dolittledobig

this issue has been there since i got the car last year. every 5 to 30 seconds there would be a mild hiccup felt inside the cabin at idle speeds. it does it randomly. my diag tool picks up no fault codes relating to the issue however checking smooth cylinder test data shows when the hiccups happen. smoothness values from 0 to 2.5 would be normal, 7.5 and up would indicate a misfire. during those idle hiccups my numbers would go to about 5.80 max, and frequently be in the 3s and 4s throughout each cylinder one or two at a time when the hiccups happen. symptoms are the same whether engines cold or warm. a thing to note as well on cold starts during fast idle the engine rpm doesnt lower smoothly but instead it winds up and down for 15-20 seconds and only after it settles down to normal idle. i checked the main pcv hoses they are still flexible and seal correctly. im not sure what would cause this, injectors, oxygen sensors, fuel filter or fuel sender unit? there is nobody local here who has experience working with petrol engines so im lost on what to do next. i caught up with the previous owners neglect below but this is the last issue on the car i want to have sorted.

jobs i done since owning the car.
-genuine 0w40 amg oil, all filters.
-8x new delphi coil packs
-new NGK IFR6D10 spark plugs //noticeable improvement on fuel economy.
-tested plug leads 2kohm
-valve cover gaskets, breather cover reseal, oil filler cap.
-new alternator
-new MAF seal both sides
-new thermostat, coolant flush.
-new oil filter housing seals, oil pressure port seal
-donor coolant reservoir, return line spout cracked on old one.

if i missed anything or need more details let me know. i can also make a video of the engine behaviour. thanks

dolittledobig

4 garages later and dont know nothing. pretend to be experts but cant diagnose a repair friendly engine.

Rahulio1975280C

I am not familiar with smooth cylinder test data, is that a standard test you can do with engines or some unique feature from the scan tool you are using? Seems neat.

I looked at your list and see three opportunities. I am going to list them in order of least cost to most cost based on this statement you made:
"instead it winds up and down for 15-20 seconds and only after it settles down to normal idle"

1. (FREE) Clean your throttle body.
Take the MAF sensor back off. This job is a PITA. Use a camera or slip your phone back and take a blind picture of the opening to see what it looks like before you start. There might be a build up of oil/slime all over the inside. Use a degreaser and wipe out the opening. You can carefully actuate the throttle body flap and clean all around the opening. Then, for extra credit, use a Q-tip and do a detail on the perimeter. This makes sure the throttle body does not get caught on grit/buildup that may be present. Refit your MAF, but look at number 2, below. (I am now in my late 30s and have owned my C55 for 13 years. Body hurts much more after working on a car.)

2. ($) MAF sensor cleaner. In the US, we have a company called CRC that makes a product called Mass airflow sensor cleaner. Some say it can bring a dying MAF back for a bit, I don't think it will destroy a MAF, maybe worth a shot to see if it helps.

3. ($$) Pre-Cat Oxygen Sensors
- I just did the front two, before the primary cats, after 168,XXX miles and the initial power off the line of my engine went up. I am curious if your oxygen sensors are still "working" but have become lazy/slow. When your car transitions from fast idle to regular idle, that up and down winding seems suspicious and maybe the oxygen sensors are giving strange data to the ECU. (without throwing a code)

Help us help you by:
1. Share a video here, it will help give more context.
2. Tell us how many miles/kms are on your C55?

dolittledobig

1. video of fast idle acting weird: https://streamable.com/hs070k

2. 118,000 miles

WadeBrookss

I looked at your list and see three opportunities. I am going to list them in order of least cost to most cost based on this statement you made:

PJmak

Check breather hoses on top of the motor that go from valve covers to the back of the intake manifold.

SyHS

Had a similar issue on a CLK500 that I sold a while back. Would stumble over itself on cold start, smooth out, then do it again after warmed up and idling. I removed the EGR valve and ran the car without it at idle -- ran much better with it unplugged. Installed new EGR valve and it was fixed for a few days, then it threw a code for catalytic converter/O2. Used spark plug foulers with new O2 sensors and the car was fixed and off to it's new home... I imagine you fixed the issue as the last response was 5 months ago.

dolittledobig

nope no fix yet. its becoming a rabbit hole. i found out the car had its crankshaft (not the sensor!) replaced before because the one time use bolts that hold the ring gear to the crankshaft were the updated ones. i was given a full service history except for the fact i cant find the crankshaft job. maybe ill pull the intake manifold and rebuild the injectors and if that wont help it ill leave it be.

SyHS

Also a bit confused on the term "hiccup". Does the car shake when it happens? is it like a misfire? Also how constant is the issue when it arises, Does it happen quickly, or last for 10-20s before clearing itself out..

I find trouble believing it is an injector issue. I am more familiar with the M113k engine, not so much the M113 w203 one... I do know the M156 have intake manifolds that crack and fail, I'm unsure if that is an issue on these NA engines though..

The CLK500 I had sold had 119k miles, and the previous owner was tracking down the same issue, it would also fluctuate RPM's on cold start between 800-1500 never balancing out until it was warmed up. It never threw me a CEL until after I installed a new EGR and the O2 sensors were too far gone(bad EGR can destroy your cats/sensors).

I have a strong feeling it's something along the lines with carbon buildup or a throttle body/plate issue. You can pull the throttle body and check the plate/flap to see if it's gunked up and not closing all the way.... A faulty EGR would cause issues inside of the cylinder walls with air/fuel mixture. EGR valve is supposed to slowly open and close on its own, if there is an issue with smoothness and it's quite constant then the EGR valve could be stuck OPEN, causing excess recirc gasses back into the engine, causing the smoothness in the cylinders to fluctuate, messing with the AFR... I would pull the EGR valve and suck on it(yes i know lol) to see if it can open with vacuum. Also helps to get eyes on it to see how much carbon buildup is inside, can even pry the egr flap with a screwdriver to see if it is stuck.

There are some really helpful videos on youtube about testing EGR valves to the M112 and M113 engines, it is a common problem.