Fuel Pump Diagnosis Keep Your Logic Clear & Simple

March 25, 2013 | Reply More

This clean 2004 BMW 325XI M54 checked out initially, but developed a no-start issue.

We have the pleasure of working on a local judge’s cars. His daughter had just turned 17 and he bought her a 2004 BMW 325 XI after having us check the vehicle out thoroughly. Before she took possession of the car, we did a tune-up, including the fuel filter, and replaced the fan clutch, which goes bad on almost all specimens of this model. The vehicle passed muster otherwise.

A few weeks later, the car was towed back to our shop because of a no-start condition. Whenever you get a no-start, check the basics first: battery, fuel, and spark (well, the internal engine parts that generate compression are traditional basics, too, but we saw no reason to suspect them at this point). The battery was good, the starter was engaging, and the engine had spark. Before we pulled out our trusty bottle of propane and attached it to the brake booster vacuum hose as we usually do to see if the engine would fire up with an artificial supply of hydrocarbons, we hit the key again and it started.  That classified this situation as an intermittent.

Immediately, we theorized (but could not yet prove) that we had a fuel pump problem, simply because the KOEO (Key On Engine Off) turns were priming the pump, at least sometimes. However, Long Term Fuel Trim was not elevated and there were no codes that would tip us off that the fuel pump was on its way out. So, how could we make sure that the pump itself was the culprit before replacing it?

The Art of Diagnosing Fuel Problems

The first thing many BMW techs will check is the fuel pressure as read at the gauge port on the fuel rail. After all, this is the factory-recommended procedure. But is this really the best way to diagnose fuel problems on BMWs?

The company publishes two fuel system specifications for the E46 model. One is that fuel pressure should be 3.5 BAR both Key On Engine Off and Key On Engine Running, which is about 51 psi on the gauge. The other is that fuel volume should be 2.25L per minute, which is 0.59 gallons per minute on a fuel volume gauge. If you do not have a specification for fuel volume for a particular model, expect at an absolute minimum that volume should be above 0.45 gallons per minute on four-cylinder vehicles, and 0.55 gallons per minute on turbo-equipped fours, and six-cylinder and up engines.

The reason we want to check fuel volume as opposed to just fuel pressure at the fuel rail is twofold. First, if we merely checked fuel pressure at the rail, we cannot rule out that the fuel pressure regulator that is internal to the vehicle’s fuel filter is defective. Being that we already replaced the fuel filter on the vehicle a few weeks previously, it would be nice to avoid a needless parts-changing routine


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Category: the bimmer pub

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