This is a story from my personal experience as an auto mechanic. I worked on 1996 Chevy Tahoe V8 5.7L. The owner of the car complained that the car was hard to start after he parked for a while. Once it started, it drove fine and never had problems such as the engine stalls and shakes. After the engine shuts off, it could start right back if the car did not sit for more than 5 minutes.
First, I suspected a fuel leak. 5 minutes are enough time to lose the pressure in the fuel system if there is a leak. I thought there was too small leak to affect the engine performance. I thoroughly checked by leaving the car on the lift for more than an hour, but I could not find any leak. I scratched this possible cause from the list.
My next target was a fuel pump: the pump delivers fuel and creates fuel pressure. I thought the fuel pump did not work hard and fast enough. I installed a fuel gauge and measure the fuel pressure, and that was it! The pump built up the pressure just before and after the engine started. When there was no pressure, the engine would not start. That forced me to think of a leak, but I had not found anything earlier, and I soon noticed that the pump was not working when experiencing the problem.
At that time, I found it a little odd. Because the symptom indicated a physical problem like a leak, but it is an electrical problem. I came up with some things, but none of them accounted for all the symptoms. Time was up. I told the customer to come back tomorrow, got off work. As far as I know, the customer never came back to the shop. The problem was not solved until I happened to see a wiring diagram of the same car.
That was a long time ago. I was a newbie as an auto mechanic, and the shop I worked for never had auto repair manuals or wiring diagrams. If I go back to the past and the same situation, I would go straight to the library and try to look for a repair manual instead of wait for more than 5 minutes every time I tried to locate the problem. There would have been enough time to fix the problem and give the car back to the customer on the same day.
Looking up wiring diagrams is a short cut for finding the cause of problems. Reading repair manuals before you start a job saves your time. This is one of my experiences that remind me how important auto repair manuals are.
In case you wonder what was the cause of the problem on Chevy Tahoe, look at the diagram.
A relay for fuel pump was bad whole time. On older GM vehicles, an engine oil pressure switch is giving power to a fuel pump while the engine is running. ECM sends a signal to the relay at the item of only start-up.