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  Tech Tips

Ford Fuel Supply Systems Can Be
Hard to Diagnose

Posted 9/9/2010
By Bob Mordorski

Many of the calls we have received recently involved diagnosis of something that normally would be a very simple subject: fuel pumps and fuel system supply issues.

Up until about 10 years ago, this was pretty simple on a Ford. There was basically a fuel pump, fuel pressure regulator, return line, injectors, fuel pump relay, inertia switch and some wiring. Then, in 1998, Ford started introducing returnless fuel systems. The Continental and Escort/Tracer were the first to use the electronic returnless fuel system (ERFS) and the Ranger was the first to use the mechanical returnless fuel system (MRFS).

When determining which system is being used, there are two simple checks to perform. The first is to inspect the fuel lines at the fuel rail. If there is only one line, it is a returnless fuel system. When consulting a wiring diagram for the fuel pump, a good rule of thumb is that if there is a fuel pump driver module between the inertia switch and the fuel pump, it is an ERFS. If the fuel pump relay and inertia switch feed battery voltage directly to the fuel pump, it is a MRFS.

To make things more interesting, the 2000 and newer Lincoln LS and 2002 to 2005 Thunderbird models not only use an electric fuel pump but a jet pump module. This is needed due to the saddle type fuel tank that is used on these models. The electric fuel pump, in the right-hand (RH) side of the fuel tank, supplies pressurized fuel (normally 30 to 55 psi) to the jet pump module in the left-hand (LH) side of the fuel tank. The jet pump supplies the pressurized fuel up to the fuel filter and fuel rail for the injectors. The additional purpose of the jet pump module is to siphon fuel from the LH side of the tank and feed it back to the RH side of the tank so that there is always fuel to the fuel pump. On this model, the jet pump is responsible for holding the fuel pressure on the system when the vehicle is turned off.

Several years ago, I had my first extensive challenge with one of these vehicles. The hotline subscriber's complaint was simply a hard-start after a hot soak. The fuel pressure would not completely bleed off quickly when the car was turned off, but would eventually bleed down 15 to 20 psi after 20 minutes or so. When cycling the key, the pressure would build back up but the car would still be difficult to start, and once it started, it would run rough for several seconds.

The first test was to determine what was causing the pressure to bleed off. This became difficult due to the fittings and lines that are used on the vehicle. But what was available was a fuel can for injector cleaning that could be pressurized. So the fuel pump line was removed from the fuel rail and the fuel can was connected and pressurized to about 35 psi. The car ran OK. When the engine was turned off, and the air supply removed from the fuel can, the system pressure appeared to hold. The thinking was that the check valve in the jet pump module was at fault, so the module was replaced. Well, to our dismay, the problem did not change - so a different approach was needed.

After the correct adapter fittings were located, I suggested that we tee in a fuel pressure gauge and then, when turning the vehicle off, pinch the section of the adapter going back to the fuel tank. This left the gauge connected to the fuel rail. Lo and behold, the pressure would still slowly bleed off. So at this time, the fuel rail was lifted up from the intake. There were no obvious signs of any injectors leaking that should cause this problem, but after sitting several minutes, one of the injectors had a very small bubble of fuel on the tip. That injector was replaced and the test rerun. Now the system held pressure the way it should, and the starting problem was resolved.

Though these systems can be more difficult to diagnose, if the right equipment and correct test methods are used, the task can be a lot simpler. If you have any questions on fuel supply systems, just give us a call.

Bob Mordorski Bob Mordorski is the Identifix Ford team leader. He is an ASE master with L1 certification.
Experience Identifix This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.

For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time.

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