Clacky di clack - fuel pump or?

Hi,
My car: 190 D 2.5 (diesel) 1987. Automatic 4 gears.
I get a "clacky di clack" at low rpm. usually when running in gear 4 at low speeds like 50-60 km/h (1500 rpm).
The sound seems to be half of the rpm. of the engine hence I guess it is the fuel pump.
What could this sound be?
It disappears and comes back again usually back when car is going down hill at low rpm. or up hill at low rpm. when pulling up the hill in 4, gear.
I have used diesel fuel additive for cleaning valves.
What additive if any could fix this?
Many regards Niels Ulrik
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I read that those additives you buy in a shop $3 for 150ml. is:
water kerosene alcohol
ie. I could use kerosen+alcohol or maybe just kerosene as an additive?
kerosene is $3 for a liter in Denmark.
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Why use any additive? The injection pump is lubricated with engine oil so no additive is needed for it and additives don't affect the combustion of the fuel. It's your money.
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Unlike gas engines, diesels don't have preignition problems - unless the injection pump's timing is too far advanced. So fuel additives have nothing to do with the noise.
You don't say how many miles or Kms this engine has gone. The noise seems to occur when the engine is lugging rather than at high engine speeds. The timing chain and the camshaft, run at half the speed of the crankshaft. I suggest you check the timing chain for wear and stretch. Chains will break after too many miles.
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435.000 kilometers has it run.
I changed the top gasket 2000 kilometers ago and had a look at the chain and it looked fine and is in tension. But if I am wrong then it might have gone a little loose.
If the timeing chain is one chain off will the engine run at all or could it produce this clack noise? It is not noticible for an unaware and seems to go away after running the car on the road for 30 minutes or less - or at least gets less.
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If the chain were off by one tooth the valve timing will be off about 16 to 18 degrees. Does that allow the pistons to hit the valves - I don't know.
A chain that's run 270,000 miles is very tired and suspect. The chain not only wears but also stretches. The rule for the old 3.0L 617.XXX five cylinder turbos is that the chain should be replaced if the crank shaft angle is 5 degrees (or more) when the camshaft is at 0 degrees.
The measurement is made on the 617.XXX engine by rotating the engine until the notch in the thrust collar that's immediately behind the chain sprocket of the cam is aligned with a groove cut into the first camshaft support's left side (0 degrees on the cam). One then reads the crank angle off the scale on the crankshaft's vibration damper (where the belt drive pulleys are located). Your engine may be a bit different but the principle will be the same. This check will also determine if the cam is correct or off by one tooth on its sprocket.
Finally, in closing, this engine is getting close to the mileage where it will need an overhaul so it may be worth considering, depending on the condition of the car, either doing a full overhaul, or doing nothing until it breaks and then making the decision to overhaul it, install a used engine or scrap the car.
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I do believe I got the chain back on right again - I used a little drop of red naile-polish to mark the chain and I assembled everything back again accordingly following all the Haynes book description.

Crankshaft and chain and everything looked fine once I had a look once the topgasket was off. Oil was leaking out into the coolant at front piston so that piston was a little dirty with sood as where the other pistons but not bad - what would be expected.

The condition of the car is really good. New laguear and all parts working flawlessly. Previous owner had it in a heated garage and was a car-fanatic in the sence he liked it being well keps but he was not a amatear mechanic and did not do any work on it himself. Had a mercedes shop look at it I can tell from the many receipts that folloed the car.
The car is really worth it.
Addtives: I was thinking of an additive that would clean the fuel pump and injectors?
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PS. About the sound: The sound is also more of a "thu thu thu thu thu..." half the engines rpm. The sound raises a little in volume and goes back down again and then disapears almost to nothing sounding like it is from the fuel pump somehow. The fuel pump is probably the original so it has had the many miles on it since new.
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I'd look at the vacuum pump before the fuel pump.
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Will do. Thanks. I did not think of it.
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I added Redex fuel addtive 50 ml. to 25 L of diesel fuel and now noise is gone :-) Me happy.
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