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Misfire exacerbated by moisture

Hello Car Talk community,

I would like the input of the group. I have a 2001 Ford Windstar Sports SE with 155K miles. I did a tune-up eight months ago. With the tune up I had to replace the fuel pump because it was defective. Later, I found that there were misfires occurring since I was getting slow acceleration. I had the spark plugs on the guilty cylinders replaced again and the spark plug wires.

Periodically, I have had to return to the shop to have the vehicle checked. I sometimes get a blinking check engine light while trying to accelerate, which now occurs only while it is raining or I am exiting the car wash. The shop is at a loss as to what it is (spark plugs and wires are fine, no lose connections noticed). I am wondering if I should take it to someone else. Of course, taking it somewhere else will cost me $100 to check on someone’s else work.

The shop replaced the ignition coil. Distributor cap is fine. They keep saying that the engine is running fine while in the shop and do not get any codes. I had them drive it while it was raining and they discovered that cylinder 4 is misfiring. They worked on the cylinder. The vehicle runs well, until it rains or I wash it.

My questions to the group, “If everything is OK, how come it is misfiring? What are they missing? What is the connection to the moisture?”

How old is the distributor cap? That would have been the first thing I would replace.

I’m unable to understand how a shop can replace only one of a related group of standard ignition parts when trying to resolve a wet weather problem. Or to claim that a distributor cap and its partner, the rotor, are fine. Moisture can penetrate a crack in the cap around a post that’s not visually obvious. And, if you used budget wires 8 months ago, they could be suspect as well.

Any good shop has more than its code reader available. And iot has a spray bottle. With an analyzer, and a “scope”, it can read countless sensor outputs as well as analyze the spark pulses for amplitude, ramp, noise, grounding, reliabillity, and numerous other anomolies.

I’ve attached a bit of information about scopes for your perusal.

The $100 you spend at a competant shop just might save you hundreds in unproductive parts swaps. I’d try another shop. Give them the whole story and see what they say.

That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

Skip the shop. Get a misting bottle with some water in it. On a dry day run the van and start misting things until you get a mess. Obviously start with anything electrical having to do with cylinder 4 - the wire & post on the distributor cap. Include the fuel injector harness. Misfire does not mean lack of spark (it only might).

My opinion is that a compression test should always be performed when any engine performance problems exists and as part of any maintenance where the spark plugs come out. Unfortunately, this is often not done.

If the engine has some cylinders going down this can lead to fouled and misfiring plugs. When moisture is involved the spark will always take the easiest route to ground and the easiest route is usually not through the tip of a fouled plug.
New plugs may help but it also may be just a short term cure.

A misfire only when there is moisture and high humidity means the electricity never makes it all the way to the plug that is missing. In the old days a new distributor cap was the best solution. A cap can look fine on the outside and have carbon tracks on the inside causing the electricity to arc to the wrong plug. If you have a distributor cap replace it and the rotor.

Next plug wires can go bad, and even the dirt and dust on them can hold the moisture causing the electricity to jump to the wrong plug or to ground. Clean the plug wires with a good solvent (kerosen) or replace with a new set of good quality plug wires. Replace one at a time carefully you don’t want to get the position of the wires wrong.

If your Windstar is equipped with COP’s (coil on plug) then you don’t have plug wires or a distributor cap. Each plug has a coil sitting on top of it. The rubber boots between the coil and the plug can go bad. Once a coil misfires a lot, it can damage the coil. Solution is to replace the bad coil and also replace that spark plug.

Thank you for the suggestions. I will give the shop one more try to fix the problem. Some of your comments brought up some intriguing points.

  1. Distributor cap: I have been told that it is fine. I can be more assertive and asked for it to be replaced, but how much does a new one cost?

  2. Spark plug wires and spark plugs: All are brand new. Possible defective ones?

  3. Rubber boots: This is the one comment that really intrigued me. While they were trying to assess what the problem is, they’ve noticed that this plastic “hubs” that connect to what I was told was the injector were loose on Cylinder 4 and 5. Shop told me that if I want it to replace it, they would have to replace it for all of them and the part cost $800. So they used a silicone based adhesive to patch them up. I am beginning to think that this is the problem.

  4. I may succumb to the ineptitude of the shop to fix the problem and take it to a dealer. I have been a customer of the shop for five years. I don’t want them to think that I doubt their skills by going somewhere else. Also, anytime I go to the dealer, they are always trying to convince me to trade the Windstar in for a newer model.

It’s almost impossible to tell if the Distributor cap is bad/good by a visual inspection.

Last cap I bought was for my wifes Accord…I think it was about $35. Replace the rotor also (another $20). Labor to replace both should be well less then an hour - although I’m not familiar with your vehicle…some vehicles replacing the cap and rotor can be a pain.

Silicon rubber patch job is not likely to hold up. The rubber will not handle heat, and it isn’t a good insulator so the spark can still jump. I don’t think this is a good fix, and I agree with you about this being a suspect area. I can’t really tell what the “hubs” as you describe them are, but still something sounds fishy about the “patch job” with silicone rubber.

My point about a compression test is this. If the compression is abnormally low this can lead to a deterioration of the spark plug and a misfire.

The spark will make it to the spark plug no matter what. The spark may not jump the gap on the plug and this is made especially more difficult with the plug tip being under tremendous pressure in the cylinder. Watching a spark jump a plug tip while the plug is placed on engine metal may or may not mean anything.

When the spark is unable to jump the plug gap it’s going to attempt to go somewhere else and that’s usually through the plug boot and into the cylinder head. New boots in an attempt to cure this is not the answer. Moisture in the air or plug wells just exacerbates the problem even more.

Many a vehicle has gotten every part in the inventory and depleted many a bank account because a compression issue was not weeded out from the get-go. The spark plugs are out so that potential issue should be taken care of at that point.
I check compression even on very low miles motors and can’t even imagine not doing it on a 155k miles engine.

New spark plug wires are often the relief for moisture related problems.

You don’t have a distributor cap or you don’t have a 2001. You have a coil pack that will most likley arc when sprayed with water as mentioned

Final result: The problem was alleviated by replacing the ignition coil (again). I took the defective one back to the original shop and asked for my money back. They eventually gave it back after I went through my credit card company to dispute the charge.

It is nice to hear a good resolution, thanks for posting back and happy travels!