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  #1  
Old 01-01-2012, 08:04 AM
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W109 3.5: engine rattle at low rpm

I have a lound chatter at low rpm when accelerating. It is just at very low rpm form idle on and the sound is disappearing after.
I don't know what it is. Could be timing chain or preignition? How can I find out what it is and how can I solve the problem?
Martin

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  #2  
Old 01-01-2012, 11:38 AM
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Again it is happening at very low rpm and my oilpressure at idle is not that great..about 20psi when hot.

I need your help here because I am about to take the valve covers off and change the valve stem seals. I could in theory also change the chain guides if you think this might solve the chatter...assuming it is the chain.
I did change the chain and tensioner a year ago but decided not to change the guides because they were aluminum with rubber, worn but won't break.

Another idea what the chatter might be is: Preignition. But here I also have no experience how to recognize preignition.
Thanks, Martin
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  #3  
Old 01-01-2012, 01:38 PM
GGR GGR is offline
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Pre-ignition and TC rattle are different noises. Hot weather favours pre-ignition, so it may it as you just arrived in Oman. To be sure you can turn the dizzy back a few degrees and see if the noise is still there.
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Old 01-01-2012, 04:47 PM
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I would put money on preignition , you should also run 1 liter of ATF per 100 liters of fuel. This helps with preignition and cleans the combustion chambers getting rid of carbon build up, saves valve stems and seats and keeps the whole injection in good order. Chains do not fail that quickly.
You need to use a good oil such as Castrol GTX 20/50 .As long as the oil pressure goes to the top at engine speeds above idle ,you will be OK. We get really hot weather here in summer too and oil pressure dropping off is normal.
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Old 01-02-2012, 09:31 AM
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Thanks mercmad,
It sounds relieving to me if it is probably preignition. Using a 1% ATF mixture for improvement is easy to do.
Questions:
1)Would you say that my shot valve stem seals (I am using a quart every 300 miles) are not making the case better. I am about to change these seals in the next week.
2) Is it true that Preignition are happening at low rpm rather than higher speeds?

Not sure if I'll get Castrol GTX 20/50 here in Oman (Middle East) where I am residing right now. But I am sure I can get a 20/50 Oil or such. With Temps above 75 in Winter I can probably use a SAE 40?
Oilpressure goes to the top as soon as 1500rpm are reached.

Finally you wouldn't change the chain guides- correct? Because the wear of the old aluminum/rubber guides doesn't influence the slack of the chain meaning new guides wouldn't improve anything.
Best Martin


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Originally Posted by mercmad6.3 View Post
I would put money on preignition , you should also run 1 liter of ATF per 100 liters of fuel. This helps with preignition and cleans the combustion chambers getting rid of carbon build up, saves valve stems and seats and keeps the whole injection in good order. Chains do not fail that quickly.
You need to use a good oil such as Castrol GTX 20/50 .As long as the oil pressure goes to the top at engine speeds above idle ,you will be OK. We get really hot weather here in summer too and oil pressure dropping off is normal.
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Old 01-02-2012, 09:39 AM
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GGR,

I am not sure if the chatter didn't occur in colder climate but it didn't happen so often for sure.
I can't ignore it any more.
If chatter is preignition the fuel quality can be a reason, maybe also the leaking oil through the valve seals (I'll change them soon).
If preignition is still there after valve stem seals change I could dial back the timing at the distributor.
Martin


Quote:
Originally Posted by GGR View Post
Pre-ignition and TC rattle are different noises. Hot weather favours pre-ignition, so it may it as you just arrived in Oman. To be sure you can turn the dizzy back a few degrees and see if the noise is still there.
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  #7  
Old 01-02-2012, 11:07 AM
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Dial the distributor back asap to check. If it is (prolonged) preignition, it will ruin your engine. The distributor is held in place with an allen key bolt to the right of it (left if you are standing in front of the car facing the engine). You will need a long extension to get to it, but I would HIGHLY recommend changing it to see what happens. Also, any mechanic will be able to diagnose pre-ignition. It basically sounds like a diesel when it does preignition.

