300tdi auto misfiring

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Hi, im new to all this !....my 96 disco 300tdi disco is misfiring, a friend of mine is looking into it, had taken no4 injector off, its been tested and cant seem to find anything wrong with it, going to take out no3 injector and put it where 4 should be, if there is still a problem ive been told that going to have to look inside....(what ever that means).... my no4 is wired and been told this could cause a problem to find a new one.....but i have managed to locate one although 2nd hand.....can anyone possibly give me some more insight as to what could be the problem if not injectors.....many thanks ........looking through some of the threads u all seem to like lots of info...so here goes my effort in trying to tell u...lol
It starts well, ticks over like a bag of nails, lumpy in acceleration but not gutless, no extra tapping noises when running....erm...white smoke on starting up.....hope this helps ?

check cam belt hasnt slipped a tooth

Injector has been tested and alls well, have now swapped injector 4 with no 3 injector, still have same problem, going to check cylinder gasket as my friend has said that can cause same problem due to lack of compression ?

Hi Lola99,did you sort your problem? As i have a very similar if not the same problem & i'am bordering on the BIG hammer technique
Any answers would be greatfully recived.

If no4 injector has been removed, were the wires cut or was there a connector which was pulled apart?
On mine somebody had cut the wires & remade connection with crimped spades, this was'nt done properly & gave an intermittent fault which was same as air ingress / crap in fuel system. Remade joint & all running sweet as hope this helps.

no 4 injector is used on the autos for pump timing as they are edc pumps so it needs to stay in no 4 cylinder
you can check the injector using a multimeter across the 2 pins in the connector from the injector it should read about 100 ohms
Also the temp sensor on top of the cylinder head can cause problems if the resistance is incorrect the engine may think its cold and tell the ecu to inject more fuel causing a missfire as its overfueling
The ecu is located in the drivers kick panel and if you unplug the multi plug you can check sensor readings there
coolant temp sensor pins 13 and 53 should give you 2280-2720 ohms at 20 degrees c
injector pins 5 and 12 should be 90-100 ohms
when looking at the plug no 1 pin is top right pin 19 is top left
pin 20 is middle right ,pin37 is middle left
pin 38 is bottom right ,pin 55 is bottom left

worth looking at crak sensor ,drivers side gear box bell housing at top