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Fuel injected 4.9 miss

  #1  
Old 07-08-2018, 05:07 PM
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Fuel injected 4.9 miss

I have a 1990 f150 4.9 5 speed. I acquired this truck about 10 years ago and It got parked about six years ago. Recently I pulled it out of the weeds and I've been working on getting it running. So far I had to replace the computer due to bad capacitors The O2 sensor because it was never hooked up since I had the truck. And the front fuel tank sending unit and pump. I've driven the truck about 300 miles now and I've started to develop a miss in the 6th cylinder along with terrible fuel economy. At first it was under heavy acceleration and now it's all the time. Emmisons equipment has been off the truck since I've had it. I also know I have vacuum leaks but nothing that has effected the way the truck runs ( at least not to the point of missing). I've swapped spark plugs and plug wires but can't get the Miss to move and I know I'm getting good spark. I'm going to compression test tonight but I'm not sure where else to look? Has anyone else had this problem?
 
  #2  
Old 07-08-2018, 06:42 PM
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You might want to check your distributor and rotor.
 
  #3  
Old 07-08-2018, 07:17 PM
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Fuel pressure? You say it has vacuum leaks? That could be the problem.
 
  #4  
Old 07-08-2018, 07:21 PM
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Roadie has a good point.

Also, I noticed that you didn't touch it for a while, and you pulled it out of the weeds.

I'm wondering if rodents didn't chew up some wiring.
 
  #5  
Old 07-08-2018, 09:35 PM
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I'd swap injectors around, #6 may be stuck open.
 
  #6  
Old 07-09-2018, 05:51 PM
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OK I did some checking today. Cap and rotor are good, compression is 120psi. Didn't check the other cylinders because I ran out of time. Wiring is all good. And the two lines that are broken have been broken since I've gotten the truck. One is a green line on the top of the motor and the other one runs above the number 6 spark plug. I will replace these when I get time but I don't think they caused this issue. Fuel pressure is unknown. It ran good when i pulled it out of Its parking spot and the last 250 miles I drove it where flawless. It was only the last 50 miles that this Miss came about. It runs the same it did as I parked it. Except for I was getting around 14mpg when I parked it and now I'm getting around 6 mpg. The only other difference I notice is that after I changed the computer it doesn't want to idle down below 1200rpm. It will after a while but it seems to fight it. If I have time tonight I'll pull codes and see what it says.
 

Last edited by MJohnston2448; 07-09-2018 at 05:52 PM. Reason: Double typed a sentence
  #7  
Old 07-09-2018, 05:55 PM
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Also I was wondering if a possible bad injector. Is it possible to clean them or just better to replace them?
 
  #8  
Old 07-09-2018, 09:22 PM
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It's best to replace them, or at least get them cleaned and rebuilt.
 
  #9  
Old 07-09-2018, 09:47 PM
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Put a couple bottles of chevron techron or Gumout with PEA in the gas and see if it helps.
 
  #10  
Old 07-13-2018, 04:01 PM
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Thanks for all the advice. I don't have any time for the rest of this week to work on the truck but starting next week ill tear into it and keep everyone posted.
 


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