Porsche 944
no-start diagnosis
Troubleshooting a non-starting 944
This page should help
you diagnose a "no-start" problem with your Porsche 944
First: Was the car
running and died, or recently running? If so please scroll down to section #4.
If not, please start at section #1.
SECTION #1
If the car was not
recently running, first check that the battery is at proper voltage. Using a
digital voltmeter, it should read 12.9-13.2v
If the battery voltage
is adequate and the engine is indeed turning over (starter works), check for
spark by pulling the ignition wire for cyl #1 at the
spark plug.
Place a medium size
screwdriver in the wire boot and place it near (not touching) the intake
manifold (as shown below). You should be able to keep this gap
between the screw-driver
and manifold in an area that can be seen while sitting in the car to turn the
ignition key. While turning the key to
the "start"
position look out the
windshield, beneath the hood and look for a spark from the screwdriver to the
manifold. If you don't have a spark,
scroll down to section
#4. If you do have spark, continue on to
section #2.
SECTION #2
Also check for fuel
pressure at the fuel rail by placing a rag under the end-cap nut, and slowly
remove it. There may be a plastic
cover over the fuel
rail, the front end of which is hinged and can be lifted up to gain access to
the end cap. Be careful as there is a ball
bearing inside this
fitting that is required to maintain a positive seal, DO NOT LOSE IT! If
possible, attach a length of hose to the fuel rail
fitting and let it drain
into a cup. If you have help, then a collection device can be held at the end
of the fuel rail to prevent gasoline from
running all over the
engine. Now turn the ignition key to the
“start" position and see if there is adequate fuel flow. If there is no
fuel flow
scroll down to section
#5 for information on making a fuel relay jumper. If there is fuel flow, and
you have spark, but the car still doesn’t
start, we need to check the fuel injectors see
section #3.
SECTION #3
The fuel injectors are
fired in pairs and control fuel from the pressurized fuel rail into each runner
of the intake. A NOID light is required
to check the impulse
signal to the injectors as it is a very short pulse. First, one injector at a
time, remove the connector and check both pins
for + voltage when the
key is turned to the "on" position. The DME fires each injector by
grounding one of the leads to ground. If you find that
there is no voltage to
one or both of the pin sockets on the connector there may be a problem with the
injector harness or the DME unit. If you
do have + voltage at
both pin sockets at each injector connector, then proceed to check for an
impulse signal on each injector connector with
a noid
light while turning the key to the "start" position. If the noid light
does NOT light up then the injectors are not getting a signal, and again
the harness or DME maybe
at fault. If the noid light DOES flash, then the
injectors are getting a signal, and the injectors must be checked.
For information on
checking injectors, scroll down to section #6.
SECTION #4
Try dis-connecting
and re-connecting the Speed and Reference sensor connectors at the top rear of the
engine. These are located at the rear of the
intake manifold and are
usually held in place (one atop the other) in a metal bracket. These connectors
are prone to corrosion, and often are
the cause of a no-start
problem. I suggest re-seating each connector one at a time!! If the sensor
cables are confused and plugged into the other
connector the car WILL
NOT START. After re-seating these connections the tach
needle should bounce slightly while cranking the engine. If the
tach needle doesn't bounce
than there may be a problem with the reference sensor. If the needle does
bounce, and the car still doesn't start continue
on to section #5.
click to enlarge
SECTION #5
If re-seating these
connectors doesn't work, try installing a jumper in the DME relay (also knows
as the Fuel relay) socket. This relay controls
the fuel pump, and often
the contacts wear out over time. Alot of 944 owners
keep a spare relay in the glove box for just that reason. For reference,
the part number for the
DME relay is: 993-615-227-00.
On early model 944s
(before mid 1985) the relay panel is located above the driver’s feet, under the
dash. In cars built after mid 1985, the relay panel is
located in the engine bay just
forward of the windshield on the drivers side of the vehicle.
The jumper is simply
pieces of wire that are inserted into terminals 87, 87b then 30 (in that order)
to provide voltage to the fuel pump and DME without
having to have a functioning
relay in place (as a side note, the three terminals used on the relay socket
for the jumper are the 3 larger terminals).
USE CAUTION and remove
the jumper when not testing the vehicle! (see photo of
jumper below) With the jumper in place, there should be fuel pressure
at the fuel rail (see
section #2). If there is no fuel pressure at the fuel rail while using a relay jumper
then the fuel pump is most likely at fault.
The fuel pump is located
under the vehicle at the passenger rear portion of the vehicle. It is a metal
"can" slightly smaller in diameter than a pop can.
It should be removed for
further testing. If you now have fuel pressure at the rail using the jumper, the
car should start, and you are in need of a new
DME relay. If the car
still does not start continue to section #6.
SECTION #6
To check the electrical
viability of the injectors an ohm meter is needed. Once the injector connectors
are removed, Each injector (on a normally aspirated 944)
should show a resistance
of 2.4 ohms. If the meter shows no resistance (open) or a dead short, then the
injector needs to be replaced. If the injectors
do ohm out correctly,
They can become clogged and can sometimes be "revived" by firmly
tapping on them with a long screw-driver near their base
where they are inserted
into the intake manifold. A clogged injector is not usually at fault for a
"no-start" issue, but can often cause a rough idle.
If the injector ohms-out
ok, and tapping on them doesn't allow them to fire, then they should be removed
to be cleaned and further tested.