Oil Safety Valve Operation

Is the primary control one that lets the burner recycle on a flame out? Check you cad cell Ohms.

@BrooklynMike said:  "I know the OSV valve needs a vacuum to open and allow oil to flow to the burner. When I turn off the burner, do I loose the vacuum in the oil line ? Is there an air leak in the oil line tubing? Is there an oil pump problem? Again, only see this problem after turning off the burner. During normal burner cycling for heating or DHW (tankless coil) , the burner fires normally. Your thoughts?"

To your question about the losing the vacuum. "do I loose the vacuum in the oil line ?" Yes you do. That is by design. The weight of the oil pushing against the OSV causes the valve to close. When the burner motor stops rotating the pump, any vacuum left in the fuel line will cause the OSV to stay open until there is no vacuum left and the head pressure of the fuel in the tank closed the OSV. The OSV will reopen once the burner motor starts and a vacuum is created in the fuel line on the discharge of the OSV.

To your question about an air leak in a fuel line "Is there an air leak in the oil line tubing?" The answer is inconclusive. You will need to perform a vacuum test on the fuel line in question. Disconnect the fuel line from the OSV and place a Flare Plug on the flare nut and tighten with the same torque you would use to connect to the fitting on the OSV. Then install the vacuum gauge near the fuel pump using an unused port or tee fitting. With fuel still in the fuel line, operate the pump and allow it to reach at least 12" vacuum on a gauge. Then let the pump stop and allow the fuel pump cut off valve to seal the discharge side of the pump. You now have a closed system and the vacuum should hold for 15 minutes. If the vacuum drops to 11" or 10" within the first few seconds, don't worry. You want to mark that spot on the gauge after about 10 seconds. Wait for 15 minutes and you should not loose any more vacuum over that time. If you do, then you have an air leak somewhere in the fuel line or the pump.

To answer your question "Is there an oil pump problem?" You can conduct that same test as above but you need to remove the fuel line and cap off the fuel line fitting at the pump. Make sure there is enough fuel in the pump to get it to pull a 12" vacuum. Once you get to 12" vacuum on the gauge, shut off the burner motor and watch the gauge. If the vacuum drops when connected to the fuel line and does not drop when the fuel line is isolated, then your fuel line is the problem and the pump is fine.

If the isolated fuel pump loses vacuum then you may have a defective pump seal, a defective pressure cut off valve on the nozzle port or even a poorly installed strained cover gasket.   That gasket can be replaced after properly preparing the cover and the matching surface on the fuel pump.  If it is a bad cutoff or a bad pump seal, then it is cheaper to replace the fuel pump than it is to replace the cutoff valve or pump seal. (labor being what it is today)

With the pump no longer leaking a vacuum, you can reconnect the fuel line and do the test again to verify that the fuel line is not leaking. You may have a leaking fuel line AND a leaking pump. Once you get the pump to stop leaking, then you can verify the fuel line is good or is leaking with the same test. Keep isolating sections of the fuel line until you have no leak. When that happens you have discovered a leak. repair that leak and test the entire line again. You may have more than one leak in the fuel line. This can be time consuming but is the only to verify the leak location on a system with fuel line connected to a fuel pump.

You and also pressure test a fuel line with no fuel in it with 20 to 30 PSI air pressure. that leak will cause bubbles to form at the point of the leak. Do not use 20 PSI on a fuel pump to test for leaks, that amount of pressure will destroy the pump shaft seal. Then you will need to replace the fuel pump.

Hope this helps.