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So that brass cap does, in fact, house the schrader valve. When I depressed the valve, however, no audible air or fuel was expressed (I did this with the key at position 2 but no crank).
This seems to imply an obstruction or issue with the low-pressure fuel line which can certainly cause misfires to cylinders 4 and 5 which are on the Bank 2 fuel rail.
When I unscrewed the nuts to the Bank 2 fuel rail, fuel flowed out.
I don't believe I damaged the fuel lines when I removed and re-installed. No visible kinks, dents or leaks.
Am I going down a rabbit hole? What is/are my next steps on this issue? (I purchased an OTC fuel pressure gauge from Amazon. Figured that might be necessary at some point.)
ETA: Since it is my day off (and I can not stand sitting still), I disassembled the fuel lines to inspect them carefully. There are some pressings into the tubing where the rubber clamps may have been over-tightened, but there are no violations of the aluminum tubing. When I force air through both the high pressure and low pressure lines, there is good return on the opposite end with no whistling to suggest leaks.
I do not (yet) have an air compressor but my neighbor does. My plan this evening is to blow air through the Bank 2 fuel rail to ensure there are no blockages that might be causing issues with the low pressure Shrader valve not expressing air or fuel. It is hard for me to imagine a significant issue as there had never been any issue with Bank 2 misfires until I reassembled the top half of the engine and I am exceedingly careful and precise.
A few years back another forum member @Westlotorn had a misfire on cyl 1, and after a lot of work/pain, he found a bit of a seal stuck at the exit of the fuel rail for cyl 1. It could be that a seal got bitten, a piece traveled through the fuel rail up to cyl 4 and 5? Basically what you have concluded: check the fuel is perfect clean throughout.
The LP fuel pump will only turn ON when engine is running or when actuated using Xentry/Launch.
It is not like other cars where when key is IGNITION ON-ENGINE OFF, LP fuel pump already running-priming.
Still that schrader valve would have to leak out fuel when you press it, because remaining fuel pressure from last engine running
will still exist, perhaps 2-3 BAR or so.
Try using your LAUCH for LP fuel pump actuation and also you can see the pressure, which is the LP fuel pump pressure at the fuel filter housing here
the pressure sensor is. If you use mechanical fuel pressure gauge to read fuel pressure at engine side fuel test port, it will be a bit less pressure.
There is head loss, usual. up to 1.1 BAR.
1) I used compressed air on the Bank 2 fuel rail. No debris, no obstructions. Buttoned her back up.
2) Took out the spark plugs in preparation of a cold compression test. Can someone walk me through the basic steps of the test on the M276. I've watched the FCP Euro video. I'm guessing I can just have someone depress the accelerator while cranking. Dumb question: Does the upper intake plenum need to be installed for this?
3) I received my boroscope yesterday. I did not rotate the engine but I took some basic photos of each cylinder, which I will post. Some of the piston heads looked uneven to me. Maybe I'm nuts...
Results of COLD Compression Test (using OTC Deluxe Compression Kit)
Cylinder 1 215 (rigid line)
Cylinder 2 190 (soft hose), 210 (rigid line) --> not a lot of room, required disassembly for rigid line to fit
Cylinder 3 215 (rigid line)
Cylinder 4 210 (rigid line)
Cylinder 5 195 (soft hose), 210 (rigid line) --> not a lot of room, required disassembly for rigid line to fit
Cylinder 6 195 (soft hose) --> no clearance for rigid line but likely same as cylinder 5
They all appear mostly similar. No large deviations to suggest valve or piston ring issue, especially given cylinder 2 was the issue given misfire code. Do I still need to do operating temp test or leak-down test based upon these numbers?
P.S. My adapter got stuck in cylinder 1 on first test. FML. What a pain to get it out. Luckily, I was able to thread it off with my index finger (very slowly).
In other news, I re-disassembled the fuel rails and blew them out with compressed air. Just fuel, no foreign particulate or broken o-rings/seals.
Re-installed the spark plugs and coils tonight. Hopefully, will re-install the plenum tomorrow AM before work and get to business on coding the fuel injectors (new plugs 3 and 5 were fouled with dry black soot so guessing fuel too rich) and resetting the codes.
