SL/R107: Fuel damper
07-18-2006, 06:20 PM
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fuel damper
Hi Jim,
I found this on a Toyota site but believe it applies to MBs as well.
Marinucci, Dan
When a normally reliable component fails, it can put a 'damper' on a
quick diagnosis, especially if the symptoms are missed or misinterpreted.
Toyota has used dampers on its fuel injection systems for years. But if
you're like most technicians, you haven't looked twice at a damper
because it's normally very reliable. Trouble is, when a damper finally
does fail, it often confuses and frustrates uninformed technicians.
This month I'll explain how a fuel damper works and well as what can go
wrong with it. There are plenty of these dampers in service, and you
don't want a bad one fooling you!
At one time or another, eveiyone has heard a water pipe in a house begin
to bang after someone opens a faucet and closes it suddenly. If the
banging pipes bother you enough, you can source a little hydraulic
damper from a hardware store or home center and tee it into the noisy
pipe. Basically, this kit acts like a hydraulic shock absorber on the
water pipe, dampening the effects of the sudden pressure change inside
the pipe.
The fuel damper on an injector rail (photo 1 at left) also serves as a
hydraulic shock absorber of sorts. Electric fuel pumps don't produce a
totally smooth, seamless flow of fuel. To the contrary, fuel pump output
pulses, and ultimately this pulsing can cause noises downstream in the
fuel rail. Of course, the risk of hydraulic noise in the fuel system
varies from vehicle to vehicle and system to system.
On older Toyotas, you might see a fuel damper mounted right at the fuel
pump outlet. But the most common location for it is at the fuel rail
inlet. The damper contains a spring and diaphragm that absorb some of
the shock of the fuel pulsing into the fuel rail. When the system is
working correctly, fuel pressure lifts the diaphragm, pushing it up into
the damper housing. Pushing back this diaphragm allows the fuel to
continue flowing into the fuel rail.
If you pry off the plastic cover that's pressed onto the top of the fuel
damper, you'll find a factory-set adjustment screw (photo 2). This screw
limits the travel of the diaphragm inside the fuel damper. Ordinarily,
this adjustment is set for life and you certainly don't want to
experiment with it. This screw also provides a quick-and-dirty fuel
pressure check. When there's no pressure in the system, the screw sits
down against the recess in the damper cover. But when fuel pressure is
present, it lifts the damper's diaphragm enough to push the screw
noticeably outward.
Visually checking the position of this screw is no substitute for a
proper pressure/volume test. It's an all-ornothing proposition: When
there's no pressure, it's recessed; when there is pressure, it
protrudes. But it doesn't tell you how much pressure, or if the pressure
is stable under load.
Leaks are the most common Toyota fuel damper failures. When you
investigate a customer's complaint about a gasoline odor, you may find
fuel is seeping out of the fuel damper. Watch for gasoline stains on and
around the fuel damper. Replace it if it's seeping fuel.
Occasionally, the adjustment screw loosens up, allowing the diaphragm to
stroke too far downward. When this happens, the damper's diaphragm
restricts fuel flow, causing a severe lean condition during
acceleration. When the driver punches the gas pedal, the engine falls on
its face with a severe hesitation (sometimes almost stalling), bucking,
chugging or surging. When he backs off the throttle, the engine runs
better because enough fuel flows to satisfy it during light-throttle
driving.
Remember that fuel pressure has to push this diaphragm upward into the
damper assembly before gas will flow into the fuel rail. Rut when the
diaphragm has "overtraveled" due to a loose adjustment screw, fuel
pressure just can't push or lift it enough to provide normal volume to
the injectors. I'm not aware of any quick, reliable way to reset this
diaphragm travel adjustment back to original. Replacing the fuel damper
itself is the safest, surest fix.
An episode at Tom Dwyer Automotive Service in Portland, Oregon,
illustrates how potentially confusing symptoms can be when that
factory-adjusted fuel damper screw loosens up. Technician Steve Poole
wits troubleshooting a 1995 Toyota V6 truck with the same lean symptoms
I described earlier. The truck's fuel Eiter and fuel pump had already
been replaced.
