M20B25 - Identifying engine for ECU

[andrew_baran]

Hey guys,

I bought an M20B25 from a zoner recently, to replace my ageing M20B20 in my Touring.

The engine comes complete with all the loom and bits, but does not have an ECU on the end of it. It is my understanding that there are various models of ECU for the B25, and I wondered if there was a way of identifying which ECU was correct for my car, so I can purchase the right one?

Cheers,
Andy

[maggspower]

I'm assuming that the m20b25 engine and loom are facelift items, the correct ECU should end in 173 on the sticker on it you could use a 380 (auto) and there may be others. What I would do is use your m20b20 ECU and get a b25 chip for it, may be off someone on here, fastfeel I think his name is

[andrew_baran]

I don't know whether the engine is facelift or not. Is there a code or specific on the engine that could identify which one it is, to get me the right ECU?

Would chipping my ECU for a 325 only work if both engines are post-facelift?

[maggspower]

Have a look at the big plug that the ECU goes on, does it have two or three rows for the pins on the ECU?

[Speedtouch]

Early 325i engines had 25 6E engine codes stamped in the block, to the right of the inlet side engine mount, just above the block:sump interface.

Later ones from mid 1988-onwards were 25 6K.

However, just to confuse matters, some 25 6E blocks were fitted with the later Motronic 1.3 setup, at around F-reg, up to around June 1988.

As above, later Motronic 1.3 ECUs are 173 or 380 and had 55-pin 3-row connectors (earlier ones were 35-pin 2 row), however, you can use any of the later six-pot ECUs (172/173/179/380/381) as they are all identical apart from the EPROM chip.

[andrew_baran]

OK, so could I potentially buy a 325 mild chip from yourselves, and fit that to my existing M20B20 ECU, if only the EPROM is different?

[Speedtouch]

Indeed you could, yes.

It's not advisable to run an M20B25 using an M20B20 EPROM, as both ignition timing and fuelling will be different.

For instance, if the fuelling is too lean, you could risk burning out the exhaust valves, or worse, holing a piston.

[andrew_baran]

Well in that case i'll source a proper B25 ECU and then get me a chip!

[Speedtouch]

Your existing M20B20 ECU will be just fine if it is a 172 or 381 type - just change the chip.

[andrew_baran]

Right. Misunderstood there.

I'll double check everything, and PM you shortly.

[andrew_baran]

So it turns out the engine was a 6E early engine, with a 3 row 55 pin connector. Are there any additional considerations I should take when fitting this to my post facelift car?

[Speedtouch]

No, it has the 3-row Motronic 1.3 connector, so you should be fine. Some of the early facelifts on the F-reg had the 6E engine along with M1.3, so you got the best of both worlds - old skool high compression meaty engine with the later and better Motronic.

[andrew_baran]

Oooh nice. That's a bit of good fortune! I'd better hurry up and get the damn thing in the car!