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Hey guys,
I hope this isnt anything too serious but lately on my 450SE ive noticed a 'knocking' noise. It's louder when the engine's cold and gets quiter as the engine warms up, to the point where u can hardly notice it. Im pretty sure its coming from the engine.
My car was recently converted to LPG , would this have anything to do with it?

Im not THAT mechanically minded, but i hope this isnt anything too serious! The car runs fine, as it always has. Ive heard of big end bearings going, causing knocking etc but surely if the car's running fine it couldnt be that?

Ryan

Is it a regular, deeper knocking sound like someone striking the engine block with a wooden mallet, or just something that happens occasional and less intense?
If it was a big end bearing it would be worse under load and you would still hear it when the car warms up.
My dad had a VS Commodore where a big end bearing bearing failed after 1500km, that was the characteristic of the sound. As you would expect, it was his last Holden.

If it only happens when cold, it could be piston slap. I don't think I've ever heard piston slap occurring in an MB motor, though. It is almost normal in Volvo red block four cylinders.

Is the engine running smoothly when this happens?

Tim

hey TJ,
its not really that intense, but its regular. The car runs perfectly... no problem there at all. Im not even sure if its anything to worry bout, but i thought id better check . I guess really i need someone to have a look at it. Might drop it into my mechanic. I Wonder if its anythin to do with the LPG?

Get it checked by someone knowledgable - soon.

It doesn't sound right.

Bill

Is the noise there on every cold start?

How long before the noise disappears, seconds or minutes?

koan

yeah i should get someone to have a look at it. It may well be just me being paranoid but id rather be sure.
Funnily enough this morning i didnt really notice it... but last night on the way home from work it was quite loud. Tends to get quieter as the engine warms up

would the engine running lean cause it?

LPG is supposed to have a much higher octane rating than petrol so knocking would only be an issue if it was running too lean.

M-B engines can knock when cold - both my previous W211 - E320 and my present 164 - ML 500 had/have a little piston slap which is only audible when I stop to open the gate when leaving on a cold morning.  They both have V6 and V8 versions of the same motor
Not audible from within the car - only when standing outside with a stone cold idling motor and disappears within minutes as the engine warms up.

I have never heard either my 450 or my 6.9 make the same noise though - totally knock free on a cold startup.

Bill

hmm okay , who knows perhaps it is lean?

can anyone reccomend a good mechanic north of the river (perth) to take a look at it, or does any one wanna have a look?

or would i be better taking it to the people who installed the LPG?

Youtube is your friend.  Sounds like a time to video the engine idling both cold and hot and post for us to have a listen.

Some things to note though...

If the knock is more like a tap, or clank, it could be as simple as a hydraulic lifter not pressurising properly.

If the engine is missing on 1 cylinder (fouled plug) then the missed stroke can sometimes sound like an engine knock.  I know whitey, my 450SE race car, sounds like its knocking when 1 cylinder (#2) fouls and then when another fouls (usually #6) the engine sounds like it running with no bearings...

Go around your 8 cylinders with a timing light and make sure all 8 are firing...

Is your Oel pressure good, and hits 3bar as soon as you press on the accelerator?.... Oel pressure (or lack of it) has a close relationship with the condition of the bearings.

Whitey also had a terrible knocking when i bought it but would subside as the engine warmed up.  After much mucking around I discovered a number of issues which have been fixed and made the engine quieter when cold....
1. the oil pickup in the sump was blocked so the oil system was slow to pressurise
2. the chain tensioner was worn and not holding enough pressure enough of the time so the timing chain would vary from tight to loose to tight accompanied by strange noises.

Some people also suggest adding a cup or so of ATF to your engine oil, the purpose of which sounds reasonable (can't think exactly what it is know) but if things become quieter after that then it means something.... i think its supposed to help unstick stuck lifters etc.  Can't hurt to try it.

Hope this helps,

I.

Yep - just cleared a varnished (BAD) 4.0 liter inline 6 that sounded like a bad rod at low rpm, especially cold

I used 1 qt transmission FLUSH and ran it five minutes, and let it sit for a week

Then used 5 quarts ATF and 2 quarts motor oil and ran it around the block a couple times,

THEN used 6 quarts heavy 15-40 oil (just like our cars - are you using high viscosity?  5-30 can be atrocious in these) and 1 quart trans fluid

Smooth and silent.

FYI

hmm well it does tend to miss a bit and run rough when its cold. Maybe thats what the knocking sound is.

Oel pressure's good...

Im using Mobil 1 fully synthetic, i think its 5w-50.
Is that okay?

Might try adding some ATF then, see if it helps at all

ATF is to act as a detergent and is usually only used as a flush and not left in for any length of time.

Mobil 1 should be a high detergent oil anyway.

I would be more in favour of getting the noise checked and see what it really is, get the gas mixture checked and if you think you have an oil problem try Castrol GTX 20-50, or Castrol synthetic 10-60 or one of the LPG specific oils.

Give us the noise as a sound or video file and see what comes up.

Bill

YOUR OIL IS TOO THIN

Mobil 1 5W-50 is in fact the recommended oil for these engines in their product data. I have used the 5W-50 in the past with no problems other than slightly elevated consumption initially. I have now reverted to the Synth S 10W40 with a cup of ATF on the changeover as the lifters were slightly noisy. I now have no problems with noisy lifters and the consumption has dropped to about 250mL/1000km with some redline activity. There is certainly no abnormal engine noise and the engine is original with 343,000 km on the clock.
One of the best places to go NoR is the "Star Shop" in Osborne Pk. Herbert should be able to help you out, just don't bring any parts from AutohausAZ with you.

Tim

Ach!  I stand corrected.

I run 15-40 high-detergent (basically a truck oil) in mine - as lighter weights seem to eat cams in 126s