1985 ford f150 problems

i have 1985 ford f50 ext. cab with a 5.0l fuel inected. it has been in the shop since before thanksgiving and the mechanic is not sure how to fix it any thoughts would be great.it had been sitting up for about 3
years before i bought and have been driving the truck for about 18 months the mieage shows 80000 which is supposed to be actual miles.
my son had been driving it for about 3 months and the truck always had a slight miss when you drive it. this may not be part of the problem but in the middle of october circuit city installed a new stereo in the truck and afterwards we took it out 3 times to add speakers and wires for a subwoofer. about 3 weeks after this the truck started to crank slowly like the battery was dead.
then on the thursday before thankgiving the truck died on my son and we could not get it started. i had aaa tow it to the local mechanic. he said the fule pump was not working. i figured it was out of gas since the gauges do not work on the truck ( dual tanks ) after putting in 10 gallons of gas and a new battery it still would not crank. the mechanic replaced the fuel filter on the frame, the modulator and pickup coil. my son drive it 15 miles and we had trouble staring it again. sounded like it was not getting enough gas. when it did start it ran really rough and blew out alot of smoke from the tailpipe. i drove it almost all the way back home and it finally died going around a corner. aaa towed it back to the mechanic shop. they said that the truck now has a new set of problems. there thoughts are that 3 of the injectors are not firing. it gets spark leaving to the injector but no spark arrives. i would think changing out thge wiring harness. but that is probally not a practical solution. it does start but idles very rough. and the mechanic calls the three cyls. as dead. he has called other shops and the ford house but they have no idea either. couple other things that should be noted. when the aaa driver was loading the truck gas was coming out of the top of the gas tank and the heater coil maybe going out because i noticed water on the passenger side floor mat and my son said that it got really smokey inside the cab the last time he drove it.
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Most of what I comment about really depends on how many extra stereo components you have added, but this may help you some with your electrical problems.
When you add a stereo of that size to an older vehicle, you really have to think about the possibility of upgrading the battery and alternator as well. If that truck only had a small 500 - 750 cranking amp battery and an alternator only rated at ~60 amps, when you add amplifiers that demand more power and heavier gauge wiring to feed those amplifiers, you have to do something to feed more juice into the electrical system. Otherwise the stereo is going to try and get more power from the other electrical components, whether it be your EEC system, power windows, fan heater, or headlights, it really doesn't matter. But it will affect the overall performance of the electrical system if there is not enough juice to go around. Depending on the size of the amplifier/subwoofer setup, you may only need a 0.5 or 1 Farad power capacitor to store extra power to feed the amplifier, and you may not have to upgrade anything else. Otherwise, if there is no cap, you should at the very least upgrade the battery to more cranking amps, or look for a deep cycle battery, and then in the future, think about having the alternator rewound for higher output.
Having said that and not knowing the size of the stereo that is installed, it could very well be affecting your starting system, fuel injection system and whatever other electrical problems you mentioned. The engine miss you described could have always been there, regardless of the stereo system and condition of the electrical system. I also agree with Backyard Mechanic in that you have a bad fuel tank(s) and that definitely both sending units are probably not working properly (known problem on older Ford trucks), that would be the fault of your fuel gauge. If your windshield is steaming up and you can smell coolant inside the truck, more than likely the heater core is leaking, look for wet spots on the floor. I can't say whether that is an easy replacement or not, but I would guess that it should be fairly easy, because it is a truck and meaning there should be more room to work under the dash.
Good luck and hope this helps some, Sharky
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on the fuel tanks was wondering if the fuel pressure at the motor checks ok (which it does) does that mean the tanks are ok. and if the tanks need to be replaced can they be fixed if rusted or should new ones be bought. also the sending units do work. when the tanks are first filled the register full for about 20 mintues so i would assume that the floats have a whole in them.
the amp is not installed yet just a 300 watt cd player and subwoofer. i will get a larger battery and see if that makes a difference.
what really concerns me is the part about the mechaic says there is no fire going to 3 of the plugs. he says it is coming out of the modulator but the fire never makes it to the plug. he says there is gas at the plugs so the plugs are fouling out because of lack of fire. this is causing the truck to run on 5 cyl. instead of 8. simon
Sharky wrote:

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Three cylinders not firing, and the rest are? I am assuming that somebody checked compression on these non-firing cylinders... On this vintage, with a distributor, it's almost certainly plug wires. They may be about 18 years overdue for replacement. I would also replace the distributor cap and rotor for good measure. You know, like they used to do every 10,000 miles in the good old days...

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