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!