Suburban SW6 lights but drops out after 30+ seconds

[GenoB]

I'm a pretty good troubleshooter but this one has me going. My 10 year old water heater has been working fine but now will only light and run for approx 30 seconds + or - a few seconds.
a) Normal sequence to start up, 5 second wait, valve actuates, igniter lights gas, runs for 30 sec
b) I replaced the propane regulator incase the gas was dropping out or weak
c) Replace the igniter thinking it may not be sensing the flame is there (2 pin)
d) Checked the gas valve solenoids using 12vdc, they both work strong and click hard
e) While the flame is working, there is 12vdc present at the gas valve. When it drops out there is still 12 vdc present at the gas valve. Sounds like gas is shutting off inside valve but its not because of electrical shutting off ?
f) Took main circuit board to RV shop and it tested OK
g) Cleaned and checked all electrical connections on gas valve and main circuit board.
h) DC power is always 12.5vdc
i) Disconnected gas line from gas valve and at regulator and blew out with 30 psi air compressor, thinking oil might be blocking lines ?
j) There is no lp switch over, single tank with new regulator. I have tried another lp gas tank, same thing

I give up. I still think its a gas issue, either weak or blocked or the gas valve has some kind of mechanical shut off perhaps when gas is low pressure or it is just bad ? I don't think its going to be electrical since the gas valve still had 12vdc when iit shuts down. sometimes it will go right into the start up sequence after the flame goes out and relight immediately for a short time ?

Any ideas ?

[BFL13]

So the flame goes out first and 5 secs later the 12v stops at the gas valve and it also shuts the gas off? The time it stays burning after first getting lit varies, and sometimes it relights and then quits again. You wait and it will fire up then quit again.

So something is getting too hot quickly and then it works again when it cools down some?

After the flame goes out it takes 5 secs for the gas to shut off, so the flame sensor system is working properly?

What does the flame look like when it is burning?

[Old-Biscuit]

Have you cleaned the orifice & burner tube?

12VDC staying on gas valve then gas is present and flowing...dirty orifice/burner tube will disrupt that balabnce between air/fuel resulting in flame out

Soak in alcohol then air dry
NO POKING or HIGH Pressure Air

[opnspaces]

Might be the thermostat not tight to the tank or eco switch. There is a service manual at Bryantrv Link
Scroll down to water heaters. There is a troubleshooting flow chart near the end of the manual.

[Roger10378]

How does it do if there is other gas usage? Lite a couple of burners on the stove and observe them while the WH starts and goes out. If there is a gas problem upstream you will see the flame get weak there as well.

[GenoB]

UPDATE

Sometimes it will run for a couple minutes before shutting off. Then fire right back up.
After it does this a couple times it has a hard time lighting until I shut if off and wait a min then it starts right up again ?

[GenoB]

Thanks for your reply !

Well I replaced the 2 prong igniter because of just that thought. I believe the sensor is in the igniter but I am no expect, just seems logical, I see no other sensor wires or devices.

Problem with all this is the 12vdc going to the gas valve coils are still on and active when the burner shuts off and stays on for about 5 seconds. So I don't think it an electrical thing ? but this is new to me, I'm an electronics tech and have worked on logic systems in the past.

The burner just stops for no apparent reason ? I there anything in the gas valve that might shut off if the propane pressure was to drop ? Like if the propane regualor is still not putting out enough ? I did replace it but the pressure on the secondary side can be held by my finger

[BFL13]

Usually the story here is that the "flame sensor" does not find any flame so the whole thing shuts down right away. (as you suspected in the OP)

You said you put in a new "igniter", but did that include a new sensor?

Newer "igniters" do not have a separate sensor electrode, but instead they measure flame between the igniter electrode and the other prong where they spark. That signal goes back to the board.

So whatever you have it is still no flame sensed by the board so it shuts off the gas to the burner.

New "igniter", board checks ok, so IMO suspect the connections from new igniter to the board are not properly connecting.

30 seconds is a long time for that though?

[RJsfishin]

Hot wire 12v to the gas valve, if it stays lit get a new board.

[GenoB]

RV shop had a what looked like an official tester box with simulating switches and leds. I checked to make sure it simulated the fire sensor and he said it did

[DarkSkySeeker]

How was the main circuit board tested?