Results of COLD Compression Test (using OTC Deluxe Compression Kit)
Cylinder 1 215 (rigid line)
Cylinder 2 190 (soft hose), 210 (rigid line) --> not a lot of room, required disassembly for rigid line to fit
Cylinder 3 215 (rigid line)
Cylinder 4 210 (rigid line)
Cylinder 5 195 (soft hose), 210 (rigid line) --> not a lot of room, required disassembly for rigid line to fit
Cylinder 6 195 (soft hose) --> no clearance for rigid line but likely same as cylinder 5
They all appear mostly similar. No large deviations to suggest valve or piston ring issue, especially given cylinder 2 was the issue given misfire code. Do I still need to do operating temp test or leak-down test based upon these numbers?
P.S. My adapter got stuck in cylinder 1 on first test. FML. What a pain to get it out. Luckily, I was able to thread it off with my index finger (very slowly).
In other news, I re-disassembled the fuel rails and blew them out with compressed air. Just fuel, no foreign particulate or broken o-rings/seals.
Re-installed the spark plugs and coils tonight. Hopefully, will re-install the plenum tomorrow AM before work and get to business on coding the fuel injectors (new plugs 3 and 5 were fouled with dry black soot so guessing fuel too rich) and resetting the codes.
Rigid vs soft and how long the hose does matter for compression value.
Also battery best to have battery maintainer ON, 25 - 30 amps is enough.
Warm engine has better compression than stone cold engine, oil help seal the rings.
210 PSi is 210/14.5 = 14.4 BAR , it is good.
Allowable difference between cylinder is 21.75 PSI. Yours all OK.
I don't think leak down test is required, unless you already have the tool.
This is what soft hose extra length can do to pressure value.
Note : I am using digital/electronic pressure transducer and not analog gauge and I do not use the check valve which
analog compression test must have in the hose kit. Thus, I can read actual pressure rise and fall and not only pressure peak.
I forgot to ask. Did you have any photo or took a closer look at how ur intake valves condition are like ? You opened up intake system completely and you can take a peek.
We GDI engines have dirty intake valves.
Your cylinder 2
My cylinder 1 - Intake valve 1 of 2
.My cyinder 1. Intake valve 2 of 2
The white vaves are exhaust valves. Black valves are the intake. Intake valves are almost always bigger than exhaust valves.
.
.
.View from intake valve INPUT side.
.
Manual hand cleaned....
My engine was ony 36,000KM or so that August 2022.
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You wrote in blue : Re-installed the spark plugs and coils tonight. Hopefully, will re-install the plenum tomorrow AM before work and get to business on coding the fuel injectors (new plugs 3 and 5 were fouled with dry black soot so guessing fuel too rich) and resetting the codes.
Dang, you did not code new injectors quantity compensation yet ?
.
Oky doky, I hope all run good with your engine. I want to hear the good news.
I posted a picture above of how bad mined looked (I will add it again here). I thought I went nuts, spending 30-45 min using a pick by hand per valve. Clearly, I just scratched the surface!
A good example of before (this cylinder was not so bad, actually)
Cylinder 1
Cylinder 2
Cylinder 3
Cylinder 4
Cylinder 5
Cylinder 6
Last edited by mdzukunft; Apr 11, 2025 at 12:53 PM.
This morning, I was able to code the fuel injectors, reset the DTCs and - voila - all but one code vanished. This time, it is a persistent Cylinder 5 misfire, which only came about after replacing all of the fuel injectors and failing to encode them prior to initial start-up (at that time, it was both a Cylinder 4 and 5 misfire). Vehicle runs smooth and quiet for 20 sec before the CEL blinks and engine starts being out of time.
Now... When I removed the spark plugs for the compression test, I noticed that the spark plugs for cylinders 3 and 5 were fouled with black soot (rich fuel ratio) which I attributed to the fuel injectors not being coded previously. I cleaned the plugs; clearly 3 was cleaned more thoroughly as its cylinder is not experiencing a misfire.
Before I left for work, I swapped that plug with an older (non-fouled) spark plug. When reassembling, the ignition coil boot separated from the coil pack. I put this back together. Cleared the DTC, fired up the engine and the code persists.
Plan: Going to recheck the fuel injector coding on that cylinder, bought a new spark plug and ignition coil.
I think I'm really close. Any other input would be appreciated.
So all these below are new, correct ? First, I bought and installed all new fuel injectors (3 had older part number and black Teflon seals, 3 with newer part number and white Teflon seals). I did not refresh the o-rings in the fuel rails. All COP/coils are new All spark plugs are new
BTW, why did you not refresh the entire injector seal kit , the o-ring to the fuel rail and the rest of the parts you get from the kit ?
.
.
If coding fuel quantity compensation for fuel injector #5 and still misfiring, start swapping the #5 COP/coil first with a say cylinder #4 COP.
