View Full Version : atmospheric crankcase venting question

03-15-2005, 06:52 PM
Years ago I removed the air injection system on my 76 tr6. After a recent engine rebuild, I also got rid of the evaporative loss system and related crap (British technical term).
I don't have a PCV valve (and don't want one) to vent into the intake manifold. I know that early tr's vented directly into the atmosphere via a tube to the underside of the car. That's what I've done and it seems to work (only about 250 new miles on engine). Does anyone know if I need a vented oil filler cap and/or if i'm screwing anything up?


03-15-2005, 07:01 PM
To be quite frank with you, a correctly operating Positive Crankcase Ventilation system is not a detrement to your engine, but rather, it is good for the motor. It will keep the internals cleaner, you will have less blow by because of pressure build-up. It sucks out the unwanted gases that seem to get by the rings. The oil stays cleaner. Engines run better with some vacuum in the crankcase, rather then pressure. I would try to retain the PCV system. Your engine will thank you for it.

Dave Russell
03-15-2005, 07:08 PM
I totally agree with you. Also, maintaining a slight vacuum on the crankcase will help to reduce oil leaks. As if LBC's had oil leaks!

03-15-2005, 07:09 PM
Martx-5 is dead right. You're not screwing anything up the way you are set up, though a proper catch-tank would be cleaner, but there's nothing wrong and a lot to go right with PCV. About the only downside is oil deposit inside the inlet manifold, though on a new engine that should be slight and in any case is a small price to pay for cleaner internals.

03-15-2005, 07:26 PM
Sounds like I want the vacuum. I don't think i can just dump it into the manifold--wouldn't that just create a huge leak? How did the old PCV valve work on the 69's? I'm looking at the diagram and it's attached to the manifold (I think mine has the spot pluged) and connected with a T in the line running from the manifold to the carb (I assume since they don't show the carb).

Dave Russell
03-15-2005, 07:34 PM
The PCV is slightly spring loaded to the open position. Engine vacuum keeps the valve closed except for a slight calibrated "leak". When vacuum drops, the valve opens to scavenge the crankcase. You do need to use a PCV that has the "leak" & flow rate somewhat calibrated to your engine displacement.

03-15-2005, 09:43 PM
Thanks Dave,

That makes sense. However, the way it's illustrated, the PCV seems to get vacuum from a center port on the manifold, but is T'd into a hose that appears to go from the rocker cover to the carb or manifold. How does it regulate vacuum if it's T'd in like that. Below is a link to Moss that shows the 69 set up.

Dave Russell
03-15-2005, 10:01 PM
The PCV doesn't regulate vacuum. The vacuum regulates the valve. The tee goes to the carb air intake.

Under high vacuum conditions, engine at idle, the PCV flow is at minimum. The vapors escaping from the crankcase (rocker cover) flow through the tee & into the the carb air intake to be burned.

Under lower vacuum the PCV lets more flow through & collects the rocker cover vapor plus a little back flow of fresh air from the carb air intake. I suspect that there is some sort of intentional restriction in the branch to the carb intake.

03-16-2005, 01:43 PM

When you said you got rid of the evaporative loss system and other 'stuff' (another tech term), were you referring to the carbon canister and its associated goodies?

By retaining the carbon canister and plumbing it as per the factory setup, you will be getting all the benefits of a positive crankcase valve, without the valve. This also provides the necessary fuel tank venting. Nothing in this system will harm engine performance and it does serve to keep the lube oil and innards a bit cleaner.

03-16-2005, 07:04 PM
Steve, you can dump the vent from the valve cover straight down and the vacumn of speed will vent your engine, and additionally lubricate the entire underside of the car. Not a good idea. I suck from my triple Strombergs from the valve cover through an oil separator which dumps the oil drippings back into the pan. Trust me, there is a fair amount of oil that goes this route. You don't want to suck directly into your carbs from the valve cover especially if you have an unbaffled valve cover, your gas will approach that used in your weed eater. The "pollution" canister used on the TR is a good thing and should be utilized. Mine was stripped away by the PO and couldn't find all the parts. GoodParts' oil separator works well in this app. Never suck directly from the manifold vacumn directly to the valve cover without something like a PCV valve to regulate. There's enough vacumn here to vacumn you carpet with.


03-16-2005, 07:31 PM
Thanks for the advice. It looks to me that an oil separator with PCV is the way to go. As to the canister, perhaps I shouln't have gotten rid of it but it had become so fuel soaked and the anti-runon valve broken which led to a crazy amount of run on. As my post above perhaps didn't clearly articulate, I'm still a little confused as to the PCV valve set up. From the '69 diagram I've seen, the PCV is mounted to vacuum source on the center of the manifold and t'd into a hose running from the rocker cover to the carb. How does this regulate the Vacuum? Won't the vacuum simply bypass the valve?

Dave Russell
03-16-2005, 09:19 PM
How does this regulate the Vacuum? Won't the vacuum simply bypass the valve?

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That's why I said I suspected that there was an intentional restrictor somewhere between the PCV tee fitting & the carb inlet. I think the restriction should probably be about .040" diameter. You could probably just block off the branch of the tee fitting going to the carb.

You DO need to have a vented filler cap. The more usual connection would be to run a hose from the filler cap vent to the carb fitting. Without some sort of vent (air intake) there is enough vacuum on the PCV to suck the gaskets into the engine.

There is likely to already be an oil separator built into the valve cover connection.

Hope I haven't confused things too much.