Poole followed the fuel line up from the frame-mounted filter and found
a threaded junction in the fuel line down on the right inner fender
panel. He teed a pressure gauge into this junction, which happens to be
well upstream from the fuel rail inlet and the fuel damper. First, the
system passed a simple vacuum-applied/vacuum disconnected test of the
fuel pressure regulator.
Poole's experience taught him to verify that fuel pressure was stable
under load. Considering how severe the lean symptoms were, Poole
expected fuel pressure to drop like a stone during acceleration.
Surprise/ Fuel pressure didn't drop at alll Then, when Foole checked
fuel volume at the pressure regulator outlet port, the truck did more
than just iail the volume test miserably. He noticed there was hardly
any fuel flowing from the return side of the system.
A quick check of injector pulse width showed that the ECM was increasing
pulse width substantially when Poole snapped the throttle open.
Furthermore, feeding the engine some propane through a vacuum port
during acceleration perked the engine up considerably. So what the heck
ailed this engine? Dirty injector inlet screens might cause a severe
lean condition, but Poole wondered if all six injectors could be
restricted that badly. Plus, that still wouldn't explain the extremely
low fuel volume in spite of a new fuel pump and filter.
Poole studied the system a little longer and noticed the fuel damper on
the injector rail. He told me he had seen a number of these dampers seep
fuel, causing gasoline odor complaints. But he also wondered if
something was somehow hanging up the damper diaphragm, causing it to
malfunction when the system was under load.
He removed the truck's fuel damper and inspected it as well as the
opening into the fuel rail. Everything looked clean as a whistle, so he
tried a new fuel damper; that fixed the truck. Hindsight is always
20/20. Poole said that if ho had been thinking inoro clearly, he could
have compared the upstream fuel pressure reading to one taken downstream
of the fuel damper.
Poole's a good sport. I told him that the next time he encounters these
lean symptoms during acceleration on a Toyota or another similarly
equipped vehicle, he would be better prepared. When the driver asks what
might be wrong with his car, Poole could respond with a straight face:
"You may have a screw loose somewhere."
Copyright Hearst Business Publishing Jan 2004
Provided by ProQuest Information and Learning Company. All rights Reserved
I found this on a Toyota site but believe it applies to MBs as well.
Marinucci, Dan
When a normally reliable component fails, it can put a 'damper' on a
quick diagnosis, especially if the symptoms are missed or misinterpreted.
Toyota has used dampers on its fuel injection systems for years. But if
you're like most technicians, you haven't looked twice at a damper
because it's normally very reliable. Trouble is, when a damper finally
does fail, it often confuses and frustrates uninformed technicians.
This month I'll explain how a fuel damper works and well as what can go
wrong with it. There are plenty of these dampers in service, and you
don't want a bad one fooling you!
At one time or another, eveiyone has heard a water pipe in a house begin
to bang after someone opens a faucet and closes it suddenly. If the
banging pipes bother you enough, you can source a little hydraulic
damper from a hardware store or home center and tee it into the noisy
pipe. Basically, this kit acts like a hydraulic shock absorber on the
water pipe, dampening the effects of the sudden pressure change inside
the pipe.
The fuel damper on an injector rail (photo 1 at left) also serves as a
hydraulic shock absorber of sorts. Electric fuel pumps don't produce a
totally smooth, seamless flow of fuel. To the contrary, fuel pump output
pulses, and ultimately this pulsing can cause noises downstream in the
fuel rail. Of course, the risk of hydraulic noise in the fuel system
varies from vehicle to vehicle and system to system.
On older Toyotas, you might see a fuel damper mounted right at the fuel
pump outlet. But the most common location for it is at the fuel rail
inlet. The damper contains a spring and diaphragm that absorb some of
the shock of the fuel pulsing into the fuel rail. When the system is
working correctly, fuel pressure lifts the diaphragm, pushing it up into
the damper housing. Pushing back this diaphragm allows the fuel to
continue flowing into the fuel rail.
