Today’s fun project is a 2001 Chevy Silverado 1500 pickup with a 4.8L V-8
The customer’s concern was hard starting. It would start but had an extended crank time. It would eventually start after the 3rd or 4th attempt. The symptom was more pronounced after sitting for extended periods of a few hours or more. Here are some diagnostic Oscilloscope captures taken during the fuel system testing procedure. You can see how the fuel pressure is somewhat low with the engine running.
The fuel pressure regulator functions okay and the pump’s RPM’s are within normal operating range for a 10 segment pump. The current (amperage) load is not within specs. Notice how the fuel pressure drops off to zero within 10 seconds after the engine is shut down.
This is an abnormal pressure drop. This is the cause of the customer’s concern. A normal pressure drop should be less than 5 PSI after 15 minutes. The running fuel pressure should also be closer to 55 PSI. Cutting the old pump open for a visual inspection reveals the commutator bars looked okay. The pump’s internal fuel return check valve had failed allowing this abnormal pressure drop after pump shutdown. The initial pump prime of 2 seconds at key on couldn’t produce a high enough pressure to start the engine. After a 3rd or 4th key cycle the pump prime pressure increase was enough to start the engine. Always verify the cause of failure before replacing parts. Also verify proper operation after all parts replacements. The fuel filter was also replaced with the fuel pump to protect it from possible filter restriction damage.
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This is the fuel pump’s armature with 10 commutator segments.
This is the current ramp (amperage draw) and segment signature of the pump while it’s operating.
This capture shows fuel pressure from key on engine off, to start up and engine running along with a fuel pump input voltage monitored channel at the pump and the relay output channel at the fuse block. This also checks for possible voltage drops in the fuel pump feed circuit. Voltage drops can cause low pump output problems that could lead to incorrectly diagnosed fuel pump replacement.
This capture shows the fuel pressure drop at shutdown. There is no substitute for a dynamic system test. This information is invaluable for proper fault verification to facilitate proper vehicle repair. No guesswork as to what’s wrong here.
2003 Cadillac DeVille DHS IPC Cluster
This is a 2003 Cadillac DeVille-DHS. The customer said that the tach and speedo had stopped working while driving. After some diagnostics and research, I found that the ignition 1 fuse had blown. Time to find the shorting circuit. It was necessary to isolate 7 circuit feeds from the ignition 1 fuse. The fault was intermittent and it took some time to isolate the 7 feeds down to the shorting circuit. Circuit isolation is the only way to pinpoint this type of fault when a single fuse feeds power to multiple components within the system. After narrowing it down to the rear integration module (RIM) and the rear object sensor control module (ROSCM) circuits, I was able to find that the harness rubbed through in the right rear sail panel pinch weld area and was shorting the power feed wire to the rear object sensor control module. The scan tool had communication with the module until the fuse would blow. 4 modules would stop communicating. The MIL lamp would also shut off. This looked like a PCM fault at first glance and very well could have lead to false PCM replacement, if not for diagnostic due diligence.
This one was a true automotive bug hunt.
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