Spark Pilot goes out in 2 secs so no time for Burners to ignite

Liberator

I am still trying to troubleshoot my furnace. I have a Lennox G16Q3-75 which is from the 80's/90's timeframe with a S8610U board. When I engage the tstat for heat, the intermittent electrical spark pilot lights and distinguishes in 2 seconds as it becomes smaller in size in the 2 second period.

The pilot flame is approx 3/4" but dances allot due to the inductor motor fan coming on. I have cleaned the pilot flame port, cleaned and then replaced the flame sensor.

Any advice please?

harleyrider

it almost sounds to me like the spark box is going bad

sleepyg

The unit is not sensing the flame, the thing that concerns me is the flame "dancing". That makes me think that you may have a crack in the heat exchanger. Also have you checked your gas pressure?

Liberator

Hi Sleepy

Thanks for the post. The flame does dance abit as the inductor fan is on. It also does appear the gas appears strong at first and quickly dimishes as if the gas was is cut off.

How do I check the gas pressure please?

Tator1076

Need to see if getting 24 volts to main on gas valve. If not then box is not sending 24 for main to come on only pilot.

Tator1076

You will have m/ c /p check between m and c

how

I too don't like the dancing pilot flame but I don't see how a cracked heat exchanger can affect a normal pilot flame before the main blower comes on.
First test to try.
It sounds like you are losing the power to the pilot solinoid if the pilot is shutting down in two seconds. I would be measuring between the gas valve's P & C to first see if that current is also dropping out when the pilot drops out. If it is still being powered when the pilot flame drops out then the gas valve is in question.

(one exception can be some very light debris inside the pilot tube that is forced by gas pressure up against the pilot orifice. Once the pilot shut down occurs and the gas turns off, the debris falls back down until the next call for gas) Two seconds of pilot flame is just about right for that!

The second exception would be if you have very low gas flow (as opposed to pressure) from a gas cock that is almost shut off?

If the power to the pilot solinoid is dropping out within a couple of seconds then the ignition board is in question, as Harley points out..