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Two-stroke outboard motors have traditionally used a mixture of gasoline and lubricating oil as their fuel. A long standing practice was to pre-mix the gasoline and oil together in the boat's fuel tank. This required careful calculation and measurement of oil and gasoline, and particularly in large tanks could be awkward to accomplish. Also, pre-mixing set the gasoline-oil mix to a fixed ratio, typically 50:1. In the mid-1980's outboard manufacturers began to offer oil mixing pumps which would blend lubricating oil from a separate reservoir tank with the gasoline as it was being pumped to the engine. This arrangement offered several advantages: the operator did not have to compute and measure oil for pre-mixing in the fuel tank. The operator just had to keep the oil reservoir full and oiling would be done automatically. Also, the ratio could be made variable to allow for different lubricating needs based on engine throttle setting.

OMC introduced an oil/gasoline mixing system for their outboard motors in c.1984, and called this feature a Variable-Ratio Oiling system or VRO. Over the years some engine failures caused by lack of proper lubrication have been mistakenly attributed to defective VRO components. As a result, some owners of OMC engines with the VRO feature have disconnected the pump and reverted to pre-mixing. This article discusses the history of OMC outboards with VRO, the theory of VRO operation, and the diagnosis and repair of common problems. Suggestions for updating older motors are included. It also dispells some of the myths that have arisen in "dock-talk" about the VRO system. Earlier versions of this article have appeared in BASS & WALLEYE and TRAILER BOATS magazines, the AM-Tech newsletter, and other online resouces. A revised and updated version is presented here with the permission of the author, Bill Grannis, whom we thank for this valuable contribution to our REFERENCE section.

VRO STORY—The Myth of the Mixer

by Bill Grannis

Whenever there is a gathering of boaters, the conversation usually turns to engines, then gravitates to stories about problems. One of the bones OMC owners like to pick concerns the oiling system. A typical conversation that we've all heard at one time or another might go something like this:

  1. "Yeah, my boat smokes like hell at low speeds and fouls the plugs," bemoans one boater. "Must be that worthless VRO pump acting up."
  2. "Uh huh, my neighbor had a blown engine," quips another. "Took out two cylinders on his V-6. The mechanic told him the VRO went bad. He didn't get an alarm or anything."
  3. "Come to think of it," the third member of the group chimes in, "my brother-in-law's offshore rig with twin outboards must have a flaky VRO; one motor always uses more oil than the other."
  4. "My engine is hard starting at times," says a fourth angler, "the VRO pump must be getting weak and the warning horn blows constantly whenever I run at full throttle on my 150."

The truth of these stories is that none of the symptoms described is the fault of the VRO, yet the poor VRO pump gets the blame. Each rig has a different problem that is blamed on the oil pump because of misunderstanding, ignorance, and misinformation. Even many "experienced" mechanics do not have the knowledge of how the system operates and are quick to blame something that is unfamiliar to them.

For instance, the first boat owner's problem (1) is an air leak somewhere in the boat's fuel system causing "foamy" fuel instead of a solid column of liquid. This aerated gasoline has less volume and less resistance causing the fuel pump diaphragm to cycle faster. Each cycle pulses oil into the foamy mix, increasing the oil:fuel ratio to the carbs creating a smoky exhaust.

The mechanic of the second guy's neighbor (2) did not understand that the VRO mixes the fuel and oil internally then delivers the mixture through the fuel lines to each of the carbs. Since each carb receives the same oil to gas ratio, the VRO cannot cause only two cylinders to fail. Since the VRO did not malfunction, there was no alarm to sound.

On the other hand, the brother-in-law's motors (3) have two different year's pumps on them. The later model VRO's pump more oil at low speeds than the original ones did.

The last guy (4) blames the VRO because he once had a car with a weak fuel pump and a replacement cured its hard starting problem. Actually, he only needs a refresher in the correct starting procedure. As for the constant sounding horn at high speed that stops when slowing down, that's a fuel restriction warning, not a VRO malfunction. A lack of oil flow is a pulsing on and off horn every second or so.

A decal is available (OMC P/N 335707) for the dashboard that illustrates the different warning signals and what they mean. This would be for 1995 and earlier motors, since 1996 the SystemCheck gauge uses four LED warning lights that illuminate to indicate LOW OIL, NO OIL, OVERHEAT, or CHECK ENGINE.


