How to test and confirm a blown intake manifold on a GM Vortech 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. The top portion is composite, the lower is aluminum. The centrally located fuel injection unit is positioned in the lower plenum that is only accessible when removing the upper plenum.

Air enters the upper portion of the composite intake manifold, in this engine, near the front of the manifold. Air is then directed down to the bottom, through am aluminum lower intake manifold. Note that the gasoline operation of this engine makes no distinction on where the air enters the engine as fuel is metered directly at the intake port. When a gaseous fuel is metered into the engine, it is biased directly at the front.

Air is distributed through a central plenum, then distributed to the intake runners. 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 36 inches!

This is a great deal of air and fuel to move between fuel mixture changes accomplished by the vaporizer-regulator.

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. Venturi systems typically have the automatic fuel mixture controls mounted in the vapor supply lines, on the vaporizer, or a combination of the two.

This photo shows the underside of the intake manifold. This manifold is sealed to the lower housing by a silicone O-ring that is very resilient. It easily conforms to the metal and provides a good seal. Careful examination of the passages at the upper edge of this image show an attempt at equalizing the PCV vacuum source.
The opening at the lower edge is the location for the vacuum assisted power brakes.

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 or other appropriate system specific test equipment, and read the air fuel mixture and duty cycle of the fuel solenoid(s).

(Note: This method of diagnostics applies directly to the Impco and OHG carburetion system, but others may be generally similar in mixture control.) The fuel mixture solenoid should be fluctuating at approximately 50%, or ranging from 5 to 9 volts. The O2 sensor should be cycling between full lean and rich positions.

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.

After the engine is reassembled and running, verify the fuel mixture settings.

If necessary, tune the fuel mixtures as recommended.

One mandatory item is to install an RCV in the vaporizer cover. This is a VERY important step and should NOT be overlooked. This RCV will only work on the Impco and OHG series vaporizer/regulator.

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.

On this engine, it is CRITICAL to remove the throttle body and drill and tap at the very front of the body a 1/8" NPT threaded hole for a fitting to accept the PCV hose. Remove the existing PCV fitting on the engine intake manifold between intake runners for cylinders #3 and #5. Plug this port and extend the hose to the above installed fitting. The purpose of this modification is to remove the vacuum leak from the two cylinders and place it towards the front of the engine at a more centralized location. This engine VERY poorly distributes air and fuel mixtures on gaseous fuels (although as previously mentioned, gasoline is not affected) and any extraneous air leak can be problematic.

On this photo, it is easy to see the forward mounted air intake (the round port) and the location for the fuel injector body (the oval port) and the small round fitting for the PCV (located just to the right of the oval fuel injector body port).

The round opening located just behind the oval port on many vehicles is not used and frequently is the location for split intake manifolds. It may be used for vacuum assisted power brakes on some vehicles. On this particular engine, the cast-in-place fitting was a convenient place to let out the backfire gasses, along with about 15 fractures located at the rear edge of the oval port. The backfire also over pressured the MAP sensor, visible just at the upper left of the oval port.

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

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Page uploaded 12-29-01.