal lugs, so a plastic filtered vented cap (in fact, the one from my OE rocker cover) does fit. That doesn't solve the problem of the positive pressure, but it's good to know.
I'm pretty sure I don't have a vented gas/fuel cap. So many MGB variations!
"
Then I'm mystified. The non-vented fuel cap depends on the evaporative loss system to allow air into the tank. Air has to get into the tank somehow, or the car will stall due to a negative pressure in the tank.
I checked Clausager and the export cars did NOT have evaporative loss (and therefore the carbon canister) - but that does leave the mystery of how the fuel tank was vented.
This is actually important because I remain convinced that, either you have lots of blow-by, or something is hooked up wrong with crankcase ventilation. But given that you don't have the evaporative loss, it does seem pretty straightforward and hard to mess up.
The tube that leads into the side cover goes through a sort of box that contains some type of filter - it can apparently become clogged. But you say you can blow through the tube so that doesn't sound likely.
I'm out of ideas.
The vented cap should not make any difference. The non-vented cap was used with the vented rocker covers. Either vent is designed to let the upper end breathe. If there's positive pressure, it blows. If there's negative pressure, it sucks. When you're doing 4000rpm, some of the oil is aerosol-ized and displaces some of the air in the engine - that's when a vent needs to blow. When you then stop and most of that oil falls out of suspension in the air, there needs to be a a way to suck air back into the engine. The vent in the top end is mostly to let oil drain back down the push rod tubes. If you have positive pressure in the rocker galley all the time - causing the vented cap to blow oil, check to make sure your hose from the rocker is attached to the intake manifold and not the exhaust manifold :). In order to get exhaust gas into the rocker galley to pressurize that area, you either need a monstrous crack in a head, a ton of positive pressure in the sump (but you say your vented side cover is working fine), or you've got some worn exhaust guides with old or bad or none stem seals. I'm betting that guides and some clean exhaust valves with some new stem seals will fix you right up. PM if you need some instruction. Basil 707.762.0974
There doesn't appear to be a vent at the fuel tank - but then I'm not exactly sure what it is I'm looking for. The cap is a locking one, with no apparent venting, and there's nothing else obvious (but I haven't climbed under the car to check the side of the tank).
I just took the black plastic cap from the 1970 and, with a tight fit, it does go on the alloy cover. Interestingly, the engine idle does drop just perceptibly when this cap removed. I'm suspecting blow-by - but is this from the valve seals, or the compression or oil control rings?
Perhaps I should just shuddup and enjoy the driving until something becomes obvious!
This is an archived discussion from the The MG Experience Forums
If you would like to post a reply, please click below to visit the The MG Experience Forums:MGB & GT Forum: Positive or Negative Pressure?
Archive Index | The MG Experience Forums | Return to The MG Experience