VRO or OMS testing

1986 Evinrude 90 HP

I've been trying to identify a rough idle and unspent fuel/oil problem. I did all the standard tests and got up to testing the VRO.

Ran clear hose on output side of VRO and pressure tested. At 800 RPMs the pressure would gradually drop from 7 to 3.5 PSI within 15 to 18 second cycles. Each time pressure reached about 3.5 PSI, I noticed a very small group of bubbles would pass out of the fuel outlet and just as they did the pressure would immediately jump back to 7 PSI. Then the cycle would repeat itself again.

Removed VRO and found it was not the original. It had part # 500455? ? can?t read last digit, but assume it was 8 or 9 (making it one of the newer OMS pumps). It has 4 wires and inlet and outlet fuel fittings held in place by a metal bracket and torx screws, and pulse limiter was blue.

I asked a retired marine mechanic I know to bench test it and will get back to him in about an hour. He said back in the day they did try to rebuild these but didn't have much success. He thinks I should replace with new. If the bench test points to a bad diaphragm, I'd like to know what folks on this forum would recommend- new or rebuild.

I'm not looking to premix, because since E10 I've decided to use up the gas in my tank at the end of each season in my truck. I do use stabilizer all year. I don't think its a good idea to let it sit 6 months.

Last question- in a correctly working pump, shouldn't the pressure stabilize and not behave like I described above? I figure as the air builds up in the pump it causes the pressure to gradually drop. Once that air bubble is released, the pressure returns to 7 PSI. As to my rough idle, I expect the air is causing it to mix too much oil, fouling my plugs, and leaving too much unspent fuel/oil in the exhaust.

Re: VRO or OMS testing

first there should not be air in the fuel system. it is sucking air between the tank and the pump. check connections.

Re: VRO or OMS testing

rough idle could be dirty carbs, link n sinc problem, idle speed set too low. ethanol poision. see the top secret file.

Re: VRO or OMS testing

Just got back from doing VRO bench test with mechanic. The test did not turn up anything wrong. The test was:

1. Pressurize pulse hose to 15 psi- does it hold - yes.
2. Holding pump in normal running position, with fuel outlet covered, rapidly remove pressure hose- pump should not click- it didn't.
3. Wait 5 seconds and then uncover fuel outlet- pump should click- it did.
4. Vacuum pulse fitting at 15 in. Hg., in normal running position, and fuel outlet covered, rapidly remove vacuum hose. After waiting 5 seconds, ucover fuel outlet- pump should click- it did.

These were the tests I did prior to testing VRO:

1. Fully charged battery, fresh 89 octane (E10) fuel and XD50 oil.
2. Fuel line holds pressure (bulb gets and stays hard).
3. No fuel restrictions (tested using new hose and portable tank).
4. Verified good ground connections.
5. Checked spark plug wire resistances.
6. All spark plugs look identical, but build up carbon quickly.
7. Verified all sparks jump ? inch gap using an adjustable spark tester.
8. Compression (warmed up/ throttle open) 135 135 135 130 PSI.
9. Cylinder head temps (warmed up/ at idle) 137 - 138 F.
10. Decarbed twice.
11. Verified all recirculation check valves pass fuel out and not in.
12. Verified clear recirculation fittings at bypass covers and main bearing.
13. Verified VRO does not leak fuel down into pulse hose while pumping primer bulb.
14. Verified primer solenoid valve does not leak in run position.
15. Rebuilt carbs.
16. Replaced follower roller, synced, adjusted cam follower, set idle to 700 RPM in gear.
17. Ran clear hose on input to VRO- no bubbles.

Mechanic suggests I might want to do a leak down test on cylinders. He says engine is probably just getting old, and when they do the first thing to go is the idle.

I'm still curious if VRO pressure should stabilize, or is it normal to see it drop from 7 to 3.5 PSI in 15 to 18 second cycles like it does?

Re: VRO or OMS testing

Regardless of what the VRO pump does, it would not directly effect the idle unless you have an air leak. The fuel pump fills the float bowls and when the engine is running provides fuel/pressure when needed to the primer solenoid. It has nothing to do with getting fuel from the float bowls to the firing chambers, that's done by vacuum from the pistons.

Re: VRO or OMS testing

My thought was that over oiling would effect idle, and a malfunctioning VRO might cause it.

Re: VRO or OMS testing

WestPoint said:

My thought was that over oiling would effect idle, and a malfunctioning VRO might cause it.

Click to expand...

Are you using too much oil?

If you have an air leak on the gasoline side incoming fuel line, you will definitely use too much oil. http://www.youtube.com/watch?v=SEbW-6eOc5I

Re: VRO or OMS testing

To prove or disprove your over oiling theory, why don't you cap off the oil line, connect up a portable tank of premix and watch your idle after a short run.

-are both throttle valves completely closed at idle?
-have you checked timing at idle? Timing at idle is a indicator for incorrect idle mixtures...

Re: VRO or OMS testing

The manual has a test to verify oil consumption. Using a 10 in. length of 1/4 inch ID clear hose that you've marked off in 1/2 inch increments, at 800 RPM the original VRO should consume a one inch column of oil in approximately nine pulses. Unfortunately this is not the original VRO and it will probably consume more. This VRO is an OMS (either 5004558 or 5004559).

I watched the over oiling VRO video on the link ezeke provided. In the video the mechanic mentions that the VRO was clicking more often than it should and that is a sign it could be over oiling.

Does anyone know where I might be able to find out the consumption figures for the OMS pumps, or how many clicks at a given RPM they should do?

To clarify about the air bubbles I see every 15 to 18 seconds- these are real small and hardly noticeable unless you are looking very closely- if combined they would probably amount to a single 1/16 inch diameter bubble or less. There is no air coming into the pump at all. Perhaps what I'm seeing is normal.

I will verify the cam follower pickup timing as suggested by schematic. I'm not real keen on capping the oil to test how it runs on premix. I read one mechanic's opinion that you could damage the oil pump side doing that.

Re: VRO or OMS testing

If you don't want to cap off the oil side, simply disconnect and cap off the VRO outlet. Then run a line from the premix tank to the carb inlet manifold and run the unit off of the primer bulb.

At least you'll know if its a mixing problem

Re: VRO or OMS testing

Hey- I like that idea. I'll give it a shot.