Diagnostic Solutions: Solving Insufficient Fuel Problems – UnderhoodService

Diagnostic Solutions: Solving Insufficient Fuel Problems

Solving cranking, no-fuel or insufficient-fuel driveability problems on late-model imports can be challenging, especially if a technician doesn't take into account the way that modern fuel systems operate. Some vehicles, for example, limit vehicle speed by deactivating fuel injectors. In some rare cases, a miscalculation in vehicle speed can cause an insufficient fuel condition.

173;sensor to starve the engine for fuel ­during start-up.

In equally rare cases, a mis-calibrated manifold ­absolute pressure (MAP) sensor might indicate a much higher intake vacuum than what exists in the intake manifold and cause the PCM to miscalculate the air/fuel ratio. Similarly, the ­engine coolant temperature sensor (ECT) circuit can be shorted to voltage, causing the ECT data to indicate 300° F ­engine coolant temperature. ­Either case will cause an insufficient fuel or cranking, no-start condition.

DIAGNOSTIC TROUBLE CODES

Insufficient fuel delivery at highway speeds generally results in a P0171 DTC (lean, bank one) and a P0174 (lean, bank two). In either case, a DTC is generally stored when positive long-term fuel trim values approach 25%. In addition, freeze-frame values will be stored along with the DTC.

Because only slight differences exist between freeze-frame data generated by insufficient fuel ­delivery and data generated by a sensor-induced miscalculation in fuel delivery, it pays to spend some time looking at the applicable freeze-frame values. If, for ­example, the freeze-frame values indicate that the DTC occurred at wide-open throttle (WOT) at 3,500 rpm and that the oxygen sensor voltages were less than 500 millivolts when the DTC was stored, it’s possible that the fuel filter is clogged or the fuel pump delivery rate is marginal.

If, on the other hand, the “calculated load” value in the scan tool data stream is less than 80% at WOT, the MAF sensor might be miscalculating airflow into the ­engine. In most instances of low calculated load values, the MAF is out of calibration or it’s dirty or clogged with debris.

I mention using the calculated load value because it’s easier to use than the grams-per-second value. While calculated load values vary among applications, it pays to track those values so that a relationship can be developed ­between normal and abnormal values.

Last, an insufficient fuel condition can be created by the driver filling the fuel tank with E85 ethanol gasoline. If the vehicle isn’t designed for E85, the fuel ­injectors can’t flow enough fuel to create the desired air/fuel ratio. In other cases, gasoline contaminated with diesel fuel might cause the same effect. And, while we’re at it, it still might be possible that the fuel injectors are partially clogged from using old or inferior-blend gasoline. Clogged fuel injectors are a long shot, but one that must be always considered when diagnosing insufficient fuel problems.

                                          Preventing Insufficient Fuel Delivery Problems

  unless you’re testing a pulse-modulated system,  always test fuel pump pressure and volume when  diagnosing an insufficient fuel problem.

 

dirty maf sensors often cause the pcm to miscalculate air/fuel ratios, which can result in insufficient fuel complaints.

 

 

when replacing a fuel pump, always inspect the  condition of the electrical connector, fuel hose and fuel hose connections.

 

 

to avoid a repeat of an insufficient fuel problem, inspect the original fuel pump inlet filter. if it’s clogged with debris, the fuel tank may require  cleaning or replacement.

 

 

when assembling a new fuel pump, make sure that the inlet filter is correctly located in relation to the pump mounting plate. a pinched inlet filter might cause an insufficient fuel complaint.

 

 

Tie down loose fuel pump wiring to prevent  interference with the fuel level sensor float assembly. In some cases, lower-than-indicated fuel levels can cause driveability complaints.

 

 

 

 

 

 

 

 

 

 

 

You May Also Like

Understanding Coolants

All-season coolant used inorganic acid technology and worked great for almost 30 years.

In the 1960s, coolant was changed twice a year. In the fall, antifreeze with ethylene glycol-based coolant was put into the engine to prevent the coolant from freezing and cracking the block when a cold front hit. Often, if the engine got hot, the antifreeze would boil off. In the spring, the engine would be drained and filled with water and maybe a small can of an anti-corrosion treatment.

Ignition System Do’s and Don’ts

Why do ignition systems give technicians problems when diagnosing ignition-related misfires? The answer is that some technicians use tests that might give inconclusive results or do damage to the coil or drivers inside a module.

Tools To Service Serpentine Belts

Servicing the serpentine belt on some vehicles is a tough task.

Battery Charging and Diagnostics

Here are six tips to use when diagnosing a vehicle with a dead battery. 

Why Do Timing Chains Stretch?

As the timing chain wears, it can change the timing of the camshaft and crankshaft.

Other Posts

High Pressure Direct Injection Fuel Systems

The main destroyer of high-pressure fuel pumps is a lack of oil changes.

Carbon Deposits and Direct Injection Engines

The primary cause of these problems is that fuel and added detergents are not hitting the back of the intake valves.

A Closer Look: Gasoline Direct Injection (GDI)

Gasoline direct injection (GDI) is used on most new vehicles and requires a different approach to diagnosis and service. GDI technology has been an integral part of helping to improve fuel economy while reducing emissions, and can be found on more than half of the U.S. fleet. In fact, the use of GDI engines has

Acura Turbo Engine Service

It is important to check the operation of the solenoids that control vacuum to the actuators.