I have a few questions about some of the noises that my 84 300d turbo makes. It has 87,000 miles and runs very well and is in excellent condition.

1. I have a 240d and I am familiar with diesel noise, but my 300d makes a racket while at idle. It is louder than any others that I have compared it to, and it seems to be coming from the back 2 cylinder area. I recently ran 2 consecutive 1tank - 1bottle of redline injector cleaner through and there was no change in the noise. It is a typical diesel knocking noise, but seems too loud. It continues until about 1200RPM and then goes away.

2. Between 1200RPM and 1800RPM there seems to be some kind of "light thudding" noise coming from the engine. Not like a bottom end because it goes away with RPM. It is most noticable when the engine is under load and all the windows are up at low speed in 1st or 2nd gear.

3. What causes the typical knock of a diesel at idle. I am just curious.

4. Not noise related, but what is the proper way to adjust the aneroid compensator?

5. Could any of these odd noises have anything to do with the poor 22 mpg that I am getting?

Any help is appreciated. Thanks


DieselNation

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82 240D - 257k
84 300d - 86k

Sounds like injector knock.

Injector knock......is that the typical diesel rattle at idle? Or is injector knock a bad thing? I am new to the diesel discussion so my questions might seem elementary.

1. Is it OK that it seems to be louder in the last two cylinders?

2. Is the thudding noise as described above typically caused by injectors or another problem? Or is it normal?

5. Would it cause me to get 22 mpg?

DN

__________________
82 240D - 257k
84 300d - 86k

Diesel knock is the sound of the fuel igniting when sprayed into the hot air in the cylinder. The later the ignition, the more fuel there is to "knock" and the louder the sound.

Three things have a large influence on diesel knock at idle: condition of injectors, compression, and injection timing.

An injector that sprays a solid stream of fuel instead of the desired cone of atomized fuel will cause a hard knock, as will an injector with the nozzle stuck open. Usually also causes the engine to vibrate excessively. Sounds a great deal like someone is driving a large nail into a slab of oak on top of the head. Cured by replacing nozzle ($50).

Low compression will also cause injector knock, and replacing the nozzle won't fix it. Low compression temp results in slow ignition. Usually causes lumpy idle as the cylinder in question will be low on output too. You will see heavy black smoke at speed, worse under full load. If the valves are out of adjustment, you will have low compression, and setting them properly will fix the problem. If the rings or cylinder walls are shot, you will need to rebuild the engine. A compression check and leakdown test will tell you which is the problem, although I'd bet valves unless you also have excessive blowby and oil consumption.

Fast injection timing will cause excessive noise and poor performance at low speeds, including excessive smoke that goes away at high speeds.

Hope this gives you a place to start! Don't fiddle the ALDA until you get the engine working!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Peter

Excellent post.

Is this type of knock also called "nailing"?
I have seen this term on additive cans-claiming to reduce nailing.

__________________
Ed
1981 300CD (Benzina)
1968 250 S (Gina) 266,000 miles!
1983 Alfa Romeo GTV6 (Guido)
1976 Jaguar XJS-saved a V-12 from the chevy curse, what a great engine!
1988 Cadillac Eldorado (better car than you might think!)
1988 Yamaha Venture (better than a Wing!)
1977 Suzuki GS750B
1976 Yamaha XS 650 (sold)
1991 Suzuki GSX1100G (Shafty Gixser)
1981 Yamaha VX920RH (Euro "Virago")
Solex Moped
1975 Dodge P/U camper


"Time spent in the company of a cat, a beer, and this forum, is not time wasted!"

I've always heard it called injector knock or diesel knock until this last year, here on MercedesShop. English is a living language!

I prefer injector knock, as it is more descriptive. After you've been around these beasts for a while, the various sounds become distinct. It is possible to distinguish between injector knock, rod knock, and a low compression knock when you have heard them enough, and know when they are loudest, etc.

Any direct injection (mechamical only) diesel will have that characteristic rattle. They newer CDI/TDI engines have electronic controls, and inject a tiny bit of fuel, then the main charge, so there is very little knock. They also have a "prechamber" in the piston -- seems to work sort of like a power cell diesel/prechamber with the power and fuel economy of a standard direct injection diesel, but without the noise and smoke.

DieselNation most probably has a combination of bad injectors and low compression knock on the last two cylinders -- that "low thudding" accompanied by a ringing noise that goes away at high speeds sounds just like the 220D, and I know that is low compression -- excessive blowby and tight valves before the last adjustment.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

psfred

Thank you very much for your help. I will check the valve adjustment and correct if necessary. The car does not burn any oil at all nor does it smoke excessively so I hope valve clearance is the issue.

DieselNation

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82 240D - 257k
84 300d - 86k

DieselNation:

If the valve adjustment doesn't fix the problem, I'd try new nozzles next, then investigate the compression and valve condition via a leakdown test -- you could need a valve job.

Good luck!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!