Oil boiler flame cuts out but no lock out and cycles continuously

[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

 

[Chalked]

Do you have a flue gas analyser and are you oftec registered. If not you shouldn't be fiddling.

But 6 inch of fuel, barely covers the outlet.

 

[oilypig]

Thanks
Yes but its home in England.
Sorry 6 inch above outlet from tank, but not a great deal of 'head' but it's been run dry before with no problem.
cheers

 

[lame plumber]

try a new photocell

 

[oilypig]

try a new photocell

Thanks

 

[jensda]

have you cleaned the filter on burner or just the line maybe coil as well

 

[lame plumber]

if the pump and fan are running ok continuously its unlikely to be capacitor/coil

 

[jensda]

would agree lame plumber but had a 3gb with similar problem last year and it was a faulty coil usually coils fail after a certain run time when hot but not this one for some funny reason def not a capacitor I would think as well as there is no lock out it does sound like fuel starvation or over airing

 

[kimbo]

Could be faulty oil pump that when gets hor loses pressure. Check with gauge. If it was photocell I would expect it to go to lockout. I would then go for control box. There could be more than one fault

 

[Roger Welton]

If photocell gives a false light reading then burner runs continually and won't lock out, as lp says photocell first, if ok then control box.

 

[Bunker]

OP says he's tested pcell but that is the obvious choice.
Is it the early model with the Mecron or the later with the RDB Burner?

 

[kimbo]

According to post the burner doesnt run indefinetly without lighting otherwise as roger says the cell is seing a false light. The burner us firing and going out repeatedly.
Sounds like a g5 burner or g3 on these

 

[Bunker]

According to post the burner doesnt run indefinetly without lighting otherwise as roger says the cell is seing a false light. The burner us firing and going out repeatedly.
Sounds like a g5 burner or g3 on these

Yeah, so if its seeing a false light, the ignition wont come on atall? COuld be worth having the pcell off and wire brushing the contacts but Im also liking the sound of the coil: runs for 20 mins before cutting out? Then repeatedly? Could be too weak to hold solenoid open.

 

[oilypig]

oops....filter on burner? where be that? it's a g5. Thanks

 

[oilypig]

Also if left an hour or so it relight and run again after a few cycles of not locking out as described originally.