Humid weather - rough idle and no power with cold engine

It sounds characteristic of deteriorated spark plug wires to me. They may look normal, but be failing when moist. Common.

Thanks!

Is it possible that the problem is related to the distributor cap like cracks in it or something? Or would such a problem manifest itself differently?

Absolutely, a defective distributor cap can do the same thing. There are more elegant ways to diagnose the problem, but you can try this...On a moist morning, take off the distributor cap and wash it out with several sprays of WD-40. Sling it more or less dry, and reinstall. Start the car and see if it runs better.

You can do about the same with the plug wires, but you dont take them off the plugs. Spray them down well with WD-40 and start the car.

If you get a noticeable improvement in either case, replace the offending part. (If you replace the distributor cap, replace the rotor at the same time)

Usually if it is a moisture related problem, this will give you a clue.

Plug wires can fail in other modes, not just loss of insulation resistance on muggy days.

I like to take a spray mist bottle of water and spray the wires with it in the dark. If the wires are bad, there will be a wicked light show....

Mike

Absolutely, they will do that. Amazing how much light is generated under the hoods of some cars when it is dark out.

Like the man wrote, WD-40, - Water Dispersant 40. - disperses water. It penetrates like penetrating oil. Handy stuff. However I do like to remove my ignition wires and wipe them clean as well. WD-40 or paint thinner cleans them. If you decide to replace the wires you can buy a set with a lifetime warranty. Just file the receipt away where you can find it and get a new set for free whenever you need one.

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This is all good stuff! Thanks!

The only concern I have is that WD-40 is relatively flammable. Will it ignite from the sparks that could still be created?

Have never seen that happen. It is safe enough, when used in this way. It is indeed a water dispersant. As a spray lubricant, it works but is not the biggest apple on the tree.

I suspect you will find the spark plug cables deteriorated.

Let us know how you come out.

Well, with the working engine in the dark I sprayed some water first on the spark plug wires (nothing happened), and then on the distributor cap, and saw a couple of these really visible nice orange sparks appearing on the distributor cap. I'll double check in the daylight to see if there are any obvious cracks there ... But the idea is clear!

This group is amazing, what percentage of car mechanics would actually figure out something like that without replacing a whole bunch of expensive stuff ?!?

Thanks to all! Mike

snipped-for-privacy@nospam.nix wrote:

A word of caution.. You may never see any obvious tracking or damage on the distributor cap so dont let that lead you down the wrong road. Sometimes there are carbonized tracks INSIDE the plastic.

Other times you may see the burn track inside the cap, perhaps between two of the towers.

There is a way to check for an internal burn track, but I wont go into it here.

It can and will ignite if sprayed on when the engine is running. Once it has been sprayed, it evaporates fast and the left over wetness won't ignite easily that I have seen.

We use it a lot in the distributor caps of Jeeps before a water crossing or just after if we get them wet. A quick spray and wipe with a rag and we are good to go.

I would be suspecting a bad cap and rotor in your case though.....

Mike

snipped-for-privacy@yahoo.com wrote:

Indeed there was nothing visible on the surface, just large patches of semi-wet looking oily dirt. The discharge was happening at the place closest to the engine's metal (~2 mm from the cap). So it appeared as if the electric current traveled some distance on the cap.

Since I did not have any WD-40 handy, I used 70% iso-propyl alcohol instead (the stuff they sell in pharmacies). I think it is even better than WD-40 since it can wash away both water soluble stuff (like salts) as well as some of the oily substance. Using more hardcore solvents is bad for the plastic.

If the cap is well washed and the conducting channels through the cap are far away from the metal, I think that keeping the cap clean from the water absorbing dirt is all that may be needed in short term.

I should add that after spraying alcohol on the cap, it dried out and looked uniformly dry without any wet patches. Worked like a charm!

I find an application of dielectric grease or 'spark plug boot protector' to name a type on all the plug and cap boots works well to keep humidity issues at bay.

Mike

snipped-for-privacy@yahoo.com wrote:

Sure, IPA works fine. You may find this is a cure, or you may find that the cap, rotor, have sustained enough oxidation that they will return to arcing when the humidity gets high.