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Automotive Car misfires when it rains

  1. May 2, 2014 #1
    My 02 jetta misfires on cylinder 3 when it rains, has anyone ever had this problem and do I fix it? Also my horn doesn't work when it rains, I dont know if its related but it seems to odd to me coincidence. Thanks for help.
  2. jcsd
  3. May 2, 2014 #2


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    Sounds like you have a short in your electrical system which occurs when it rains.

    The plug wire to Cyl #3 could be bad (I'm assuming you have a gas Jetta). Plug wires don't last forever, and a replacement set shouldn't be too expensive.

    (After 12 years, you've gotten your money's worth out of the original plug wires, IMO)

    Check the wiring to the horn. There may be a place where a short circuit is created if the wiring gets damp.
  4. May 2, 2014 #3


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    With a 12 year old car, several different faults that occur when it rains could be a sign that the whole electrical system is getting old, rather than faults closely related to the symptoms. The extra stress of humidity or water spraying around pushes it over the edge from its "just about working OK" state when everything is dry.

    I would start by checking the battery terminals and earthing straps to the engine block, chassis, etc, for dirt and corrosion causing poor connections, and clean up any other electrical connectors that are dirty. It might be worth getting the battery and alternator checked. If the alternator brushes are just about worn out, a bit of dampness could make a significant difference to the operating voltage of the entire electrical system. I once had an old car where the ignition warning light sometimes stayed on dimly for a few minutes if it had been standing outside in the rain, until the alternator warmed up enough to dry itself out.

    A new set of plug leads won't do any harm, but they are unlikely to fix the horm problem, and they might not fix the misfiring either if the cause of both problems is somewhere else.
  5. May 3, 2014 #4

    Ranger Mike

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    These days the electric system is really touchy. When we ran into this situation the first thing to do is run a ground wire from the negative side of the battery to the engine. Try grounding it a the 4 coil ignition pack. I suspect you have a coil pack going out on you as the #3 coil is sandwiched between two other hot coils. Maybe has had oil leak. Check for this oil around the body of the coil pack. Run another ground wire to the body. Years of body flex and vibration will cause loss of ground. If this does not cure things go for the in depth. I would check the wire connection at the horn as this is at the front of the car where all the slush and road muck rally corrodes things. Again a bad ground. Electrical resistance gores up when things are wet.
    Last edited: May 3, 2014
  6. May 4, 2014 #5
    One thing I have always found helpful when diagnosing wet weather symptoms is a cheap spray bottle full of water. With the car at operating temperature I would spray around each component (coil, plug wires, etc.). Do these issues start at the same time?
  7. May 5, 2014 #6

    jim hardy

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    Run it in the evening with hood up. Look for tiny sparks jumping around #3 sparkplug and its wire.
    But i'm more suspect of the coil pack - they will develop cracks that allow moisture ingress.
    If you can see the coil, look for a small black line on it where case has split.

    If it's the coil it will get worse,
    and if it's a setup where one coil serves two cylinders(which i think it is) you'll get misfire codes on that cylinder too.

    Try a search on "2002 VW coil pack recall"
  8. May 5, 2014 #7
    Thank you for the advice. My mechanic said to try spraying some silicone on the distributor and see if it helps and it has a bit so im thinking its probably a crack. The odd thing is that the parts are all pretty new as I had a new engine out in about a year ago.
  9. May 5, 2014 #8

    jim hardy

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    distributor ? or coil pack?

    6 cyl obviously, 4 would be 2/3 of that.

    See if your misfire codes are on two cylinders served by same coil.

    Reason i think that is i just helped a neighbor with his 4 cyl Chevy that uses similar coil pack. We found a split on the side of mis-behaving coil that served #1 and #3, the two that reported misfires.

    On the GM coil pack an ohmmeter between the two high voltage leads for a good coil shows a few thousand ohms, and no connection to ground( the metal frame..) Bad coil showed a few thousand to ground. Yours might be similar.
    Last edited: May 5, 2014
  10. May 5, 2014 #9
    Yes your right it is a coil pack, you are almost certainly right that it is a crack in the housing but sadly it is very hard to see it as in is tucked under and behind the air intake. Is there a risk in using a spray bottle to check for shorts? I cant imagine that its good for the car to cause a short on purpose.
  11. May 6, 2014 #10
    By using a spray bottle, you will be duplicating any rain getting on a component. I wouldn't spray items such as an ECM (I'm not saying your ecm is bad, I'm referencing a component that you wouldn't want to intentionally spray ), but the bottle will help find a coil or plug wire that might be broken down and causing the spark to short (this works well on the secondary side of ignition). You might spray around the coil or plug wires and look for spark. As Jim mentioned, if you do this in the evening, you might be able to see this a little easier. I have used this method for years when diagnosing someone’s car that has an older ignition system. Also, in some cases those coils can break down on one cylinder, it is common to lose two at once, you might need a scanner to see the misfire on the second one. I actually have a VW coil in my toolbox that lost one cylinder.
    Last edited: May 6, 2014
  12. May 6, 2014 #11

    jim hardy

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    hmmm that's very interesting.
    My friend's car gave misfire codes , ~8 times out of ten on cyl #3 and ~2 out of ten on cyl #1. That of course told us it was something common to those two cylinders. Coil pack was a lot cheaper than the ECU.