If still misfiring, no choice but swap #5 injector to no 4 cylinder or no 6 cylinder, whichever easier for you and see if misfire follows new location.
If indeed injector is the root cause....where did you buy them from and what P/N are they ?
Had a dream last night that I reversed the ignition coils in relation to their cylinders. Guess what? I did! Re-coded but unfortunately the cylinder 5 misfire persists.
And my understanding is that the cylinders are numbered thus:
Bank 1 / Passenger:
1 --> 2 --> 3 (front to firewall)
Bank 2 / Driver:
4 --> 5 --> 6 (front to firewall)
Plan remains the same. New spark plug and ignition coil. Then maybe swap the new FI for one of the working old ones.
Had a dream last night that I reversed the ignition coils in relation to their cylinders. Guess what? I did! Re-coded but unfortunately the cylinder 5 misfire persists.
And my understanding is that the cylinders are numbered thus:
Bank 1 / Passenger:
1 --> 2 --> 3 (front to firewall)
Bank 2 / Driver:
4 --> 5 --> 6 (front to firewall)
Plan remains the same. New spark plug and ignition coil. Then maybe swap the new FI for one of the working old ones.
In blue is correct.
Igition coil need no coding, I think u meant injector ?? Yes.
1) My battery died. My NOCO equipment has - as yet - been unable to desulfate or revive it. Bought an Interstate H8 from Costco this AM. Installed.
2) The re-placed, older (but working) Bosch spark plug fouled on Cylinder 5 (dry black soot). Replaced with new Bosch plug, fouled (on one side) as well after one minute of engine idle. See pics below. Appears fuel-rich. This would indicate at least 3 options (correct me if I am incorrect): (1) faulty fuel injector, (2) leaking valve seals (but why all of a sudden) or (3) leaking valve cover around Cylinder 5's spark plug hole. I really hope it isn't 3...
3) New Bosch ignition coil went in with the new spark plug. No change in misfire.
Plan is to swap out Cylinder 5 fuel injector and re-test. (Just waiting for engine to cool down before I remove fuel lines.)
If that does not take care of the misfire, what is next?
1) My battery died. My NOCO equipment has - as yet - been unable to desulfate or revive it. Bought an Interstate H8 from Costco this AM. Installed.
2) The re-placed, older (but working) Bosch spark plug fouled on Cylinder 5 (dry black soot). Replaced with new Bosch plug, fouled (on one side) as well after one minute of engine idle. See pics below. Appears fuel-rich. This would indicate at least 3 options (correct me if I am incorrect): (1) faulty fuel injector, (2) leaking valve seals (but why all of a sudden) or (3) leaking valve cover around Cylinder 5's spark plug hole. I really hope it isn't 3...
3) New Bosch ignition coil went in with the new spark plug. No change in misfire.
Plan is to swap out Cylinder 5 fuel injector and re-test. (Just waiting for engine to cool down before I remove fuel lines.)
If that does not take care of the misfire, what is next?
If swapping injector #5 and misfire still exist in cylinder #5, you then need to trace the command and/or command wire of both injectors and coils from ECM.
Leaking valve seals I do not think so as you did not touch the intake valves.
Even if you tried to clean the carbon builld up at intake valves, I doubt you can poke and damage the seal which you can't access from intake port 2 small openings when you clean
the carbon build up.
Black soot at spark plug meant rich.
Rich can mean a few things.
01. Coils did not fire properly, either coils not healthy, plugs not healthy or the command ( power ) to coils did not arrive at coils ....which can be wiring issue or worse ECM driver for that coil went bad.
You have swapped other coils and new spark plug and no improvement.
02A. Injector leaking fuel or spraying too much fuel.
02B. If injector stuck close, you will have lean burn.
03. Too much fuel and/or not enough air = rich burn.
04A. There is a hydraulic lifter for each valve. That thing can collapse/leak and cause valve to open less. If intake valve hydraulic lifter say it collapsed, that can mean not enough opening/air
into cy #5 and cause RICH burn. This thing usually makes some sort of ticking noise if it is not healthy. This thing uses oil pressure to work.
04B. Hydraulic lifter co-worker is the rocker arm. That can also goe bad and make valve opening less. It will make noise when gone bad, usually.
ONE WORD OF CAUTION FOR OUR PIEZO INJECTOR
Do not ever ever un-plug a piezo GDI injector we use in M276/M278/M157 while engine is running. This can cause injector to stuck open.
Piezo based injector has no spring like oldie fuel injector, which will come home ( closed ) on spring power when power is removed.
