Lennox Pulse trouble

scooter

Hey guys,
I have got a pulse that is kicking my a$$
Fires and runs for a random period of time, sometimes 10 seconds, some times 5 minutes and everything in between.
it is a 1992 g21, upflow, natural gas. I installed a new air diaphram and inspected/cleaned the gas flapper assy. Removed and inspected spark plug and sensor, both very clean. reinstalled. checked for obstructions in vent and combustion air and made sure the drain was clear.
when running I get O2 levels that drop pretty quickly to 5-8% but remin fairly stable there. the CO levels out around 3ppm, but then while it is running my O2 will start to drop steadily, fairly slowly, but steady and around 3% O2 the CO starts to go through the roof, 440 ppm one time. then the flame goes out, inducer kicks on and it fires right back up.
I am running it with the combustion air disconnected at the furnace.
I have no drop in pressure in the basement during running and popped open a window just to make sure I had adequate combustion air. I am suspecting the gas valve as I can turn the adjusting screw through its entire range and nothing seems to happen. I did not jump at this as I have never really been sucessfull at accurately adjusting the gas pressure on theses things.
Also, is there any way to tell if the heat exchanger was ever replaced?
Thanks guys.
Scott

pacnw

have you pressure tested the HX?

I had one that did similar, after jumpering all the safeties to eliminate those, it failed the pressure test.

it could also be a plugged HX.

scooter

No, I have not pressure tested the HX, and I Really dont have a clue how! I have heard that this is a standard procedure but until now have had no reason to try to perform it.
Today I grabbed a complete gas valve, accumulator, flapper assy and subed it out with no change in the performance.
I guess I would not even have a clue how to determine that the secondary is restricted.
I did take a vacuum that can be made to blow, and hooked it up so that it would blow into the air intake with the diaphragm chamber removed. I measured the pressure increase in the HX at the gas oriface and the pressure only went up slightly, 5-6" water column, and the air was coming out of the exhaust very freely. it did not seem to put a very high load on the vacuum either. Pretty rube goldberg but I am pretty confident that the HX is not restricted. I extracted some water out of the exhaust trap and there was some granular soot in the water but I have nothing to compare it to so I have no idea how much is an idicator of a major problem.
If the heat exchanger is compromised, is it possible to see the crack, hole, comprimised area, visually from the top of the furnace with the evap coil removed, and what are you looking for???
Thanks guys
Scott