Baffling radio interference

[Punomatic]

I have a 2001 Ford F350 crew cab, long box pickup, with a Triton V10 engine, which we picked up in October. We love our new-to-us truck, but we have an issue with the radio.  We live in a rural area, in case that makes a difference. 

So, here's the problem: when we tune in an AM station, there is interference in the radio reception.  I understand that ignition interference can cause static.  The strange thing is that the interference is like a popping sound that becomes louder when I step on the accelerator, AND the popping sound DECREASES in frequency as the RPMs INCREASE.  I am far from conversant with radio signals and pickup ignition systems, so I am baffled. What would cause the frequency of the popping to decrease with increasing RPMs? Intuitively, I am thinking that as the cylinders fire more frequently, the radio interference from the ignition system should increase in frequency.  I guess my two questions are:
[list type=decimal]
[*]What causes this noise?
[*]How can I eliminate the noise?
[/list]
Thanks in advance for any insight you can give me for this problem.
Tom

 

[donn]

How many miles on the truck?
My first guess is you have a spark plug wire that had broken down.  What I suspect your hearing is RF interference from one or more plug wires.  This evening after it gets dark out, go out start the truck and let it idle while you open the hood and look carefully at the wires.  You might see some sparking and arcing around in there.

 

[Punomatic]

How many miles on the truck?
My first guess is you have a spark plug wire that had broken down.  What I suspect your hearing is RF interference from one or more plug wires.  This evening after it gets dark out, go out start the truck and let it idle while you open the hood and look carefully at the wires.  You might see some sparking and arcing around in there.

The truck has 115K miles on the odometer.  According to the previous owner, whom I trust, he had the 100K tune up done. I would imagine that checking all the ignition components would be part of that service, but I don't know. I will try your "arcing test," if I can figure out how to turn off the motion activated light in my garage. LOL. Still, I would think that if an arcing ignition wire were the problem, the popping sound would increase in frequency with increasing RPMs, not decrease.

 

[Lou Schneider]

Spark plugs require more voltage to jump the gap at higher RPMs or when the engine is under a load.  I think the spark plug wire diagnosis is a good one, but if a wire is only breaking down under load you may not see anything at idle.

 

[carson]

I would think that if an arcing ignition wire were the problem, the popping sound would increase in frequency with increasing RPMs, not decrease.

  We all seem to be overlooking the point he is making in the quotation above. I agree with him.

 

[jje1960]

EMI can (obviously) be caused by any number of the electronic components.  One very basic trouble shooting technique would be to disconnect one plug wire at a time, then see if one of them could isolate the problem.  Again, this issue could be caused by any number of components.

 

[carson]

My point, Jim.  Let's come up with a few "alternate components"  I can't think of any off-hand without putting on my thinking hat right now.

 

[John From Detroit]

Other things on an engine that cause RFI (Radio frequency interference) are the fuel pump and the emission controls.. Now some of those may "Slow down" as speed increases.

FORD may be able to help you. They do (or should, I know both GM and Chrysler do but am not so sure about Ford) have divisions that deal with RFI issues.

 

[dsrtrdr]

Impulse noise - Impulse noise is created whenever a flow of electricity is abruptly started or stopped. Most impulse noise affect the lower frequencies. There are many possible sources for impulse noise it is important to keep an open mind when analyzing RF noise. Some causes may be over torquing a 7/16 DIN connector (this causes the center pins to move back into the cable a bit, then reopening the connection, perhaps as a test, then reconnecting the cables at the right torque, but now with the pair of center pins that do not make a firm contact. As I mentioned there are many possible sources for impulse noise. IF your unit had filter before remember that filters do fail.

 

[jje1960]

I actually had a very similar situation on a Ford ZX2 car a number of years ago, it actually turned out to be the wiring that was feeding and/or around the radio itself.  If I remember right, I did isolate the interference, which was a harness behind the radio, just checking connections and wrapping black tape around the loose wires in the harness ended up fixing the issue.  However, could be so many different reasons... Sorry, don't have any Jim silver bullets to offer! 

