Rough idle and LOUD knock while cold! Please help

[PshlosE30]

It all started suddenly some days ago when i start the engine after sitting overnight.
It idles and runs rough for about 7-8 minutes after startup. Accoring to narrowband O2 gauge it runs LEAN, vacuum is -0.2 bar (normaly it was -0.5 to -0,6 bar) and every 5-6 secs i can hear a LOUD knock (like hitting an iron part with a heavy wrench using much force,actually you can hear it 20 meters away) coming from the head or somewhere near the intake manifold. By the time i hear the knock, vacuum goes 0,then it jumps instantly to -0.2bar. At range of 1000-3000 rpm its still lean and stalls,sometimes it shuts off but if u fully press the throttle,mixture is rich and car pulls fine without knocking or hesitation.
Could it be detonation or knock from lean mixture?I checked the hoses and manifold for leaks and find nothing.I tried another ICV and TPS,also check the coolant temp sensor with multimeter and it has 800ohms after shutdown the engine and 1500ohms 5 hours later... Could it be blown HG? havent see water or oil on plugs and no mayo in oil. Also i tried to unplug one by one the coils while idling rough,nothing change,loud knock its still there

Engine is a M50B20 NV supercharged with Eaton M65 running 0.4bar at 6900. Map is stock,havent mess with ignition or fuel yet.

[PshlosE30]

UPDATE
Today i removed the head.No signs of piston touch with valves,also HG was clean,not blown or rust or something.I washed all of the lifters,removed the old oil and top up them with new fresh oil.Also camshafts looks in very good shape,no scratches or rust.Tomorrow i ll remove the valves to inspect them too,hope to find something there,otherwise i ll look at crankcase for deffects.

[jbontke]

I wold rule out the internal engine. I think knocks are rhythmic and occur much faster than every 5-6 seconds. Maybe the flywheel? Bad harmonic balance? Maybe water pump? Or something on the accessory drive?

[PshlosE30]

I really thought that too,but my big concern is the low vacuum,and everytime it knocks, vacuum goes to atmosperic pressure,thats why i suspected the cams (especially the intake cam)... If vacuum where at optimal level,i would suspect the flywheel..As for accessory drive, i have remove the air con and power steering and installed at custom pulley from Mercedes to spin the charger..Actually you made me to think,what if the bolts from the accessory pulley (supercharger pulley in my case) are too long? Dont remember how's down there,i ll check at realoem..One day after installing that pulley everything happen..

[AlpineS50E30]

i had the exact same issue and it had me scratching my head for a bit, it's actually a simple issue. You are correct in asking if it's lean mixture/ detonation. In my scenario i went through the intake pieces one by one, and it ended up being a small vacuum leak that fixed my issue. Listen closesly for the sound on the drivers side of the engine, if it is then check all intake hoses and gaskets and then check your maf. Good luck!

[PshlosE30]

i had the exact same issue and it had me scratching my head for a bit, it's actually a simple issue. You are correct in asking if it's lean mixture/ detonation. In my scenario i went through the intake pieces one by one, and it ended up being a small vacuum leak that fixed my issue. Listen closesly for the sound on the drivers side of the engine, if it is then check all intake hoses and gaskets and then check your maf. Good luck!

Really thank you mate for your answer! Wish to told me that a week ago :P Right now head is torn apart, installed new valve seals, cleaned and polished valves and seats, tomorrow ill send it to machinery shop for planning and vacuum test... Still wait for the HG to arrive (online stores are affected by corona virus too :( ) When i assemble everything up,ill update.... Thanks again mate! Stay safe!

[AlpineS50E30]

no problem man, i'm sorry i haven't been as active as i could be on this forum but i saw the post and i was like let me try to help this guy fix this issue before he goes too far into the motor!, lol...sucks that you went as far as you did already but hopefully the car runs like new for you, any other questions feel free to pm me if the issue persists, i know it can be frustrating sometimes! Good luck bud and you and your family stay safe as well!

[PshlosE30]

UPDATE
Yesterday i bolted everything up (except the supercharger) and started it,but knock was still there. So i triple ckecked everything for leaks and found again nothing..Then i remembered what AlpineS50E30 said, so i unplugged MAF and no knocks! with the MAF unplugged it idles way better,and is more driveable.Maybe little hesitation past 5000rpms until 6900 but i guess its because it runs the stock map for fueling and ignition due to MAF lack (SS headers, 2,5in single pipe custom decat exhaust) One thought is that MAF send wrong signal at DME and it to adjust idle,open allways the ICV (low vacuum) and leaned the mixture so it was knocking... Is it possible?

[PshlosE30]

Finally found the solution some days ago... IAT sensor was stuck reading 60 celsius all the time. So,DME leaned the mixture and fully opened the ICV (low vacuum). I suspect that supercharger cooked it,so i installed an intercooler. Now runs like new and cool!

[AlpineS50E30]

awesome!!! i'm so glad you figured out the issue, so it technically was a lean condition knock if the iats were off and opening up the icv! congrats on finding and fixing the issue man!