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CEL comes on for a few weeks then goes off

Discussion in 'Stock 1995 - 2001 Explorers' started by Softballdad, August 7, 2019.

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  1. Softballdad

    Softballdad New Member

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    So, my 2000 Explorer threw a PO171 OBD code about a year ago. System too lean-Bank 1. After researching this I'm leaning towards it being the O2 sensors. I've already checked the MAF and haven't found any other vacuum leaks. I have hesitation while going up hills and long slow climbs and my gas mileage sucks. This seems to be worse the hotter it is outside because it doesn't seem to happen in the fall and winter. ( Hot air is less dense than cold air? So the air/fuel ratio is different? ) My CEL comes on for a few days and then will go off for weeks it a time. If one or more of the O2 sensors were bad wouldn't that code always be happening? And the hesitation is always there, even on the flat roads. There is no trouble starting it or at idle. Everything feels fine. If it were something other than O2 sensors, like a fuel pressure issue, I would have major problems bigger than hesitation, correct? The thing that doesn't make any sense is that whatever the problem actually IS, it's not throwing the code all the time. Just two days ago I put fuel injector cleaner in, and today is probably the coolest day in at least a month and Bam!, my CEL comes on. What the ? Anyone have a similar problem or advice?
    I've been driving it like this for a year (terrible and lazy, I know), and again, whatever the problem is, it hasn't gotten any worse.
     
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  3. toypaseo

    toypaseo Flunked daycare Elite Explorer

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    My check engine light used to do that, with the same code. Over time, it would be more frequent, or stay on longer. Now, it’s on all the time. I haven’t pulled the codes, so unsure if it’s the same code, or more...
     
  4. fast_dave

    fast_dave Active Member

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    @Softballdad,

    Help us help you; what specific engine is in your Ex - 4.0 OHV or 4.0 SOHC or 5.0?
     
  5. Softballdad

    Softballdad New Member

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    It is the 4.0. Shamefully, i do not know if it is OHV or SOHC. It is an XLS. I should add that the guy I bought it from installed an aftermarket cone style air filter so I wonder if that's messing with the Air/Fuel ratio.. But I've had the car for at least 5 years and it never coded until last year.
     
  6. Runnin'OnEmpty

    Runnin'OnEmpty Well-Known Member

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    The SOHC engine will have those letters stamped on the top engine cover. The OHV will have a smaller triangular shaped cover. The most common cause of lean codes in these engines are vacuum leaks, usually from deteriorated plenum O-ring gaskets. Since you're only showing a bank 1 lean code, I'm leaning (pun intended) toward a faulty O2 sensor - upstream bank 1. The erratic MIL and poor performance also points to the sensor.

    That's just a semi-educated guess though...

    The plug to that sensor is located behind the passenger side valve cover/head. You might have to remove the wheel and inner fender splash shield to access the sensor.
     
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  7. Softballdad

    Softballdad New Member

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    That's exactly what I'm thinking. Thank you so much! It is my goal to at least get to the sensor and get it off this weekend. What's funny is that now that the light is on, it seems to have a little better throttle response. Makes no sense to me. And If I'm in park and hit the gas, there's no hesitation or stumbling at all. Does that make sense?
     
  8. Softballdad

    Softballdad New Member

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    So, instead of just buying things and wrenching away, I hooked up the code reader this morning. It's giving me P0304 Cylinder 4 misfire. The CEl flashes at me when I'm on the long uphill coming home and then when I get to the top, it stops or goes off completely. It's hesitating pretty hard at that point. This would make sense, and I'm glad I didn't run out and buy o2 sensors just yet. So I opened the hood and the first thing I noticed was the spark plug wires were marked by the previous owner and unless he labeled them wrong, they are not in the correct order. They show on the left side front to back 1,2,and 3 which I believe is correct. The right side front to back shows 5,6, and then 4. All the diagrams I can find show it should be 4, 5, and 6 front to back. The wires are so tightly jammed up in and around everything I can't follow their paths without tearing everything apart. I don't really want to do that. What's my next step? Replace all the plugs and wires?
     
  9. Mike65

    Mike65 Elite Explorer

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    If your Explorer has the 4.0 OHV cylinders 1, 2, & 3 are on the passengers side starting with the front cylinder as # 1. So 4, 5, & 6 are the drivers side again starting with the front cylinder. Pull the #4 cylinder spark plug & see what it looks like, is it fouled out?. If it is fouled out put a new spark plug in it, clear the code & see if it comes on again & if it does how long before it came on again. I was experiencing the same problem with my 2000 Ranger with the 4.0 OHV & it turned out to be cracked cylinder heads.
     