 

[John From Detroit]

That sounds like the radio problem I had Jim... After spending a considerable time saving money on hair cuts (you don't cut it if you pull it out).  I finally pressed on a MOLEX connector and heard a nice sharp SNAP as the latches latched it to the socket it was SUPPOSED to have been plugged into.. Radio has been 100% since.

 

[Mavarick]

I'm just guessing here based on some of the older systems I remember back when am was a big deal. Right now of course the easiest fix would be to plug in your mp3 and not to worry about the am side of things! 
I agree your popping noise is high voltage cross over or arcing as Donn and others have pointed out. It can be caused by a number of things including where the power wire for the radio comes from (aftermarket install?). Another words you could replace the electronic ignition system and still have some noise. Most used to clear up this noise by installing a noise filter on the radio from most auto stores or radio shack.
You probably have a bad cap or plug wire as is suspected. Why the rpm difference, think of it this way. When you watch a hub cap on a car barely moving you see the rpm of the hub cap. As the car speed increases and reaches a certain point the rpm of the hub cap seems to blur and slow down in rpm, not increase with the speed of the car. The popping noise increases a bit when you step on the accelerator, then reaches a point where it blurs and won't get any faster, as it seems to see a slower arc, then when you take your foot off the accelerator, the rpm of the engine slows to a certain speed where the popping actually starts to speed up again, then the popping slows down once again to match rpm of the engine. This is what I remember in some of the situations I ran into. Listen closely to your noise and see if it matches this explanation a bit better now. If so, look for a filter if you want to listen to am. Otherwise, go to FM or get your mp3 player out. Good luck with it.

 

[1joester2]

Just for grins....
Start it up at night when you can easily see sparking around the coil packs. Tune the AM and set the volume so you can hear the popping over the engine noise and see if you find rouge sparks from a failing wire. If not, make sure the plug cups are clean, then retest. If that proves fruitless, put in a set of plugs.


My '02 Escape had a popping on AM that was pretty random but consistent. I noticed late one night I could see sparking on one of the front coils, so I replaced all 6 and the plugs while I was in that far (Yay Ford, remove the intake to access the rear spark plugs... really?). When we got to Florida, it was back. The popping was also consistent with a developing miss while driving and was getting progressively worse. I had taken a spare coil so I swapped things around thinking one of the coils was bad, but found nothing. My dad suggested we meter the spark plugs and sure enough one metered wide open. We replaced it and it's been great since.


...just sayin'...

 

[Punomatic]

Thanks for all the thoughtful replies. I really appreciate how everyone is trying to solve this problem for me.  Yesterday, I had a thought. We have an RV garage that is long enough to keep our camper and the pickup, provided I park the pickup with the front end under the cabover of the camper. In order to make that work, I changed out the antenna from the OEM to a shorter mast, shortly after we bought the truck. It occurred to me that the antenna might be the problem.  With engine running, I swapped the antenna masts and found that the noise on my favorite stations was the same with either mast. There was also no discernible difference in the quality of the reception.  Just for fun, I hit the seek button on the radio.  It quickly found a "local" station (remember, I said we live in a rural area), and the reception was perfect, with no noise! So, perhaps, I just need to find stations that are closer to home. Only problem is, I don't WANT  to learn Spanish at this age!! 

 

[driftless shifter]

How old are the plug wires? If over 50k or 5 years they are probably breaking down. They may pass an OHM's test, but the rest of the wire begins to fail. The popping may change because spark plug resistance changes with the amount of fuel not air. Resistance is highest when accelerating with a heavy foot, lowest at idle or coasting. The resistance is lowered at cruise because the throttle isn't at or near wide open.

 

[carson]

Me again.... One thing we still haven't answered is the pulsing/popping rate at differing rpm's of the engine.

If the same rate is present at all speeds and only the volume changes, then the spark plug wire theory doesn't make sense.

Being in a rural area with only weak signals cause the AGC (automatic gain control) of the AM radio to go to high gain and will try to make weak signals readable. Any trace of a signal will be received...hence the dreaded static noise.

  This of course will not find the source of your popping sounds. Consider this just more informational data.

AGC description here>>>  http://en.wikipedia.org/wiki/Automatic_gain_control

 

[donn]

Move into Portland or get yourself an XM/Sirius radio for CHRISTmas.  Either should cure you problems.