A question for the Motronic gurus

[martinclan]

In a further attempt to diagnose the missfire I have on my M20 engined Marlin I would like to understand more about the operation of the Motronic 1.3 ECU.

Could someone tell me which of the sensors are essential for the engine to run and which others are there to tweak the operation according to conditions?

Crank Position Sensor - Essential
Air flow meter - Essential
Temperature sensor - non essential
Oxygen sensor - non essential
Throttle body fully closed switch - ??????
Throttle body fully open switch - ??????
Sensor on cyl 6 spark plug lead - ?????? (not really sure what this does anyway?)

Cheers, Robin

[BenHar]

Throttle body fully closed switch - Tells the ECU to switch to tickover control. You won't get a decent tickover if this is not working or not adjusted properly.

Sensor on cyl 6 spark plug lead - not essential. Tells the ecu to fire the injectors three at a time instead of six at a time.

Ben

[Grrrmachine]

Put Engine Management into the Wiki, and all is explained.

[HairyScreech]

Hi Robin.

Someone built a sportster on here a while back, from what I remember it takes the E30 rear sus and engine/drive train complete right?

All the engine wiring is pretty much identical to the E30.

Regarding the ECU, while not 100% in tune with the details of the operation of the 1.3 I can tell you this:

CPS is critical, this is unlikely to be your issue but is easily checked if you have a spare one, Where in the country are you? I have a sensor that I extended the lead on to use as a diagnostic sensor, can be fitted and the cable looped over the rocker cover in about 2 minutes.

Air flow meter is how the ECU senses load, the ECU is relying on the AFM to get the timing and fuelling right at partial load.

Air and Coolant temp sensors - these more or less act as a fuel and timing adjustment to the base map, If these are dead it will cause over or under fuelling at nearly all engine temps and loads bar full throttle.

Oxygen sensor only functions at partial load and tries to trim partial load fuelling to stoichiometric.

Throttle switch has 3 function - closed for idle control (should click as it closes) Partial throttle and full load, full load is at about 80% throttle or more.
It is a very simple TPS and is not variable, its only really on or off so either works or it doesn't.

The inductive sensor on the plug lead is there to act like a cam sensor.
The dizzy is driven directly from the camshaft so detecting the high frequency pulse done HT lead 6 tells the ECU where in the cycle the engine is.
Not sure what the ECU does with this though.

The motronic 1.3 is really bloody simple in its function, not much more than a high powered Casio calculator. That said it's simplicity does make it reliable provided the inputs and the mechanicals of the engine are ok.

[daimlerman]

Is your misfire one cylinder or all six?

As the engine sensors will not isolate one plug,a single cylinder miss could be duff plug/HT lead/sticky valve/piston ring etc....compression test will tell us more,of course!

Missing on all six is more likely to be ECU related in some way.I guess that the obvious will have been checked,dizzy(for tracking)rotor arm and so on!

[HairyScreech]

http://www.e30zone.net/modules.php?name ... c&t=261365

There's a more detailed thread on this problem here Malc.

[jimbom30cab]

^^ proper engineer right there !

[martinclan]

The HT lead sensor tells the ECU which bank of 3 injectors to fire, if it is missing, the ECU will fire all 6 instead of 3. The sensor is not essential to the running of the engine.

What spark plugs are you running? The best/most reliable in the m20b25 motor using the motronic 1.3 ECU are NGK BP6ES.

Back in the days before fancy technology took over, when diagnosing my 2stroke racing bike motors for issues with fuelling/mixture/running, I would take the bike for a run, get it to operating temp, then run it in the particular rev range I wanted to check, for a distance of at least 1km, then simultaneously hit the kill switch and put it into neutral and stop as quickly as possible. I would then quickly remove the spark plug/s and read them to determine fuelling mix, timing, overheating, miss-firing etc. This never failed to diagnose and sort out any issues. (I would also then trailer/tow/push the bike home and remove the cylinder head and check the squish readings etc, but that is another story..)

This could be used here to see what is happening with yours when it is miss-firing badly. Just run it for a good distance with it banging and popping away, then kill engine/slip into neutral/stop and remove all plugs and see what they tell you. If you are skilled in reading spark plugs, you will quickly see what issues are present like lack of fuel/wrong mixture/lack of spark on certain cylinders/intermittent spark/ timing/detonation etc. The spark plug is the soul of the engine, it tells all...

If you are not skilled in reading spark plugs, still do the above and take good close up pics and post them on here. I am sure that there are others, apart from me, who can help read them for you and help you get closer to solving your issue seeing that other methods dont seem to be helping.

OK cool. Wish I had known of this idea before as last time it conviniently (only just) made it back onto my drive.

I am going to try next time it occours. I'll post some pictures as although I know some basic spark plug signs I certainly don't have the knowledge to diagnose a problem like this.

Cheers, Robin

PS - last time I checked the plugs they were text book perfect - but that was of course after a short run when the problem didn't occur....

[HairyScreech]

That's the one, the 6th lead signal IDs the cylinder firing for the bank or batch fire.

So not really something worth worrying about and unlikely to cause your issue, bank or batch fire will make little difference over all.

[Speedtouch]

I would check the resistance of the HT leads first; all should read around 10 kilohms or so.

Then, I would do a compression test to make sure all six cylinders have roughly equal readings of around 180 psi; any significant variance indicates either failed head gasket or valves/piston rings not sealing properly.

Also, check all engine and ECU earths - particularly important on a kit car.

[HairyScreech]

^^ as above, on engine swaps or specials I always try to run an earth to the body and one direct back to the battery. The second earth back to the battery will not hurt but can save a lot of pain hunting bad earths.

[martinclan]

What spark plugs are you running? The best/most reliable in the m20b25 motor using the motronic 1.3 ECU are NGK BP6ES.

Back in the days before fancy technology took over, when diagnosing my 2stroke racing bike motors for issues with fuelling/mixture/running, I would take the bike for a run, get it to operating temp, then run it in the particular rev range I wanted to check, for a distance of at least 1km, then simultaneously hit the kill switch and put it into neutral and stop as quickly as possible. I would then quickly remove the spark plug/s and read them to determine fuelling mix, timing, overheating, miss-firing etc. This never failed to diagnose and sort out any issues. (I would also then trailer/tow/push the bike home and remove the cylinder head and check the squish readings etc, but that is another story..)

This could be used here to see what is happening with yours when it is miss-firing badly. Just run it for a good distance with it banging and popping away, then kill engine/slip into neutral/stop and remove all plugs and see what they tell you. If you are skilled in reading spark plugs, you will quickly see what issues are present like lack of fuel/wrong mixture/lack of spark on certain cylinders/intermittent spark/ timing/detonation etc. The spark plug is the soul of the engine, it tells all...

If you are not skilled in reading spark plugs, still do the above and take good close up pics and post them on here. I am sure that there are others, apart from me, who can help read them for you and help you get closer to solving your issue seeing that other methods dont seem to be helping.

Picture of spark plugs 1 to 4. (5 and 6 difficult to remove as they are obscured by brake servo!) Taken just after a bigtime session of missfiring - only just mde it home....

DSCF2034 by Robin Martin, on Flickr

[BenHar]

Bosch, rubbish.

Replace them with NGKs.

Ben

[HairyScreech]

Yep, first of try a set of NGK BPR6ES.

Bit pink on the plugs there, you don't have leaded fuel or octane booster in the tank do you?

[Brianmoooore]

Haven't you just discovered an incorrect (2.5 bar) FPR fitted to your car?