Accumulators for EFI systems
Accumulators for EFI systems
(OP)
Greetings,
First, the question:
Do you know of any EFI car that operates at 28-30 psig that uses an accumulator
in the high pressure side of the fuel feed system?
The background:
I have a 76 Jaguar XJ-S with hot start problems. On a warm day, upon restart after
a shutdown of perhaps 30 minutes to an hour there is a bit of rough running
and popping (mild backfire). In extreme cases it is really hard to get going, like it's
running on 3 (out of 12) cylinders. After a bit it clears up and runs fine.
The car has a Bosch-Lucas D-jetronic EFI system.
I have done a lot of research on this, and a lot of work on the car, like rebuilding
most of the fuel system (all but the pump, actually). I am pretty sure it's due to vaporization
in the fuel rail. I've done lots of tests and am pretty sure that what happens is that
during heat soak back the fuel rail gets hot, the fuel expands, and is released through the
regulators (see below for the plumbing details) as they maintain the 30 psi regulation pressure.
Then, as it begins to cool, the fuel contracts, causing the pressure to drop. During the period
when the pressure is low and the engine compartment is still moderately hot, vaporization occurs.
Now, in a car with a single rail and single regulator, starting the pump would quickly clear the vapor.
However, the early XJ-S cars have 2 rails, one for each bank, each of which is shaped like a race track.
Fuel feeds in at the middle of the outside straightway, and ingectors tap off along the inner
straightway. There is a regulator on each rail, taking return flow from the middle of the inside
straightways. Consequently, there are many ways for the fuel to flow, and if one path has vapor
in it, it will offer a higher resistance to flow and is therefore in some sense blocked, i.e., a vapor-lock
of sorts.
To try to fix this I first rebuilt the fuel system to be absolutely sure the rail pressure was not decaying
down due to check valve or regulator leaks, or external leaks. Both regulators and the check valve were
replaced, as was all hoses, and the injectors were refurbished professionally. More recently,
I have installed a air temperature thermostat to keep the auxiliary fan on after shutdown. It is currently set
at about 135-140 degrees F. After driving on an 85 F day and parking in a closed garage,
it will cycle on and off for an hour or more. I believe this might have fixed the problem, but have
not driven enough to be sure.
If the problem persists, the next thing I want to try is to install an accumulator near the fuel pump.
This will keep the regulators from releasing fuel during heat soak back, thus preventing rail
pressure decay during the cooling period.
So, the question to you is where can I get such an accumulator? I have learned that Jettas and
a lot of other late 80s cars with CSI fuel injection do use accumulators, but these operate
at 80 psi. Consequently they MAY be useless on the D-jetronic systems at 30 psi.
I'd appreciate any and all discussion on any of the points above, or related matters
First, the question:
Do you know of any EFI car that operates at 28-30 psig that uses an accumulator
in the high pressure side of the fuel feed system?
The background:
I have a 76 Jaguar XJ-S with hot start problems. On a warm day, upon restart after
a shutdown of perhaps 30 minutes to an hour there is a bit of rough running
and popping (mild backfire). In extreme cases it is really hard to get going, like it's
running on 3 (out of 12) cylinders. After a bit it clears up and runs fine.
The car has a Bosch-Lucas D-jetronic EFI system.
I have done a lot of research on this, and a lot of work on the car, like rebuilding
most of the fuel system (all but the pump, actually). I am pretty sure it's due to vaporization
in the fuel rail. I've done lots of tests and am pretty sure that what happens is that
during heat soak back the fuel rail gets hot, the fuel expands, and is released through the
regulators (see below for the plumbing details) as they maintain the 30 psi regulation pressure.
Then, as it begins to cool, the fuel contracts, causing the pressure to drop. During the period
when the pressure is low and the engine compartment is still moderately hot, vaporization occurs.
Now, in a car with a single rail and single regulator, starting the pump would quickly clear the vapor.
However, the early XJ-S cars have 2 rails, one for each bank, each of which is shaped like a race track.
Fuel feeds in at the middle of the outside straightway, and ingectors tap off along the inner
straightway. There is a regulator on each rail, taking return flow from the middle of the inside
straightways. Consequently, there are many ways for the fuel to flow, and if one path has vapor
in it, it will offer a higher resistance to flow and is therefore in some sense blocked, i.e., a vapor-lock
of sorts.
To try to fix this I first rebuilt the fuel system to be absolutely sure the rail pressure was not decaying
down due to check valve or regulator leaks, or external leaks. Both regulators and the check valve were
replaced, as was all hoses, and the injectors were refurbished professionally. More recently,
I have installed a air temperature thermostat to keep the auxiliary fan on after shutdown. It is currently set
at about 135-140 degrees F. After driving on an 85 F day and parking in a closed garage,
it will cycle on and off for an hour or more. I believe this might have fixed the problem, but have
not driven enough to be sure.
If the problem persists, the next thing I want to try is to install an accumulator near the fuel pump.
This will keep the regulators from releasing fuel during heat soak back, thus preventing rail
pressure decay during the cooling period.
So, the question to you is where can I get such an accumulator? I have learned that Jettas and
a lot of other late 80s cars with CSI fuel injection do use accumulators, but these operate
at 80 psi. Consequently they MAY be useless on the D-jetronic systems at 30 psi.
I'd appreciate any and all discussion on any of the points above, or related matters





RE: Accumulators for EFI systems
from a 1980 VW Rabbit. It does indeed function as an
accumulator in my system, storing about 1/2 cup of fuel.
So fan, it seems to have cured the symptoms, but only
time and more hot weather driving will tell.
Ed Sowell
RE: Accumulators for EFI systems
RE: Accumulators for EFI systems