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Low vacuum

Discussion in 'Stock 1991 - 1994 Explorers' started by DasBaldDog, June 21, 2011.

^^Searches ExplorerForum.com^^





  1. DasBaldDog

    DasBaldDog Member

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    Figured out my orphan wire and faulty starter problem and ran to Autozone to pick up some random stuff.

    While i was there, I borrowed their vacuum gauge and am getting a VERY steady 14 inches of hg. This is the same reading I was getting before I embarked on my long mission to replace all the intake gaskets. On said missions, I found a fairly deteriorated intake manifold gasket and figured that was it. Apparently not.

    Like I said the vacuum is steady at 14 inches. Warm idle is about 700 rpm or so.

    All the vacuum guides say late ignition, plugged exhaust. Definitely not the exhaust... New cat, new muffler. Shouldn't be the ignition, it's set by the computer and I should get a CEL if it retarded the timing, right?

    I sprayed out the IAC with carb cleaner but have no idea if it's leaking. Sprayed the entire engine with carb cleaner, no leaks so far as I can tell.

    What's next to check? Am going to go buy my own vacuum gauge after work today so I can more easily check at home.
     
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  3. Roadrunner777

    Roadrunner777 Well-Known Member

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    Have you checked to see if someone removed the octane jumper? That, by definition, retards timing.

    Plug every vacuum hose on the tree that you can. You need to leave the one hose to the fuel pressure regulator, but I think you can leave the rest off.

    I mean, vacuum is everything from the throttle plate to the intake valves. Hmm... dirty throttle plate not allowing complete closure?

    I'm trying to decide it the IAC could be involved too. I never thought of this, but the IAC must get air from some place before the throttle plate, no? I guess there must be a passage in the intake manifold I have not noticed.

    I think, in the earlier post, you had sprayed carb cleaner around to find vacuum leaks. If not, then that's a good step, but I think you did already.
     
  4. DasBaldDog

    DasBaldDog Member

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    Where/what is the octane jumper?

    Going to try and close off vacuum lines after I go buy thegauge this afternoon.

    Yep I used alot of carb cleaner.

    The Iac gets air from a passage just in front of the throttle plate I believe. When I removed it to clean it, I could see one of the air passages went forward to the throttle plate.
     
  5. Roadrunner777

    Roadrunner777 Well-Known Member

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  6. DasBaldDog

    DasBaldDog Member

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    Okay, will try that tonight.
     
  7. DasBaldDog

    DasBaldDog Member

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    As a side note does anyone know what normal vacuum is on a 4.0 OHV? L


    All the vacuum guides I read say that while 17-21 is normal, some motors run lower than normal.
     
  8. DasBaldDog

    DasBaldDog Member

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    Checked that jumper at lunch, it's still there.

    I tried a few other things.

    Warmed it up and unplugged the IAC, no affect. Not sure if that means anything.

    Also fully warmed up I shut off the engine, waited 10 seconds and pulled out the brake booster vacuum line, there was a quick "woosh".
     
  9. Roadrunner777

    Roadrunner777 Well-Known Member

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  10. Maniak

    Maniak Moderator-Stock 91-94 Staff Member Moderator Elite Explorer

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    I can't answer that, but I can give you a tidbit that "might" help.

    When we were checking the vacuum on the new motor in the Van I was also seeing low vacuum.. It was about 3 psi low.

    While I was talking to Lunati (I'm using their cam) they asked if we were high altitude. I thought "no", but apparently 3500 ft is considered high altitude (even for making a cake :))

    For every 1000 ft you go up, you lose about 1 psi of vacuum.. Which explains why my reading were 2-3 psi low from what they said I should have.

    So, are you 3000 ft or higher?

    ~Mark
     
  11. DasBaldDog

    DasBaldDog Member

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    About 1000ft or so.



    I am thinking.... If the O2 sensor was old and thought the engine was lean, would it retard the timing? I am not sure when the O2 sensors were last replaced/how many miles they have, or how well they are running.

    Do you know if this engine has a MAP sensor?

    Could an old air temp sensor cause any sort of issue?


    Pulled the codes, nada. Nothing in KoEo or koer.
     
  12. DasBaldDog

    DasBaldDog Member

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    Probably concerned over nothing.

    Checked it with the new vacuum gauge today and its reading ~19-20 inches, so basically perfectly.



    Might still replace the O2 sensors, they look to be fairly old and knowing the rest of this truck, may never have been replaced.
     

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