oil burner cuts flame goes out after 20mins tries to relight

[oilypig]

Hi
Any ideas please?
The boiler runs fine for 20 minutes approx then flame cuts out the oil pump continues to run The flame relights for a couple of seconds and cuts out again. This then carries on indefinitely, pump running all the time flame relighting and going out every few seconds.
It does this in or out of boiler chamber.
I do not have to press reset to get as the pump does not stop running.
I have emptied/cleaned the filter/water trap.
I have bled the pump.
There is about 6" of fuel in tank.
It is an old system been working for years.
I have measured resistance of flame sensor with light on and off and it seems to work ok going from high to low resistance when light shined on it. Also checked nothing blocking detector from seeing flame.
It is a Grant euroflame 50/90
Thanks

 

[Tipper]

Sounds like a fuel starvation problem to me.

How high/low is the tank above/below the burner?

 

[oilypig]

Thanks tipper
not much head, the fuel level at the moment is maybe a couple of inches above the pump.
Difficult to tell exactly due to positioning.
It has been run dry previoulsy and restarted ok, not sure if it needed bleeding that time though.
it actually has 6 inch in the sight tube not tank.
ta

 

[oilypig]

Have cleaned pump filter and checked good flow from tank.
Am going to replace coil I think
Cheers

 

[xr4x4]

New Nozzle.

And check oil pressure.

 

[oilypig]

This an interesting and helpful find!
got the 50v so going for coil
http://www.rielloburners.co.uk/lumo...rtfaultfindingfor535seldrdbseriesburners2.pdf

 

[oilhead]

6" in sight tube may mean empty tank, but running 20 mins or so suggests coil breaking down. If a G5 burner with three wire coil, then holding coil may be failing.

 

[oilypig]

ok so coil didn't do nor photo cell. put switch across cell and it runs fine if I mimic what photo cell should do! any ideas?
thanks

 

[xr4x4]

New Nozzle.

And check oil pressure.

 

[oilypig]

Thanks 4x4
Can you explain how it can run fine with photo cell shorted if pressure wrong or nozzle problem? Flame looks ok but sure it wont harm to replace anyway
Thanks

 

[oilhead]

Simple. your switch ignores the flame completely.
Is your boiler balanced flue?

 

[xr4x4]

Give it a service and make sure you get the right nozzle, not a replacment of the one fitted.

A proper service can sort most problems.

As its run out of fuel, it can take a tiny spec of dirt to affect the nozzle.