Really high idle after tightening spark plugs

Question

I had a very nasty tick in my engine after doing a bunch of work to it, I just redid my chain tensioner which went a long way to fixing it, then I found that spark plugs could cause a tick, so I tightened them. It fixed the problem.

However, the reason for this post is that I'd like to understand why the car behaved how it did (both the ticking) with loose plugs, and also it idled really high after tightening the plugs.

Here is a YouTube video showing the ticking

When we tightened the plugs down I started the engine and it really struggled, then revs shot right up I had my hand on the keys (didn't see tacho) but figured it was very close to red line (and still going up) so I cut it. When I restarted it sitting in the car the revs sat at about 2k for 3-4 mins, putting it in gear dropped it to normal (ca. 900 rpm) putting it back in park put it back to 2k. After 3-4 minutes it dropped the idle down.

Now I've restarted it three or four times since and it's behaved normally. I figure it might just have been the ECU learning new tolerances but I'd like clarification.

The vehicle is a BMW 740 with an M62 motor.

Answer 1

Loose plugs will probably need a higher voltage to run properly, the ignition system has no problem supplying spark, but, the reverse EMF from ignition coil pack is fed to ECU, the voltage is probably outside normal parameters so 'brain' thinks it's running lean. Also, just removing and re-seating plug caps can fix issues caused by arcing over ('ticking' you can sometimes hear) Also, not sure if you have crank or cam trigger (probably both but could use IAP for cam position?) Re-setting timing chain tensioner will take out slack which was causing retarded cam timing and probably faulty sensor input to ECU (10 degrees or more retarded isn't too unusual with a loose chain as cam runs 'half speed') As you surmised, ECU has 'learned' the new settings and is probably using a standard 'map'

Answer 2

Some days without answers, so here is what i am thinking, please notê i am not bmw specialist.

This is a bad noise: It has happen after changing the tensioner as you said, if this kind of noise come slowly this is important but could sounds like "normal" old engine, but if it appear after this kind of works around the timing chain, don't look after sparks plugs thee are not in fault, and with the video there is no asynchronism, the ignition is fine.

For me the noise is from the cameshaft or a moving part around: Not sure but i believe your car got VANOS which needs specials adjustement for get them working fine, and this probably a problematic parts for car with somes years.

If it happen to me like you describe, there is no way to drive like this, this need full check of the timing belt, calibration, with the special constructor recoimendation.

Take note all this kind of bmw motors had this noise after some time, the problem is it come brutally after human works, this is no good, something got to be done for sure.

Light bad timing can cause high idling, for sure.