By Mark Giammalvo
Here is a shot of the engine diagnostic connector and relay box.
Here is the new fuel pressure adapter that we purchased to test the van. The old one "grew feet." and apparently walked away.
VEHICLE: 1993 Mazda MPV Van
Mark's First Law states that when business flow begins to increase,
problems and headaches also increase in direct proportion.
Fuel tap? What fuel tap?
The first thing to do was to connect a fuel pressure gauge to the
vehicle and continue driving it. I popped the hood and looked for the
fuel tap. Fuel tap? Oops, there I go again thinking of my past GM repairs.
The Mazda 2.6 engine does not have a separate fuel tap or Schrader Valve
connection. However, with our trusty master fuel pressure gauge kit, we
can hook up a fitting that will allow us to "T" into the fuel system. Well,
at least I thought I could. A look in our fuel pressure kit revealed that
the adapter I needed was missing. I was experiencing something we are all
too familiar with in this industry: Missing tools that were either "borrowed"
and never returned or were put on a car for testing purposes and never
removed. After a frantic call to our tool distributor, we had the replacement
fitting that same day. Normally, I tape the fuel pressure gauge to the
windshield, but in this case it was easier to read when it was attached
to the radio antenna. It wasn't the best duct tape job, but it stayed on
the antenna at highway speeds so it worked for me. According to our technical
information service, normal fuel pressure (regulated) on this engine is
30 to 37 psi with 18 to 21 inches Hg vacuum at the fuel pressure regulator.
Although I went through five test drives with no running problem I noticed
a trend in the readings. During the road tests, the gauge would bounce
at times from 24 to 32 psi. Then, the pressure would be a steady 34 psi.
Although there seemed to be an abnormality on the fuel pressure circuit,
the car was not symptomatic. Also, something else interesting was occurring.
As the fuel tank got lower on fuel, the fluctuations periods became more
frequent. I have found this can often be the initial symptom of a tired
fuel pump. There is some data to support the theory that the heat of fuel
pump operation is lessened when the pump is completely submerged in fuel.
As the fuel level drops, more of the pump becomes exposed and the internal
pump temperature rises. What is not an issue on a healthy pump becomes a
major factor on a failing pump.
There were still some problems to overcome. No testing had been
done to see if the fuel pump was receiving adequate supply voltage. Another
trip to our technical service information proved interesting. Fuel pump power
on this van comes from the circuit opening relay (COR). The COR is mounted
next to the engine ECU on the right front floor under the rug below a metal
protection plate. The COR is a five-wire relay. The black wire is ground
to the relay's coil when cranking. That same coil gets power from the starter
circuit on the green/red wire during crank. The blue/green wire is ground
for the relay's coil when running. That coil gets its power from the ECU
on the black/white wire when tachometer signal is present for normal running.
Finally, and most importantly, the solid blue wire sends power to the fuel
pump when the relay contact closes. After another two test drives the car
was finally hesitating. During these events, the fuel pressure dropped to
12 to 18 psi and the voltage to the pump remained a constant 12.4 volts.
Even though all the variables had not been eliminated, our tests pointed
to an intermittent fuel pump problem. Obviously, the best diagnostic test
would be to hook up power and ground test leads as close to the pump as possible.A
fuel pressure volume test and an amp draw test can be performed as well.
Given a busy day's time constraints combined with this industry's escalating
labor cost per hour, it is often more acceptable to perform tests at a remote,
more accessible location, hence the method to my madness. As a final protective
measure we installed a new fuel filter. It was a good thing we looked up
the procedure to change the fuel pump; otherwise, we would have dropped the
tank for nothing. As typical with Asian engineering, the pump access plate
is located within the cabin under the #2 seat.