B230ET stopping.

[rogerthechorister]

The symptoms are identical to when the CPS was failing on one of my Sport Editions, and I was convinced that the OK running at over 40 mph was related to lower temperatures due to higher airflow - but of course in that case it was the temperature on the bellhousing and at the back of the engine that mattered.

There will be some frontal airflow from round the headlamp, the holes in the front panel next to the radiator, sideways from the radiator aperture, and possible (depends on the inner wing panel that I have not looked at this week) from the hole in the inner wing that some people modify to get a cold air intake into the airbox.

[Laird Scooby]

Originally Posted by rogerthechorister

She is dumb Motronic, so has an air flow meter (spring loaded flap) rather than an air mass meter.

Original L-Jetronic then. Had the same system on a few Datsuns i owned when i was a young pup and also a resto project that never happened. Pretty bomb-proof because of its simplicity although in the late 70s (when the Datsuns were made) it was real cutting edge stuff and considered very complex!

[aardvarkash10]

Hey Dave - In the spirit of friendly discussion and to agitate my grey cells a a little, here's my tuppence in response - sorry for the threadjacking everyone else!

Originally Posted by Laird Scooby

Airflow??? On a component buried under the air cleaner box behind the headlamp in a place where airflow is probably at a minimum?

Yup. Any airflow will reduce the under-bonnet temperature overall.

Originally Posted by Laird Scooby

All coils will have a resonant frequency regardless of whether they are electronically switched or not. What i was referring to is this :

https://en.wikipedia.org/wiki/Resona...ctive_coupling

This is of course correct. The issue is whether its relevant. Its not.

Originally Posted by Laird Scooby

This still enhances the efficiency of the coil to produce a better spark. If the module is weakened due to being close to an overheat condition, the spark intensity will be reduced.

Perhaps, perhaps not. Since the switching is effectively digital, the output transistor is either on or off. Sans a temperature affected junction or component in the output section and resulting loss of current, the output voltage of the coil will be unaffected.

Originally Posted by Laird Scooby

A much higher spark voltage is needed to ignite a weak mixture so at a cruising speed where the revs coincide with the resonant frequency of the coil, the output will be that much better even with a weak input. Hence above 40mph where the coil is in it's most efficient band the misfiring is minimised.

This makes two assumptions - the mixture at cruise is weak/lean (probably not - it should actually be near ideal 14.7:1) and that there is a resonant frequency issue with the coil at a specific road speed - there isn't. In any case, the coil is probably capable of around a 25kv output. At stoichiometric ratios, the ignition voltage required is typically between 5 and 8kv. Lean, this may double (at worst). All this from personal experience with lots of different vehicles, oscilloscopes, and problems to be fixed.

Originally Posted by Laird Scooby

I'm not saying mine is the definitive answer and wholly agree with your normal caveats about proper, methodical fault finding but even proper methodical diagnosis needs a starting point. In this case all the evidence is pointing towards the module being the most likely cause making it a good place to start.

Totally agree.

[Laird Scooby]

Originally Posted by aardvarkash10

Hey Dave - In the spirit of friendly discussion and to agitate my grey cells a a little, here's my tuppence in response - sorry for the threadjacking everyone else!



Yup. Any airflow will reduce the under-bonnet temperature overall.

My point was there will be very little airflow over the ignition module so won't provide much, if any cooling effect. Also we've been having a heatwave here recently (30+++C) so it seems very much temperature related.



This is of course correct. The issue is whether its relevant. Its not.

Funny you should mention it but it is relevant - the fault disappeared above 40mph when the coil is most likely to be in the resonant "window"



Perhaps, perhaps not. Since the switching is effectively digital, the output transistor is either on or off. Sans a temperature affected junction or component in the output section and resulting loss of current, the output voltage of the coil will be unaffected.

This was my point in the first section - elevated temperatures are causing the problem

This makes two assumptions - the mixture at cruise is weak/lean (probably not - it should actually be near ideal 14.7:1) and that there is a resonant frequency issue with the coil at a specific road speed - there isn't. In any case, the coil is probably capable of around a 25kv output. At stoichiometric ratios, the ignition voltage required is typically between 5 and 8kv. Lean, this may double (at worst). All this from personal experience with lots of different vehicles, oscilloscopes, and problems to be fixed.

