Dr. Crankshaft And The Weird Fuel Pump
Dabney was driving his 1933 Pierce Arrow one afternoon, when he started having a lack of power under acceleration. He noticed that if the car was driven slowly and on a level road it ran fine, but as soon as it needed to climb or accelerate to any degree it lost power and couldn't regain it.
He arrived at the Dufus garage just as the Doctor was walking by, so as the Doctor opened the door for Dabney he drove the Pierce Arrow into the work bay. As he was laying fender covers over the paintwork, Dabney explained the symptoms to the Doctor. Immediately, the Doctor started ruminating about the cause.
"You say the car runs fine on level ground but can't pull power when needed, eh?" asked the Doctor. "Sounds like it might be a fuel delivery problem. Did you check the pump recently?"
"I put a new fuel pump on the car six months ago, and it ran fine until now," said Dabney.
"Well, let's check it anyway," said the Doctor. They connected a vacuum/fuel pressure gauge to the line at the carburetor and cranked the engine. The fuel pump was, in fact, putting out 2 psi of fuel pressure, approximately what the specifications called for.
"Hmmm," said the Doctor. "Looks like the fuel pressure is good, so we can pretty well eliminate the pump. Let's check the filter just to make sure.
They inspected the fuel filter, a device that had been placed in the fuel line some years ago by the previous owner. Early cars like the Pierce Arrow frequently only had clear glass fuel sediment bowls rather than filters. The bowl was there, of course, but served no purpose other than decoration.
The pair concluded that the fuel filter was good and there were no obvious issues with the carburetor, so they went out for a test drive. Once again, the car exhibited the symptoms, so back they went to the garage.
"Okay, let's agree that the engine is starving for fuel," said the Doctor. "I suppose we might shine a light into the gas tank to see if there's debris around the pickup tube."
"I'll go get a flashlight and mirror so we won't have a potential explosion by using the electric shop light," said Dabney.
They were sure they'd find problems in the gas tank, but after snaking a mirror into position to see the pickup tube they were surprise to see that the inside of the tank was in pristine condition.
"Now this is a puzzler," said the Doctor. "We've got good fuel system integrity and yet we're starving for fuel. Maybe we'd better hook up the gauge again and drive the car. We can put in a "T" fitting and run the gauge's hose into the passenger area so we can monitor fuel pressure as we drive."
They did so and again brought the car up to speed. To their amazement, as the car was under load the symptoms appeared again, simultaneous with a serious drop in fuel pressure.
"Now that's weird," said the Doctor. The fuel pump is losing pressure as engine speed increases, and yet at low speed it meets pressure specs. Let's take 'her back to the garage.
They parked the Pierce Arrow and opened the hood once they were back in the garage. The Doctor brought the gauge where they could see it and instructed Dabney to rev the engine. Sure enough, the pressure dropped.
They repeated the experiment several times and verified that the conditions were correct. During the last engine run-up, Dabney happened to be leaning out of the car. As the fuel pressure dropped he noticed hundreds of bubbles in the fuel bowl next to the engine.
"Hey, Doc," Dabney yelled, "look at the fuel bowl!
The Doctor did just that and suddenly realized what was going on. He told Dabney to shut off the engine and the two of them stood in front of the car. "Dabney, old boy, we've got ourselves an air leak. Somewhere in these fuel lines is a loose connection that is allowing the pump to suck in air under higher speeds. We've got to find it."
They started looking, and eventually found a section of rubber hose that had been inserted between the gas tank's fittings and the fuel line along the frame. It had a small cut in it, so they replaced the hose and that was that...