Wet weather misfires

[sdg3205]

Recently I've experienced a misfire when driving in the rain. It's a pain in the ass on flat roads, but we're doing a rally in May up in the mountains and I'll need all 6 cylinders to be on their best behaviour.

I spoke with toby and he has some good insights. One possibility is the blast resistor is getting wet and causing voltage drop at the coil, another is that water is finding its way into the distributor. Yet another possibility is my wires are toast, but they were new Bosch from DMC in 2010 and have only about 15k miles on them.

The really bizarre thing is that this has only begun recently, which would actually lead me to believe it's due to "wear and tear." Has anyone else experienced this sudden wet-weather misfire?

[RamblinDMC]

I had a similar problem and found out that my distributor cap had cracked in half!

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[sdg3205]

I had a similar problem and found out that my distributor cap had cracked in half!

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Damn! so with the crack it was fine when dry, but misfired when wet?

[RamblinDMC]

It misfired sometimes when dry, but became even worse when wet.

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[Bitsyncmaster]

Condensation inside the cap is one cause of sudden misfire problems. It may or may not be worse in the rain.

[cis6409]

Mine was the same , driving in the wet was a pig it got so bad I had to pull over , eventually took it off the motorway and got on to secondary road . Nursed her back home but thankfully the rain stopped and the roads dried off considerably and it was like a switch going off , as soon as it got to dry off power was restored and she flew home ..

Changed the plug leads cause found 3 of them with tiny cracks and tears from vibration rubbing off the manifold(5 years old or so ) .
Also replaced distributor cap and rotor and found the cap had a crack on one side and was letting water In , you could see the black scorch track down the inside of the cap where it was arcing affecting 2 of the 6 points but only in that area ..

Since then all good in the rain thankfully

Cheers
Shane

[David T]

Condensation inside the cap is one cause of sudden misfire problems. It may or may not be worse in the rain.

Doubtful it is the ballast resistor but it could be bad/dirty/rusty connections to it. Remove each wire one-at-a-time and clean up any rust. It is most likely a problem in your secondary ignition system and that means replacing the dist cap, rotor, and ignition wires. If the spark plugs haven't been done in a while, them too. The wires could have been damaged and that is causing the problem. When installing them you must be careful not to bend or pull on them. Rare for a cap to crack but if mishandled or not fit to the dist properly it can happen.

[AugustneverEnds]

I experienced a misfire several times this past summer in wet weather. This winter it struck again when the car was parked in the garage. I needed to take the intake off for another issue and that's when I noticed the Bosch wires (installed in 2013) were slipping badly off the distributor cap. I did buy the Bougicords I posted about in another thread (cross referenced with a Volvo 262) and they fit fine especially over the spark plug well, very snug. I still need to put my intake back on so I haven't fired it up yet with the new wires. I will let you know how it goes.

I personally think Bosch's wires are a mediocre product and will only last 3 or 4 years when the car is being driven on a fairly regular basis.

[DMC5180]

I had an issue with arc jumping on my Original Coil. I replaced the coil and the coil wire. The coil wire was the original one with EMI shielding. The replacement wire is not shielded. I often wondered If that 30 year old wire was also a contributing factor. If the rubber insulation deteriorated on the lead it would be a very short jump for an arc to reach the Shield ground.


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[Josh]

Although I do not have the prv, I had a misfire under similar conditions. Turns out it was a damaged plug wire.
It seems hard to find a nice quality plug wire. Ive had multiple instances there the wire just falls apart, slides off the crimped terminal. Worse yet it partial slides off the crimped terminal and you are none the wiser that there is a poor connection.

[sdg3205]

I'm replacing the wires with Grady's AC Delco. A new rotor, cap and plugs from DMCNW. They're all 7 years old now and given this is a new issue for me I can only guess that it is due to parts wearing out.

[Rich]

I'm replacing the wires with Grady's AC Delco. A new rotor, cap and plugs from DMCNW. They're all 7 years old now and given this is a new issue for me I can only guess that it is due to parts wearing out.

Those would be typical parts to replace without more information. Might skip the rotor since it's inside the cap. You did not mention the coil but if you are running with the plastic cover in place over the coil it may not be getting very wet in the rain.

If you want to try something before swapping parts here's quick assessment method: take a look at the entire secondary ignition (from the coil all the way to the plugs) in the dark with the engine misfiring. You may see telltale sparks (exterior of dist cap, coil, coil/plug wires/boots). It will not show internal oversparks, only ones on the outside. Will not show any plug problems, either.

An extension idea from that method. In case the car isn't misfiring fill a hand-spray bottle with plain water. Use that to "rain" on selected parts of the system with engine running in an effort to isolate the problem area.

Both of these only work with the car stationary, of course.

[dmcnc]

I also had pretty bad misfiring kicking in over the past week or so after the big storm that swept through the Southeast. I found and corrected two issues:
1. Cylinder # 6 was full of water. Dried it up, but I still had some light misfiring, which led me to
2. Spark Plug wires arcs all over. I had no idea how bad it was until I did the water mist test on the wires at night with all lights turned off. I had about 6 points where you could see sparks jumping from the wires to the nearest ground, i.e....stainless fuel lines, auxiliary air pipe, and intake corners facing the interior. I insulated the wires with electrical tape and reduced or eliminated most of them. These wires are only a year old, but I'm going to change them out soon.

The car runs very smooth now.

[David T]

Those would be typical parts to replace without more information. Might skip the rotor since it's inside the cap. You did not mention the coil but if you are running with the plastic cover in place over the coil it may not be getting very wet in the rain.

If you want to try something before swapping parts here's quick assessment method: take a look at the entire secondary ignition (from the coil all the way to the plugs) in the dark with the engine misfiring. You may see telltale sparks (exterior of dist cap, coil, coil/plug wires/boots). It will not show internal oversparks, only ones on the outside. Will not show any plug problems, either.

An extension idea from that method. In case the car isn't misfiring fill a hand-spray bottle with plain water. Use that to "rain" on selected parts of the system with engine running in an effort to isolate the problem area.

Both of these only work with the car stationary, of course.

Good practice is to replace the rotor and cap together since they wear to each other. The other reason to replace the rotor is I have seen rotors that have arced to the distributor shaft leaving a carbon trail on the rotor. Difficult to diagnose and verify but it happens. Coils can also arc but it seems to be rare for the Bosch coils to fail that way. Bad wires and caps seem to be the main causes for arcing in the Delorean. Make sure you gap the plugs properly or that can lead to premature failure of the wires. If the gap is too big it might be easier for the wires to arc than for the spark to jump the gap on the spark plug. Also handle those new wires tenderly. The core is carbon and it doesn't take much to damage it. Tightly bending them or stretching them is all it takes.

[ssdelorean]

Damn! so with the crack it was fine when dry, but misfired when wet?

Exactly what happened to me a couple years back. My cap's crack was so small it could barely be seen without flexing the cap to see it open up.


Rare for a cap to crack but if mishandled or not fit to the dist properly it can happen.

Figures that it is a rare thing and happened to me. But if I recall correctly, I think I was over the recommended replacement millage by about 15k miles or so. That may have a factor in it's fatigue. :-)

[AugustneverEnds]

Put the intake on and got everything back together today, first time starting and driving with Bougicord spark plug wires. Started right up, no misfiring or other odd ignition behavior. Felt good on a short test drive, so far so good is the initial verdict. Interested how the AC Delco's compare to the previous Bosch's.