Rheem "Imperial 90 Plus" forced air furnace - short cycling?

Hi all,
still over at my friend's house... cut on heat last night for the first time this year. He turned the heat on and went to bed; I realized after a while that the inducer blower was running but the air handler was not. Investigated; found that the flame was not lit, inducer was sitting there grinding away. Popped cover, found a cracked hose between the inducer fan housing and a pressure transducer. Cut it shorter, reconnected, OK furnace works. But I did notice that it seems to only run for about 5 minutes at a time, then will kill the flame, AH fan will run for a while then shut down. Almost immediately the inducer fan will start again, flame will relight, lather rinse repeat. My immediate thought was that it was restricted airflow due to a gunked up A-coil for the AC.
This evening I got over there and ripped it apart, sure enough the A-coil looks gunky, however I could not clean it properly because I am still fighting one rusted screw for access to it. The access panel to the underside of the A-coil is in two parts; I removed one and replaced the access panel on the evaporator so that some air could bypass it if that were the problem. Unfortunately, it seems to be exhibiting the same behavior. I didn't time how long it took it to cycle last night (actually, early this AM - I got the thing working in the first place about 1 AM) so I can't say if the time between cycles is longer now with better airflow.
Now here's the weird thing. There is absolutely no nameplate, data plate, whatever with any kind of part number that I can find on this furnace to give me any sort of clue as to where to find a manual online for it so I can delve deeper. Can anyone give me any hints as to where to look?
So, I guess the question is, without knowing exactly what model furnace this is, can anyone tell me if this is this normal behavior for this type furnace, e.g. am I worried over nothing? I just don't want him to have to pop for a new heat exchanger due to constant overheating.
I sadly do not have an IR thermometer - I own one, but I'll be damned if I can tell you where it is right now.
Finally, do any of those A-coil cleaner sprays actually work well? The A-coil is quite filthy, probably due to some ductwork pulling loose and allowing air to bypass the filter. I'm a little concerned as this A-coil is well past its "best before" date and when looking on the Big Box's web site to see if my local store carried anything for this purpose, the reviews seemed to indicate that a few people had problems with their fins disintegrating after application of said cleaning spray. Is this a real concern? (when I said it was past its "best before" date, I meant it.)
thanks much,
nate
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wrote:

I can't answer your question, but is it possible that it is somehow being starved for combustion air? Recent weatherization?
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On 10/04/2011 08:16 PM, Malcom "Mal" Reynolds wrote:

Don't think so; air intake appears to actually be inside house and only exhaust goes outside (I can only find one pipe anywhere,) I have not however checked exhaust for airflow as it has been raining for what feels like about a year straight now...
nate
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How is the internal air handler thermostat set, maybe the high limit switch is set toward the low side. What the unit is doing is actually the most efficient way to run the furnace.
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Nate Nagel wrote:

No trouble code?
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On 10/04/2011 10:35 PM, Tony Hwang wrote:

This thing isn't that new... Just on appearance this looks older than the one that was in my last house which was installed in the late 80's.
nate
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On Oct 5, 7:45am, "Stormin Mormon"

First thing I would do is make sure the thermostat is constantly calling for heat. Could do that by temporarily bypassing the thermostat and connecting the wires directly.
As Stormin said, I've found on the furnaces I've dealt with the tag with the model and other data has been located inside where the gas valve is, on the upper inside surface of the right side of the unit.
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wrote:

good ideas, I will try both tonight.
nate
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