I recently bought a 2008 R320 with the OM642 engine 240k km/ 150k miles. I bought it cheap as it had a bit of a miss that I assumed was just a flaky injector.

The PO had taken the car to the dealer and they recommended doing a compression test and if that was OK, to replace injector #5.

I took the air intake off and first thing I noticed was damage on the compressor wheel. Not sure if it was related to the miss or not, I removed injector 5 and had it cleaned/flowed/tested. It came back OK. I used my borescope and found no evidence of damage in cylinder #5.

So far I’ve tested compression on the front 4 cylinders and they all come in at around 350 psi (24 bar) using a Chinese compression kit. The compression tester adapter isn’t an exact replicate of the glow plug so if the readings are low, it could be explained by that.

When loading the engine using the brakes in drive and rev to 1500 RPM you can feel the miss come and go. Full throttle the miss seems quite regular. The area around cyl 2/3 seems a bit noisy. Not sure if it’s mechanical, an injector, or some minor injector seal issues. No CEL (check engine light). No pending codes. Which I find strange given how noticeable the miss is.

No smoke, though the DPF may be making issues?

Fuel economy is poor. I did one 20 minute drive with the EGR valve unplugged and fuel economy seemed better (~9.5L/100 KM / 25 MPG). The past 500 km was 14.5L/100km (16 mpg) but that was under previous ownership, so who knows under what conditions. I thought it would set a CEL but didn’t. Engine would rev high, but no boost.

While working on it, I’ve also noticed the left valve cover has been resealed. And around cylinder 3 there are a bunch of aluminum shavings that weren’t cleaned up. Like someone fixed a stripped thread and didn’t clean up afterwards.

The plan is to finish compression testing the rear two cylinders. Assuming they check out OK, what’s next??

Remove injectors one by one and borescope the remaining cylinders? Send the remaining ones out for cleaning? Or swap in a brand new one, one by one, to maybe identify a bad injector?

Remove valve cover and have a look?

I have more experience with gasoline engines. I’d describe it as a plug with too large a gap, or a leaky ignition wire. It’s not a ‘hard’ miss if that makes any sense.

Unplugging an injector will set the CEL, so the ECU can detect that. But could it detect a connection with say poor connectivity? Or a weak driver?

Any troubleshooting tips would be greatly appreciated!

Check the injectors have had respective code numbers entered and correctly for each cylinder (front of vehicle .. left side front is number 1 and behind it 2 and then 3…right hand side is 4 at front then 5 and 6 )
On my om642 engine Jeep Grand Cherokee removing the original worn injectors and replacing with new , the ecu defaulted the original codes to AAAAEE and it ran awful and misfired / jumped about when idling until the new were coded in.
Get a good code reader and look at the live data and see what amount of fuel the injectors are putting in , its measured in millimetres squared not time duration.
My Jeep is low geared (120mph max in 5th at redline) it averages 25uk mpg , the same engine in an e class will be giving over 32-35 mpg possibly more.
Do not unplug an injector to cut out a cylinder as above it will need recoding , you can purchase blank plugs that thread on fuel rail injector line outlet to cap it off and deny an injector fuel and ensure no drop in rail pressure.
Do run the return line into a can just to check no air coming out of system
Valve covers are specific fit / sized to fit so you cannot just take one from another engine and put it on without checking clearances as cams run in them

https://youtu.be/w90fMas4Jrk

Got a new scanner (Thinkcar Pro). Took it for a highway cruise today. Ran great even though I only erased a soft transmission code (low voltage from a dead battery a month ago). No miss but still has a slight vibration (weak cylinder?) when brake torqued to 1200 RPM.

After about 20 minutes, headed home and it started running rough. Quite a noticeable miss on one hole.
Parked in the driveway, and one cylinder started knocking quite loudly. I popped the hood and by the time I got out, the cylinder stopped knocking. It was loud enough that I should have been able to identify which one was the culprit. I jiggled some injector wires and couldn't reproduce it. Definite leaky injector? Or could it also be electrical?

If the scanner had the ability to deactivate cylinders then I would have identified the offending cylinder. It can code injectors so I recoded them to the existing markings, resetting the correction values. Maybe it will identify something. The previous corrections:

I managed to borrow a Xentry and run some tests.

The live injector corrections show injector #2 typically at 4 to 5 mm3/HUB, with a max of 5.82 after a 20 minute drive. Limit is 5. Injector 4 showed similar max correction (5.77) but it’s less of an outlier when driving around.
A video showing the roughish idle and testing: https://********.com/photos/0409_guNiCQhXhU82JVyRRA-Q

Are there any other diagnostics that I should do before just replacing it? Still no codes.

Test of compression was 158 +/- 1.
The “Quantity correction values for smooth running control over several pressure levels has a limit of +3 mm3/HUB. I ran it several times. Warm engine, not much to report. I tried once putting it in drive and managed to get values of 4-5 for injectors 2 and 4 with the additional load.

With a hot engine (97C), I was eventually able to get cyl #2 to give results around 2.0 for all pressures (600-1600 bar). Cyl 3 registered 3.9 at 1600 bar which is a ‘failure’. It was running pretty rough at this point.