355 Motronic 5.2 help identifying bulging component?

My ECU has a bulging component (green cylinder marked RO51K or R051K). Does anyone know what this is? It is along the right side of the picture. There's two - only the upper one is bulging. Both ohm out at ~0 ohms, but I suspect (hope?) the upper one is misbehaving at operating current.

Symptoms:

My car cranks but doesn't start (and run) reliably, and my OBD reader no longer connects to the ECU (immobilizer disarmed, ignition on... my reader used to connect just fine under these conditions). Fuel pressure is good, but no spark or fuel injector signals. I was hoping it was the crankshaft position sensor - its wiring is intact and I changed the sensor which allowed the car to run for a few minutes once, but now it's back to not running - and OBD still doesn't work.

Thanks very much
Image Unavailable, Please Login

 

Thanks for taking a look!

BTS132
GDS
XG23

Bulging: the lower R051K device is a straight cylinder while the upper one has a larger diameter in the middle.
Image Unavailable, Please Login Image Unavailable, Please Login

 

Thank you very much for weighing in. I will continue the hunt!!!

 

Thanks for all the help.

I checked the crankshaft position sensor gap - it was 0.53mm with a shim installed, which is within spec (WSM calls for 0.35-0.90mm). I removed the shim, which adjusted the clearance to 0.50mm. No change.

I now suspect the immobilizer. I have three symptoms:

1) fob intermittently works
2) OBD2 won't connect
3) cranks but often won't start - or will start and then dies shortly after

It seems like a defective immobilizer could be the root of all these problems.

When I removed the immobilizer trim cover, I discovered three wire taps on the immobilizer wiring harness. They led to an aftermarket alarm / shock sensor (that was partially disconnected). Ugh. I cut out the wire taps and repaired the wiring. That didn't fix it, although it seems to have slightly improved - the fob seems more reliable and the engine will briefly start more often. Tapping the immobilizer case seems to make a difference when the fob is having a bout of unresponsivemess.

The aftermarket alarm worked by shorting a white/black wire in the immobilizer harness to ground. *sigh* Does anyone know what signal this white/black wire carries?

The other wire taps were to the yellow and red/green wires. They fed relay inputs that would conditionally short the white/black wire to ground.

Immofixer.com says common failure modes of the immobilizer are cracked solder joints and failed relays. I'll take a close look at these - any other suggestions?

 

You all are amazing.

It was the white wire with black stripe. It is connected the called out pin (see attached picture) on plug E. My 3 plugs are marked E, F, and D. F is the antenna / middle plug and D is the plug with heavier gauge pins.

I measured 21 ohms to ground on the white with black stripe wire (which was the one that was wire-tapped and conditionally shorted to ground by the aftermarket alarm system). I measured ~1 ohm to ground on the black with white stripe wire. Should both these wires be hard grounds?

There were two relays installed. The coil inputs to relay 1 were immobilizer yellow (ignition key power) and ground. The normally connected outputs were used, with one leading to the aftermarket alarm and the other output leading to a relay 2 coil input.

The other coil input on relay 2 was connected to immobilizer red/green (direct battery power). Someone had already disconnected this other input when I discovered it, thus disabling the alarm. The normally open outputs were used on relay 2, connected to ground and immobilizer white with black stripe.

It seems the alarm forced the white with black stripe wire to ground when the ignition is off AND the alarm activated. With the ignition on, the alarm does nothing.

If this doesn't end up being something trivial, I'll be in touch with Michael / immofixer for sure!
Image Unavailable, Please Login

 

Some updates here.

Michael at Immofixer helped me find a leaky capacitor on the immobilizer interface unit. He was super helpful!

Earlier this spring, my problem actually went away by itself. I was able to enjoy most of the summer until it returned again (car either would either (1) crank without starting or (2) start for a split second and then die - fuel pressure is good, but no spark or fuel injection). Then a few weeks later, with no action on my part, the car behaved normally again.

Michael / Immofixer noted that a failed immobilizer interface unit has the same symptoms. However, the symptom is normally permanent instead of intermittent.

With that guidance, I opened up my immobilizer interface unit and lo and behold found a leaking electrolytic capacitor. One of the circuit board traces underneath the capacitor was darkened. I desoldered the capacitor and tested all nearby traces - fortunately all were good, including the darkened trace. I cleaned up the leaked fluid, soldered in a new capacitor and so far so good.

I unfortunately pulled out one of the leads on the original capacitor while removing it, so I couldn't confirm its performance was degraded. But I'm cautiously optimistic that it was the cause of my problems.

Time will tell.

Many thanks for all your help