Misfiring when warm - why?
 

I stuck new coil packs and plugs in the car a few months back and everything was running fine. when I fired it up on the driveway.

I haven't been using it much due to various reasons but took it out for a longer run and suddenly, after about 15-10 minutes it started to lose power and struggle to rev as if the fuel mix was wrong.

I suspected a leak in the air intake as that is bodged together to fix a 2.5 air intake manifold to a 2.8 engine and sure enough I found a leak. I have now fixed this and brushing soapy water reveals to obvious leaks anymore.

Each test drive show the car is perfect, revving smoothly and pulling strongly until suddenly, like a switch is flicked, once it warms through the splutter and power loss happens. Park up for a while to cool down and it is fine. Warm up again and it happens.

MAF broken? Something else?

Help please as it has MASSIVELY failed the MOT on emissions only.

If it's emissions, that sounds like the MAF. I diagnosed a duff one today by swapping in a known good one. That's an advantage of having 2 Z3s to play with.
Do you have a water leak that is spraying the electrics somewhere?
Does it go limp after 15 minutes idling on the drive?
Any clues in the OBD? Does your scanner do recordings? You'd think something radical would show up at the moment of change in output.

I suspected an air leak initially but am now fairly sure it is not that. The car runs absolutely fine when cold, and even when initially warming up. It idles smoothly and revs freely. I would expect an air leak to show up all the time, or at least once everything had warmed up a bit.

The change is almost like a switch being thrown between smooth running and spluttering once it is thoroughly warmed through. It is this switch like change that makes me think it is a sensor that is playing up.

ECU read temp and air flow, decides it cold and uses one mapping. Then decides it has warmed up and wants to adjust the settings but is getting bad info about airflow so it over-leans the engine.

I will have a play tomorrow when I will hopefully have some daylight. I plan to drive it until it starts spluttering and then return home to try
1) Disconnect MAF, start car and see if the problem stops.
2) Reconnect MAF, start car and see if the problem returns.
3) Swear profusely when nothing makes sense and torch the fricking thing!!!

Any better suggestions?

As it starts to play up when it gets warm I would say it is the camshaft sensor.

Could make sense, it does seem to me to be something triggered by a sensor rather than an underlying mechanical fault. It can be challenging to track exactly which sensor.
Suggestions on testing the camshaft sensor welcome

Decide to replace the £2.99 ODBC reader I bought years ago which basically just read and turned off the service light but wanted something that wasn't specific to the BMW round connector so bought one of these first

Hakkin OBD 2 Cable Round Adapter 20 PIN to 16 PIN Car Diagnostic Connector for BMW E36 E46 E38 E39 E53 X5 Z3

And then a cheap scanner that still seems to cover most the stuff I will be trying to fix

OBD2 Scanner Code Reader TOPDON AL300, car Auto Diagnostic Tool with Full OBD2 Functions, Engine Fault Code Reader CAN Scan Tool, HD LCD Display Screen, for Cars After 1996

Lets see what turns up Monday. I will tr to get a video done today / tomorrow to record the issue and post on here in case it is useful. At least it will help you lot diagnose my problem. :-)

Morning WCA i have a Foxwell Automaster scanner , i paid £120 for it so it is a reasonable one , do you want me to post it down to you? could send this morning if that would be of any use , Geoff .

I am okay for now casamolino but may take you up on it if mine doesn't help

My scanner of choice is a Creator C110 with a "pacman" 20 pin adapter. The later C120 (in colour!) doesn't cost much more. They are BMW/Mini specific devices. I have a "universal" cheapie scanner which won't read the BMW Z3 OBD which I believe is a bit basic- an early implementation.

Yes no problem just get in touch , Geoff .

...and the problem was???

Forgot to update, sorry. It was a knackered lambda sensor. The garage diagnosed and repairs as it was sending no error messages but staying on Full cold start choke - or what ever the modern equivalent is - so ran fine cold without errors but got messy when warm.

Clean MOT and ready for a spring respray and sale.

Found this on youtube WCA !

https://www.youtube.com/watch?v=RzS77AY3tNc

My commute to work on the B road route used to be like that. Rather more oncoming traffic made it interesting as well.