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

 

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

 

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?

 

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

2. 118,000 miles

 

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

 

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.

 

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.

 

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.

 

the first symptom is when i do a cold start (tried 10-12c, 32-35 celsius ambient), it holds the fast idle (1100rpm ish) for about 25 seconds, then it settles to normal idle (800rpm) then winds up to fast idle again and repeats this for a minute or two, until it settles to 800-900rpm. i noticed the colder the engine the worse this symptom as it would take longer to clear.

second symptom is the misfire-like shake felt inside the cabin when idling. on my diag tool it, the smoothness cylinder test doesnt reach the value range for it to count as a misfire. but every 2-5 seconds i can feel a slight lump or a hiccup. sometimes it takes 15 seconds between, or one after another.

never had a moment where it would idle clean for at least 30 seconds. the engine can never idle smooth.

on cold engine its more noticeable and id says happens a little more frequently. when its warmed up it still quite frequent.

car revs are healthy and does pull, wheels spin on blipping throttle. no power loss.

throttle plate was clean when i checked it, i only wiped off a thin dusty layer of soot. i replaced the two MAF seals on the plastic elbow fitting. breather hoses all looked supple when i squeezed them. i'll have a look at the EGR

 

Could your intake swirl flaps be stuck? Not common on the M113, but neither is carbon buildup.

 

They aren't stuck, I can move the actuator with my finger

 

Idles should be higher on a cold start, but they shouldn't oscillate back and fourth from 800-1200. It sounds like it could be an AFR problem. What's weird is normally bad AFR trims will cause idle surging constantly and not every 15-30s.

A smoothness cylinder test really isn't going to help you track down a problem besides alerting you that measurements are off.

If you are able to check your AFR (air fuel ratio) and those are in the correct spec then you can rule out nearly everything fuel/air related. Personally I feel like the car is running lean.

Could you reupload the video on Streamable for me to see? Or take a cold start video again along with the engine bay while it's happening.

 

yes ill do it the next morning. ill record a cold start and letting it do its thing for about a few minutes

 

here's a video of a typical cold start.
There aren't any cracks in the intake manifold and nothing obvious with the vacuum leaks. I checked multiple hoses around breather covers and they arent damaged or perishing.

For the lean mixture on the AFR I'd add the car always stinks of fuel on the cold start and its very obvious for anyone standing near or sitting inside with the windows down.

i have an icarsoft MB v3.0 and checked ECM and the only code showing is refrigerant pressure too high because my a/c doesnt work for a different reason. im not sure where i can find the ignition control module however. i dont know if the m113 has an air control valve. i cant find information online.

my next job is definitely pulling the intake manifold and see about the egr and throttle plate.

 

So it's running rich, which is usually normal on a cold start depending on how strong the smell is... Next time you cold start it try and see where the fuel smell is coming from. For a leak it would be on the backside of the block on the rail, or underneath the rear bench seat. If the smell is coming from either of those places it's likely a leak that occurs when cold but when the car warms up the lines expand and pretty much self-seal for now. AFAIK none of my W203's have had a faulty gas tank, but I've come across a handful of W211's with the gas tank problem. I would likely rule that out, but you never really know.

EVAP leak could cause excess fuel into the engine causing all sorts of problems. Exhaust leak could also cause the car to run rich and smell of fuel, especially when cold. A small exhaust leak would 100% cause a slight misfire that is undetectable by the ECM and affect the cylinder smoothness. EVAP would do the same, but I find it hard to believe an issue with EVAP system would not throw a CEL as they are really sensitive to changes compared to other components/systems of the car.

Do you know if the car has been tuned?

 

no i dont think it ever had a tune. all original including exhaust system. the rough idle is also present when fully warmed up after hours of driving. at stop lights id still feel the shakes happening frequently just a little weaker. the gas smell definitely is coming from the exhaust. near the rear of the car or mid reversing is the strongest.

edit: there is one time i remember there was little to no shake but i only gave it thought now. i was parked pointing slightly downhill, mild but noticeable slope with the engine running, handbrake on in neutral. ill try replicate that again see if its consistent.

 

I have exactly the same issue of random hiccups.
No codes, none of the mechanics wants to deal with me anymore in my area. Everything that could be replaced was replaced or rebuilt but it still shudders from time to time.
At idle in P or N from time to time it sends some vibration impulse through the seat (like when you pee in winter outside and then whole body shudders )
No error codes.
Some days no vibration at all and some days it’s more often.
During my investigation I was able to get scenarios when these vibrations go away but they do not make any sense and mechanics tell me that I’m crazy:
1. When you drive it hard (couple of hard pulls)
2. When you do not go higher than 2.5k rpm
3. When you put in reverse and drive like 10 meters then there is no vibration.

The latest update was 2 cans of seafoam ( 1 per tank) and vibrations got much less noticeable.

My mechanic said that it’s either carbon buildup issue or dirty gas tank/gas filter.

Car has 60k and set of a while with previous owner.