Hi All,

I could really do with some help troubleshooting a suspected ignition and/or timing issue with my 2003 W211 E320.
To give you the back story, up until now its been a great reliable car and has been well looked after. I bought it with only 27k miles on the clock back in 2017, and we're almost at 112k now, so we've done our fair share of trekking together!

Last week on the way to work, on a completely normal drive with no issues, while cruising on the motorway at 70mph, the tachometer suddenly started bouncing up and down and then seconds later the engine died.
I managed to pull over and once stopped, cycled the key. The engine turned over just fine, but didn't light off. Nothing, not a few hits and pops, nothing at all. I tried this a few more times with the same result. OBD diagnostics came back with a Pending P0341 - camshaft position sensor "A" circuit range/performance, and Stored P0335 - crankshaft position sensor "A" circuit.
I kept trying to troubleshoot at the side of the road (clearing the codes, idiot checks, diagnostics) but with no luck, it just kept cranking over but with no hits. Eventually the (old and weak) battery ran low and I had the car recovered back home.

After work I started to troubleshoot, threw a newer and charged battery on, and it was pretty obvious from the raw fuel smell out the exhaust, and complete lack of it lighting off, that there was no spark. Of course I checked under the hood, all the coil packs, plug wires, wiring, relays etc looked perfectly fine. Nothing burnt out or loose. Then I found that in the engine bay fuse box, next to the ECU, that fuse 58 (15A - Cylinder 1-8 ignition coils) had blown, which would explain the lack of spark, but not why it had blown.

The next morning I put in a new fuse, and it started up first time! Being sceptical of this actually being the root of the issue, I first drove the half mile to the local shop. No problems. After buying a couple of bits in the shop, I got back in the car and made the brave decision to try to drive into work. It started up first time again, but just 3 miles later, the engine died on me again. I pulled over and expected fuse 58 to be blown again, but nope, this time it wasn't! Same result though, it would crank over, but no spark. I tried this a good few times, but with no luck. ODB diagnostics had the Stored P0335 - crankshaft position sensor "A" circuit code showing again, but not the camshaft sensor this time. I tried clearing it, but nope, no spark. I had my wife tow me home, and just as we were getting to the house I thought I'd try one more time, and it started up first time. On the driveway, I went to park it up after unhitching, and just as I put it in drive it died again and wouldn't spark.

I called my go-to independent MB expert and he suggested that with the codes and the pattern that it only died when the engine was hot, that the CPS was a good place to start. I then set about changing out both the camshaft sensor (easy) and crankshaft sensor (easy, but a real pain to get to), after which, it started up first time! I took it on a test drive of about 17.5mi, including sitting idling for a while which let the engine come right up to 90c, in case it was a temperature related issue. I made a few stops on the way round the route, and at each one I'd check the ODB diagnostics, to find no codes. It was driving just fine, however, when I would turn the engine off, wait a bit, and try to restart it, it cranked for an extended period of time (oh no!) and then backfired down the exhaust a couple of times, and did not start. On the next cycle of the key it would fire right up and run just fine. The ECU is not throwing any codes now.

I've since taken the car to and from work twice (2x 51mi round trips) and it's completed them just fine and without shutting off on me. The first day it started up the 1st time, and drove in flawlessly. On the second day, it backfired (from ambient 20c) and didn't start when starting up for the first time for the day, but then started and ran fine after the second key cycle. On the way in on the second day, I stopped at the services about half way to work, and when I came back to the car 5 minutes or so later, it backfired and wouldn't start for three key cycles, then lit off and ran fine on the 4th attempt. On the way home that evening it started up the first time.

I'm completely stumped as to what is causing this issue. The ECU still is not giving any codes, it's not down on power at all, and the fuel economy is as good as ever. I can' think of why the timing would be so far out that it can't start on one key cycle, and fires down the exhaust, but can then start up just fine and run smoothly on the very next try. If anyone has any ideas of what's going on, or any suggestions of things to try, please do let me know!

 

this sounds like your ECU module is compromised by... :
leaking capacitors
oil in the harness
water in the harness

You did good freshening up your CKP + CPS

Your coil fuse got burned when ECU energized one or more coil with solid DC Voltage. In DC voltage the coils act like a short circuit.

 

Thanks for the input! I have one bit of the loom that I’m a bit suspect of and will have a look at, and then the connectors to the ECU, and then inside the ECU itself.

Are there many electrolytic capacitors in there? I didn’t think there were, but happy to be told otherwise.

I’ve seen a good few ads for ECU testing, repair, and cloning services, so might give that a go.