  10. Softballdad

    Softballdad New Member

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    Yes, I think I'll try that first. What other symptoms were you having with the cracked heads?
     
  11. Runnin'OnEmpty

    Runnin'OnEmpty Well-Known Member

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    Sounds like the previous owner marked the wires as to their coil pack positions, which are 5,6, and 4 (driver's side of coil, front to back). If the wires were wrong, you'd have a dead miss and not intermittent. You could have a weak plug or high resistant wire on number 4 cylinder. Easy to check with an ohmmeter, or you could just replace the one wire and plug (as a test) until you can replace them all. It's easy to access the number 4 plug, and that's what I'd change first, then drive it and see if the misfire is gone (as Mike65 mentioned)...
     
  12. Softballdad

    Softballdad New Member

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    Could it be that originally the P0171 code was because it was when we were camping up in the mountains and the air/fuel ratio was thrown off by the high altitude? And if that problem had gone away, would I be able to drive on a misfiring cylinder 4 for over a year without something else happening? I have noticed it hesitates more the hotter it is outside. If that wire or plug is already in bad shape, the extra heat would make it worse. It didn't seem to happen last winter, it's just gotten worse as the weather got warmer.
     
  13. koda2000

    koda2000 Explorer Addict

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    A cracked head is a good possibility on the OHV 4.0. Try changing the spark plug and swap/replace the plug wire and see if that helps. Some here have had good results with adding K-Seal to the cooling system.

    Cylinder order for the V6 is 1-2-3 on passenger side, 4-5-6 on driver's side.

    3 6
    2 5
    1 4
    Front
     
  14. Softballdad

    Softballdad New Member

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    Ok, thank you for the info! What other symptoms would i have if there was a cracked head?
     
  15. koda2000

    koda2000 Explorer Addict

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    Maybe no other symptoms. Maybe a slight coolant loss. Maybe sightly less compression on the effected cylinder. The crack(s) tend to be very small, hairline cracks and occur between the valve seats. Cracked heads are the most common problem with the OHV V6. Otherwise these engines are pretty bullet proof and can easily go 300K - 400K. A cooling system pressure test on a cold engine might tell you that you have a crack if the pressure drops over time.

    New OHV heads are not too expensive, relatively easy to replace and the engine is worth savings if the truck has no other issues. Do the diagnostic tests starting with the easiest, least expensive things first (K-Seal will cost you around $14-$15, wont hurt anything and might buy you years of additional service. No guaranty of course).
     
  16. Softballdad

    Softballdad New Member

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    Thank you so much for all the help! I'm going to get new plugs and wires later today. Will see if that makes any difference and go from there.
     
  17. lobo411

    lobo411 Active Member

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    Sounds like the car is relatively new to the OP, so I'd do the basics first. OP has to assume that the previous owner slacked off on the maintenance before he sold it, because that's what everyone does. Replace the spark plugs and wires, and see if that doesn't take care of the misfire.

    P0171 can be caused by a vacuum leak. OP mentions that this code and his hesitation is tied to the ambient temperature. When it's cold, the problem appears. When it's warm, the problem disappears. I'd suspect the lower intake manifold gasket. When it's warm the lower intake manifold gasket swells and develops a good seal. When it's cold, the gasket shrinks and causes a big vacuum leak. IME the lower intake manifold gasket lasts about 100-150k, so it might be worth throwing the dice and just changing it.

    If the OP wants to save the cost of a gasket (which is like the cost of a pizza...not worth saving imo, but hey....to each his own), he could test the theory by torquing the lower intake manifold bolts. If the problem goes away for a while, then the OP will know that when it returns he needs to replace the gasket.
     
  18. Softballdad

    Softballdad New Member

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    Hi lobo411! Actually the previous owner took pretty good care of it, more diligent than I have been. The problem is worse when its hotter outside, not colder. You have it backwards. But still great info! I love this forum! Heading out now to replace the #4 plug and see if that changes anything. I really appreciate everyone's advice!
     
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  19. Softballdad

    Softballdad New Member

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    Well, replacing the #4 plug made a huge difference! Not sure if the computer may be re-adjusting to compensate for actual spark on 4becuase it felt different last night and it seemed to run like a champ today! Thank you everyone for all the ideas and advice!
     
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