2001 Chevrolet Suburban fuel system case study

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

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.

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 we’re ready for the next step…

What’s 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!

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)

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…

Time to dig deeperWhen 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%).

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.

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.

We’re 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 doesn’t provide pump volume specs. In it’s 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.

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.

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 aren’t that difficult, if you follow a logical sequence of events, and don’t 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.