If you pry off the plastic cover that's pressed onto the top of the fuel
damper, you'll find a factory-set adjustment screw (photo 2). This screw
limits the travel of the diaphragm inside the fuel damper. Ordinarily,
this adjustment is set for life and you certainly don't want to
experiment with it. This screw also provides a quick-and-dirty fuel
pressure check. When there's no pressure in the system, the screw sits
down against the recess in the damper cover. But when fuel pressure is
present, it lifts the damper's diaphragm enough to push the screw
noticeably outward.
Visually checking the position of this screw is no substitute for a
proper pressure/volume test. It's an all-ornothing proposition: When
there's no pressure, it's recessed; when there is pressure, it
protrudes. But it doesn't tell you how much pressure, or if the pressure
is stable under load.
Leaks are the most common Toyota fuel damper failures. When you
investigate a customer's complaint about a gasoline odor, you may find
fuel is seeping out of the fuel damper. Watch for gasoline stains on and
around the fuel damper. Replace it if it's seeping fuel.
Occasionally, the adjustment screw loosens up, allowing the diaphragm to
stroke too far downward. When this happens, the damper's diaphragm
restricts fuel flow, causing a severe lean condition during
acceleration. When the driver punches the gas pedal, the engine falls on
its face with a severe hesitation (sometimes almost stalling), bucking,
chugging or surging. When he backs off the throttle, the engine runs
better because enough fuel flows to satisfy it during light-throttle
driving.
Remember that fuel pressure has to push this diaphragm upward into the
damper assembly before gas will flow into the fuel rail. Rut when the
diaphragm has "overtraveled" due to a loose adjustment screw, fuel
pressure just can't push or lift it enough to provide normal volume to
the injectors. I'm not aware of any quick, reliable way to reset this
diaphragm travel adjustment back to original. Replacing the fuel damper
itself is the safest, surest fix.
An episode at Tom Dwyer Automotive Service in Portland, Oregon,
illustrates how potentially confusing symptoms can be when that
factory-adjusted fuel damper screw loosens up. Technician Steve Poole
wits troubleshooting a 1995 Toyota V6 truck with the same lean symptoms
I described earlier. The truck's fuel Eiter and fuel pump had already
been replaced.
Poole followed the fuel line up from the frame-mounted filter and found
a threaded junction in the fuel line down on the right inner fender
panel. He teed a pressure gauge into this junction, which happens to be
well upstream from the fuel rail inlet and the fuel damper. First, the
system passed a simple vacuum-applied/vacuum disconnected test of the
fuel pressure regulator.
Poole's experience taught him to verify that fuel pressure was stable
under load. Considering how severe the lean symptoms were, Poole
expected fuel pressure to drop like a stone during acceleration.
Surprise/ Fuel pressure didn't drop at alll Then, when Foole checked
fuel volume at the pressure regulator outlet port, the truck did more
than just iail the volume test miserably. He noticed there was hardly
any fuel flowing from the return side of the system.
A quick check of injector pulse width showed that the ECM was increasing
pulse width substantially when Poole snapped the throttle open.
Furthermore, feeding the engine some propane through a vacuum port
during acceleration perked the engine up considerably. So what the heck
ailed this engine? Dirty injector inlet screens might cause a severe
lean condition, but Poole wondered if all six injectors could be
restricted that badly. Plus, that still wouldn't explain the extremely
low fuel volume in spite of a new fuel pump and filter.
Poole studied the system a little longer and noticed the fuel damper on
the injector rail. He told me he had seen a number of these dampers seep
fuel, causing gasoline odor complaints. But he also wondered if
something was somehow hanging up the damper diaphragm, causing it to
malfunction when the system was under load.
He removed the truck's fuel damper and inspected it as well as the
opening into the fuel rail. Everything looked clean as a whistle, so he
tried a new fuel damper; that fixed the truck. Hindsight is always
20/20. Poole said that if ho had been thinking inoro clearly, he could
have compared the upstream fuel pressure reading to one taken downstream
of the fuel damper.
Poole's a good sport. I told him that the next time he encounters these
lean symptoms during acceleration on a Toyota or another similarly
equipped vehicle, he would be better prepared. When the driver asks what
might be wrong with his car, Poole could respond with a straight face:
"You may have a screw loose somewhere."
Copyright Hearst Business Publishing Jan 2004
Provided by ProQuest Information and Learning Company. All rights Reserved
