Engine misfires with dampness
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Thread: Engine misfires with dampness

  1. #1
    SOPHOMORE ROVER james's Avatar
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    January 18th, 2010
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    Engine misfires with dampness

    Long tale of woe here with my 2001 4.6, appreciate you reading.

    I’ve had two ongoing and possibly related issues, but putting all the details in in-case they’re related.

    Late last summer, had the HT leads replaced due to engine misfires, apparently one lead had worn through, and was arcing against the block. Problem apparently rectified. Also moisture dependent. If I recall, cylinder 7 was affected.

    The latest issues started when I got a stainless steel exhaust put on last fall at a local chop shop. I got new cats and muffler, all Magnaflow. After that I started get CEL and my local mechanic replaced one of the O2 sensors (bank 2 sensor 1). After sporadic periods of no-CEL/CEL all pointing to O2 sensor Bank 1 sensor 1 I went to replace it, but realized the exhaust people had put the B1S1 02 sensor in the wrong place and it was being hit by the front drive shaft. Exhaust people moved the location, and I put the new 02 sensor in, no more codes for 02 sensors (at this point).
    But now, the main problem, if there is any level of moisture in the air, I now get terrible engine misfires and flashing CEL. It was dry last week, put 20 or 30 miles on the car no problems. Sunday was a rainy day, got about 2 miles from home and the trouble started, bad misfires, bad eggy smell.

    I got home and read the codes:
    P0307 – Cylinder 7 misfire detected
    P1300 – Catalyst Damaging Misfire Multiple Cylinders drive cycle C; Signal out of Range – Above Maximum
    P0304 – Cylinder 4 misfire detected
    P0300 – Random Misfire detected
    P0307 – Cylinder 7 misfire detected

    I haven’t tested with the spray bottle when dark to see if there’s arcing, but it’s almost certainly moisture related, but very immediate. I’m thinking perhaps coil packs going bad?? Does anyone have any input on this where to start looking? It’s entirely possible the 02 sensor is a red herring, as I had random moisture related misfires both before and after the exhaust work.
    Thanks,
    J
    1989 RRC
    2001 RR HSE

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  3. #2
    JUNIOR ROVER
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    Re: Engine misfires with dampness

    It certainly sounds electrical and cylinders 4 and 7 share a coil in the coilpack (coil pack 2). Damp will make it worse, especially if the top of the coil is dirty and allowing the spark to track to ground along the surface of the coil.
    96 4.6HSE
    98 4.0 Police spec
    and a number of others I maintain for the owners.

  4. #3
    JUNIOR ROVER
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    Re: Engine misfires with dampness

    If the misfires were happening on the same bank, I would have guessed upstream O2 sensor, but in this case, I would also guess that the coil pack may be bad. Looking at coil pack layout, 4 and 7 are next to each other, so makes the coil pack more suspect.
    02 P38
    SoCal, USA

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  6. #4
    FRESHMAN ROVER
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    Re: Engine misfires with dampness

    Are you absolutely sure you (or your people) got the leads the right way round?
    1998 Defender 50th 4.0L V8 Auto
    1998 Range Rover 50th 4.6L V8 Auto
    Toylander Series 2A

    Previously
    1980 SIII 2.25 petrol
    1997 200 Tdi CSW

  7. #5
    SOPHOMORE ROVER james's Avatar
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    Re: Engine misfires with dampness

    Thanks for all the replies so far.

    Quote Originally Posted by romanrob View Post
    Are you absolutely sure you (or your people) got the leads the right way round?
    Do you mean the HT leads, or the leads to the o2 sensors? In either case I'm pretty certain, as if there is no moisture, it will generally run OK (aside from the single o2 sensor issue).
    1989 RRC
    2001 RR HSE

  8. #6
    FRESHMAN ROVER
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    Re: Engine misfires with dampness

    If this unit has a distributor, it is possible for the humidity to enter the distributor when the engine cools, it then creates an issue as the engine warms, causing cross firing. I have also seen carbon tracks in either the cap or rotor, cause misfire in high humidity. These carbon tracks are typically caused by the higher than normal plug voltages caused by defective plug wires. Carbon tracks can typically be seen as shiny tracks using a very high intensity light, like a LED flashlight. Just a thought. Ray

  9. #7
    JUNIOR ROVER
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    Re: Engine misfires with dampness

    But it's a P38? Of course it doesn't have a distributor, it has coil packs.
    96 4.6HSE
    98 4.0 Police spec
    and a number of others I maintain for the owners.

