Date: Thu, 10 May 2001 19:38:12 EDT
Reply-To: Wolfvan88@aol.com
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Robert Lilley <Wolfvan88@aol.com>
Subject: Re: Gas Milage and the Fuel Pressure Regulator
Content-Type: multipart/alternative;
>>Mark Keller wrote:>Also, I've mentioned in the past, but it's worth
restating. The vacuum>produced by the engine also affects fuel regulator
pressure, because the>strength of the vacuum is what keeps the fuel pressure
LOW. <<
>>>This is wrong. ...Because the injectors are exposed to fuel pressure on
one end and manifoldpressure on the other, the fuel regulator maintains a
constant pressuredrop across them and therefore a constant flow rate
regardless of engineconditions.<<<
I disagree.
The amount of Vacuum present at the Fuel Pressure regulator (FPR) does
directly affect the fuel pressure. The higher the vacuum the lower the fuel
psi until the preset limits of the stock FPR kicks in.
The FPR's job is to match the fuel pressure delivered to each cylinder with
the load on the engine to what VW engineers thought was correct for the
engine to maintain power, cooling and emission outputs.
The position of the throttle plate butterfly determines the speed at which
air enters the manifold then on to each piston. There are two types of
vacuum: High and Low. Low is when the air flows past a port as in the
throttle body and the vacuum goes from low to high as the engine speed
increases. This is good for vacuum advance distributors. The spark advances
in direct response to the engine rpms. A vacuum retard on the opposite side
of the distributor diaphragm going to the manifold decreases the spark
advance as the load increases on the engine. The two vacuum sources balance
the spark to provide the perfect spark at all rpms AND load conditions for
the engine. The DJ uses this type of distributor setup.
The other source is High vacuum. It is high and goes low as the air moving
past the port slows down. The vacuum here is generated form the piston. As
it goes through its intake stroke, the intake valve is open and the air is
drawn into the cylinder. The position of the throttle body butterfly valve
determines the volume of air that can enter the cylinder. When the engine is
idling, the piston exerts a powerful force in trying to suck air into the
cylinder. The cam timing, air filter construction and cleanliness and other
obstructions to flow limit the volume of air that actually can enter.
NOTE of power of Vacuum:
I once had the tip of my rubber glove sucked off my finger into the brake
vacuum port on the manifold when I tried to block the port with the engine
running during a test. It probably was melted into the CAT...and the engine
died due to too much air and not enough fuel.
The VW engineers has measured the volume vacuum created by the engine design
and put a FPR to limit the psi to 29 to 36 psi based on X vacuum volume.
When there are other sources of air entering into the manifold, the volume of
vacuum DECREASES and thus corresponds to less vacuum and then less pull on
the FPR and therefore a higher psi than designed. VW Engineers built in
tolerances, a few PSI either way but not MUCH. Vacuum Leaks cause idle
emissions to increase because the limits of what the CAT and engine can burn
are exceeded. I know because I failed a State emissions test due to a
cracked vacuum line to the AFPR. It was split on the end that created a
vacuum leak and therefore HCs were too high. I fixed the hose and passed the
test a second time.
The less vacuum present, the higher the fuel pressure will be until the Max
limit built into the stock FPR unit kick in. I worked on a van that would
run rough on idle, but perfect after warm. I had just done a top end rebuild
with new rod bearings and trying to trouble shoot the problem. The FPR was
shot and at 60 psi. I replaced it with another working unit and it was
within spec. Engine started perfectly and idled cold for the first time
since they owned it, they bought it used...
When revving the engine with a fuel pressure checker, I noticed that when the
engine was revved, the fuel psi briefly increased to 34 psi then dropped to
29 psi at 3000 rpm.
I have an adjustable dual diaphragm pressure regulator on my engine and the
same thing but the psi dropped to 20 psi at 4000 rpms. I thought that this
was low so I put some holes in one of the diaphragm and the pressure dropped
to 26 from 36 no vacuum setting. I have not been able to test the new
configure much because after I decided to punch holes in my expensive
adjustable FPR, I got a tank of Non metallic sediment that has damaged my
fuel system and put my van on the disable list until I can win my suit in
court.
My dual diaphragm FPR was too sensitive to the vacuum, when I first was
deciding on parts to change for the modifications, I wanted to change the cam
profile (I did and it has worked good) Since increasing the duration (both
valves are open longer) can decrease manifold vacuum I was told.
My thinking now is that if both valves are open AND the exhaust does not have
scavenging and too restrictive an intake, then the piston can draw from the
exhaust and decrease the vacuum volume and affecting the FPR to the point
that it does not operate correctly.
I think my cam modifications work because I have increased the exhaust
extraction with the ceramic coating and free flowing muffler and increased
the intake volume so it is balanced.
The fuel exiting the injectors is pushed when the injectors are opened by the
pressure behind nozzle. Not by being pulled out. This is controlled by the
vacuum the pistons generate balanced by the throttle butterfly position and
load on the engine.
I think Mark was correct...vacuum leaks affect the fuel pressure regulator...
Robert Lilley
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