Hi
I know this has been covered, I did a search. I also read all the 700 informational sticky. My wifes 88 wagon has a fluctuating idle problem. It moves up and down about 2-3 hundred rpm. These are the things I have done.

Cleaned throttle body.

Ran a can of BK thru the car.

Major tuneup last year before smog. Plugs, cap, rotor, fuel filter, air cleaner.

Replaced fuel pressure regulator with another used one.

Checked vacuum hoses. Replaced most of them.

Checked injector seals, intake gasket,throttle body gasket, air intake hose connections , breather hose (fire trap) by spraying throttle body cleaner around them. No change in running. Doesn't appear to be any vacuum leaks.

Checked and replaced aux idle air valve with good unit from turbo parts car. I cleaned it before installing it.

Replaced fuel pump with another unit

The idle still fluctuates. After I replaced the main fuel pump today, I started the car. For about the first 10 minutes, I thought I had fixed the problem. The engine idled perfectly. Then the fluctuation started. It didn't seem as bad as before, but it was still there. The fuel pump came out of a turbo car and was only about a year old when the car was totaled.

Has anyone have any other ideas on the cause? The car has 280,000 miles.

Thanks Brad