Strange knocking sound

Hi guys, got this strange problem. Seems that whenever my car is started on a cold engine, initially I would hear a knocking sound coming from the engine. Around 5 to 7 knocks, after that, all would be okay. Only happens when the engine is cold. Any ideas?? Thanks, any help is appreciated.

Andrew
Car: MB E200 W124
My: 1994

[Benzmac]

Sounds like your timing chain tensioner is weak, have it checked.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

Wouldn't the tensioner make that noise regardless of the engine temperature? I was thinking the air pump instead.

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)

Carge,

Any resolution? I just had my timing chain replaced and now have the same knocking. The tensioner was also replaced.

Chris

[intruder]

two things I can think of:

1. too high oil viscosity

2. weak "check valve" (correct term?) at the oil filter. I've heard it described two ways; one where the check valve is INSIDE the filter and one where the check valve is in the oil filter attachment assembly. Regardless, since the filter is "upside down", oil will tend to flow out when the engine is off without a properly operating check valve holding the filter full of oil so that there is no immediate flow of fully pressurized oil at startup. As pressure rise the flow is restored and the noise disappears.

A tech may be able to give a more accurate answer but, when my 1991 300E did what you described my mechanic told me to switch to MB OEM filters and the problem resolved immediately.

for what it's worth
Intruder

[pentoman]

Mine has done that every time I start it, since I've had it (12 months), I think the chain does not get oil for the first few revolutions. Anyone care to correct me?

later

Russ

[ymsin]

Does weak tappets have anything to do with it?

carge,
I sincerely hope the knocking sound you hear is NOT a deep knock, because many times a deep knock upon starting with an engine that has been sitting overnight, especially on a cold day, could indicate overly worn crankshaft main bearings. If the mains are worn excessively, the oil that normally stays between the crank surface and the bearing will leak out and upon starting, the engine will knock several times until the oil pump pressure builds up. I hope, if it is a deep knock, that your problem is not nearly as drastic and may be engine mounts or even an exhaust pipe against the frame. A sure-fire test for bad main bearings requires a decent oil pressure gauge, meaning one that shows a meaningful range of oil pressure from 1000 TO 2000 rpm. At normal operating temperature and at a steady cruising speed of about 1500 RPM, put a load on the engine without downshifting and observe the oil pressure. As RPM increases, pressure should increase toward its normal upper limit. If it drops, your main bearings are badly worn. Now, remove your foot from the accelerator and observe the oil pressure. The pressure should stay steady or drop. If the oil pressure rises, your main bearings are worn past limits. Unfortunately, I have not yet owned a MB with a oil pressure gauge that would allow for this type of test, but thankfully, I have not yet worn out my main bearings either, so I guess I should not complain, since I have owned a '75 240D, an '85 300SD and currently have an '85 500SEL and a '84 1000SEL AMG.

[adamrei]

does the knock go away upon achieving full oil pressure?

[adamrei]

Hi again. Our research at www.wetstart.com has dealt with this. We researched the oil pressure lag build-up behavior of engines uppon start (hot/cold). One of our research vehicles has been my own 91 300E. Occassionally it would make the noise you described (other vehicles will have hydraulic tappet noise) and it was correlated for the most part to oil pressure lag. As part of our research we kept that engine permanently primed with oil and all noises are gone. Using this method we can make fossil oil "move faster" than the best synthetic (flow speed that is) by virtue of the "fossil" coming to rest inside the oil galleries (primed). So based on our observations:

Check your oil filter for proper anti-drain back valve function (just replace with a new one); Sometimes the pump (????) would loose partial prime and take longer to pump; Obviously viscosity will make the fillup of galleries more difficult (a reason to: change it hot and change it often......I think somebody has said that before many times). More research results on this at www.uspto.gov (#5,743,231; 5,782,315; 6,349,692) if anything this site is geat for fun and research. Later.