Loud "Thunk" upon start up on 08 S65
Thread Starter
Junior Member
Joined: Feb 2010
Posts: 22
Likes: 7
From: Dallas TX
W221, S65
Loud "Thunk" upon start up on 08 S65
Hi Guys,
I have a 2008 S65 with 60K miles that has been impeccabley maintained. It grew up in Las vegas so there are no rust, corrosion or salt issues. All wear components have been regularly serviced or replaced with the exception of the ABC accumulators, filters and fluid, but that's next on the list. It drives and rides like the car it was designed to be. Recently, about two months ago after it started getting colder here in Dallas, I started hearing a very loud "Thunk" upon start up. It sounds like it's coming from the left front wheel well or undercarraige beneath and to the front of me. I can feel it in my left foot. It is a single loud thunk and only occurs intermittently, about every third or fourth time I start the car. It happens about 3-4 seconds after the engine spools up. It doesnt occcur more than once, after the car is warm. It only seems to occur after the car has been sitting like overnight. I've looked under the hood and tried to find something that's loose but could see nothing obvious. I have to admit I haven't torn things apart as I'm not sure where to start. I also have not pulled any DTC's as yet because this sounds mechanical in nature and no component has apparently failed (that I can tell). Does anyone have any idea what this might be? I can think of nothing that occurs so soon after starting and could be temperature related unless there may be something loose in the exhaust system after it builds pressure from the turbos. It runs fine, pulls strongly, no other symptoms to speak of. Any help would be appreciated as I dont want to go digging unless Ive got some idea of where to start. I also dont want to let this go until it turns out badly. HELP!!
Regards,
Rick
I have a 2008 S65 with 60K miles that has been impeccabley maintained. It grew up in Las vegas so there are no rust, corrosion or salt issues. All wear components have been regularly serviced or replaced with the exception of the ABC accumulators, filters and fluid, but that's next on the list. It drives and rides like the car it was designed to be. Recently, about two months ago after it started getting colder here in Dallas, I started hearing a very loud "Thunk" upon start up. It sounds like it's coming from the left front wheel well or undercarraige beneath and to the front of me. I can feel it in my left foot. It is a single loud thunk and only occurs intermittently, about every third or fourth time I start the car. It happens about 3-4 seconds after the engine spools up. It doesnt occcur more than once, after the car is warm. It only seems to occur after the car has been sitting like overnight. I've looked under the hood and tried to find something that's loose but could see nothing obvious. I have to admit I haven't torn things apart as I'm not sure where to start. I also have not pulled any DTC's as yet because this sounds mechanical in nature and no component has apparently failed (that I can tell). Does anyone have any idea what this might be? I can think of nothing that occurs so soon after starting and could be temperature related unless there may be something loose in the exhaust system after it builds pressure from the turbos. It runs fine, pulls strongly, no other symptoms to speak of. Any help would be appreciated as I dont want to go digging unless Ive got some idea of where to start. I also dont want to let this go until it turns out badly. HELP!!
Regards,
Rick
MBWorld Fanatic!
Joined: Apr 2009
Posts: 1,264
Likes: 367
'06 MB S55 AMG; '04 Audi Allroad 4.2; '05 BMW M3 Conv.; '92 MB 500E
Yes... get that ABC fluid, filter and accumulators out of there ASAP. What I fear is that’s the sound of ABC fluid running into a failed (or failing) accumulator, most likely the pulsation damper. If that’s what it is, what comes next likely isn’t good. I’m not 100% but I’d start there. Car starts, pump starts, fluid moves, thunk.
GL
maw
GL
maw
MBWorld Fanatic!
Joined: Apr 2009
Posts: 1,264
Likes: 367
'06 MB S55 AMG; '04 Audi Allroad 4.2; '05 BMW M3 Conv.; '92 MB 500E
I don't think so but don't know for sure on W221 cars... mine is a W220 and they throw no codes... it's why people stay away from these cars so I'm pretty sure all that hardware goes without warning. You might be able to see pressure spikes on your machine, but that's completely besides the point. IT NEEDS TO BE DONE IF IT HASN'T ALREADY NO MATTER WHAT THE MACHINE SAYS. Think of changing that fluid and filter as an every 3yr/30k mile thing (5/50k if you want to press it), and the accumulators as every 10y/100k mile thing. Your car is overdue no matter how you look at it, unless it's been done already. Pressure spikes from bad accumulators lead to blown struts, lines and pumps, ALL of which cost more than the work we're discussing.
And judging how this conversation is going, if you haven't done transmission fluid and filter, do that too cuz that'll be next. You'll be back talking about how the car doesn't shift correctly. Consider that a 5yr/50k mile service item. Finally, engine and transmission mounts as well if they haven't been done.
