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Hi all. I've recently started noticing that when I start up my E55 from cold, after a few seconds once the RPM's have settled down, the car occasionally shudders. This is acompanied by a change in noise from the engine, like a dip in the RPM - but there is no noticable fluctuation in the RPM needle when this occurs.

If i pop open the hood and watch the engine, it does seem to noticably jerk around in the engine bay a lot more during this. But if I pop the car in gear with the brake on, it goes back to being perfectly smooth.

Some thoughts i've had are...

-Dirty injectors. car has done 110k, so maybe the injectors aren't flowing properly at lower duties. Not sure how I can test this, other than pulling them out and getting them cleaned. Maybe I'll throw some seafoam through them before?

-One of the accessories (alternator, water pump, or maybe SC?) is starting to seize and placing a load on the engine. I'll disconnect the SC belt as a starting point to see if this helps, but can anyne confirm whether it is OK to start the car without the serp belt and let it idle for a few minutes?


I'm not getting any CEL's or mis-fires and the car runs smooth as silk around town, so I have a feeling it might be a bugger to diagnose unless anybody can throw in some pearls of wisdom?
 

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2004 E55 AMG
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dirty injectors are unlikely but if you want to run something through the system... make sure it is something mercedes approves. some fuel injector cleaners and other fluids for that matter will hurt more than help.

it is ok to start with the belts off, just make sure you watch the temps and only run for a minute or two. i dought something is trying to lock up tho.

have you ever done spark plugs??? helped out my car and i have 76k on it.

motor mounts can also be shot and make the engine seem twitchy... they are common to go bad a long with the trans mount.
 

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Discussion Starter #3
dirty injectors are unlikely but if you want to run something through the system... make sure it is something mercedes approves. some fuel injector cleaners and other fluids for that matter will hurt more than help.

it is ok to start with the belts off, just make sure you watch the temps and only run for a minute or two. i dought something is trying to lock up tho.

have you ever done spark plugs??? helped out my car and i have 76k on it.

motor mounts can also be shot and make the engine seem twitchy... they are common to go bad a long with the trans mount.
Just an (overdue) update. I ran some fuel injector cleaner through the system which made zero difference, so I think we an probably tick that off.

I replaced the sparks only 2k ago and used the OEM ones, so I'm pretty sure that it wouldn't be that unless something is loose (but then I'd think it would be lumpy all the time).

I'm leaning towards your suggestion of the engine mounts, as I would expect some squeaking or lots of belt dust if an accessory was seizing.

Question is, how hard are the engine mounts to change? Is an engine lift required, or can it be done with a car jack for example?

At this point I'm not sure if I'm keeping the car as I can see a world of pain on the horizon, so I just want to get it saleable.
 
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