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How did you test the AMM? There is no valid test anyone other than a AMM rebuilder can perform to make sure a 240 AMM is working properly.
For the AMM I tested it by unplugging it and seeing if there was a change in the idle.
Both the rotor and the cap are Bosch that i bought from FCPEuro.
Was there a change?
Yes, the engine was really struggling after I unplugged it.
That doesn't mean the MAF is 100% good, just 80% sure it is good. Is your check engine light on?
The check engine light is not on, and there are no codes.
That literally means nothing. I have about 10 AMMs that are defective that will pass that absolutely useless test. Be careful what you read on the internet. That so called test has been in circulation for many years and it is complete BS.
And down the rabbit hole you go. A bad AMM will almost never trigger a code that means anything. If you break the hot wire it will fail the hot wire test. If you unplug it the code for a missing signal from the AMM will be triggered. Short of drowning the engine in fuel and triggering a rich mixture code there isn't anything helpful in Volvo's OBD1 code that will help you decide whether your AMM is working correctly.
What is the best way to test the AMM, or should I go and buy a new one?
The only valid test you can do is put the AMM on another 240 that is running properly and see if the symptoms follow it, and/or install a known good AMM from another 240 and see if suddenly all the problems with your 240 suddenly disappear. I always keep a known good AMM on hand as a test tool. There are so many different problems a failing AMM can cause it is a must for me to have a good one on hand. I would not buy a new AMM unless it was the only option. Many of the new AMM offerings are junk right out of the box. A surging idle is one of the symptoms of a bad AMM. I'm not saying your AMM is bad, just that what you describe is one of the many problems a bad AMM can cause. I don't want to see you chasing your tail.
Ask for a known good one here, there may be someone in your area with one that can help. Do not just buy a new one, as those are remanufactured, and are not actually known good. Lots of people have gotten new ones bad out of the box
Especially if you buy one of the new design Chinese manufactured AMMs. IPD sells them. Someone on here recently bought one from IPD that was junk right out of the box. What I was told is when they wanted to return it as a defective part IPD refused to accept the return claiming that since it was an electrical part that had been plugged in it could not be returned. AWESOME. How would a customer know the had bought a piece of defective junk by just looking at it? What a F'ing cop out. They bought a good used AMM from someone I know and are still motoring on. Two other customers of mine also bought the "new" design AMM and they were bad out of the box.
Junk yard AMMs are a crap shoot. Try to borrow one to start with. It sounds like you can still drive your car. If you determine you need a replacement, put up a wanted ad on here and someone should be able to sell you one that they have confirmed works properly.
junkyard is good if they will let you return a bad one. MAF you want is 016.
I cleaned it out about a week ago when I was trying other things.