Resonant frequency - see above. Cruise mixture - almost certainly higher than stoich, between 16:1 and maybe even 22:1 - very lean in other words!
Part of the "raison d'etre" of manufacturers using the L-Jetronic was its ability to be retro-fitted easily to enable more engines to be "lean-burn" which was all the rage here in the mid-late 80s before catalytic converters became mandatory in 1992

https://en.wikipedia.org/wiki/Lean-burn

If you have a read of that you'll see the point i'm getting at.

Totally agree.

That's better - got to the end of the "quote" bits! Haven't sussed how to do individual quotes like that on this forum yet, hence the italic typing!
We've both arrived by slightly different routes at the same probably conclusion for this, neither is right or wrong in the methods and/or clues used. We're just interpreting some things in a different way and from a different perspective but still seeing the same picture.

For now though, i'm going to put my brain cell back in its jar and hit the pit

[aardvarkash10]

I hate discussions that avoid referencing background info, so here we go.

Note especially pages 7 and 8 where typical fuel mixtures in spark ignition engines are outlined.

L-jetronic in its various formats is designed to operate in a range that minimises exhaust emissions without compromising performance. Lean mixtures increase NOx output, rich mixtures increase CO and HC. L-jet avoids both situations by holding the mixture as close to lambda as possible ESPECIALLY at cruise - the only time the lambda (O2) sensor is included in the fuel calculations.

We still agree on the ignition temperature thing btw!

Sleep well

[baggy798]

Motronic has no lambda sensor, so none of that applies. It runs quite rich at all times.

Motronic also has no hall effect sensor on the dizzy either. All it has is the 2 crank sensors reading the RPM and TDC from the flywheel. They are both the same and the part number is 0261210003. They were also used on some Porsche 928 and 944 models. Thankfully they are readily available on ebay:

http://www.ebay.co.uk/sch/i.html?_fr...10003&_sacat=0

To test them unplug from the harness and using a multimeter you should get a reading of 1000 ohms between two of the three connectors if it is OK, otherwise it is stuffed.

[Laird Scooby]

Originally Posted by aardvarkash10

I hate discussions that avoid referencing background info, so here we go.

Note especially pages 7 and 8 where typical fuel mixtures in spark ignition engines are outlined.

L-jetronic in its various formats is designed to operate in a range that minimises exhaust emissions without compromising performance. Lean mixtures increase NOx output, rich mixtures increase CO and HC. L-jet avoids both situations by holding the mixture as close to lambda as possible ESPECIALLY at cruise - the only time the lambda (O2) sensor is included in the fuel calculations.

We still agree on the ignition temperature thing btw!

Sleep well

Very useful - thanks! Page 4, paragraph 3 references the "lean-burn" of 15~18:1 as the minimum fuel consumption for "contemporary engines" and cruise is the condition where you want minimum fuel consumption. Also i'm 99% sure there isn't a Lambda sensor on this particular car so i would say it's fair to assume it relies on a map that takes into account the air-flap movement Vs engine speed to calculate load and therefore mixture requirements.
Also the date of the publication makes me wonder if it's taking everything into account. The English translation was 1994 by which time L-Jetronic had "morphed" into Motronic in most applications although there were still a few around.
Experience has taught me that manufacturers such as Bosch only release information like this to the general public when they already have a much newer product selling well in the market place for the simple reason they no longer want the monopoly on repairing it. A cynical point of view perhaps but in the main, true.

Slept ok thanks - your turn to sleep now! Have a good 'un

[rogerthechorister]

Please do not apologise for threadjacking - I see no threadjacking - all of this is very relevant to getting the grey lady running properly again!

[aardvarkash10]

No advice from here on - we have an America's Cup to win this weekend as well as some shabby provincial ex-european rugby team to thrash, so I will be somewhat distracted...

hehehe...

[Laird Scooby]

As long as freak-weather conditions don't occur or you could be playing rugby on the deck of the racing yachts!