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