  10. #8
    SOPHOMORE ROVER james's Avatar
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    Re: Engine misfires with dampness

    Correct 2001 4.6 HSE with coil packs. I suspect my profile pic caused confusion, as it's my classic.
    1989 RRC
    2001 RR HSE

  11. #9
    FRESHMAN ROVER
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    Re: Engine misfires with dampness

    My apologies for the confusion. I have seen coil packs deteriorate with age, and allow moisture into the pack itself. You might try using a spray bottle of water all over the ignition system. You might get water into something that causes a miss, which will assist in locating the issue. One other thing you might check is the condition of the plugs in cyls 4, 7. A cracked insulator or fouled plug might be the culprit. I suspect that the other codes are sympathetic to the 4,7 misfires. If you clear the codes, which ones come back? Ray

  12. #10
    JUNIOR ROVER
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    Re: Engine misfires with dampness

    It would have to be a pretty big coincidence if there were two faulty or fouled plugs (I fail to see how a fouled plug would only misfire in the wet though) on the two cylinders that share a common coil. The GEMS had four double ended coils each running two cylinders on a wasted spark system (the plugs fire on both the firing stroke and at the end of the exhaust stroke, hence wasted spark as it does nothing) while the later Thor/Bosch has these same 4 coils only in two coil packs, so each pack contains two totally separate coils. Cylinders 4 and 7 share the same coil so that is almost certainly where the problem lies. Packs are interchangeable so if you really wanted proof, swap the packs over and the misfires will move to cylinders 2 and 3 as they share the same coil in the other pack.
    96 4.6HSE
    98 4.0 Police spec
    and a number of others I maintain for the owners.

  13. #11
    FRESHMAN ROVER
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    Re: Engine misfires with dampness

    I had a misfire in two cylinders on my 2000, which I attributed to a bad coil. It turned out that the wires were swapped (connected incorrectly) when the previous owner messed with it. I installed new Bosch coils anyway, I got them from Rock Auto, they were reasonably priced.

    I found a long 1/4 extension to be a huge time saver for doing the coils. It has to be a 1/4 inch, 3/8 and larger won't fit in there. Powerbuilt makes a decent set of 1/4 in extensions that can be had for short money. You will need 1/4 in drive sockets, 8mm and probably 10mm. Larger ones won't fit.

    Coil replacement requires you to undo are some tiny nuts (8 or 10mm) buried deep inside. They are hard to reach and virtually impossible to see. Thankfully, the factory manual shows the location and explains the procedure. I used my 6mm endoscope to locate all the fasteners prior to doing the work. On my truck half the nuts were missing, so I had to get new ones. The coil setup on Bosch Rover engines is poorly designed, working on it is very inconvenient. I have seen youtube videos of owners who relocated the coils into a more accessible spot. There was a link to a bracket you could buy. I installed stock coils and plug wires, didn't bother with relocation.

    P.S. The factory manual shows how to open the hood wider than it normally does. I didn't do it, which in retrospect was a mistake. Do what the factory manual says.

  14. #12
    FRESHMAN ROVER
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    Re: Engine misfires with dampness

    Though rare, I have seen fouled plugs to act as a short, causing that plug to drain all of the energy from the coil, causing the other plug to either get very little or none of the spark, causing an intermittent miss. When wet, the coil may deliver even less energy, causing a rare miss, which might go unnoticed, to become more pronounced. I have also seen a defective coil foul a plug to the point that, when repaired, showed no improvement until the plug was replaced as well. Experience can be a task master. Ray

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