https://youtu.be/1UhOnEHIB6s

My 940 manual lpt runs rough (link to video of car runing) and vibrates a lot at idle and even more at 1500rpm. I've changed plugs, cleaned PCV, changed all filters, ran injector cleaner, etc.
The car does have a crack both on front muffler and cat, but why would it only vibrate at idle and 1500?
I'm suspecting a dirty MAF or distributor issues but i really dont know. What do you guys think?

Seems worse when it rains and today the engine cut out in the morning while idling cold.

Hi,

Failed engine mounts (or engine pads in Volvo speak) could be one of the culprits. Some vibrations which still persists despite replacing parts could be due to engine mounts. I've watched your video - the RPM needle still looks smooth to me.

Regarding the rough idle when wet/cold weather try spraying WD40 inside the dizzy cap to repel moisture.

Amarin

Yes, if it’s worse when raining or very damp outside then I would first focus on the condition of the ignition wires, coil wire and distributor parts.
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Will I buy another Volvo??? We'll see....

Doh! In my other post the word is harmonics not harmonica.These are vibrations due to speed and imbalances in all engines.

How old are your ignition wires, coil wire and distributor parts?

Examine all these parts carefully - once I found a rip in the insulation of the coil wire in a 740 - the wire was ripped inside the clip that held the wire to the firewall - I think this prpbably happened when some work was done but the wire was not removed from the clip, thus it got torn in a hidden location.

When I buy a used car, I wash the ignition wires in soap and water - in one 740 there was a burnt spot in one of the plug wires the size of a pea - the wire broke into two pieces when I tried to remove it,

Do your wires have all their looms and clips designed to keep them apart - some of the clips are made of nylon and attached to the cam cover - these clips are often broken or missing entirely.

You might observe your engine running in the dark - a light spray of water may produce a light show.

As for vibrations at a certain RPM - this might be due ti misfires added to natural harmonica in the engine - even 1500 RPM is not very smooth for a 4 cylinder engine.

I also suspect that engine management with a manual transmission is different than that for an automatic - I read about this when someone wanted to use a computer from an automatic transmission car in a manual transmission car - the advice was not to do it!