The piezo crystal is the spring instead but it has unique properties and must be electrically triggered to expand or contract ( like a spring ).
What about a leaking valve cover around spark plug hole? Is that a possibility?
Is there any way to test the hydraulic lifter and rocker? I do not think there is an appreciable ticking noise but I'll have to record sound when engine is running and ask for an opinion from you all...
What about a leaking valve cover around spark plug hole? Is that a possibility?
Is there any way to test the hydraulic lifter and rocker? I do not think there is an appreciable ticking noise but I'll have to record sound when engine is running and ask for an opinion from you all...
Valve cover holds no compression. Its for oil not to seep out, that is all.
Attached how to inspect 01- hydraulic lifter and 02-rocker arm, which MB calls 01 as Compensating Element and 02 as Cam Follower.
But its no fun to do this, as your engine is already buttoned up.
Finally got my air compressor set up and my new OTC Leak Down testing kit.
Results of Cold Test:
================
Cylinder 1. 0%
Cylinder 2. 0%
Cylinder 3. 0%
Cylinder 4. 0%
Cylinder 5. 10%, air hissing through oil cap --> piston ring failure
Cylinder 6. 0%
Buttoning back up the engine to repeat with warm engine to see if piston rings "seal."
My understanding is that 10% is usually upper limit of normal for age. Is Mercedes different? Is this the most likely culprit for my ongoing misfire?
If it is, I guess I'm looking a new car. I don't have loads of time or money to throw at an engine with 180k miles. I will say this: If anyone is in the San Francisco Bay Area and is willing to tackle this with me as a fun project, I'm all down. I'd still like to increase my wrenching knowledge and (god willing) get this thing running again.
Results of Warm/Hot Test:
====================
Cylinder 5. 0%
All misfires resolved. Cylinder 5 spark plug no longer fouling.
DIAGNOSIS: New (old stock) Bosch fuel injector stuck in open position leading to fuel rich state. Resolved by replacing with previous, functional fuel injector.
Doing an LIQUI MOLY engine flush for 100 mi and then will perform oil service and replace Mobil-1 High Mileage with Motul.
PRAISE THE LORD. Happy Easter!
Last edited by mdzukunft; Apr 17, 2025 at 04:37 PM.
Finally got my air compressor set up and my new OTC Leak Down testing kit.
Results of Cold Test:
================
Cylinder 1. 0%
Cylinder 2. 0%
Cylinder 3. 0%
Cylinder 4. 0%
Cylinder 5. 10%, air hissing through oil cap --> piston ring failure
Cylinder 6. 0%
Buttoning back up the engine to repeat with warm engine to see if piston rings "seal."
My understanding is that 10% is usually upper limit of normal for age. Is Mercedes different? Is this the most likely culprit for my ongoing misfire?<<--- it will eat oil , not causing misfire if this low a leak.
If it is, I guess I'm looking a new car. I don't have loads of time or money to throw at an engine with 180k miles. I will say this: If anyone is in the San Francisco Bay Area and is willing to tackle this with me as a fun project, I'm all down. I'd still like to increase my wrenching knowledge and (god willing) get this thing running again.
Results of Warm/Hot Test:
==================== Cylinder 5. 0%<<<<<<---------sweet !!!
All misfires resolved. Cylinder 5 spark plug no longer fouling.
DIAGNOSIS: New (old stock) Bosch fuel injector stuck in open position leading to fuel rich state. Resolved by replacing with previous, functional fuel injector.
Doing an LIQUI MOLY engine flush for 100 mi and then will perform oil service and replace Mobil-1 High Mileage with Motul.
PRAISE THE LORD. Happy Easter!
DIAGNOSIS: New (old stock) Bosch fuel injector stuck in open position leading to fuel rich state. Resolved by replacing with previous, functional fuel injector
Dang 1 out of 6 is 16.7% bad luck ratio. Quite high.
What most important is, you got to the bottom of the culprit.
Happy Easter and enjoy ur W212 and thank you for the speedy update.
Drove about 75 miles. No issues. Quiet, smooth acceleration. Drives like when I purchased it back in 2018.
Cleaned up the remaining oil leak from the bottom. Changed oil to Motul after the flush. Just need to button up the bottom fairings and I think - in the words of the Jessica Fletcher (Angela Lansbury) - "that's all she wrote."
A big thanks to one and all! I am so grateful to everyone who helped during this process, and it is my hope this thread helps others in a similar position.
https://mbworld.org/forums/e-class-w...eaning-p2.html
