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Common Rail Diesel stystems
The Common rail Bosch Diesel fuel system, has an electric
pump in the fuel tank, and filter in or out of tank, the primary
fuel system is pressurised to about 2.5 bar (45 lbs psi) plus or
minus a bit, the secondary system comprises of a high pressure
pump that runs off the cam belt, and pressurises the system
to approx 1350 bar (about 15,000 psi) which is fed to the fuel
rail, the rail has the four injectors on, and a temp and pressure
switch, the whole system is controlled by the vehicles on board
computer here by known as the ECU, the injectors are fed with
a 12 volt supply and are switched to earth by the ECU, which
gets the information from a crank sensor, when you turn the
ignition on it primes up the system for about 2 seconds, if the
primary pressure is low the engine will be very hard to start
from cold, as on most modern systems the glow plugs do not
work until ambient air temperature is below 0 degrees C, the
whole management system works via relays and fuses, it
also has a warning light on the dash, make sure this light
works correctly, if it stays on there is most likely a fault
code stored, and you need to have a code reader to be put
on it to check for stored codes and clear them, if the light
is on, and under some other circumstances, the management
system may revert to "Limphome Mode" that's where some
thing on the system is wrong or not working and the ECU
gives the system an average set of parameters to get you
home, to avoid total breakdown.

If the vehicle fails to start...
Or runs poor, first make sure you don't have wrong or
contaminated fuel, then undo pipes on filter and crank to
see if fuel pump is working, if not suspect, pump, relay,
fuses or Inertia switch, if that bit is ok undo union on input
to fuel rail, crank over and check it squirts out, be careful
very high pressure, just look, the system has a fuel
return on it from high pressure pump and fuel rail, which
lets any unwanted fuel to return to the tank, if you have no
supply when cranking suspect high pressure pump fault,
if ok check pessure at either end of the injection pipes,
should run out liberally , if not could be pressure switch
allowing fuel to go back to tank, if you have fuel coming
out of pipes when cranking and still wont start, check the
power to plugs on injectors, should switch on and off 12
volts when cranking, if nothing there check fuses and relays
also crank sensor and ECU, pin 1 on Bosch ECU should
be live with ignition on. also if it won't start may want to
think about Imobilizer fault and /or ignition Key fault.
although if the key is faulty it normally won't crank over.
may want to check the system for trip switches that might
have come out, might be wise to run through all fuses
with circuit tested with igniton on. it may also fail to start
if the temperature sensor is faulty... Don't forget these are of course compression ignition engines. You need to establish a few things, like is there an electric pump in the tank, if so does it work, if not the hight pressure pump is made in two sections, the bit that sucks up the fuel from the tank, make sure there are no restrictions and no air leaks on that bit, and the second bit is the high pressure pump that supplies the injectors with fuel, a lot of these new electrical diagnostic machines can check fuel pressure electronically, also check O rings and stuff on the high pressure pump, make sure there are no leaks and not drawing any air in anywhere. The main things to check if it wont start are, has it got compression, has it got a fuel supply and are the injectors triggering, a drop of easy start can prove a point, but that will fire up engine even if it has low compression so consider that, make sure the valve timing is right and the air and fuel filters are clear.

Also if the seconday pressure is low it will cut off the
current to the ECU pin 1 and the Injectors, so remove
all the leak off hoses from the injectors and block the
final hose as to stop fuel pouring out of it, crank the engine
for several seconds, the leak off ports should just drip a
bit, if it runs out any of them that injector is faulty, if
it runs out of all of them renew all injectors, one is enough
to make the pressure in rail fall as to cut off all injectors
if it tends to fire on one cylinder and won't pick up the
bores might be full of diesel, try removing fuel pump
relay or fuse, take top off filter, make sure no new fuel
comes in, and crank it over to try to get rid of any excess
fuel, then see if it will run on easystart, if it won't run up on
easystart it prob won't run on diesel, of course the injectors
themselves might be squirting fuel in instead of spraying
it in, in which case you will need to get the injectors tested.

Another way to check injectors is to take all the leak off hoses
off as above, block final, get 4 small glasses, and 4 pieces of
plastic pipe, put the pipe from leak off ports to glasses, may
have to block off the other side of the leak off on injector,
crank or run engine to see how much goes in the glass, the
less the better, should just drip, they should all be even, if
it runs in, that injector is faulty, the longer you crank the better.

Also it might be worth removing the pressure valve on the
end of the high pressure pump, sometimes on fuel rail,
the one i have seen is held on with 2 10mm bolts, remove
these and pull the valve out, inspect the 2 O rings for splits
and with a magnifying glass look at the tip, there is a very
small opening which could get blocked. if looks ok refit
it and refit plug. Could try tow starting it. problem was with mine
blocked injectors, prob muck got through filter. Make sure
the exhaust isn't blocked, or inlet. Just one comment, if
the MIL, (Malfunction indicator Light) doesen't work and it's
not the bulb or wiring etc, suspect faulty ECU or it's wiring,
specifically the 5 volt reference circuit, also will prob find the
code reader wont wont communicate with onboard computer

If you have problems getting car through smoke test, best
approach is put a ton of diesel injector cleaner in the fuel
tank, clean or change air filter, wouldn't hurt if you changed
engine oil and oil fiter and fuel filter, but unlikely to make
much difference, give the vehicle a good thrash up the road
for about 3 to 5 miles before you take it in for test, don't let
it get cold or idle too long, if it still fails will prob have to take
it to a diesel specalist and maybe think about injectors or
injector pumps etc...

Note: Above where i put 4 injectors on rail, it does of course
depend how many cylinders the vehicle has, might be 6.

Fred G4VVQ. 2016.
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