Help me troubleshoot my Mac Pro Startup / Shutdown Issues

[Cottonsworth]

I purchased an used, Apple refurbed Early-2008 Mac Pro (8-Core 2.8ghz x 2, ATi 2600XT, 2GB ram basically completely stock) and Snow Leopard. It is currently out of warranty. At the time of purchase the previous owner told me that the computer wouldn't start and that a power supply replacement would remedy that. After getting the computer I took it to the Apple Store and viola, it turned on fine. It seemed to work fine for a few days but then had trouble starting.

Here are the symptoms:

-The computer powers on, the video card fan turn on and it would reach the Apple logo with the spinning thing below the logo and then shut off. OR
-The computer powers on, the video card fan turns on and then the machine shuts off before the monitor comes on.

-If I leave the computer completely unplugged for a couple of days, it does boot fine and reach the desktop. I'm able to maybe work for a few minutes (sometimes it does go for a few hours) before the machine shuts off without warning. Oddly, this seems to only happen if the computer is off for a few days, leaving it off for a few hours doesn't seem to do the trick.

I have already tried resetting the SMC and removing the memory and video card and putting it back in both the original and different slots. While the computer was able to stay on, I did run disk utility and the hard drive checks out ok.

I tried reading different threads on this issue and it seems like it is one or a combination of the following scenarios:

1. Bad logic board
2. Bad power supply
3. Bad video card
4. Bad hard drive

Based on my symptoms, can anyone offer me some knowledge and advice on what the issue may exactly be? If it is a bad power supply, would it even come on occasionally, etc etc. Thanks in advance for any help.

 

[englishman]

No-one seems to know so looks like a repair jobby.

 

[CaptainChunk]

To narrow things down, I'd say you're looking at either a bad PSU or bad logic board. But it's most likely the PSU. Thankfully, this part is a lot cheaper than the logic board. They usually run for around $200 or so at most places. Here's one.

 

[nambuccaheadsau]

Another vote for a dodgy PSU. It will get worse and fail eventually.

 

[zilberas]

I have a same problem with my Mac Pro early 2008. Apple support already changed Power Supply 2 times for my mac while warranty was valid. Now, when I have no warranty, I just look how the third PSU is dying in front of my eyes. Symptoms are exactly the same as you described. I really don't know what to do next, because warranty for new PSU is valid only for 3 months, as apple said, it costs money, and approximate working time on those was 6-8 months, as I know from my experience.
My solution at the moment is to heat PS with hair dryer from back of my mac. I noticed, that after this procedure is done, I can keep computer working for all day. In the morning it will boot up for couple of minutes, I'll try to heat it with hair dryer and so on.
I wonder, if anyone knows what kind of problem is with PSU. Maybe it can be fixed by replacing electrical parts in Power Unit itself? because I really can't explain that heating phenome.

 

[CaptainChunk]

I have a same problem with my Mac Pro early 2008. Apple support already changed Power Supply 2 times for my mac while warranty was valid. Now, when I have no warranty, I just look how the third PSU is dying in front of my eyes. Symptoms are exactly the same as you described. I really don't know what to do next, because warranty for new PSU is valid only for 3 months, as apple said, it costs money, and approximate working time on those was 6-8 months, as I know from my experience.
My solution at the moment is to heat PS with hair dryer from back of my mac. I noticed, that after this procedure is done, I can keep computer working for all day. In the morning it will boot up for couple of minutes, I'll try to heat it with hair dryer and so on.
I wonder, if anyone knows what kind of problem is with PSU. Maybe it can be fixed by replacing electrical parts in Power Unit itself? because I really can't explain that heating phenome.

It's pretty abnormal for Mac Pro PSUs to prematurely fail like that at the frequency you're describing.

I'm not totally following what you're describing - the PSU is doing the opposite of overheating and you have to heat it with a hair dryer for it to function normally? That's really bizarre. I'd say if PSUs are failing that much for you, there's probably an SMC issue (try resetting the SMC) or a bad temp sensor on the logic board. While you're at it, you can try resetting the PRAM, too.

If the above procedures don't make a difference, it's probably a faulty logic board.

 

[zilberas]

It's pretty abnormal for Mac Pro PSUs to prematurely fail like that at the frequency you're describing.

I'm not totally following what you're describing - the PSU is doing the opposite of overheating and you have to heat it with a hair dryer for it to function normally? That's really bizarre. I'd say if PSUs are failing that much for you, there's probably an SMC issue (try resetting the SMC) or a bad temp sensor on the logic board. While you're at it, you can try resetting the PRAM, too.

If the above procedures don't make a difference, it's probably a faulty logic board.

I know that this is bizarre, that PSU's are failing at that frequency. Apple guys agree with that. I've already tried all those SMC, PRAM, etc. reseting tricks, hardware tests for several times... Neither worked. If it is faulty Logic board, why do Apple didn't changed it, but changed PSU 2 times instead? Now, leaving me with no warranty it would be insane to invest almost a 1000$ to a thing that maybe will not solve the problem at all.
I thought about heating workaround after I noticed, that computer works as expected only after a few boot ups. I can get computer working if I "play" with power on button for a some amount of time. With a hair dryer it will boot up much quicker and stays on. My conclusion is, that heat produced by processors and other hot components helps to keep PSU alive. It mostly causes pins, relay contacts or other electrical components to change it's size a bit, and hold current flowing.. Of course, I'm just guessing.
The main question is what exactly fails in those Power Supplies? I hear people on forums that replacing PSU always solves the problem. But why I have to buy a whole new unit. Is it impossible to repair existing one?