There is some intermittent noise the car is making, possibly from the DM flywheel?! I think it has the original DMF because it revs up slowly. It’s a 1991 with 147k miles.
It seems to run fine at higher rpm. When idling has some slight misfiring so the there is negative acceleration of rotation of the crankshaft, the noise is in sync with this intermittent deceleration.
Any idea?
You can hear it in the video attached
Thanks

 

That is likely ZF6 gear rattle as you correctly figured. Whether your dual mass flywheel is worn out or not I don't know. It would take a physical look at the DMF and measurement to see if it's within tolerance for play in the friction surface; i.e. the clutch would have to come off and you'd take measurements.

These cars are really sensitive to uneven idle due to things like an injector not firing properly or a bad ignition wire. If you have an unsteady idle due to things like those, you're more likely to have gear rattle that the DMF can't damp enough to prevent hearing the gear noise. When my car's injectors started acting up (Multecs) I changed them out for new Bosch IIIs and put in new plugs, wires, etc., then re-tuned the idle settings, throttle body minimum air, etc. Unsteady idle went away, no more gear rattle noise. Car ran much better through the gears also.

147K is a lot for a DMF according to some people here on the forum. But first I'd work on fixing your lumpy idle if you have an issue like those mentioned above. That would cause gear rattle even with a solid flywheel and conventional clutch most likely.

 

Thank you ajp01
I’ll see what I’ll check the things you mentioned.

 

I think the engine actually runs well, you can hear the exhaust bangs of every piston very evenly and it seems that the slight trembling of the engine is actually caused by the rocking of the flywheel. I’m planning to replace it. Around 147k miles being the usual failure point I think it needs at least to be checked as soon as possible. There seem to be no more DMF’s available on the market at this time so I’ll have to go for a single mass. It will probably be tricky to balance it. I was wondering if I measured the way the original DMFW is balanced whether I could simply match the weights or gaps on the new wheel. for example if the same material I can measure the volume of the holes drilled, the distance from the axis and the angle in relation to the crankshaft would it work? The problem is the FW might be initially balanced out of the crankshaft for its own imperfections and when you transfer the information to the new FW that is self balanced it would result in a slightly different behavior idk

 

ajp01
you were right about the engine. I replaced the flywheel and now there is rattling when the clutch is engaged because of the noisy gearbox but the engine misfires and makes it worse, I suppose it’s because of the backlash and rotational acceleration (+-)
Anyway, I couldn’t evaluate the condition of the DMFW while it was on the car and I thought it was the original one and the car has 140k miles and that would’ve been a lot for a DM flywheel. However, I’m not sure if it was replaced before, but looks in good condition, there the steel face has very small signs of wear, so it seems in a very good condition. The new flywheel I installed I like more because the engine can respond faster. The clutch springs are stronger than on the stock flywheel which I think is better because in case of a shock softer springs reach the end of the travel and after that they can’t do their job anymore.
Currently I’m trying to diagnose the misfiring. I’ll do different kinds of testing. I prepared pulse sensors (exhaust, blow-by), ignition spark current sensor, vibration sensors.
I’ve managed to check the ignition timing and it’s 4deg btdc not 6. For some reason someone changed it or the harmonic balancer is degraded and doesn’t show the correct timing.
Anyway, I’ve noticed another thing that is very likely the cause of intermittent misfiring that gets worse with temperature. While the engine is running with the hood open I can hear electric discharges on the driver side of the engine, moreover they seem to happen exactly when the engine is misfiring.

 

Yes, having experienced it myself in the same way I was suspecting that uneven idle was at least part of the problem. I replaced the injectors, distributor components, ignition wires, plugs, and reset the throttle body and sensors after cleaning everything up in the area of the Idle Air Control valve and the Throttle Position Sensor. It was successful as the engine idles smoothly and revs smoothly also. I did not have to replace my dual mass flywheel, it has 127k miles on it now and I'll keep it as long as possible. I have a backup used flywheel on standby.

Good luck with the fixes remaining.