Bell-like noise at cold idle; timing chain guide broken?
 

I recently noticed that my 1979 240D has developed an erratic noise at cold idle when first started up. Every few seconds there will be a ringing noise in among the idle clatter, like someone striking a large resonant bell with a small hammer, kind of like one of those Japanese bells. It seems to go away once the engine has warmed up; if fact, I had taken my car in for a coolant pressure test at MBI here in Portland, and they alerted me to the condition, but with the car warmed up at the time I couldn't even detect that something was wrong. The next time I started it up I heard it clearly, however, and to be cautious I have stopped driving it. MBI was of the opinion that one of the chain tensioner guides has broken, though when I ran this by the shop that rebuilt the engine in 2011 (Bearing Service) Brian seemed doubtful.


I figure that if I pull the lower oil pan I'll either see plastic pieces or I won't, and that may settle the diagnosis if I do.


Other random facts:
The 2011 (2012?) rebuild was very complete: new pistons, rings, oil pump, full head work, valves ground, crank polished and balanced, connecting rods offset bored to size - basically I told them not to cut any corners, not wanting to have to go back in for a long time. At the time of the rebuild I put in rebuilt injectors and had the injection pump professionally rebuilt. Well, it's been a fairly long time, but not very many miles (30k!), and here we are. I've given it a yearly tune-up every fall with valve adjustment, change of oil (synthetic), change of fuel filters, change of air filter. Were it not for this worrying sound I would say the engine is in tip-top condition, with plenty of power (well, plenty for a 240D :D). Thoughts?

No chance to pull the oil pan yet, but I've had an ominous thought: I wonder if that bell noise is a valve hitting a piston because without the guides properly keeping tension on the chain the cam shaft advances or retards enough that a valve is still open enough to interfere when one of the pistons hits top dead center... that would be very bad. :eek:

The first thing do to do with a noise like this is to remove the belts. Start the engine and run it awhile. It will take a several minutes to overheat, so you have some time. If the noise goes away, then it's related to the belt system. Otherwise, it's internal. Once you know that, your next steps will be clearer.

I'll second this. My grey car had a "bell sound" at idle also.
The alternator had split in half on the bottom. Was still charging fine, just randomly made a little "ting" at idle.

I swapped in an AL129X from a 93 Saab 9000 aero with a 4 cyl 2.3L engine as that is a 115A alternator vs the little 55A one that comes factory on our cars.

Headights no longer dim at idle, voltage is more stable across the board.

check your crank pulley bolts (they can loosen up/shear sometimes, happened on my 240D with a similar noise) and check the torque on your main crank bolt. Balancer failures are common on om61x engines

i've had one of those long case screw on a Bosch alternator back out far enough to be 'ting'd' by the stamped fan on the pulley of the alt.

Super helpful suggestions, thanks guys!

Get 3 foot of wooden broomstick (ear on one end other end against what you want to listen to) or a mechanics stethoscope and try to locate where the noise is coming from. To include the Vacuum Pump.

Watch out for moving parts.

Sound Diagnosis
 

Subscribed to find out what it is .

I too have a 240D and it's very interesting how easily it goes the slow speeds it goes ~ I rarely have to use even 1/2 throttle, I just have to plan my entrance to freeways very carefully .

It has zero problems keeping up with in town traffic, prolly why it was so popular in taxi service .

Take a look at the injectors before going deep. Get them tested at least.