DM4 took a bath, now won't boot

My son took his HP DM4t-1000 to college and proceeded to spill a cup of water along its left side. Apparently, it ran a little for a few minutes, then died. He didn't shut it off immediately. He took it to camput tech support and they pronounced it DOA and put the hard drive in a case--it works fine as an extra drive.
I got my hands on the thing and decided to see what happened inside, and tore it apart down to reseating the processor with instructions off the internet. Nothing looked in anyway abnormal. I reassembled it, and tried to turn it on. The power, caps lock and wireless lights come on, the screen starts to lighten, and then it shuts down. Hardly a surprise. But, I found that if I hit esc, or F2 or 9, or 10, or 11, it enters its diagnostic modes with no problem, and can run all the tests, and I can change boot order and all that. The memory checked out ok on one test, though the computer shut down on another test. I reinstalled the HDD, and it checks out on the short test, but the computer shut down on the long test. I don't think the processor is overheating, as it does not feel excessively hot, and the fan runs and the vents are clean. Also, I don't think temp would make is shut down after a few seconds.
Any ideas?
 
Could you clarify...

I am not sure how to do that--I did remove the small battery and leave it out, and drain the reserve power by holding down the start button. At one point, a screen came up saying the checksum was wrong and asked if it wanted something reset, which I said yes to. But nothing changed. Pardon my ignorance about most of this--I did not come willingly to the laptop repair world!
 
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