How to test and confirm a blown intake manifold on a Ford 4.6 or 5.4 V-8 engine.

Symptoms:

The vehicle may have exhibited a backfire, then a stumble when operating on the alternative fuel.  Gasoline operation MAY be normal, or near normal.  Drastic conditions may result in an engine not running on either fuel.

 Also, if the manifold is split open, the engine may have ingested plastic manifold debris!

 

What causes the backfire?

This engine design has a very large volume intake manifold that is shaped like a nautilus shell.  Air enters the upper portion, near the center of the manifold.  Air is then directed down to the bottom, then allowed to flow through a composite shell, forming the lower intake manifold.  Intake runners then pick up the air stream from this plenum, allow the air to flow up, then turn 180º, then into the cylinder head.  Total intake runner length from the bottom of the intake runner to the intake valve may exceed 20".  The distance from the bottom of the throttle butterfly to the plenum may be another 20 inches!

The accumulated distance can be as much as 40 to 54 inches!  This is a great deal of air and fuel to move between fuel mixture changes accomplished by the vaporizer-regulator.  To complicate matters, this manifold has an additional device to tune the intake manifold to varying load conditions.

Remember that all fuel mixture changes are performed at the vaporizer regulator, using a conventional Impco or OHG system.  Other systems with a venturi mixer have the same obstacle, but usually have the air fuel mixer directly mounted to the throttle body.

The process of combustion:

  • Air and Fuel is metered into the combustion chamber
  • The mixture is ignited, then allowed to flow into the exhaust manifold
  • The burnt mixture reaches the O2 sensor, where the amount of residual oxygen is measured
  • An electrical signal is sent to the fuel mixture controller, where a calculation is made
  • The fuel controller sends an electrical signal to the vaporizer-regulator, where an electrical vacuum solenoid is triggered
  • A varying amount of air-valve vacuum is commanded, to either release vacuum to allow the vaporizer-regulator to allow more fuel to enter the engine, or to apply more air-valve vacuum, to reduce the amount of fuel enter the engine
  • Fuel is then allowed to flow from the vaporizer-regulator to the air valve mixer where it is mixed with incoming air
  • The mixture is allowed to flow from the mixer-through the throttle body-through the intake manifold- then into the cylinder head, where the process is repeated.
This process is called the "Fuel Transport Delay", or the amount of time (measured in crankshaft degrees of rotation).  In some cases, the crankshaft may rotate as many as 20 revolutions before the cylinder ingests the new changed air-fuel mixture!  The delay varies by the amount of throttle opening, engine RPM, air density, air temperature, intake manifold design, etc.
In addition, this author has seen mis-matched spark plugs, improperly gapped plugs, crossed secondary spark plug wires, no mixer mounting gaskets, loose mixer adapters, O2 sensors that are "flat line" (burned out), throttle body gaskets leaking, and broken valve springs.  All of these vehicles exhibited backfires that were blamed on the use of alternative vapor fuels!
Assuming that all tune and mechanical problems are properly serviced or repaired, backfires are most frequently caused by a lean air-fuel mixture, usually encountered when the engine is started with an open throttle, but can also be caused by a tip in (throttle stab).
Note:  In many cases, the engine will run roughly on gasoline, and not at all on the gaseous fuel.  During cranking, the engine may have a very loud "roaring" sound.

Diagnostic steps:

(Assuming the engine runs.)  Connect the Autotronics Controls pn #4041, and read the air fuel mixture and duty cycle of the fuel solenoid.

The fuel mixture solenoid should be fluctuating at approximately 50%, or ranging from 5 to 9 volts.  The O2 sensor should be cycling between the green and red lights.

If the fuel mixture is locked full lean, attempt to enrichen the mixture by gently depressing the primer button on the vaporizer.  If a gentle pressure is all that is required to bring the fuel metering solenoid into spec, attempt to make an adjustment to the idle mixture.  If more than 1 turn of a mixture screw (with the Impco 300-A) or 3 or more turns (with the 200-225 or 425) is required to bring the fuel mixture into range, suspect a significant vacuum leak.

Underside of intake manifold

 Follow accepted industry procedures and remove the intake manifold.  Invert the manifold and remove the lower plenum.

Removable composite plenum

Outline identifies the composite section that has blown apart.

This view shows the bottom of the manifold.

This engine utilizes a diverter door which changes the effective length of the intake runners by adding additional volume of the other runners.
After the engine is reassembled, run the engine and verify the fuel mixture settings.

Tune the fuel mixtures as recommended.

Using the Autotronics 4041, set the fuel mixture at idle where the analog meter needle just kicks up at the "5" mark.

One mandatory item is to install an RCV in the vaporizer cover.  This is a VERY important step and should NOT be overlooked.

Either:

  • Remove the black plastic elbow and install a tee fitting, then reinstall this RCV with the opening pointing to the rear, or pointing down;
  • Remove the cover, drill a hole to accept a 1/8" NPT fitting, deburr the threads, then reassemble.

For additional information, see the section on Diagnostics and Troubleshooting.

This page represents the views of the author only.

All rights reserved, Franz Hofmann