In order to find a vehicle problem, a technician must be prepared to interrogate the vehicle owner and address their concerns.
The following should all be a part of your diagnostic game plan: a vehicle test drive with the vehicle owner (if possible), a visual inspection and a check of all the basics — battery, fluid levels, complete vehicle system scan, and service information, including TSBs. Asking the right questions such as those to better determine when does the vehicle exhibits the problem — when cold, hot, after sitting awhile, on a rough road, at a certain speed or while backing up — all will help you narrow down your search for what maybe the problem.
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Putting a stop to the problem
Years ago I had a vehicle that was dropped off from another shop with a complaint of a rough-running engine only when the vehicle was at a stop or on bumpy roads. This vehicle had a bunch of new components in it including new plugs, plug wires, air and fuel filters, fuel pump, injectors and even a PCM. While I performed a visual inspection, I noticed that the brake pedal was worn on the side facing the throttle pedal, along with a hole in the carpet. This visual indicated to me that the owner (who I never met or spoke to) most likely wore work boots or a shoe with a hard sole. The next thing I observed was that the engine started right up and ran at a normal high idle before dropping down to a smooth normal idle. After applying the brake pedal while I was selecting the D range on the shifter, the engine began to run rough until I started to drive the vehicle. With my foot off the brake, the engine ran smooth; but once I pressed on the brakes, the engine ran rough again. I thought that this may be a problem with fuel delivery even though the vehicle already had a new fuel pump and filter installed. Since this vehicle had an accessible fuel pump relay, I thought that I would current ramp the fuel pump to see if the amperage was at the normal level of 6 amps. Once I installed my amp clamp on my labscope I found that the pump current ramping waveform was normal at idle. When I pressed on the brake pedal the engine ran rough and the amperage dropped to about 3 amps. I tried applying and releasing the brake pedal multiple only to come up with the same exact results. My next step was to look up the wiring diagram for the vehicle circuit to see what the common denominator was. What did the brake pedal and the fuel delivery have in common? It was G402 (Figure 1) ground, which had 16 loads (light bulbs), including the fuel pump, on the same ground circuit.
I used the wiring diagram like you would use a map to find what road to take. I found that G402 (Figure 2) was located in the trunk on the left rear wheel housing. When I located the ground, I noticed that the area it was attached to was badly rusted. To confirm that the ground was the problem, I connected the ground pigtail that is attached to the Power Probe handle to the existing ground and depressed the brake pedal. The result was that the engine no longer ran rough with the brake pedal depressed. Since connecting a good ground confirmed that the engine ran smooth while the amperage stayed at 6 amps, I knew that all I had to do was clean up the ground terminal ends, along with the star washer and bolt and relocate the ground to a rust-free area. After I located a good rust-free area, I cleaned it down and installed the bolt, star washer and ground wires followed by applying the brakes again. Now there were no signs of a rough-running engine.