M50 knocking when warmed up

[RenaissanceMan]

Hello-

I have a '95 525it and the motor has been exhibiting knocking once the car is up to temperature. When I first start to drive it when cold, the symptoms are not there; only after approximately 10 minutes or so of driving. Interestingly enough, the knocking only seems to occur under moderate acceleration, and not under hard acceleration. It is most pronounced when the automatic transmission is in mid-shift into the next gear for some reason. I usually run 89 octane fuel. Running 93 octane fuel helps to diminish the level of knocking somewhat, but the problem is still there, especially noticeable in warm weather.

I am not getting any fault codes for the knock sensors. So, I was wondering if it is possible for the knock sensors to go bad without triggering faults to the computer? Before I jump in and change them, I wanted to get some opinions from those of you in the know. Could I have another problem?

Thanks in advance-

[LunatiC]

Could be hydraulic lifters sticking or collapsing. My M50 525i sounded like an old VW beetle under acceleration before I went and replaced my lifters. Although lifters would most probably still be knocking when the engine's cold....

I dunno, I'm not a mechanic, others in here would probably be able to give you a better reply than I...

[t_marat]

Have a 100% same problem. But in my case I would call it pinging, did you mean that?
It also happens when the engine is warm and under mild acceleration. If I floor the pedal, no pinging.
Last year I posted the problem on this forum and advice was to do the "Italian tuning". The pinging seems to occur because of carbon build up in the combustion chamber. Drive in 3rd gear at 5000 rpm for 5-10 minutes. Engine simply burns out the carbon buildup. And it did help, pinging dissapeared, for a while. Now its back again.

[RallyD]

gotta do a top end cleaning.
i've used Sea Foam (most auto stores) and Mopar Combustion Chamber Cleaner spray (dodge/chrysler/jeep dealer)
Basically get the engine hot, suck or spray the stuff into the intake manifold through a vacuum line (brake booster), turn the engine off and let it sit for 15-30 minutes. Fire it up and beat the snot out of it. It will smoke like out the tailpipe but burns the carbon out real good. On my car I did 2 treatments and picked up some more power, lost the ping, and got 5mpg. Must have been really bad.

The reason you aren't getting the ping at WOT is because the ignition pulls timing when the pedal is at ~80%+ throttle It happens most under load at mid throttle. Also check your spark plugs for any defects. A sharp metal edge can be a hot point and pre-ignite the fuel vapors trapped in the carbon deposits even before they are compression ignited. Temperature will definitely affect this condition so the hotter it gets, the worse it will be.
Fuel is expensive but I found better mileage with premium. It offset the highe cost of the gas. THese are relatively higher compression engines and need the higher octane fuel to run properly. Low octane fuels burn more quickly and will trigger the knock sensor which in turn retards timing, and kills power, resulting in the driver requesting more power and using more fuel. The knock sensor can only pull so much timing after that its up to the driver to lift.

try the top end cleaning and add some octane boost. should notice a big difference.