Is It Really the Fuel Pump?

The zero fuel pressure no-start is a common issue for today’s EFI vehicles. Fortunately, for technicians, it’s rarely a complex issue to repair. Diagnosis, however, is another issue. Often, stored codes simply tell you what you already knowl: no fuel pressure. Many techs rely on past experience to dive straight into the tank and pull the hangar assembly. Why not? The last three times you saw this problem with this vehicle, it was the pump, right?

There are several good reasons to pause for a moment before attacking those straps, reasons that can not only save time but can teach a lot about the fuel delivery system, knowledge that can pay off in the future. Here are a few points to ponder:

1 What “drives” the fuel pump? Current, yes, but there are two kinds of current, signal current and drive current. Drive current is the heavy stuff spinning the pump, but signal current has two types. The first, I’ll call it direct, are the inputs used by the ECU to signal the fuel pump relay. The second is the current line that energizes the relay coil. Typically, the ECU detects a spark to confirm the ignition is operating, and then allows engine fuelling. It might also trigger oil pressure, fuel pressure or a number of other parameters. While this is obvious to most techs, what isn’t is that the current needed to drive the fuel pump relay is considerable, sometimes requiring a hefty power transistor (and heat sink) on the ECU’s main board. Power transistors are extremely heat dependent, and don’t always have the courtesy to fail outright, but simply quit or operate erratically when overheated. Electronics techs often troubleshoot bad power transistors (and their drive circuits, usually themselves transistors) by shooting freeze spray and watching for proper operation. It can also help if the wiring diagram shows whether the power or ground is switched at the relay. It’s a small thing, but if the relay is grounded through the ECU’s “drive” circuit, and you attempt to jump the relay with a hot wire, you will have zero potential across the relay’s coil windings, and nothing will happen. You may change the relay and still have a problem.

2 What is “ground?” This is another basic that’s surprisingly overlooked. We’ve been trained to assume that the chassis is always at zero potential. But is that always true? Key grounds worth probing to the chassis are ECU, fuel pump relay and the pump itself. There should be very little voltage between a ground terminal and the chassis, less than a tenth of a volt. This is not a continuity test: don’t forget to power the circuit in question and use a millivolt scale on your voltmeter. Is the fuel pump relay grounded to an under dash bracket? I’ve seen bad grounds caused by loose or missing screws and bolts under the dash, partially isolating the dash structure from the chassis. Your meter helps here too, and if it’s questionable, a good fix is often to simply add an additional ground to a known good chassis point.

3 What about the fuse? Naturally the first step was to check the fuse, but remember that the fuel pump drive circuit is a key-on engine running circuit, so it has a magnetic attraction to do-it-yourselfers and backyarders installing accessories like driving lights. Pull the fuse, sure, but check that it’s the proper rating. Many amateurs and hackers tap a heavy current line into the circuit, blow the fuse, then replace with a higher rating. This works for a while, but overheats everything and can cause erratic relay operation. Relay hate marginal pull-in current, and can arc themselves to death in a short order. In fact, serious current draw can cause erratic ECU operation in general, so if you’re desperate, consider pulling the fuses for all non-essential circuits and see if things improve. Don’t forget to deal with stored codes and save radio presets beforehand. Unfortunately, there’s no code for “current-drain-through-teenage-thousand-watt-trunk-mounted-stereo,” so if the vehicle looks like a pinball machine, you’ll need every brain cell you can muster.

4 Mechanical factors. Changing the fuel filter is still a given for this kind of troubleshooting, as is a cursory glance at the lines. Sloppy repairs to lines are a sign of trouble, especially if the patch involves flex line. Hacks rarely worry about kinking a line, rubber or metal. And don’t forget the inertia switch. Many drivers, especially of older vehicles with little residual value, will pop a parking lot barrier or wall and say nothing about it. “I just came out of the grocery store and it wouldn’t start.” You can be a real hero by pushing a reset button, but don’t forget to do a thorough fuel system analysis and change the filter. If they won’t tell you about a minor collision, they likely ignore a lot of other maintenance issues too.