BMW M50 Engine Knock
Discussion
Over the last 3,000 miles or so (now on 110k), my BMW 525i's M50 engine has developed an unusual knock;
-It does it only when 'warm', so after approx 10 minutes running.
-It will do it if the hot engine is re-started within a few hours
-The knock sounds like it's coming from the cylinder 4/5/6 area
-It's audible from inside the car
-It is most pronounced at low revs, seems to go away after 2,500RPM
-It sounds bottom endy to me, it's not a tappet or a timing chain
-It is an irregular, random knock- If I listen for it at idle, it'll be quiet for a few seconds, knock a bit, go quiet again... and there is no perceptible fluctiation in engine speed.
-I thought I might just be hearing the gearbox first motion shaft, however it knocks the same with the clutch in.
-The engine has VANOS but the knock is coming from a different area entirely
-Recent work includes a new thermostat and housing after an overheating episode.
The engine has no other symptoms, doesn't appear to be down on power, isn't breathing excessively or getting excessively hot. The oil level is correct, and it looks a nice colour- although I haven't changed it since I got the car so I have no idea what grade is in there. These engines are more succeptible than most to overheating and blowing gaskets or cracking heads, so my next course of action will probably be a compression test.
I know it's hard to diagnose stuff like this which is why I have given you as much info as I can in a concise list.
-It does it only when 'warm', so after approx 10 minutes running.
-It will do it if the hot engine is re-started within a few hours
-The knock sounds like it's coming from the cylinder 4/5/6 area
-It's audible from inside the car
-It is most pronounced at low revs, seems to go away after 2,500RPM
-It sounds bottom endy to me, it's not a tappet or a timing chain
-It is an irregular, random knock- If I listen for it at idle, it'll be quiet for a few seconds, knock a bit, go quiet again... and there is no perceptible fluctiation in engine speed.
-I thought I might just be hearing the gearbox first motion shaft, however it knocks the same with the clutch in.
-The engine has VANOS but the knock is coming from a different area entirely
-Recent work includes a new thermostat and housing after an overheating episode.
The engine has no other symptoms, doesn't appear to be down on power, isn't breathing excessively or getting excessively hot. The oil level is correct, and it looks a nice colour- although I haven't changed it since I got the car so I have no idea what grade is in there. These engines are more succeptible than most to overheating and blowing gaskets or cracking heads, so my next course of action will probably be a compression test.
I know it's hard to diagnose stuff like this which is why I have given you as much info as I can in a concise list.
Tango13- Yes, I checked the spark plugs for tightness at the weekend. The random nature of the knock suggests it's not a loose plug anyway, because presumably with a loose plug you'd get a knock on every compression stroke?
The closest sound I can compare the knock to - bear with me on this - is an egg being hard boiled at a rolling boil in a small saucepan. The egg is bobbing about in the water and occasionally knocking against the bottom/sides of the pan.
The closest sound I can compare the knock to - bear with me on this - is an egg being hard boiled at a rolling boil in a small saucepan. The egg is bobbing about in the water and occasionally knocking against the bottom/sides of the pan.
I haven't tried that yet, not sure I have a long enough screwdriver! I will get the car over a pit next time I'm at the garage and have a good listen. The car has a stamp in the service book from 5,000 miles ago but I'm tempted to bring the next service forward; it has been sitting around for a while and I think it could run sweeter.
Driving to work this morning I turned the stereo and fan off and closed the windows, the sound seems quieter and less regular than it has been. It's very strange. If it wasn't coming from the back half of the engine, I'd expect it to be the timing chain slapping about...
Driving to work this morning I turned the stereo and fan off and closed the windows, the sound seems quieter and less regular than it has been. It's very strange. If it wasn't coming from the back half of the engine, I'd expect it to be the timing chain slapping about...
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