    Listen carefully while you're under the hood, for often you can hear a spark jumping through air where it doesn't belong, it makes a "snap" sound.

    not much. Don't go hog-wild with a garden hose.

    Do you have an inexpensive ohm-meter?
    Resistance readings from my friend's spark plug connectors would have told us what was wrong had i known how to interpret them. I described that above.

    Maybe z51vette knows -

    if your VW pack is is like this
    you should read several thousand ohms between the two plugs served by one coil
    and no continuity at all from any plug to chassis ground.

    A plug wire itself should read a few thousand ohms. So use a high ohms scale at least RX1k and X10k would be better.
    Plug wires themselves can fail open circuit but that'd probably just kill the cylinder entirely. Two to ten thousand ohms per foot is ballpark for them.
    Last edited: May 6, 2014
  13. May 6, 2014 #12
    I'm not 100% sure which engines they used waste spark on. However, I believe the 2.0l isn't waste spark, but i could be wrong. It might be worth looking on the VW forums, I'm sure there are experts on there.

    As for testing coils the primary side is generally low(less than an ohm) the secondary side shall will be higher (several thousand ohms). Even if it test good, it could still be breaking down with moisture.
    Last edited: May 6, 2014
  14. Mar 14, 2015 #13
    yes damp weather will cause weak ignition system parts to missfire, or even fail, and even stall the engine in dangerouse driving conditions (heavier rain) especially on 4.3 motors and especially on Astro's as the dog house will hold in mousture. You'll get about 150,000 miles out of the factory ignition parts, but dont be confused about aftermarket replacement stuff not lasting long, and you'll only get about 20 to 30 thousand miles out of the oem dealership/delco stuff before something needs replaced again, so when it starts doin the same thing again after 20 to 30 thousand miles its time to replace an ignition component again, thats just the way it is. I keep one extra of everyrhing in the vehicle bcs when these replacement parts go bad they go bad fast and i travel alot for camping n concerts n stuff. My glovebox cup holder thing is long gone in a bunch of broken pcs...lol, just as well its easier to get to the motor now. No prob tho i've got a real nice cooler with cup holders that fits between the buckets. Love my astro that thing goes like a mountain goat in any weather.
    Last edited: Mar 14, 2015
  15. Mar 14, 2015 #14
    if you want an easy way to find out what ignition component is bad, pull off the coil wire that connects to the top of the coil post and hold it about a quarter inch away from the coil post (use insulated pliers) and turn the engine, a spark should jump from the exposed metal part of the wire to the coil. If not, you have a bad coil ( replace the coil and the control module right next to it) if you do have spark, replace the coil wire to the coil post and remove the other end of the coil wire from the distributor cap and hold the exposed end of the wire next to anything metal and turn the motor, if no spark you have a bad coil wire, but if you do have spark reattach the coil wire and pull a spark plug wire off at the distributor cap and hold it next to the distributor cap post and do the same thing, if you have no spark there your distributor cap and rotor may be bad, and if you have spark there, reattach that plug wire to the distributor and remove the other end of the wire off from the spark plug and hold it near something metal,if no spark there it means your plug wires are bad, if you do have spark there pull the easiest spark plug out and with the plug wire attached to the spark plug hold the metal end of the spark plug next to something metal and do the same preceedure again, and if no spark at the plug itself yout plugs may be fouled. But if you are getting spark to your plug you probably dont have an ignition component related problem. But remember weak ignition components may work ok on dry sunny days, but they may fail on wet damp days, so... you definately want to check these components on a wet rainy day when the vehicle does not want to start at all. WHEW..LOL remember these replacement ignition components dont last long these days, so when your engine starts running bad you may just want to replace everything all at once bcs the components may start failing one after the other, and it'll just be a big pain. my other vehicle has coil plugs and i just change everything all at once on that as soon as it starts running bad, its expensive, but your motor can last you a very long time with proper maintenance,
    Last edited: Mar 14, 2015
  16. May 23, 2015 #15