All of the above is SClass AMG Maintenance 101.
GL
maw
And judging how this conversation is going, if you haven't done transmission fluid and filter, do that too cuz that'll be next. You'll be back talking about how the car doesn't shift correctly. Consider that a 5yr/50k mile service item. Finally, engine and transmission mounts as well if they haven't been done.
All of the above is SClass AMG Maintenance 101.
GL
maw
Last edited by maw1124; Feb 2, 2021 at 11:49 AM.
MBWorld Fanatic!
Joined: Apr 2009
Posts: 1,264
Likes: 367
'06 MB S55 AMG; '04 Audi Allroad 4.2; '05 BMW M3 Conv.; '92 MB 500E
For starters, buy some Pentosin and check the fluid level in your garage... top off as necessary... plenty of YouTube stuff on that... that should help until you get the service done. If my theory is correct the fluid is low and that's why there are air gaps in the system. Even if my theory is incorrect the services need to be done anyway so it cost you nothing extra to start there. This fluid and filter (and the transmission fluid and filter) were originally No Service Interval from MB. That has subsequently been proven foolish. The fluid coagulates over time, and of course it's worse when its cold. After it warms up it flows better. Make sense?
maw
maw
Last edited by maw1124; Feb 2, 2021 at 11:58 AM.
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MBWorld Fanatic!
Joined: Jan 2011
Posts: 1,181
Likes: 59
CL65
Hi Guys,
I have a 2008 S65 with 60K miles that has been impeccabley maintained. It grew up in Las vegas so there are no rust, corrosion or salt issues. All wear components have been regularly serviced or replaced with the exception of the ABC accumulators, filters and fluid, but that's next on the list. It drives and rides like the car it was designed to be. Recently, about two months ago after it started getting colder here in Dallas, I started hearing a very loud "Thunk" upon start up. It sounds like it's coming from the left front wheel well or undercarraige beneath and to the front of me. I can feel it in my left foot. It is a single loud thunk and only occurs intermittently, about every third or fourth time I start the car. It happens about 3-4 seconds after the engine spools up. It doesnt occcur more than once, after the car is warm. It only seems to occur after the car has been sitting like overnight. I've looked under the hood and tried to find something that's loose but could see nothing obvious. I have to admit I haven't torn things apart as I'm not sure where to start. I also have not pulled any DTC's as yet because this sounds mechanical in nature and no component has apparently failed (that I can tell). Does anyone have any idea what this might be? I can think of nothing that occurs so soon after starting and could be temperature related unless there may be something loose in the exhaust system after it builds pressure from the turbos. It runs fine, pulls strongly, no other symptoms to speak of. Any help would be appreciated as I dont want to go digging unless Ive got some idea of where to start. I also dont want to let this go until it turns out badly. HELP!!
Regards,
Rick
I have a 2008 S65 with 60K miles that has been impeccabley maintained. It grew up in Las vegas so there are no rust, corrosion or salt issues. All wear components have been regularly serviced or replaced with the exception of the ABC accumulators, filters and fluid, but that's next on the list. It drives and rides like the car it was designed to be. Recently, about two months ago after it started getting colder here in Dallas, I started hearing a very loud "Thunk" upon start up. It sounds like it's coming from the left front wheel well or undercarraige beneath and to the front of me. I can feel it in my left foot. It is a single loud thunk and only occurs intermittently, about every third or fourth time I start the car. It happens about 3-4 seconds after the engine spools up. It doesnt occcur more than once, after the car is warm. It only seems to occur after the car has been sitting like overnight. I've looked under the hood and tried to find something that's loose but could see nothing obvious. I have to admit I haven't torn things apart as I'm not sure where to start. I also have not pulled any DTC's as yet because this sounds mechanical in nature and no component has apparently failed (that I can tell). Does anyone have any idea what this might be? I can think of nothing that occurs so soon after starting and could be temperature related unless there may be something loose in the exhaust system after it builds pressure from the turbos. It runs fine, pulls strongly, no other symptoms to speak of. Any help would be appreciated as I dont want to go digging unless Ive got some idea of where to start. I also dont want to let this go until it turns out badly. HELP!!
Regards,
Rick
Thread Starter
Junior Member
Joined: Feb 2010
Posts: 22
Likes: 7
From: Dallas TX
W221, S65
Turned out to be low ABC fluid. Topped it off and no more thunk! I have since flushed the old fluid with new and replaced the filter. In the process I noticed that the left rear strut was leakng, hence the source of low fluid levels. Replacing the strut as we speak. Thanks for all the input.
