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Popping through stereo after replacing spark plugs and wires

Hi All! I have a bit of a weird problem that has popped up over the last couple days after I took my 1998 Mercury Mountaineer 5.0L V8 into a local shop for some repairs. A bit of a back story first… Over the last weekend the engine started misfiring on me on the way home from work. I won’t go into details on the misfire because it was easy enough for the shop to diagnosis and fix when I took it in (This is a local shop here in Austin that I’ve taken the vehicle to a couple times now and always been happy with… They also get quite good reviews from most people on the internet). Anyway, they call to tell me that when they read the codes, #7 was the main culprit with a couple other cylinders having a random misfire, I don’t remember which. They said they got to digging around and somehow my #7 plug wire had melted in half, their theory being it had come in contact with the exhaust manifold.

Long story short, I had them replace all the plugs and wires and do a tune up and the car is running great! Here’s where the weirdness comes in. I have an aftermarket stereo that I installed (a Pioneer if it matters) and since I picked up the car there has been a moderately loud popping noise coming through the speakers when I have the receiver set on certain inputs. The noise is sort of a popcorn popping… It increases in speed with higher engine RPM and decreases at idle sometimes going away almost altogether but it is always the same volume no matter the RPM. If I lower the volume on the receiver the popcorn volume drops accordingly.

This might be getting too far out there, but the last thing I’ve been able to figure out is that the popcorn noise is there when I have the receiver input set on “CD” or “USB” (such as with an ipod plugged in) but if I set it to “Radio” or “AUX” (the little headphone style connection) there is no popping at all.

I wanted to get some advice before I go tearing apart my dash/engine compartment… My first thought was that I need to check the grounding on the stereo receiver, however it was perfectly fine before the plug repair and started as soon as I got in the car from the shop. My other thought was that perhaps somehow the new plugs/wires are sending a spike in voltage through the electrical system, but I’m no electrician so I don’t know how this could happen, or if it was happening why it would effect the stereo.

Does anyone out there have any idea what is going on in my car?!


Spark-plugs for cars usually have a resistor in them to prevent engine noise in the radio. Sounds like the one or more of the plugs resistor isn’t working right.

I’ve had this problem with Cheap Auto-lite plugs and also with expensive Nippendenso (Now called Denso) plugs. Never had this problem with OEM plugs (AC-Delco or NGK).

But the spark plugs are the first thing to be checked.

Thanks Mike! I know the wires are Auto-lite, not sure what brand was used for the plugs. Is there any easy way to figure out which cylinder might be causing the problem?

Almost always the best performing, least problem-causing plugs and wires are OEM.

It is possible one of the plug wires is not fully seated onto the plug, or at the distributor or coil end, and the sound you hear is from a spark jumping that gap.

Nope…one at a time.

Auto-Lite…Some people here have had no problem with Auto-Lite…Others like me have.

The BEST plugs and wires you can use are OEM. That doesn’t mean you have to buy Ford plugs. You can buy Motorcraft (which makes the plugs for Ford). For GM use AC-Delco…Most Asian use NGK or Denso.

I just stay away from Champion and Auto-lite.

Since you say the engine is running well and you don’t hear the noise on the AM radio band I have to think the work was well done but wonder why the other inputs you mentioned are picking up the noise. Normally the AM radio band is the first thing to pick up a problem since it is fairly sensitive to noise problems like this. I assume the USB function uses an external cord to make connection but does the CD input do that also? I am thinking the noise is being picked up by an external connection cord that is acting as an antenna and picking up the noise. Cleaning the grounds to the engine, chassis, and battery may help reduce or eliminate the noise level.

The problem isn’t under your dash, it’s under your hood. Since the interference wasn’t there before you had the vehicle worked on you should bring it back and let them ake a second look, while realizing that they were focusing on correcting the operating problem and not the reception. In other words, understand that even if something is amiss, that does not mean they caused it or even should have caught it. They probably didn’t check for radio interference.

They know exectly what they did under the hood. they can probably correct your radio interference problem post-haste. Honestly. an oscilloscope might be in order here. Spurrious ignition signals can be seen on a scope, as can signal anomolies.

Thanks for all the tips… I have already made sure of a good seating between wire and distributor, I will check the plug end tonight. As far the AM band goes, I hadn’t checked that but I just ran out to the car and there is a definite ticking static while the engine is running, however it’s not the sharp pop that I’m getting on the other inputs… Still definitely something going on there.

The USB input is just a port on the face of the receiver. Everything is enclosed in the one unit, i don’t have any external CD changers or anything like that.

I think taking it back to the shop will probably be the best thing… In people’s opinion here should I expect to pay for shop time if I bring it back in or should they correct the problem themselves?

I think you are into a gray area there. They did their job and made things work better. I think they would be justified in charging you again to try and fix this new issue but hopefully they will work with you on it for the sake of good customer relations.

If the radio interference was caused by some error they made, like not fully seating a wire or using an incorrect sparkplug, they should correct it for free. I’m sure they’ll have no problem doing so.

If the interference is being caused by something additional to what they repaired but they simply didn’t catch it because they were focused on the operating problem (I usually turn the radio off when looking at an operating problem just to hear the engine better), than they’d be fair in charging you to fix it.