It's sad to hear that the problems with some 2.4JTDms still persist. My car is long since gone, but I've been watching this thread to see if anyone ever came up with a solution.
I am not an Alfa mechanic and I have no direct evidence that any of the following is true. However, I have a few thoughts which _may_ help someone that knows far more than I do track down and fix the issue once and for all. Apologies for the long and meandering post, but I figure this is best written down rather than in my head...
Misfire on a diesel: as far as I know only happens through a) lack of compression of b) lack of fuel. Over-fuelling or dripping of an injector won't usually cause a misfire but will result in a lack of power and clouds of black smoke.
Compression is not an issue here. So, let's assume the cause of the misfire is lack of fuel.
How can this be?
Multijet injectors fire several times on each power stroke. This is cutting-edge technology, but is usually reliable. Nevertheless, what if the injector was intermittently "sticky" and failed to inject fuel every time it was told to? Say, instead of injecting 9 times it only fired 7 of those times. What would that feel like? My guess is that it would feel very much like the misfire we have experienced. Not a total loss of power, not a severe misfire but just a roughness and lack of power that mysteriously disappears whenever the car is within one mile of an authorised Alfa dealer.
OK, my guess is that the injectors are sticking intermittently. What could cause this? It seems that certain methods of driving will provoke the misfire and also some fuel brands. The most repeatable way to get my car to misfire was to use full throttle during a DPF re-gen. This would invariably cause my car to cough, stumble and struggle to maintain 60mph...
What is special about DPF re-gen? During the re-gen, excess fuel is put into the exhaust flow. This ignites, raising the exhaust gas temperature considerably and causes the soot in the filter to burn off. This is why you get the notably high under-bonnet temperatures if you are stationary when DPF re-gen is ongoing. Modern Alfa diesels use a rather nasty method to add excess fuel to the exhaust: the injectors fire again during the exhaust stroke, injecting raw fuel into the cylinder. This raw fuel then enters the exhaust stream. It's a cheap and easy method, but does absolutely nothing to preserve fuel economy or the integrity of the engine oil. Oh well, I guess some accountant somewhere must have thought it was a good idea.
Anyway, the upshot of this is: during the re-gen, the injectors are working twice as hard as normal, firing on every stroke. Full throttle during re-gen is probably the heaviest possible injector duty. So if there are any problems with the injectors, this is when they are going to appear. Funnily enough, those owners that have been lucky enough to get a new set of injectors out of Alfa have noticed that the problem goes away. For a while.
Next up: the EGR valve. Many owners have reported problems with the EGR valve, having it replaced under warranty. This is quite a common issue on modern diesels.
What does the EGR valve do? One big problem with high performance diesels is that they tend to produce NOx pollution. This is because under some part-throttle conditions there will be too much oxygen in the fuel mix combined with high combustion temperatures. This makes NOx. The way to reduce NOx production is to reduce the amount of available oxygen. How can you do this, given air is 20% oxygen? Answer: redirect some exhaust gas (which is low in oxygen) back into the intake. That's what the EGR valve does. It opens to let exhaust gas back into the engine when needed, but closes on full throttle. It works pretty hard.
What happens if the EGR valve sticks? Well, this will allow exhaust gas into the engine intake at all times. Very, very hot exhaust gas. Which is even hotter during an DPF regen (see above). Where does this this hot gas go? Well the first thing it meets is the Air Shutter Valve - a plastic throttle-type thing that is used to manage the air going into the engine. Funnily enough, these fail quite often. Ever noticed that faint smell of burning plastic during a DPF re-gen? Ever had an Air Shutter valve fail (and cause the engine warning light to come on)? Well, now you know what might have caused it.
OK, now this super-hot exhaust gas has slipped past the EGR valve, damaged the Air Shutter valve and is on its way into the engine. Where it meets the injectors. That are already working to the max during a DPF re-gen. The injectors aren't getting the usual cool blast of intake air between each ignition - they're getting a blast of super-hot exhaust gas. They're not designed for this. So they (very) briefly stop working.
Hey presto: misfire!
Is the above scenario 100% correct? Probably not. Are the injectors being permanently damaged during the misfire? I have no idea. It's possible.
Here's what I think is going on:
1. Something causes the EGR valve to stick occasionally. Could be bad/sooty fuel. In my case I think it was the "turbo flutter" caused by a leaking air intake (see my posts early in this thread).
2. The Air Shutter valve gets damaged and is replaced (easy - the engine light comes on).
3. The misfire continues until the EGR valve is replaced (takes a bit longer, you have to moan at the Alfa dealer until they fix it)
4. Problem goes away (for a bit). But something is no longer "right" and the injectors just aren't working the way they should.
5. Eventually the injectors are replaced and the owner goes away happy.
6. The underlying problem hasn't been fixed though, and the EGR starts to stick again. And so the cycle begins again...
- Replacing one component just isn't enough to fix the issue. Alfa will hate me for saying this: my guess is that cars affected with misfire need new injectors, EGR valve, air shutter valve and "anything else that might have been damaged".
- To stop the problem recurring, something needs to be fixed. Whether it's a different engine map or uprated injectors, I have no idea. That's Alfa's problem. But I don't think Alfa should have ignored this issue for so long.
In closing, I wish you guys all the best in getting the issue sorted.
-clanger9, ex-Alfa Romeo owner