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94 Dodge Ram Fuel Pump Problem

I have owned a 94 Dodge Ram 1500 LST with a 318 and 2wd for 15+ years and have had the following recurring problem (as many others also have had).
<br/> I have had to replace the $200 fuel pump 4 times due to the following:
<br/> As the fuel level approaches 1/4 tank the engine will begin a gradual decline in performance then rapidly falling off to barely running on 2 or 3 cylinders (occasionally this can start happening just below 1/2 tank). It starts with a barely noticeable roughness and may go for many miles before getting worse. Then suddenly within just a mile or two the misfiring starts, one, two then most of the cylinders misfiring. It has never completely died on me and in a several situations I have had to limp 20 or 30 miles to get to a service station.
<br/> Here's the kicker: Once I put 5 or more gal. of fuel in it I can restart it rev it a little and within a minute or so it will be running fine and I can go on down the road.
<br/> I have stopped many times and checked the fuel pressure (always around 40psi), checked for bubbles (none noticeable), checked for water or crap in the fuel - from the check valve (none noticeable).
<br/> I have taken the old (replaced) pumps and bench tested them and have found no problems and they have good pressure (they make good pressure washer pumps for washing parts w/diesel or kero).
<br/> I examine the contraption that the pump, filter, and float sender are mounted in and cannot find any defects.
<br/> No fuel line problems are ever found and always replace the stupid little $20 plastic line that attaches to the h/p feed line.
<br/> I have talked to other mechanics that have no clue either but said they bought a lot of beans w/the money from replacing these idiotic devices. (Oh the good old days of spending $20 and 20 minutes once every 10 years replacing a fuel pump out in front of the parts store with a 1/2inch wrench a grease rag and a screwdriver.
<br/> My cheap solution is to keep at least a 1/2 tank of gas and a 5 gallon can in the back.
<br/> Except for the multiple water pump changes (which are also a lot of fun) I have had an excellent truck over these many years.
<br/> Any explanation would help me retain what little sanity I have and a permanent fix for this problem would be greatly appreciated.
<br/> Thanks,
<br/> bcudamatt


  • edited February 2010
    Have you ever checked the fuel pressure and volume while the truck is missfiring?
  • edited February 2010
    I have stopped many times and checked the fuel pressure (always around 40psi), checked for bubbles (none noticeable), checked for water or crap in the fuel - from the check valve (none noticeable).
    I have taken the old (replaced) pumps and bench tested them and have found no problems and they have good pressure

    When you pull over and check the fuel pressure, the engine is idling and under no load so the injector pulse width is minimal = great restriction on the pump. Under those conditions, it is easy for the pump to maintain pressure. I would be curious to know what happens to the rail pressure when you drive off, the pulse width widens, the injectors open more and therefore the restriction goes way down. Can the pump maintain the same pressure as a new one under those conditions? I would rig up a gauge I could see while driving or design a bench test with a variable restriction I could adjust and compare new vs suspect pumps.

    It would also be good to know, based on the tank design, where the various components are situated relative to the gas level when the problem begins to occur. For example, is the pump body exposed but the pickup tube completely immersed and so on. This may be a clue as to what could be degrading over time. Pinholes, leaks etc. These pumps are good at making pressure at the outlet but their design makes them poor suction devices. The inlet conditions are important and this design could be very susceptible to changes or variations in operating conditions. Just some thoughts...
  • edited February 2010
    Yes, as stated above I have stopped several times and checked the pressure, I would think if pressure is adequate the volume should also be OK. Have yet to rig up a fuel line and run it into the cab so I can test at speed.
  • edited February 2010
    You both have a good point. Volume/Pressure at speed would be the critical test. What rags me is that the giganto enourmous tank still has 15 or more gallons in it when the prob occurs. It seems that there should be plenty of fuel to pick up - no cavitating or slosh problems should occur at that level although if a small air leak would occur it should also present some bubbles in the fuel rail. One screwy thing about the oddball geegaw that holds the pump requires a substantial level of fuel before it enters the pump 'chamber'. There is no pickup tube just this chamber where the pump sumps the fuel up thru a screen. Stupidest design I could imagine. I was told you never want to run one out of fuel.
    Now that I think of it though the old pumps would shoot a good full (unrestricted) stream 15-20 feet when testing. As I stated they make a good parts washer pump.
    I thought for some time of replacing the entire p.o.s. with an external pump but a mechanic freind told me of the Ford rollover probs in the early days of these high pressure systems when the pump kept running and caused hellish fires. So I changed my mind!
    Thanks for the info.
  • edited February 2010
    Did you remove the fuel tank, clean it out, and change the fuel pump sock (screen/filter) when you changed the fuel pump?
  • edited February 2010
    Wow, that does sound like a lousy design to me too. If some small degradation in performance immediately affects the driveability then I'd be ticked off as well. I had one other thought- it would be good to insure that the power supply to the pump is not only delivering the full bus voltage but that it is also capable of delivering the necessary current for the pump to operate properly. The connections near the pump would be most suspect. I'd go so far as to extract the pins from the housing and inspect the crimps. Measuring the voltage directly across the pump leads might reveal if there are any connection integrity issues along the way. Ideally, this should be done with the pump under maximum load. A struggling pump is not good for longevity.

    I don't like the idea of it requiring so much fuel to begin sumping. They may have done that to insure any particulate settling in the tank would be below the pickup screen but that sounds excessive. You sound like a sharp guy and persistant enough to find a solution. I would certainly be interested in hearing what you find and any fix you devise.
  • edited February 2010
    Yes, several times. The same problem has always come back after replacing all.
  • Just an update on this old post.
    Never have found the problem but now I have a clue.
    Since it has been so Very Very hot the last couple of months the problem has gone away (temporarily I'm sure).
    Now I'm wondering:
    - bottom of tank had accumulated water and now its remixed and gone?
    - regulator working better in hot weather?
    - brain fried in hot weather and im inaginin tings?
    Just wondering.
  • bcudamatt, was anybody ever murdered in the truck? Just a suggestion.......
  • edited July 2011
    I have a '96 Dodge 2500 Van. At about 100k miles it acted alot like you say your's is doing. Finally I got a new pump and when we removed the old one we found out that it was like 2 cans, one inside the other with screws in elongated holes. The bottom half seemed to move down as the fuel level went down. However, it was hanging up. When it hung up it wouldn't pump fuel. Putting in more fuel would solve the problem for a while. The new pump was different and hasn't failed in the last 39k miles.
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