What's the deal with Fuel injectors?
How would I know if I need to replace them?
Are there any HP gains by upgrading to "Performance Injectors"?
What are your recommendations for them?

These are for a 1991 V8 GT Foxbody (stock engine).

 

You will not see gains by adding bigger or "high performance"inj on a stock motor.

 

you replace them when you are making too much hp to support 19lbs injectors or they are thecause of leaking down/loosing fuel pressure

 

So there's no real reason to replace them then? Do they go bad?

My Fox is vibrating (badly)at idle at about 1k. When I rev it'll smooth out and not vibrate. At first I thought it was the Harmonic Balancer, but then I got a tip to check to make sure my cyliners were firing, they seem to be ok, I had a mechanic come by and look at them and he said it's the fuel injectors that's not working and they need to change.

 

Wow that mechanic is awesome!!!! Diagnosis of rough run through eye sight and or osmosis....wish I was that good!!!!.......if you have a misfire then you narrow it down to a cylinder then from there you check to see if it has spark if it does either that cylinder has one heck of a vaccum leak at the intake port which I doubt then from there you make sure that injector is firing via a noid light..if it is then you can chec the resistance spec on the injector if it is open then replace it ....it could also be plugged up.....but also if a single cylinder is missing it is always a good idea to verify compression and that cylinders ability to seal.......so if your mechanic did all this then I'd say he's on the ball but if he just walked out and said it must be that and guess I wouldn't neccessarily trust a guess..proper troubleshooting techniques yield full proof results

 

I was standing right there when he was under the hood and I can tell you that he did not do ANY of that. All he did was ask me to start the car, then pulled off the spark plug wires one at a time. Then he twisted the distributor, which btw, made the car not vibrate any more. But he put it back to the location it was and the car went back to vibrating. I asked why he just didn't leave it at where it wasn't vibrating but he said, it would backfire in that position.

I just want the car to stop vibrating.

 

well, since he has messed with the timing it may be off a few degrees now....but that doesn't mean your fuel injector/s were bad ....don't know how or why he drew such a conclucion from that......if he retarded timing it could cough and hiccup bad but advancing it to far would cause it to ping/detonate......you'll need to pull the spout connector and start by getting a timing light and set the base timing to 10 degrees and no more then 14 and at 14 you'd want to run premium gas......after base timing is set with spout removed and rotating distributor......tighten holddown for the distributor and but spout back in and drive and see if problem is solved....you may have to get a wire brush and clean off the markings on the balancer and using a paint crayon or tire marker....put a mark at 0 and one at 10 degrees btdc