RFI and the radio


I am getting a huge amount of RFI noise in my radio, surprisingly not related to RPM but related to the amount of depression of the throttle. This is most easily heard while driving up our hill. Depress the throttle and the noise increases in frequency and volume with the depression of the pedal and before the RPM picks up. Some times it is worse than at other times. The noise is not as bad down town. The car is an 03 Ford Focus wagon. The CD player works well. Does anyone have any suggestions as to the cause and solution??


Plugs or plug wires.


I was thinking that there must be some sort of throttle position sensor that is radiating as the RPM is only a very minor contributor to the noise. Plugs and wires are in good shape through a visual inspection. Maybe some sort of a filter is needed but I don’t know where, the radio is only fairly clean down in town. We are only a couple of miles out of town and reception from local stations are affected also.


If the freq and volume really does not increase with RPM, that argues against the source being ignition (plugs/wires). If freq/vol vary with throttle position, I wonder if the noise is from the fuel pump, or (weird and less likely) maybe something related to fly-by-wire throttle (if this car has that). I think other people here will know more the liklihood of about this, and how to fix it.

Alternatively, since the noise appears to be coming in as RF (CD player works), and since it is not as bad downtown where the radio stations are stronger, perhaps the antenna connection or its ground is going bad or the trimmer capacitor (for AM) needs to be adjusted. I.e., maybe the noise is at the same level it has been all along, but the radio is getting less of the desired signal. If the problem came on suddenly, I’d look at this first.


Try changing the plugs and wires anyway. A visual is not definitive. Insulation breaking down, or even a poor connection at the plug end clip (I’ve had these) can be easily not readily visable.

Plugs and wires account for the overwhelming majority of radio interference problems.


You have not told us what make model and year car you have. That could make a big difference.

My 1970 VW Beetle was mostly mechanical; my 2002 Diesel Beetle Has a drive by wire system so it may all kinds of electrical noise related to were my accelerator is.


The original post says it is an 03 Ford Focus Wagon.


Does it do this with the engine not running, i.e. if you turn on the ignition and press down on the accel pedal without starting the car? Does it do this with the engine running and the car sitting still (i.e. reving the engine) or going in reverse?


With the ignition on I get only a slight buzz from the radio with no throttle effect, upon starting the car there is only a slight increase in the buzz, revving the car adds no additional components to the buzz. Upon driving home today the buzz was definitely controlled by the position of the throttle, especially noticeable while going up a hill. It is not linked to the rpm but the position of the throttle. I think that when the rpm catches up the buzz is reduced a bit. I have no idea about a fly-by-the-wire throttle, probably not on this car as it is not an expensive one. There is definitely less noise downtown, but our truck radio seems to be working fine up here. Thanks all for these answers though.


I doubt seriously if the ‘03 Focus is fly by wire. A good ol’ mechanical connection to the throttle is more likely.

Which leads me to wonder if it’s somehow associated with the throttle position sensor…the theory is wild, but the problem is unique.


As an amateur radio operator my tendency with a problem like this would be to hit a fly with a sledgehammer. Assuming you have the stock radio in the car there must have been some failure (bad ground perhaps?) that’s causing your problem. If the radio is heavily integrated into the rest of the car’s electrical system then your only option is going to be to find the problem and fix it. If it’s more isolated then further isolation might stop the symptom, though, probably not the cause. With the disclaimer out of the way let’s talk about sledgehammers!

If you cut the radio’s stock power supply and wire it instead directly to the battery both positive and negative with something like a 14 gauge wire that ought to make your audio crisp and clean on its own. Don’t skimp and get your negative from a chassis ground, yeah it’s negative, but it’ll be carrying most of the RF noise from the many electrical components of the car. If that still doesn’t work an inductive filter on the power supply would be the next step. This is simple, get yourself a small piece of iron and wrap the power supply wires tightly around it a few times. If you can wrap them through the inside of a donut shaped one, all the better. Install this as close as possible to the radio. Use something that’s not already part of your car or connected to the chassis ground. If you do noise contained in the ground may be injected into the power supply wires by your core.

You might only want to consider my advice if all else fails. You’ll lose neat features like a radio that turns off when you shut down the car, but isn’t clean sound worth it?


I thought of another weird candidate: electrically noisy fuel injector. I guess that those things have solenoid-operated valves, but I have no idea how they suppress the “inductive kick”. If the supression failed, you might get quite a bit of RFI. But if the suppresion had failed, I’d expect the electronic driver (computer?) to get zapped. Also, injector firing rate is related to RPM, not to throttle position.

Which leads me back to a question I did not ask before: How are you differentiating between related-to-RPM and related-to-throttle-position?


When I go up the road and slow down to go around a curve then coming out of the curve I step on the gas to accelerate, the more I press down the louder the static and the faster the buzz. If I let off then the loudness recedes and the buzz slows down. My Hayes repair manual says that there is indeed a throttle position sensor but not the cost to replace same.


When I go up the road and slow down to go around a curve then coming out of the curve I step on the gas to accelerate, the more I press down the louder the static and the faster the buzz. The rpm meter does not show rpm coming up as fast as the buzz. It is a fairly steep hill, at least for bicyles. If I let off on the gas then the loudness recedes and the buzz slows down. My Hayes repair manual says that there is indeed a throttle position sensor but not the cost to replace same.


Throttle position affects the combustion chamber pressure, and therefore the electrical load on the spark plug as well as the RPMs. That is to say, that if the RPMs don’t go up, the combustion chamber load still does, and makes it more difficult for the spark to jump the electrode gap. This means the wires are going to reflect more electrical noise.

Replace the plugs and wires.


Jay is correct. You are varying cylinder pressure with the throttle. This causes voltage demand (the voltage needed to fire the plugs) to vary also. As you depress the gas pedal, the voltage in the ignition system increases. That’s what you hear in the radio. Change the plugs. If no improvement, change the wires.

Is the noise the same on AM and FM?


Good point, Jay WB and Caddyman, but OP says that freq of buzz increases with throttle position, rather than with RPM. And he seems to have a handle on that; see his 10/31 7:59PM reply above. However, maybe his tachometer reading lags the actual RPM a bit, giving a misleading appearance that the freq tracks throttle position.

Replacing plugs and wires would be the simplest and maybe cheapest thing to try – after making sure the antenna connection and ground are OK :>)


I doubt that the TPS is causing the noise. The sensor works on 5 volts or less.

If you truly believe it’s the TPS why don’t you just unplug it and see if the noise stops?


Yeah, I knew it was a wild guess when I posted it. But it was the only thing I could think of that fit the parameters.

I second the suggestion.


Thanks for all the suggestions. My wife picked up a new antenna but did not try it coming home. I’ll try it tomorrow, and then on to plugs and then a wiring harness. Also will see if I can unplug the TPS thing. I’ll have the results in a day or two.