OMC introduced the VRO on the V-4 and V-6 engines in 1984. The pump consisted of a combination oil pump and fuel pump actuated by crankcase pulses through an air motor. Gasoline reformulation in those early years contained alcohols and solvents that softened the internal rubber components and caused pump failures and damaged engines. Back then, no company had alarm systems for a pump failure or loss of oil flow. OMC took care of the blown powerheads and improved the pumps considerably.

The VRO2® was introduced in 1986 and included a "NO OIL FLOW" alarm. Changes included an electronic circuit to compare the oil pulses with the engine's RPM, a better oil pump piston, alcohol resistant seals, and a brown fuel outlet for recognition. Unfortunately, the damage was done. Public perception fed by uneducated mechanics put the blame for almost every engine problem on the VRO. No matter what happened to a motor, someone would utter "...must be a bad VRO."


Actually, the much-maligned pump is very simple and reliable. It consists of four basic sections:

Cutaway View of VRO Pump

Cutaway View of VRO Pump Showing Major Components

The air motor converts the pressure and vacuum pulses from the crankcase to a linear motion. These pulses are routed through a pair of check valves above the engine crankcase pulse fitting. One directs pressure cycles to one side of the air motor's piston (white chamber) and an inverted check valve directs the vacuum cycles to the opposite side (tan chamber).

Each engine revolution moves the piston (dark green) incrementally to one side of the air motor chamber (tan) and compresses the large spring. When the piston (dark green) nears the end of its travel, a small spring unloads a poppet valve located in the middle of the piston assembly. The pressure (white area) on one side of the piston rushes into the vacuum side (tan area) as the large spring quickly pushes the piston back to its starting point.

As the throttle is advanced, the pressure pulse gets stronger because of the increased airflow through the crankcase. This back and forth motion of the air motor's piston (dark green) is what powers the fuel pump and the oil pump. At idle and low throttle settings, the weaker crankcase pulses cause shorter piston movements, which in turn, pump less oil with each cycle. As the throttle opening increases, the stronger crankcase pulses cause greater piston travel resulting in longer oil pump strokes. That means, more oil is delivered increasing the amount of lubricant in the fuel up to a 50:1 ratio.

The fuel pump, which is located in the middle chamber of the VRO, consists of a push-pull piston (orange) and diaphragm with a flapper valve and an inlet check valve. A rod connects the fuel pump piston (orange) directly to the air motor piston (dark green) and the two chambers are sealed from each other by an o-ring. As the back and forth motion is generated in the air motor, the double-action fuel pump's piston (orange) and diaphragm is pushed and pulled by the rod cycling fuel to the carburetors. Gasoline is drawn into the blue area of the chamber, travels through the piston's flapper valve, and is pressurized in the green section on the following stroke.

The oil pump is directly connected to the fuel pump piston (orange) and diaphragm so they work in tandem. As with the fuel pump, oil (yellow) is drawn in through the inlet fitting and a check valve as the air motor starts its travel. On the return stroke, the oil pressurizes, exits directly into the fuel chamber (blue area), and is blended while enroute to the carbs.


Of all the traditional two stroke outboards produced worldwide, OMC products have the only automatic lubrication systems with a "no oil flow" alarm. The VRO's electronic circuitry compares the RPM of the engine to the number of pressure pulses from the oil pump piston. As the oil flows into the fuel area, the pressure moves a small piston that actuates a steel pin in the alarm module. The pin's movement is detected electronically and "counted." If the engine revolutions exceed the preprogrammed oil pulse ratios, the warning horn is activated sounding a rapid on and off sequence. Since 1993, the VRO is actually called an oil metering system (OMS). The ratio averages about 60:1, slightly leaner at idle and slightly richer at full throttle. The older pumps idled between 150:1 (prior to 1990) and 100:1 (1990-1992), then richened up to 50:1 as the engine load increased. They are recognized by the brown fuel outlet fitting and/or a wiring harness attached to the alarm module.

