Rough idle at 1/8th tank

Whenever our (2004) gas tank is around 1/8th empty the engine runs VERY rough. Could this be dirt or water in the gas tank, or maybe a air polution device that plugged? If it's bead gas, how should we fix it? There used to be a fuel additive that you added to the gas tank and it would get rid of it, but it also used to mess up the sparkplugs. Hoping we could get around that somehow.

What does the fuel pressure measure when this starts to happen? The electric fuel pump in the tank relies on fuel to help cool the motor. You may have a fuel pump that is starting to fail.

-Rod

I had the same problem on my 97 -- I replaced the fuel filter and it went away. Have you changed the fuel filter? If not, try some dry gas, etc, to see if it goes away then swap the filter if the problem still exists. If that doesn't fix it, it could be the pump, could be the pump strainer is clogged, could be dirt in the tank... Any number of things, but the first two things I'd do is dry gas then filter, unless I had more than 12k miles on the filter, in which case I'd swap that out ASAP.

P.S. Dealerships rarely actually swap the filters under their "X0,000 mile service" packages unless you're taking the car to a good one. If it's been "serviced" according to the schedule, you could have 30-50k on the filter by now.

What does the fuel pressure measure when this starts to happen?
I don't know, I've only noticed it at red lights. Would I have to pull the gas tank to test or replace it?

Have you changed the fuel filter?
Yes, a few months ago.

try some dry gas
Won't this mess up the spark plugs?

could be the pump strainer is clogged
If it was clogged wouldn't it show symptoms all the time, instead of when the tank was very low in gas?

The fuel pressure you can measure using a fuel pressure guage attached to the Shreader valve on the fuel rail near the intake manifold.

If you don't own a fuel pressure guage, you can probably borrow one from your local chain auto parts store. Make sure it has the smaller "Ford adapter."

-Rod

Fuel pump going bad or clogged fuel filter ... but you shouldn't be running at 1/8 tank.

This is definitely getting a lot worse. Ordinarily I would just go out and buy the new fuel pump, but another symptom has started up, and now it makes me wonder how it could be the fuel pump. It no longer acts up just when the tank is low. It also acts up now when there is any kind of a load on the engine (air conditioner on, going up a steep hill. etc.). OK - so far this all makes sense for the fuel pump, BUT I've also noticed that everytime the engine stutters there is a loud popping static noise in the radios speakers. At first I didn't see how it could be associated with the engine problem, but now that the problem has gotten worse there's no mistaking it, they are both definitely associated with each other :frown: I have verified this many times. For example; I could be at a red light and the car would be running fine, turn on the AC and INSTANTLY the engine would stutter and the popping would happen at the same exact time. Turn the AC off and both would stop at the same time. As far as I can tell it has to do with engine load. So here's my question, how would a bad fuel pump cause electrical problems?

If we forget about the low gas tank symptom, what else could cause something like this? If I went to Auto Zone or another parts store, would one of those testers that they plug into the car help diagnose this? Ordinarily I would just go there and ask but the car is running so bad that I was worried that it might not make it home :frown: I would definately take the chance if I thought it might help locate the problem though. What do you guys think the cars problem might be?

FWIW, my A/C does the same thing - that's part of the reason I don't use it; it takes a lot to run the A/C. But the static in the radio might be caused by some sort of ignition leak or bad ground somewhere.

But the static in the radio might be caused by some sort of ignition leak or bad ground somewhere.
That's what I was thinking, but unfortunately I don't know what parts on a modern day car would cause this. Back in the 70's if the coil had a bad capacitor it would act up similar to this, but on this car each sparkplug has it's own coil and I doubt that they all went bad at the same time. All I know is that it has something to do with putting a load (or stress) on the engine and then it acts up like this :frown:

Have you measured the fuel pressure yet? It sounds like you shouldn't have any problem duplicating the concern, so getting an accurate measurement of fuel pressure should be easy, and could certainly give evidence for or against a faulty fuel pump.

Keep in mind that a fuel pump is an electric motor, and electric motors are horrible for causing noise. Often times filters will be installed on the power line for electric fuel pumps for exactly the reasons you describe. Check some TSB databases from the mid to late 80's and you will find many for retrofiting filters to the fuel pump supply lines.

Under differing engine loads, the fuel pressure regulator will potentially allow the load on the fuel pump to change as well. This change in pump load can potentially cause the noise on the fuel pump supply line to change as well.

-Rod

Have you measured the fuel pressure yet?
Can't find a place that has one for rent.

Keep in mind that a fuel pump is an electric motor, and electric motors are horrible for causing noise.
That's true. Tomorrow I will call the dealership and see how much a new fuel pump costs :( I was told not to buy an aftermarket one because they are usually no good. I'll have to see if I can find this Shreader valve that you were talking about. I couldn't find it in the service manual but I know it's supposed to be around the intake manifold somewhere.