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    Does the horn stop working when the wipers are turned on? or when it is wet?
    "Wipers on", would suggest a wiring fault or short between the steering column cables.
  17. Jul 11, 2015 #16
    HELLO !
    I will continue if this message is accepted. New member!
    Thanks for interesting read.
  18. Jul 11, 2015 #17
    Many thanks for the good read about car ignition systems when it rains. We have recently 'aquired' a Chevrolet Tacuma 2.0 Auto (seven years old) which is magic when good weather. Tows a large caravan with no problem. When it rains however, it simply will not start. Engine cranks over no problem but no fireing. Leave it for a few hours in fine weather and Hay ho..starts at first time. I am wondering what you think of WD40 or Maplin spray silicone grease or Holts Damp Start perhaps on the coil pack and spark plug leads ? I will try the water spray on parts as suggested. tried looking under the bonnet in the dark for 'sparks' but I guess not dark enough where we are.
    Nice comments about the subject on this site, best Ive seen on the subject so far.
    Please excuse any spelling...never been able to spell and now at 75 probably never will.!
  19. Jul 11, 2015 #18


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    @ norcimguy. Welcome to PF.
    You should not apologise for your spelling, it is excellent. You only missed the c in acquired and put an e in firing.
  20. Jul 11, 2015 #19

    jim hardy

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    The trouble you describe sounds like something water soluble has got on the high tension circuit somewhere. It might be salt from winter roads, or antifreeze from a summer overheating incident.
    My Oldsmobile developed that trouble after a spider spun a web inside the distributor cap.

    Thorough cleaning is a good start. I'd use a solvent like denatured alcohol that's miscible with water, or maybe engine degreaser..
    I've even washed them with laundry detergent & distilled water then dried in the sun.
    Don't overlook inside of distributor if it has one.

    Hint - set your radio to AM between stations with engine off.
    Set volume so you can hear the hiss.
    Start engine - you should hear ignition noise. A single wire arcing will announce itself as distinct "pops" at engine speed, above the general interference...
    Place in gear and load engine. Spark voltage goes up as cylinder pressure increases, so you can judge the condition of your wiring by how much the spark noise increases. Compare to another car that runs well.

    Get and learn to use an engine code scanner. In US we have a standard called OBD2, applies to everything post-1996. I find the basic fifty dollar el-cheapo works just fine.
  21. Jul 14, 2015 #20
    Sorry for late reply and thanks for the 'excellent' spelling bit. That's a first! Had to call out roadside assistance RAC (UK) on Sunday. Guy plugged in a laptop and ECU was OK. Then proceeded to bang a spanner under the fuel tank. The car then started. Outcome was ''get the fuel pump changed ASAP''.
    Good weather here and car has started no problem. I have tried the AM radio test Jim and so many thanks for your detailed input. The off station noise is pretty uniform at tick over. There are no abrupt 'clicks' or anything which leads me to think that it may not be a HT problem.
    Two things come to mind:
    Did the banging on the fuel tank disturb electrical connection to the fuel pump? So the car started. OR, is there some wiring in the engine bay that when it rains, it shuts down the supply to the pump.
    At the moment (dry weather) I can hear the pump fire up for a few seconds at switch on.
    I am waiting for rain to see if I can hear the pump (or not) at switch on.
    Will report back and many thanks.
  22. Jul 14, 2015 #21

    jim hardy

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    That banging on the tank changed something suggests your trouble is physically near wherever he banged.

    Fuel pumps today are very high speed rotary type with very small clearance. Most common failure is it ingests a tiny piece of grit that locks it up. Sometimes it'll break free for a while. I had one do that at only 4,000 miles.
    Often the electrical connection is atop the tank and while they can get wet, auto manufacturers have learned from marine industry how to make weathertight connectors. Still, anything can happen.

    IMHO a more likely spot is where the power return wire(ground) connects to the car's chassis. That'll be a bolted connection and if it corrodes, power gets to the pump but cannot get the rest of the way back to battery - it's blocked at the corroded connection. So the pump is starved for power.

    Anyhow - to get to it one usually must either drop the tank or remove an access cover. When you replace the pump ask your mechanic to remove, clean and put Fel-Pro conductive grease on the ground connection.

    I recently helped a friend get his pickup truck running. His fuel pump was "grounded" to the bed okay and that connection was clean, but a braided ground link tying truck's bed to frame was corroded away. So power got to his pump just fine but couldn't get back to the battery/alternator . He was on his third pump in as many days. Giveaway was the engine barely ran (we measured low fuel pressure) and wouldn't run at all when taillights were on - they too were "grounded" to the truck bed. Without that braid the only return path was via rusty bolts holding bed to frame. Those rusty ground connections blocked (though not quite completely ) power return path for both fuel pump and taillights.

    Your mechanic should have access to wiring diagrams that'll show where the fuel pump's ground connection is.
    I trust you're not having any other symptoms like funky taillights or turn signals.

    If you want to experiment underhood, swap the fuel pump relay with the one from something innocuous like sunroof .

    my two cents, and that's overpriced.

    Have fun and learn alot.

    old jim
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