Fuel Injection
troubleshooting
(for a '91)
Simplified
fuel circuit diagram.
The Townsend write-up says to ground pins
5 & 17 of the ECU and see if the fuel pump and injectors run, but this is
incorrect; pins 5 and 17 are grounds. They mean pins 20 and 21.
First, check all the fuses; if they are
OK, proceed.
I would start by replacing
both the system and fuel pump relays and then seeing if the car will
start.
If you want to troubleshoot first, pull
the ECU and remove the connector from the ECU. Ground pins 21 and
20; even without the ignition being on, the fuel pump should then run (you can
hear the fuel swooshing through the system and you can faintly hear the pump
running at the back of the car). If the pump doesn't run, it might
be the pump or it might be the fuel pump relay or it might even be the fuel
system relay (it has to actuate before the fuel pump relay can actuate); the relays are a lot
cheaper than the pump, so
try them first. Actually, the pump and system relays are identical, so
it's a good bet that you can just buy one and try it in both positions.
If the pump checks out OK, remove the
ground from pin 20 of the ECU connector but keep pin 21 grounded. (
Ungrounding pin 20 will keep the fuel pump from running, but residual pressure
will still dump raw fuel into the cylinders if the injectors open on the next
test. Raw fuel in the cylinders can potentially reach the catalytic
converter and cause it to overheat catastrophically. I did not know this
and did not have a problem with it, but it might be a good idea to carefully
loosen a fuel fitting to bleed off residual fuel pressure before trying to
actuate the injectors. If you do try to loosen a fitting to bleed
pressure, wrap it in cloth to prevent spraying. Make sure there are no
heat or ignition sources around and take extreme care. Don't forget to
retighten the fitting afterwards.) Now ground pin 18;
when you ground pin 18, you should hear the fuel injectors click. If you
don't hear them,
you probably have a bad System relay (or perhaps corrosion at pins 13 or 15 in
the large round connector in the aquarium). If the injectors do click, it's looking bad for the ECU,
although there are a few other things you can check.
With pins 21 and 18 grounded, there should
battery voltage to pin 5 of the air mass meter.
With everything connected normally, you
should see some small voltage (300 mV to 1V) across the two fuel injector wires only
when the engine is cranking or running. Although it's a pulsing
voltage, you should be able to read it on the dc scale. This voltage is
due to the ECU pulsing pin 18 to pull the injectors to ground and allow them to
open. The pulse duration depends on a number of factors, but you should
see something.
Bear in mind that when the ECU and all
else is hooked up normally, the fuel pump will run and the injectors will
pulse only when the engine is turning (cranking or running).
This is because the ECU must get pulses from the crankshaft sensor
before it will provide grounds for the System and Pump relays.
Distilled
step-by-step checklist
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