94 VW Golf hates the rain

During rainy/misty days, esp when a little cool and/or quite humid, my car gets very cranky. It will start, but run rough, sounding very much like a misfire. To keep it from bogging down or cutting off while accelerating, I have to give it a heavy load of gas. And if I cut it off while it’s running this rough, it won’t start again for about 10-15 minutes – even if I leave the car and come back after that time, it won’t start for another 10 minutes after trying once or twice.

On hot and sunny days, it NEVER does this. Runs fine. And just to make things muddier, it occasionally runs fine on rainy days.

The only recent work done on the car has been: a) new clutch about 2 months ago, b) adjusted the idle up slightly after car was cutting off when braking/stopping, but only on hot, dry days. (The rain problem started before this adjustment was made.)

Car is a manual, with 140K on the engine.

It needs at least the replacement of the spark plug wires, and possibly also the replacement of the distributor cap. The insulation on the wires is cracked and degraded from years of engine compartment heat, and these cracks admit moisture during very humid conditions, thus leading to a problem getting enough electrical energy to the spark plugs. Similarly, distributor caps can become cracked and damaged over time, causing essentially the same symptoms.

The old, dried out spark plug wires on this car are likely to be the cause of the rough running when it’s wet outside. Old wires develop small cracks in the insulation. Moisture in the air allows the current in the spark plug wires to short to ground against the engine, and the plugs don’t fire correctly.

You can see this phenomenon at night. It looks like tiny little bolts of lightening arcing from the spark plug wires to the engine. It’s cool to watch, but doesn’t do the car any good.

Try a new set of spark plug wires.

How did you adjust the idle speed?

Mechanic adjusted it. Can’t quite remember what he did, but it wasn’t just changing the idle speed.

As for the wires and cap – simple fix, right? Something I can do? Nothing trickier than making sure the right wires go back in the right order? Thanks for the replies.

Yes, this is a DIY job. If you replace one wire at a time you can’t do it incorrectly. If you get the wires on the wrong spark plugs the engine won’t run very well at all.

One way to make sure the wires are correct is to take some digital pictures before you do anything, that way you can confirm it’s right when you’re done. Also, replace both the cap and rotor while you’re at it.

Replace the spark plugs too. Most plug wire and coil arcing (worse with aged wires and coils) is often caused by worn and/or misfiring spark plugs.

An old spark plug is simply the first domino to tip over and start the chain reaction.

(The idle speed should not be tampered with except under certain, very limited circumstances. Simply cranking the throttle stop screw in to cover up something else is a no-no. Occassional dying at stops can be caused by a faulty/dirty Idle Air Valve if it does this when the car is apparently running well.)