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RPM Jump to 0 and Back Randomly with Misfire Feeling

Discussion in 'L: Horizon/Omni, Rampage, etc' started by 88horizon5speed, Jul 17, 2015.

  1. 88horizon5speed

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    For those who haven't read in my clutch thread, my car has been and is continuing to have a feeling of a misfire along with the tach going from driving RPM to 0 and back (doesn't always go to 0, sometimes on less of a stumble it goes down less).

    - has only happened when car is warm
    - replaced HEP, cap and rotor
    - checked fuses 10 and 11 (fuel pump and tach)
    -planning on checking grounds next

    any other suggestions or comments? I appreciate it!
     
  2. GLHS60

    GLHS60 Well-Known Member

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    Faulty HEP. Try a known good used one if possible, there are plenty of faulty replacements.


    Thanks
    Randy
     
  3. ImperialCrown

    Level III Supporter

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    OEM parts are best. Look for Mopar part# 5227271 or equivalent.
    If it is a good OEM part and you are having the same issues, look for wiring or PCM issues.
     
  4. Bob Lincoln

    Bob Lincoln "CHECK FAULT CODES"
    Level 2 Supporter

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    Did you change the plug wires? They should be changed whenever the cap is.
     
  5. 88horizon5speed

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    the plug wires and plugs are good. I just checked my codes and the only stored code is 1-2

    http://www.allpar.com/fix/codes/faults/code12.html

    I haven't disconnected my battery in a long (definitely not within 15 engine starts) so like this code says I am going to look for wire problems going to the ECM. Hopefully its not actually the ECM that's going bad.
     
  6. 88horizon5speed

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    All the parts I put in were SMP, non T series. Ill have rock auto send me a replacement to try another new HEP
     
  7. GLHS60

    GLHS60 Well-Known Member

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    Faulty HEPS don't necessarily set any codes, the falling tach is the clue some of us have learned from past experiences.
    Some of us, like me, have several vehicles so isolating an intermittent HEP problem is somewhat easier as I can rotate a HEP and see if the problem follows the HEP. Not very scientific but effective all the same.
    I have a bag of Turbo Heps that have proven reliable and encourage others to grab any spares when ever possible, from scrap yards etc. Due to the suspect reliability of replacement HEPS, I cant remember ever buying a new one. Shiny clean replacement ones were often the reason cars were in the scrap yard in my opinion, often accompanied with other shiny new coils etc.
    Although I'm not active now, in the past whenever I would stumble across a HEP, I would try it in one of my drivers to verify its reliability. Some proved good others, not but the falling tach was the most reliable indicator.
    I don't mean to "harp" at you, but I.C. is a top quality diagnostician and has also urged you to try and start with a known reliable HEP. Chrysler HEPS, although more costly have a far greater reputation for quality.

    Best of luck!!
    Thanks
    Randy
     
  8. 88horizon5speed

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    I appreciate any advice I can get, I have no way of getting access to a known working HEP but I can try a mopar one. Out of curiosity I don't understand how a new part can be non working as often as you make it sound? I'm not saying I don't trust what your saying but when I buy a new part i am under the logical understanding that it should be a new and working part, no?
     
  9. ImperialCrown

    Level III Supporter

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    One of the greatest early HEP-killers were cheap distributor rotors that didn't have the reluctor vanes grounded through a center tab to the distributor shaft.
    112527.jpg
    Inside the distributor on a running engine, it looks like a 'lightning storm' going on with spark scattering around and all over the place with the spinning rotor. Some of this high-voltage impinges on the reluctor vanes and should be taken right to engine ground. Without a grounding tab on the rotor, some of this voltage jumps from the rotor vanes to the HEP. This can eventually kill the HEP. The high-voltage can also travel up-harness from the HEP to the PCM. While the PCM has some protections against transients, reverse polarity and over-voltage, it might get through the protections and damage sensitive things.
    If you pull the rotor off, look at the underside where it fits over the distributor shaft. Make sure that there is a metal tab that connects the reluctor vanes to the hole that slips over the distributor shaft for a ground connection. The rotor should fit snugly to the shaft.
    Later distributors mounted the vanes underneath the HEP and that greatly reduced this failure from happening.
    0900c152802514e1.gif
    As for bad new parts out of the box, no that may not happen that often but non-OEM parts may not meet the specs or standards required to operate very long, or in some cases not operate at all. It may or may not even fit. This has happened to me with 'no-name' brand parts. I won't go through that again.
    How can they get away with selling you an insufficient part? If it is non-OEM or made for a different application, they seem to be able to. They seem to have had no problem with me returning the part for a refund when I explained the problem either.
    It doesn't have to be Mopar brand, but it should definitely be described as an OEM-replacement part somewhere on the box.
     
    floridaman2013 likes this.
  10. 88horizon5speed

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    I have checked the rotor and it is as you describe. I haven't been buying no name parts, I always buy name brand such as SMP or ACDELCO and I do always make sure they say meets OEM specs. Do you happen to know how many and where all the major grounds are?
     
  11. ImperialCrown

    Level III Supporter

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    The 3rd volume (Wiring Diagrams) would show ground, splice and connector locations. I don't have that manual. It is actually a 3-volume set that covers: Engine/chassis, Electrical/Fuel and Wiring Diagrams. Shop around and check a local main library? It may be worth having this set if you are going to do your own work and it may even pay for itself the first time.
    An example:
    http://www.factoryrepairmanuals.com...odge-fwd-car-factory-service-shop-manual-set/
    The PCM grounds should be in the left, front quadrant of the engine compartment (around/under the battery tray, on the radiator support and inner fender panel).
     
  12. GLHS60

    GLHS60 Well-Known Member

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    You can't beat factory service manuals for sure but if you know someone with Mitchell or something similar you might ask them to print off the wiring diagrams. I had some printed and enlarged when installing turbo engines in my Rampage and Omni's.

    Thanks
    Randy
     
  13. 88horizon5speed

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    I have all three manuals but to be honest its kind of confusing to try and chase down the ground locations from looking at the wiring diagrams. I wass hoping someone could have a list of the major ones but I can try to chase them down
     
  14. ImperialCrown

    Level III Supporter

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    There should be a later chapter like 8W-90 that shows ground, connector and splice locations as they appear in the vehicle harness routing?
     
  15. 88horizon5speed

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    Ok so the problem has gotten worse. I replaced the ignition coil, no dice. Car would drive great one day, the next it was almost undriveable. Replaced the MAP sensor and so far I have gone some time without problem. Im hopeful it was the MAP sensor and so far it seems that is the case. Just want to keep everyone updated incase it can help with someone elses similar problem.
     
  16. Dave Z

    Dave Z It's me, Dave
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    Did you replace the HEP again as IC suggested?
     
  17. 88horizon5speed

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    yes I have replaced it and still had the problem after that. So far I have driven problem free for quit a while. After the problem got worse I changed the MAP sensor that day and havent had the problem since then. If anything else comes up about it I will post it here. Thanks for the help every one
     

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