Fuel Injection Problems

Mechanical fuel injection operation is simple compared to most other motorsports fuel systems.  Good performance depends on a good setup.  However, many different problems can occur from an improper setup that are discussed throughout our books, MFI tech, and air density tech publications.  Here is a summary of several.

Baseline Setup Challenge

Many mechanical fuel injection suppliers set up high-end operating pressure at less than 100 psi.  Another practice is most often to set up the jetting combination for a rich mixture.  This combination is a common practice to be safe.  Tuning involves leaning out the engine to get the best performance.  This usually involves increasing the size of the main bypass to reduce fuel to the engine.  This increases overall jet area reducing fuel pressure.

Often, we see new setups where the larger main bypass reduces fuel pressures in the low operating range.  For example, fuel pressures for methanol setups below 50 psi are common in these cases.  However, they tend to be sluggish on the low-end.  A reduction in the nozzle and main bypass size combination is the only solution to get fuel pressures to higher, more responsive values.

Tuning Difficulties from Port Nozzles

A second row of nozzles is often added, especially in supercharged engines.  The purpose is to better control fuel distribution.  Additional port nozzles increase system complexity, especially when fuel pressures are too low.  Tuning becomes a difficult task of trial-and-error.  We saw several applications where simply increasing the fuel pressure with smaller nozzles without adding port nozzles was a better alternative.

For example, our bracket / match racing blown methanol funny car could run mid 6 second quarter mile elapse times with ease.  It was equipped with only 8 hat nozzles that were on the small size with the proper main bypass.  This resulted in higher operating pressures.  Fuel pump life remained great for the duration of our drag racing.  Most of our competitors all used two sets of nozzles with lower fuel pressure and a lot more trial-and-error tuning.

Wrong Spool in Barrel Valve

Inside the barrel valve is a spool.  It rotates with the throttle blades to control different fuel delivery needs from idle, part-throttle, to top-end.

Bypass openings and ramps are machined into the spool by the manufacturer for a specific fuel and power level.  Using the wrong spool can be trouble-some.  For example, an Enderle nitro spool for high percentages and volumes of nitro will not work very well in a straight methanol setup.  The transition circuit for nitro bypasses a great amount of fuel in the throttle position from idle to full throttle.  For methanol fuel, that transition is too much.  The engine on methanol with that nitro spool may lean out at part throttle and backfire.

Contaminated Fuel

We saw two categories of fuel contamination: humidity and foreign fluids.  Humidity can come from storage of end-season fuel in plastic containers.  It is also reported to come from some suppliers from time-to-time.

Foreign fluids can come from suppliers as well.  Shipping tanker rail cars, for example, are often used for other fluids when they are not shipping methanol.  Necessary flushing of a large tanker rail car may not be done adequately.  Remnant fluids from previous shipping may not be adequately flushed.  One fuel system supplier reported that it is common that 1% of racing fuels is an unknown contaminant.

When we drag-raced:

  • we were careful to isolate new fuel purchases with any older fuel, left over from a previous outing to isolate fuel turnover from contaminated fuel if it occurred.
  • We paid attention to our fuel supplier reputation with other racers. We stayed away from supplies that were not well established.
  • Sometimes we would check specific gravity of our fuel to confirm the methanol contents.

We experienced humidity contaminated fuel only once in many years of racing.  It was apparently from long term storage of methanol in our plastic fuel jugs. .  After that, we switched to metal containers for fuel storage.

We optimized our MFI tune-ups with air / fuel ratios in the center of the operating window.  In the mostly bracket and match drag racing that we did, our tune-ups were not sensitive to the 1% unknown contaminants that may have occurred from the suppliers

When fuel is contaminated, the engine may run too lean.  There is simply not enough methanol in the volume for a proper fuel mixture.  The engine may run poorly from reduced power.  It may also backfire from too much leanness.

Wrong Fuel Pump Size

Once at a race, we saw a fuel injected racecar that ran poorly.  We checked out the setup and determined that the fuel pump was marginally on the small side.  The tuner said he knew that and was trying to compensate with jetting changes.  Unfortunately, if the fuel pump is too small, no amount of jetting changes can compensate.  There is simply not enough fuel. 

Wrong Fuel Pump Flow Tag

On two different occasions, we saw rebuilt fuel pumps that were delivered with the wrong flow bench tag on the pump.  The mistakes were apparently made in shipping.

One of these was a replacement pump for a proven combination.  The race tuners said that the engine suddenly ran overly rich when the fuel pump was replaced.  It was a tough situation after the racer lost about 4 months of competitive racing.  An analysis of the setup on ProCalc for the symptoms suggested that the fuel pump was too big.  The fuel pump was dismantled.  Components were measured, and the supplier confirmed that it was a larger fuel pump.  ProCalc was used to determine a larger main bypass size to properly compensate for the larger pump.  Performance was fully restored.

Drag Racing Launch Pedal Control Problems

A throttle pedal control is often installed in drag racing to hold the engine at part-throttle for the launch, then transition into full throttle on the run.  If the part-throttle location set up by the pedal control device is near the barrel valve transition point between idle and full throttle / full fuel flow, fine tuning around that can be trouble-some.

For example:

  • reducing the throttle opening to reduce engine speed for the launch may open the idle or pump saver bypass, leaning out the engine. That can cause a more dramatic reduction in launch power.
  • Increasing the throttle opening to increase engine speed for launch may shut down the idle or pump saver bypass dramatically increasing engine enrichment and also a reduction in launch power. That would not be expected from the increase in engine speed for the launch.

Launch characteristics would then be affected by both the change in RPM and a dramatic change in launch fuel.  Tuning may be trouble-some.  We instruct all of our customers who use throttle pedal controls to examine the barrel valve spool angle at various throttle launch positions.  That is needed to determine where the bypasses are on-or-off in the throttle rotation.  Then either note the rotational positions reference for tuning or adjust away from the transition points.

List of Additional MFI Problems

Here is a list of added problems:

  • fuel line leaks under pressure
  • fuel line vacuum leaks under fuel flow
  • reversed inlet / outlet fuel lines
  • wrong high speed / idle poppet installation switch
  • Enderle spool upside down
  • Hilborn spool over-center
  • idle surging
  • too many measurements
  • too much data
  • wrong fuel pump rotation for proper outlet flow.

Further discussion is beyond the extent of this newsletter.  Added information about these problems plus more info about the prior examples is throughout our MFI publications, MFI articles, blog, newsletters, & air density tuning articles