The original VRO from 1984 and 1985 had a black fuel outlet fitting and no wiring. OMS pumps have a blue/gray outlet nipple or a blue/gray pulse nipple, or a black fitting held in place by an oval metal bracket and two Torx headed screws. To simplify things just remember that the air motor converts the crankcase pulses to a back and forth motion. Directly connected to the air motor is the fuel pump and the oil pump. The travel of the air motor pumps the fuel and the oil together to the carburetors and the alarm system monitors the oil pressure counts. Just like manually premixing the oil and gas, the VRO (OMS) automatically does the same thing and sends that fuel mixture to the carburetors.


There is no mechanical maintenance that has to be performed on the pump itself, but the VRO system should be inspected occasionally for cracked lines, leaks, and broken clamps. One of the most common causes of engine failure is water in the remote oil tank. Condensation and spray can build up over time and, like water in a fuel tank, it stays on the bottom to be sucked up at the worst possible moment. Water pumps through the system as easily as oil, so several time a year use a turkey baster to draw off some fluid from the base of the tank and squirt it into a clean jar. Look for sediment or water settling to the bottom of the container. If you see any signs of contamination, the tank should be flushed clean, the filter (OMC P/N 174377) changed, and the oil line purged. Be sure to read and understand the service manual before doing any of these procedures.

If an oil tank is exposed to the elements such as in an open boat, installing the tank inside a covered battery box will protect it from spray. You can also replace the clear plastic cap with the later model's solid black filler cap (OMC P/N 176217) that provides much better water intrusion resistance. The oil line should be OMC's 1/4-inch I.D. inner "rubber" hose (OMC P/N 333485) without any splices. Early systems had a smooth vinyl oil line that hardened after a few years and sometimes caused air leaks. (Some installations even had 5/16-inch fuel line squeezed down over the smaller fittings.) These air leaks can also trigger "NO OIL" alarms, commonly at startup, idle, and trolling speeds.

Another service tip is to avoid Ty-raps or worm gear hose clamps on this system. The ratchet clamps (OMC P/N 322654) give the best seal and full 360-degree clamping action. Late-model OMC's use a double-wire wound spring clamp (#339277) that also works well in keeping lines air tight. Air leaks and/or fuel restrictions will affect the VRO and usually show up as excessive oil consumption, smoking, and fouling plugs.

If air gets into the fuel, it causes a foamy solution, similar to a head of beer in a tall glass. (A vapor lock situation does the same thing ). The volume of liquid fuel is less and the oil delivered is the same as for a full chamber of fuel. Now you have richer gas oil mix with its resulting symptoms. A fuel restriction such as from a bad anti-siphon valve, kinked line, or a partially plugged filter does not allow a full charge of gas to be drawn into the fuel pump cavity. Again, you get the full shot of oil mixing with less gas. This is also the reason to not disconnect the gas line and run the engine "dry". As you run out of fuel, the oil still pumps, filling the lines and carbs with your favorite TC-W3 lubricant.

Since the VRO depends on crankcase pulses to operate, it is susceptible to backfires from a lean running cylinder or an out of tune engine. If your motor is older than a 1993, make sure it has the blue colored pulse limiter to protect the air motor and the check valves in the pump. Follow the pulse line from the VRO to the engine block and look for a hex shaped fitting threaded into the crankcase. If it has a black face on it, replace it with a blue style (OMC P/N 435009). The crossflow V-6's have a pulse limiter with a fitting at each end and spliced in line with the VRO to crankcase hose. The inline replacement is OMC P/N 435010.

Since 1986 (and on updated 1984's and 1985's), the pumps have an alarm system that operates off the alternator or tachometer circuits. Occasional checks should be performed to make sure the charging system, warning horn, and related parts are operational. The quickest and easiest test is the grounding of the tan overheat temp switch wire to the engine block. To do this, turn the key on, motor not running, and use a jumper wire to ground the tan wire from the motor harness and listen for the warning horn. It should be a loud steady tone. On 1996 and later engines with a SystemCheck® gauge, only the "hot" light will come on, but no horn unless the motor is running. With the SystemCheck® equipped motors you will have to disconnect the OMS (VRO) four wire connector and carefully ground the tan motor harness wire lead to illuminate the "no oil" light.

