Air Injectors and Direct Injectors Tech Tips for Mercury Optimax Outboards

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Optimax DFI Air Injector Removal Tech Tips

A faulty Direct Injector can cause the following symptoms:

1. Rough Engine Idle.
2. Low Engine Power.
3. Excess Fuel Consumption.
4. Plug Fouling.
5. Alarm beeping or Descriptive Text warning on SmartTach.

Have you had an Air Compressor Failure?
Your compressor can deposit debris in your air rails
and your Air Injectors. Always flush your air hoses,
fuel rails and DFI injectors after an air compressor failure.
Read more about Air Compressors.

Using a DDT (Digital Diagnostic Tester) or a CDS (Computer Diagnostic System) is the best
way to properly diagnose your DFI Injector(s).

Since most customers don't have these expensive tools, the next best thing is an ohm meter.
The Ohms test is not always conclusive. A DFI Injector can test OK with the Ohms test yet it
could still be faulty.

NOTE: Read your Service Manual for instructions to remove your Fuel Rail Assembly.
The Fuel Rails must be removed to service the Air Injectors.
See below for some tips on removing your Fuel Rail Assemblies.

The Air Injectors must be removed for proper test results.

A good digital Ohm Meter must be used for accuracy.
You should get a reading of 1.3 ± 0.3 ohms between the two pins.
Confirm these specifications in your Service Manual.
If your injector reads outside these specifications it must be replaced

The Injector should show NO continuity between either pin and the case body/ground.
If the injector shows ANY continuity between either pin and the body ground it must be replaced.

Removing Your Fuel Rail(s)

The fuel rail(s) must be removed to remove and/or replace your Air Injectors.

1. Turn off your battery switches and disconnect the negative battery cables from your batteries.

2. Your fuel rails may be pressurized. Fuel can cause fire, explosion, damage, injury or death so
please use extreme caution when working with fuel.
It is best and safest to use Fuel/Air Pressure Gauge (part 91-16850--1) or Gauge Set (91-852087A1/A2) to depressurize
the fuel rail. The starboard fuel rail has a schrader valve at the top of the rail.
Hook up the gauge to the schrader valve and expel the excess fuel into a container.
Some experienced techs simply cover the schrader valve in a rag and depress the center pin to release the pressurized fuel.
It is much like letting the air out of a bicycle tire valve only much MORE DANGEROUS.

3. Unsnap all the red fuel injector harness plugs by depressing the wire retainer and pulling off the plugs.

4. You do not need to disconnect any of the air hoses or fuel lines if you don't need to
completely replace the Fuel Rail Assembly.
Instead, remove the two rail retaining nuts and gently pry the rail away from the head.
The Air Injectors might pull out with the Fuel Rail.

5. Disconnect each Air Injector harness plug.
Gently pry out each Air Injector from the cylinder head.
NOTE: Once removed, the injector seals will swell. The seals should always be replaced.

6. If you are going to leave the rails loose make sure you cover the rails in plastic bags to prevent debris from getting into the rails air and fuel passages.

7. Use new seals and use the special seal installation tools to reinstall your Air Injectors.

8. The Fuel Rail Bolts must be torqued to 33 ft pounds.

A Note About Fuel Pressure and Air Pressure
Fuel and Air Pressure Gauge Kit

Fuel Pressure and Air Pressure is critical.
The Fuel Pressure Regulator and Tracker Valve maintains exact fuel and air pressures.
There must be exactly a 10 PSI difference between the fuel pressure and the air pressure.
If you don't have exactly a 10 PSI difference the engine will run bad or not at all.

There are a few different items which can effect these pressures.
A clogged fuel filter(s), bad fuel line, faulty lift pump, faulty high pressure pump,
bad pump grommets in the VST (Vapor Separator Tank), bad air compressor, faulty pressure regulator,
faulty fuel rails or rail fuel lines, a bad tracker valve, bad fuel injector or a bad air injector are
just a few of the items which can cause improper pressures.

It is standard practice for an experienced Optimax tech to check the fuel pressure
and the air pressure early on in the diagnosis procedure.

Most engine performance problems are fuel related.
Fuels containing Ethanol can ruin boats fuel lines.
Fuel lines can collapse and cause fuel restrictions.
We now recommend replacing your boat-to-engine fuel lines every year.
Large amounts of Ethanol can also ruin your electric fuel pumps.

Typical Optimax Fuel Pressure and Air Pressure specifications are listed below: Fuel Pressure at Starboard Rail Schrader 88-92 PSI.
Air Pressure at Port Rail Schrader is 78-82 PSI.

Please refer to your proper Service Manual for your model Optimax for exact specifications.