2001 Chevrolet Suburban fuel system case study

1
2001 Chevrolet Suburban fuel system case study

• Prepared for Standard Motor Products by David W.
  Patrick Jr.

2
2001 Chevrolet Suburban

• 5.3 liter V-8 engine, 4spd. Automatic trans and
  4WD.
• 219,000 miles on the clock.
• Check engine lamp on, with codes P0171,and P0174
  stored.
• Customer reports occasional rough idle, and
  intermittent power loss.

3
Where should we start?

• We have recently seen a number of these vehicles
  in our shop with these same codes and symptoms.
  Replacement of the intake manifold gaskets has
  been the common cure.
• Since we already had the scan tool connected, we
  decided to take a quick look at our O2 sensor
  voltages, and our current fuel trims.
• We isolated and graphed the individual PIDS we
  wanted to view. This simplifies the screen and
  speeds up the sampling rate. Now were ready for
  the next step

4
Whats the next logical step?

• We broke out our propane wand, and monitored our
  scan values while dousing the intake.

• We saw no change in any of our scanner readings
  not what we expected!

5
Time to get serious! Lets check fuel pressure!

• We had to let go of our vacuum leak theory. We
  now connected our fuel pressure gauge.
• Using the bidirectional controls on the scan
  tool, we operated the fuel pump, and recorded our
  readings.
• After bleeding all of the air out of the system,
  our readings were as follows
• 62 psi with a 1-2 psi drop within 5 minutes. Well
  within specs.(55-62 psi less than 5 psi drop in
  1 min. with pump off)

6
Now what?

• Now I decided to try an old mechanics trick. I
  taped the fuel pressure gauge to the windshield
  so I could view it while driving.
• I still had the scan tool connected, so I
  configured it to read O2, fuel trim, and MAF
  sensor PIDs.
• Now driving in real world conditions, with two
  good windows into the system. I drove the vehicle
  hard for 20 minutes. Finally, climbing a large
  hill it hesitated

7
Time to dig deeper

• Back at the shop, I wanted to get a better look
  at the mechanical condition of the fuel pump. I
  removed the fuel pump relay and installed a
  fused jumper between terminals 30 and 87 in the
  relay box. I then connected my low amps probe
  around the jumper wire.

• When the vehicle hesitated, the fuel pressure did
  drop some, but was still within specs. MAF sensor
  appeared normal when compared to the specs. We
  had. Fuel trims moved to full lean correction
  (25).

8
Finally some answers

• With the amp clamp connected to a good scope, and
  measuring fuel pump current I captured this
  waveform. This scope capture clearly shows
  damaged brushes and/or armature in the fuel pump
  motor.
• It was only now that I realized ,that I had
  missed an important step in the diagnostic
  process back to basics.

9
Were almost home!

• Since our initial pressure reading was at the
  high end of the specification, we made the
  assumption ,that the volume was probably O.K.
  Wrong!
• The manufacturer doesnt provide pump volume
  specs. In its test procedures (a four page long
  flow chart yikes!). Here are some generic
  specifications I dug up from a fuel system class
  I attended a few years back
• 1 pint in 30 seconds 4 cyl. small 6 cyl.
• 1 quart in 30 seconds large 6 cyl., V8, or any
  turbo or supercharged engine.

10
The patient was starving

• As the last slide clearly shows, our fuel pump
  provided less than half of the minimum required
  fuel volume for this potent V-8 engine.
• The replacement pump comes in the form of a
  module/assembly, complete with pump, fuel gauge
  sender , and even a new fuel tank pressure
  sensor. The installation of the pump was
  straightforward, and no problems were encountered.

11
The cure is at hand !

• After replacing the pump, our fuel trims bounced
  back to normal. This scope capture of the new
  pump in operation, as well as a good long road
  test, confirmed our medicine had cured the ailing
  truck.
• Fuel system diagnostics arent that difficult, if
  you follow a logical sequence of events, and
  dont skip any steps. We made a rookie mistake by
  not testing fuel pump volume, when we tested
  pressure. This definitely cost us time, but we
  did fix it right the first time, and without
  replacing unnecessary parts.