Good luck,

Bert
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  #8  
Old 01-02-2012, 05:48 PM
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Oil leaking from seals or rings causes preignition too because it becomes a low octane fuel .Because this is happening at low revs when the engine is working harder this is a clear sign. Any good 20W/50 will work fine. Do not bother changing the rails unless the bonded surface has worn through (rare on these engines) . Normally I only change the banana rail on the tensioner because the nylon cover wears out.
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Old 01-02-2012, 06:45 PM
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[QUOTE=mercmad6.3;2856635]I would put money on preignition , you should also run 1 liter of ATF per 100 liters of fuel. This helps with preignition and cleans the combustion chambers getting rid of carbon build up, that quickly.



Does it build up any deposits on the spark plugs during city running ?
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  #10  
Old 01-02-2012, 07:30 PM
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Hi, never, as far as I have seen And I run it in all of my mercs (13) . I have pulled heads from engines I have run it in and everything has been perfectly clean with no carbon in the combustion chambers. One thing I have learned though is the Aston martin DBS V8 owners are cautioned against using many of the octane boosters' sold by auto shops because it builds up on the intake valve stems and this build up causes valves to stick . I have never seen this in Engines running ATF.
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Old 01-03-2012, 07:55 AM
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I assume that none of the MBs you use it in have catalytic converters, right? I'd imagine it'd foul them up, or cause premature failure?
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  #12  
Old 01-03-2012, 08:04 AM
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Vintage Mercedes don't have no sticking cat cons....LOL!

But I do know guys with post 1985 Cars here who use it and it doesn't worry the cats.
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  #13  
Old 01-03-2012, 09:43 AM
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Thanks everyone for all your wisdom

So now there is the option that the chatter is preignition coming from shot valve stem seals. I am prepared to change the seals and wanted to figure out the reason for the chatter at low revs...because I am there anyhow.
I started to remove the airfilter, removing the plugs. First thing I saw was cylinder 8 was very 'wet', engine oil was dripping down the block coming from the plug and the plug inside was completely covered with sludge and oil. Looking into the hole I saw a lot of engine oil ans oilt soot there. I guess a lot of oil is leaking down the valve guide there at cylinder 8 (which had not great compression when I tested a year ago, 120psi versus 160 at the others) so there is a great chance that the valve guides are shot and killed the valve stem seal.
I'll see when I'll remove the rockers and springs. I think a radial play of 0.5mm is the max. If I have more I should remove the head...which I don't want to at this point.
Can this very oily cylinder with dripping engine oil cause preignition?
I was also wondering why the oil is dripping down the block (and causing these ugly spots in the Garage and driveway). Could this be a loose spark plug?
The spark plug is totally covered with oily black soot and I assume I need new plugs? All the other cylinders are 'dry' on the outside.
Martin
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  #14  
Old 01-03-2012, 03:50 PM
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I suspect you have a case of broken rings actually. if the valve seals are reasonable and not hard (compare them with the new ones) and the valves not overly loose you certainly need to pull that piston and check it out.
But yes,that will cause preignition and a lot of other things such as the oil leaks which will be exacerbated by the blowby from the sick cylinder as crankcase pressure will overcome the seals and the idle air suction system.
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  #15  
Old 01-04-2012, 12:37 PM
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Sounds like a loose valve guide to me, or a very work valve guide. Replacing valve stems seals rarely works because the guides are usually worn by the time an engine starts using oil. That head has to come off to fix this problem and I would do both sides if the compression is low in other cylinders. I doubt it's a piston ring problem - the bottom end on these engines is very strong.

A cylinder with 120 PSI isn't likely to ping, that's more likely ignition timing. I've found that the throttle valve can have blocked vacuum ports on these engines and the distributor will not work properly when that happens. Blow through the port/s with some compressed air and you should be able to feel the air coming through inside the throttle bore.

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