Kenmore Dishwasher, intermittent weak wash and rinse

I have a Kenmore Ultrawash III dishwasher. Approx. 15 yrs old. SN: FD3334919 Model No. 1564990 (I think it's 99. Those numbers are a little worn on the plate.) Part No. FD3334919 (From the Sears installation manual. May not be useful info.)
Sometimes the dishes don't get very clean and there is clearly food residue left on the dishes. Some "stubborn" food as though the wash cycle wasn't very effective, and some loose food residue as though the rinse cycle wasn't very effective. Other times the dishes appear clean except for a white residue left on the dishes, as though just the rinse cycle wasn't very effective. Sometimes the dishes come put fine, but the described problems seem to be getting worse over time.
I've noticed that sometimes, starting at the beginning of the quick rinse cycle, I can hear the unit humming but don't hear any water swishing around. When I open the door and look in immediately, the water isn't moving. When I close the door again the water starts swishing. Perhaps the same thing is happening during wash cycle and I just haven't noticed it.
Any idea what's going on, and what the fix might be? Pump getting vapor locked? Solenoid not switching properly?
Advice on fix versus replace? Again, the unit is about 15 yrs old. We will be selling the house in another couple years.
Thanks for any advice,
Paul
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On Sun, 13 Apr 2008 21:52:08 -0500, Paul Stivers

Before starting a wash, do you make sure that no dishes are blocking the rotating sprayer arm?
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AZ Nomad wrote:

Thanks. We do try to check for that but sometimes could do better about not cramming stuff in.
The thing is that sometimes the water doesn't circulate at all during at least the first rinse cycle (just that humming noise I mentioned,) so I don't think blocking the rotating sprayer is the main problem.
Paul
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The problem is 15 years old. Pumps tend to wear and not give the same pressure, spray arms clog. Rather than be miserable for the next couple of years, get a new one for $500 and keep yourself happy and the new buyers happy.
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Edwin Pawlowski wrote:

Yes, that would be the altruistic thing to do. But what if it's just a solenoid valve ;).
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clipped

Our dw leaked, so I called the serviceman. He checked it out, only problem he felt was soap residue causing excessive suds which kept the water from draining. The solution was to dump about a quart of vinegar into the dw and run it through a cycle empty of dishes or detergent. It worked.
I sometimes get cloudy residue on glasses, which is from too much detergent. Wash them in full- strength vinegar to remove the film. I really don't like dishwashers - rather do them by hand.
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Norminn wrote:

Thanks. I read about the soap residue (from a google search) and the vinegar application (from the service manual.) We used 2 cups of vinegar, though. I'll try more. I also cleaned the soap residue from around the door opening and checked that the water level float was working.
Paul
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