Great note!
Question. Does this mean that there is some sort of flap valve or whatever
hidden behind that metal cover over the outpipe in the TR6 valve cover? If
so, how is it changed?
The idle in my TR6 is a little high and rough, but drives beautifully under
any throttle above idle. Now I am going over the system looking for carb
leaks etc... Perhaps this valve you speak of is a source of some of the
problems...
Mark Hooper 72 TR6
-----Original Message-----
From: Peter Macholdt [mailto:vze2846b@verizon.net]
Sent: Monday, February 25, 2002 12:01 PM
To: 6pack list
Subject: Re: Emission Control
Thanks Larry. I'm posting this to the list because I thought it might be of
interest to others.
The answer below is a great explanation of the EVC system. It addresses my
question about the valve in the base of the EVC unit. However, I still
don't know what the upper spring, plunger, diaphragm and plate do. Based on
the explanation, why is the upper part even needed? I would think that the
lower valve would handle all the functions of the emissions system as
described.
BTW, this is an academic exercise only. I'm not going to change anything, I
only want to understand it.
Thanks,
Peter
'68 TR250
From: LaJoMor@aol.com
Date: Sun, 24 Feb 2002 23:24:17 EST
To: vze2846b@verizon.net
Subject: Re: Emission Control
Peter...
The ECV (better known as a PCV valve-positive crankcase ventilation valve)
captures what is commonly called 'blowby'. That is the fumes from combustion
that blows by rings or valves and ends up in the crankcase/valvecover.
These emmisions are piped either to the ECV valve or directly into the air
fillter box theoretically to be reburned through the intake manifold. This
and the timing and carburetion settings on the car are about all the
emission control there was on the 250. The 6s got more complicated with air
injection and fuel vapor canisters etc. See the below paragraphs for a
little more detailed info about PCV valves in general.
Question:What is the PCV valve and what does it do? Answer:The blowby vapors
that end up in an engine's crankcase contain moisture as well as combustion
byproducts and unburned fuel vapors. The crankcase is sealed to prevent the
escape of these gases into the atmosphere, but the vapors must be removed to
prevent oil contamination that leads to sludge formation. The positive
crankcase ventilation (PCV) system siphons these vapors from the crankcase
and routes them into the intake manifold so they can be reburned in the
engine. The main component in the PCV system is the PCV valve, which is
usually located in the valve cover. A hose connects the PCV valve to the
intake manifold. A second hose between the air cleaner and crankcase or
other valve cover (V6 or V8 applications) provides fresh air to help flush
the vapors out of the crankcase. Some engines have a separate air filter for
the PCV breather hose located inside the air cleaner. The PCV valve is a
spring-loaded valve with a specific orifice size designed to restrict the
amount of air that's siphoned from the crankcase into the intake manifold.
This is necessary because air drawn through the valve from the crankcase has
a leaning effect on the fuel mixture much the same as a vacuum leak. So air
flow through the valve must be controlled within certain limits. At idle,
air flow is reduced because little blowby is produced. When the engine is
cruising and vacuum is high, airflow through the PCV valve is at a maximum
to purge the blowby vapors from the crankcase. It's important to note that
PCV valves are sized for specific engine applications. The wrong PCV valve
for an application can flow too much or too little air causing driveability
problems. Varnish deposits can clog the valve, so replacement for
preventative maintenance is recommended (every 50,000 miles usually).
Hopes this answers some questions...
Larry Morrison
TR250 CD5182L
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