Date: Thu, 10 May 2001 09:28:53 -0800
Reply-To: Mark Keller <kelphoto@islandnet.com>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Mark Keller <kelphoto@islandnet.com>
Subject: Gas Milage and the Fuel Pressure Regulator
Content-Type: text/plain; charset=us-ascii
Listmembers
The general idea of this post, is to associate some symptoms of too high
of a fuel pressure. I own a 91 with a digifant fuel injection system.
The information pertains mainly to digifant although. Some information
is valid for digijet-- any additional information regarding the digijet
reactions to fuel pressure being too high that would help list members
should be posted too.
I've ben watching the posts concerning fuel economy and those about Fuel
pressure regulators that read a few pounds high. The regulator's in
these vehicles are under constant pressurized use and have a service
life. I believe the regulator gets far too little attention considering
an overrich mixture and it's associated symptoms: bad O2, Temp II, Temp
I, or maladjusted AFM.
What really seems to compound the troubleshooting process is the engine
seems to misfire, hesitate or malfunction the same no matter what is
really the problem. Many say this is because the computer gets confused
and thus similar observed engine symptoms can have radically different causes
A bit of background on the fuel pressure which is listed to be 29-36
PSI. The computer ASSUMEs the fuel pressure to be within specs when it
sets any and all mixtures via a fuel injector pulse. The computer has no
way to change the fuel pressure. There is no adjustment to fuel
pressure. Fuel mixture can be adjusted somewhat via the O2 sensor which
feeds data back to the ECU regarding mixture, but the ability to adjust
the pulse to compensate for a too high fuel pressure is limited. Below
are some clues when that limit is being reach But when the fuel pressure
is too high, the regulator must be replaced.
The first clue that the fuel pressure regulator is setting too high a
pressure is cold mornings when temp I, an air temp sensor, determines if
the computer richness the mixture to start the engine. A pressure
regulator that is beginning to get too high will cause the engine to
sound like it's running on three cylinders, or chug, and then as it
warms up and the Temp I compensation is removed, the chugging smoothes
out. I've had this, and was related too high a fuel pressure.
The second clue is when fuel pressure is higher than Clue I above and
results in a overrich mixture during general warm-up and all other
operating conditions. This COULD look like a bad TEMP II, or a poorly
adjusted AFM. Here the mixture is too rich because of The computer's
assumption of fuel pressure at 29-36 is incorrect and thus the
computer's base pulse calculation results in a rich mixture and leads to
step 3 below.
Step 3 The O2 sensor gets coated with soot from overrich mixtures. Then
the computer get a "too lean" signal and increases the injector pulse.
Then you really get an overrich mixture. I've removed the soot coating
by heating the O2 with my propane torch, and the sensor worked normally.
Since high fuel pressure can all of the above symptoms, I know I had
them, it should be considered among the first things to check when
trying to eliminate overrich fuel mixture symptoms
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. When an
engine is worn, has vacuum leaks, poor timing and etc., the vacuum
created is below the needed amount. This translates into the fuel
pressure regulator setting a higher fuel pressure. High fuel pressure is
possibly a more prevalent problem than we are used to taking a account
of when solving over rich conditions.
I believe that the testimony of those who have replaced the regulator
with good results would benefit those who are facing this overrich
issue.
Sincerely,
Mark Keller
91 Carat
Cowichan Bay, BC
