on the end of the fuel injector there is a plastic cap called the pintle.. it has a smal hole in it. that sort of surrounds a sharp thin pin shaped part. if you drop them or bang against that it might do damage..
I cleaned up some volvo injectors and posted a thread on that, what I did was ceated a pressure bottle that I can add cleaner to and pressurixe then open upon the injector to backflush , forward flush and test the spray pattern. Ive seen them stick open and dump the fuel ril into the cylinder. Ive also seen them stick closed then it runs on 3 cylinders with no fuel to 1 of them..
If creating a bottle like that is work, then I have seen a video where the guy just took a can of cleaner and adapted the nozzle to a piece of about 1/2" fuel hose then stuck the injector on and sprayed the can. you can activate them to open and you need to to get any cleaner through. I just used a 9 V battery.
what Ive done as well to test them , just put the car outside, find a proper fire extinguisher, be very careful with this. we dont want you starting a gasoline fire...
if you pull the fuel rail connect the wires all back, put clean dry cloths under each injector, get in, crank for just a few seconds, stop!
run around, look and see. If they dont shut off one will be spurting fuel, it will soak the rag.. if they dont open then you will have a dry cloth there. if everything is basically ok the you will have a neat little circular spray pattern of fuel on each cloth..
a shop can test or rebuild them, none of this addresses a perfectly formed spray pattern.. it just shows if it should work well enough to run the car. you can get rebuilt ones or there are cheap made in china replacements. I'm not sure how well those work.
each injector has a little gasoline screen on the fuel rail side, I just backflushed and found them clear enough but there is a tool to remove them. I think an appropriate sized sheet metal screw on a slidehammer might do ok, never tried. a short bit of chain and a weight can be an improvisational slidehammer. the tool screws down pulling the sheet metal screw to extract the screen, if you need to you can order that tool..
my suggestion is order new o rings, the pintles, there are a couple of plastic washers, its not expensive. If you dropped or damaged the pin on one I'd replace that injector.. I think that pin is pretty sensitive, much like a carburetor jet in a SU carb..
the ECU or MCU changes the duration of power on to each injector. Its giving very short specific pulsations, as the system detects your revs going up and more air going in, it reacts by increasing the dwell time .. It changes the duration of time increment of power to open the injector.. to the injectors to maintain the air fuel mix with a varying amount of air entering. they are normally quite reliable but if the car get parked or you get bad fuel it can plug things up basically..
the fuel tank has a screen on older ones its inside the tak at the outlet.. Its a bit expensive so rather than pay 100 or more I might use some gasoline screen and make my own up. it needs ot be there to keep particles out of the pump, after the pump any little particles shoudl get trapped int eh fuel filter which is working under fuel pressure.
If yours is older like mine is early 85, check the short hose between the tank and pump. mine rotted out and it dumped all its remaining fuel in the previous owner's driveway, that bit of line does rot and it's cheap.. so at minimum check if yours is old and bad you dont wan thte tank to dump your fuel unexpectedly..
heres the link to the thread i did when cleaning injectors. you can see the tool I made for myself, nothing too complicated.. I like that better as I can test them outdoors away from sparks, please don't play around wiht open ful inside your shop.. don't start a gasoline fire. Make sure you have an extinguisher that is rated for a fuel fire right there handy.. just in case.
open fuel can be very dangerous so you wont see a lot of people suggesting you do this stuff yourself, take proper precautions of the dangers if you do any of this. I just wanted to help but I'd feel horrible if you start a fire by doing anything unsafe.. some opt to just take the injectors for a rebuild or repalce..
I think this stuff is a way to just check if there are serious problems, you may or may not find a difference in performance by going ahead and replacing them..
if the pintle is missing or cracked it will mess up the spray pattern.. they can fall off if you arent; careful. best to renew those few small injector parts if you choose to mess with them at all.
if you were pushing hard o the fuel rail you might examine. I don't think its too hard to bend stuff and maybe mis-align them.
i experimented with a tepe deck motor figured I could create short pulses by making a cam and a set of contacts. it sort of worked but ran too fast so I wasnt seeing the injectors open and close instead I simply touched the wires to power the injectors intermittently, less complicated,
here's a link to my older thread thread on injector cleaning..
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/1145509-cleaning-your-own-fuel-injectors.html