The warning horn tone sequence tells if a problem is occurring while operating the engine. A rapid on–off sound is a "no oil flow" indication. One beep every 20 to 40 seconds is a low oil level in the remote tank. Of course a steady tone at all speeds is an overheat condition. On V-6's a constant horn at high speeds that disappears instantly when the throttle is pulled back indicates a fuel restriction. Loose connections can also cause erroneous "beeps" and the most common open circuits are battery cables with finger tightened wing nuts instead of stainless steel hex nuts and lock washers tightened with a wrench. Incorrect spark plugs occasionally cause false warning signals, too. Use only the "Q" style suppresser plugs recommended by OMC for your motor. Resistor plugs are not the same thing and may be the source of problems. The factory does not endorse spark plug manufacturer's conversion charts.


In 1996, the SystemCheck® warning gauges were introduced. A tach with four warning lights or a dash-mounted gauge with the same lights show the type of malfunction that is occurring by which LED is illuminated. The horn sounds for 10 seconds when a fault occurs and the proper light stays on until the problem is corrected. There are indicator LED's for HOT, NO OIL, LOW OIL, and CHECK ENGINE (which means a fuel restriction in a V-6 carbureted motor).

By the way, the SystemCheck® can be adapted (with accessory kit P/N 176709) to any engine with a four-wire VRO pump. [For more information on the SystemCheck upgrade kit, please see a separate article and discussion.]Even better, there's a new VRO (OMS) pump that can be used to upgrade all the previous years and model outboards. The new pump (P/N 5004558) contains various fittings for different outboards, and each internal part is replaceable for easier and less expensive servicing. Part number 5004559 updates all early three-wire VRO's except those found on the two-cylinders. For those outboards, kit number 5004562 is used. OMC also makes a wiring harness adapter (P/N 174710) for the 1984 and 1985 engines that did not have any warning systems. Each OMC (VRO) kit contains the parts needed including a new pulse limiter to install the system correctly.


Since 1991, the OMC factory service manuals include a troubleshooting chart and a section on how to test, take apart, and service the VRO pumps. Besides Torx screwdrivers, you will need a pressure and a vacuum source to test the integrity of the check valves and diaphragms. CDI Electronics' #551-34PV gearcase vacuum/pressure tester work well, as does a SnapOn YA-4000 or the old Stevens pumps.

Even though they are not listed in the parts catalogs, most of the diaphragms are interchangeable. The air motor diaphragm kit is part number 435921 and the fuel pump repair kit is 436095. For those who pre-mix their fuel and oil, these kits allow the repair of the fuel pump section, avoiding the high cost of a new assembly or the re-plumbing and rigging of one or two traditional style fuel pumps.

Do not interchange any of the springs with other pump assemblies. They are calibrated for each model of pump. The latest P/N 5004558 pump is a universal fit that can be used as a replacement for all VRO equipped motors and each part is available for servicing.

When working on a fuel system, check for any restrictions or air leaks which would affect the engine. You can use a clear piece of fuel line connected to the VRO inlet and "T" off to a vacuum gauge. Run the motor at all speeds while inspecting for air bubbles in the line and for any vacuum restrictions above 4-inches Hg. A system with 3/8-inch fuel lines, a clean "spin on" filter, and a quality anti-siphon valve should show between 2.5 to to 3-inches of vacuum at full speed.

Repair and update your VRO or OMS system methodically and by the service manual. Check and test the boat's fuel system for restrictions and/or air leaks, and make sure the oil tank is clean. If you follow these recommendations, you will have a reliable and economical outboard that will keep your boating experiences pleasant.

So, contrary to boat ramp "urban legends", the VRO pump is a simple and reliable oiling system that has been standard equipment on most OMC outboards from 40- to 300-HP for the past 20 years. Most of the misinformation and "scare tactics" about its reliability originate from those who do not understand how the system works, or fail to keep it maintained.

DISCLAIMER: This information is believed to be accurate but there is no guarantee. We do our best!

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Portions Copyright © 2005 by James W. Hebert. Unauthorized reproduction prohibited!

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Last modified: Sunday, 29-Jun-2008 12:12:41 EDT
Author: Introduction by James W. Hebert; Article by Bill Grannis
This article first appeared January 16, 2005.