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Stock 93 Explorer runs as if it has a vacuum leak but

Discussion in 'Stock 1991 - 1994 Explorers' started by ddenton749, December 24, 2018.

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    1. ddenton749

      ddenton749 Member

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      I can't find a vacuum leak anywhere.

      It all started suddenly. The engine started running very rough. It would almost stall at traffic lights and I had to drive two-footed to keep it running to nurse it home. It is running extremely rich to the point where it blows black soot out of the tailpipe when I start it.

      It will idle ok for several minutes then it will start to gag and sputter, then if it doesn't stall it will clear up and run ok for a few minutes. The exhaust smells rich. The CEL will come on and then go out again. It runs better when the engine is cold but then when it warms up it idles rough.

      When it idles rough I can open the throttle a bit and the engine clears up and the power comes back. Then it will start running rough again.

      I had trouble accessing the computer due to poor connections. When I did get it connected there where no codes stored. Weird since the CEL did come on.

      I took the upper intake apart to look at the gaskets above and below the fuel rail - both appeared good.

      416,000 miles on the original engine.
      Compression test - good 180 psig in all cylinders.
      Vacuum - 14-15 inHg.
      Spark plugs black and dirty but all are the same.
      I checked the vacuum components: hoses, brake booster, cruise control, interior vent system, cold air valve, trans modulator, fuel tank vent, etc., - all good.
      Tuned up a couple of years ago but I didn't drive it at all last year.

      Injectors are very old. I replaced them many years ago with junk yard injectors. I applied 25 psig of air pressure to them and they don't leak.
      Coil pack is original.
      MAF sensor is original.
      Air tube between MAF and throttle body new.
      IACV original.
      TPS original
      Fuel pressure regulator is original.

      I have been having trouble with electrical connections lately. Most recently a dirty fuel pump fuse prevented it from starting. The cruise control will drop out sometimes when I hit a bump. Those kind of things.

      Any and all suggestions and ideas are welcome.
       
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    3. RangerX

      RangerX Elite Ranger Staff Member Moderator Elite Explorer

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      One place to look for a vacuum leak is on the air intake tube, right at the accordion pleats. Take the tube off and flip it over and check the bottom. Those pleats are known to disintegrate.
       
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    4. 15Limited

      15Limited Elite Explorer

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      Have you done a smoke test to see if/where you have a leak? Have you tested the O2 sensors?
       
    5. Roadrunner777

      Roadrunner777 Well-Known Member

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      Well, two quick things that you may have ruled out during the work you have done already, but for what it's worth:

      1) A bad fuel regulator does this. There is a vacuum port on the regulator and if the diaphragm in the regulator breaks, you end up sucking gas through the vacuum line into the vacuum tee. You can disconnect the vacuum line to test... it won't run great but it would prove or eliminate the regulator as a cause.

      2) A failed transmission modulator will do this too, but not as severely. You end up sucking air and trans fluid into the vacuum tee. You can look at the line from the modulator and see if it is wet with trans fluid.

      So, two things you may have already ruled out, but if not, the fuel regulator fits your symptoms exactly.
       
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    6. ddenton749

      ddenton749 Member

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      Yes. This was actually the very first thing I replaced.
       
    7. ddenton749

      ddenton749 Member

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      I used good old ether to look for a leak. I found none.
       
    8. ddenton749

      ddenton749 Member

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      1.) I checked the fuel pressure regulator with a vacuum tester. It held vacuum.

      2.) I checked the transmission modulator in the same way. It held vacuum.
       
    9. ddenton749

      ddenton749 Member

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      How do I test an O2 sensor?
       
    10. ddenton749

      ddenton749 Member

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      I took the upper intake and the fuel rail off to look at the gaskets. They appeared to be intact.

      I used my air hose and 25 psig air to check the injectors. They all held pressure.
      Is a bad injector(s) a possibility still?
       
    11. 15Limited

      15Limited Elite Explorer

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      Several videos on YouTube can show you how to bench test the sensor(s)...if the sensors are original, probably time to replace anyway.
       
    12. ddenton749

      ddenton749 Member

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      The O2 Sensors are only a few years old.
       
    13. Turdle

      Turdle Has Pleasure Chamber Staff Member Moderator Elite Explorer

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      To me, rich burn combined with vacuum loss = bad fuel pressure regulator passing fuel through the vacuum line.
       
    14. ddenton749

      ddenton749 Member

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      I put a Mighty-Vac vacuum tester on the fuel pressure regulator. It holds vacuum.
       
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    15. AGodlyCanuck

      AGodlyCanuck Member

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      Have you tried disconnecting the MAF to see if it runs better or the same?
       
    16. ddenton749

      ddenton749 Member

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      I did not try that. I have the upper intake off at the moment. If new injectors don't help I will try that.
       
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    17. ddenton749

      ddenton749 Member

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      So I got it all back together with new injectors, plugs, and wires and it still runs like crap.

      I DOES run better with the MAF sensor disconnected. (Not perfectly but it clears up and the idle calms down.)

      Does that imply a bad MAF?
       
    18. ddenton749

      ddenton749 Member

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      **Update**

      I replaced the upper intake and fuel rail gaskets. I replaced the injectors, plugs and wires. It still runs the same.

      At the suggestion of AGodlycanuck I unplugged the MAF and it runs better disconnected.

      Any ideas?
       
    19. RustyMacintosh

      RustyMacintosh Elite Explorer

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      MAF disconnected I believe it will run richer
       
    20. ddenton749

      ddenton749 Member

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      With the MAF connected it runs so rich the exhaust burns my eyes and my clothes smell like gas. With the MAF disconnected it runs leaner and better.
       
    21. RustyMacintosh

      RustyMacintosh Elite Explorer

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      you sure your 02's are working and are not aftermarket crapola from tinbucktooland?
       
    22. Turdle

      Turdle Has Pleasure Chamber Staff Member Moderator Elite Explorer

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      Just a hunch.
      Some ECM's will develop leaking fuel pump control capacitors. You might take the ecm out, remove the cover and take a look at the circuit board.
       
    23. ddenton749

      ddenton749 Member

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      Please explain the fuel pump capacitor issue further. Isn't the fuel pump simply on/off via the relay?
       
    24. ddenton749

      ddenton749 Member

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      I am not 100% sure if the O2 sensors are working correctly. There are no codes that reference O2 sensors.
       
    25. Turdle

      Turdle Has Pleasure Chamber Staff Member Moderator Elite Explorer

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      The ecm commands on-off cycles of the fuel pump. Relays just accomplish this.

      It's not uncommon to find bad capacitors on the ecm combined with fuel control issues.
       
    26. ddenton749

      ddenton749 Member

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      The codes I got:

      Several regarding the fuel pump voltage but I had unplugged the fuel pump relay.

      The others were 157, 158, and 159 referencing the MAF voltage.
       

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