Got a copy of the Bentley manual? Follow the directions for mode 3 diagnostics on 241-8. In fact, do all the tests for mode 1, 2 and 3. Do the Ignition tests too. You might have one or more stored trouble codes that could put you on to the problem.
Or see the General Notes on On-Board Diagnostics in Volvo RWD Cars on the 700/900 FAQ and then do the tests as described a little further down the page.
For mode 3 FI tests the FAQ lists several tests that aren't performed on the 240's. On my car it tested only the injectors and the air control valve.
I think the instructions in Bentley are a little clearer than the FAQ.
Bear in mind when you are doing mode 1 checks, with the exception of 1-1-1 (no fault codes stored), if a code is returned it indicates a problem. In mode 2 and 3 when the code is returned it means the component being checked is okay.
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'80 DL 2 door, '89 DL Wagon