99 V10 runs rough with 1/2 tank of gas
 

My truck started running poorly with an intermitent miss about 6000 miles ago.
The miss gets more consistent when the tank is below 1/2. Fill it up and the miss goes away.
No codes, shop that does a lot of V10 camper work could not figure it out.

My guess is that the fuel pump is going out and it overheats when the tank is low.
The problem is getting progressively worse. Now it misses slightly with 3/4 tank and even a full tank sometimes.

Plugs have been changed, fuel filter has been changed, and the shop found 3 COPs that were bad before all this new intermitent miss started.

The truck runs ok at wide open throttle.

Am I just nursing a bad fuel pump that will leave me standed soon?
Truck has 170,000 mile with original fuel pump.

Have you ran any seafoam or injector/ fuel system cleaner?

Not sea foam but I did use Lucas fuel system cleaner and Red Line fuel system cleaner.
Both made no difference.

By the way no black smoke and no change in MPG when it is misfiring.

Update:
Drove all day with a fuel pressure gauge connected, sticking out of the hood so I can see it. Stays between 28 and 38 psi.
Problem is the truck ran without missing today.
I will keep it connected and hopfully it will run bad while the gauge is connected.

Add some water remover like HEET and see if that makes a difference.

Did you change all the COP boots?

Don't waste time or money on fuel additives. If it were COP boots, the level of fuel in the tank would not matter.

Continue driving with the fuel pressure gauge attached. You need to determine if you're suffering a loss of pressure when the misfire occurs or if the misfire is independent from fuel delivery.

I would indeed be suspicious of the fuel pump, but I would not recommend it's replacement yet unless you just want to get it out of the way. OTOH, replacing the fuel filter, if it's not been done recently, would be a prudent measure to take in any event. It might be partially restricted.

Projectsho89- original post stated fuel filter was changed. I personnaly ran seafoam through my 99V10 and it went from a rough idle to farely smooth idle. To say they do not work is false. It may not solve every thing but it does work.

Tessera- have you noticed if your fuel tank vent is in tact?

The boots were changed with the spark plugs by a specealist mentioned on this site who does only sparkpugs and blown plug repair.

How do you check the gas vent, and if it is clogged doesnt it make it hard to fill the tank all the way? Filling is no problem.

I have noticed that the fuel gauge is not reading as linear as it used to.
ie: first 1/4 tank empties much faster than last quarte tank.

The only reason I mention this is I had a Suburban that had the baffles in the tank come loose and they were slamming against the fuel pump/fuel gauge and broke it off. Had to replace the pump AND the tank.

Any baffles in the Ford tank?

At this point there is notdefinitive proof that the level of fuel in the tank is an issue. I have personally seen and experienced water or other crap in fuel do some weird stuff. Including one case with an Explorer which would just out of the blue misfire and shudder. Turned out to be water in the fuel from storing the vehicle in an A/C garage and then going out in the humid weather.

And one of the top causes of misfires in a modular are plug, boot or COP related. It makes absolutely zero sense to just throw a bunch of parts at the problem and it end up being a case of some contaminated fuel. Simple and cheap first. And I agree keeping the fuel pressure gauge on for a while. I will say however that considering the age and miles of the truck (if original pump) it is on borrowed time.

The gas tank in the SDs is plastic. I don't know for sure if they have baffles however I would assume if they do they are molded in. There is a carbon canister in the engine bay that the tank vent line goes to. It is an evaporative emissions thing.

He's had the problem for 6000 miles! Please explain how that can be contaminated fuel over the course of 20-30 fill ups. Like I said, adding supplements in the lack of any indication that they are needed is usually unproductive.

If it is a tank venting issue, it would be most likely to be most pronounced when the tank was full. Simply stopping, opening the fuel cap (and listening for any inrush of air), would equalize pressures and would allow proper operation until the airspace in the tan went into vacuum again.

The above problem I mentioned with the Explorer went on for over a year until it was figured out what the problem was. With the low miles it had on it at the time the plugs, cops and boots were immaculate thus never changed. Neighbor across the street got a tank of bad gas at a local station that was later found out to have tank issues.(my dad got a bad batch from the same station in his work truck) It took him several trips to the dealer and several bottles of dry gas to get the truck straightened out.(5.4l truck) Then there are the boats. Fuel is not what it used to be and if you do nothing ethanol/water mix can stay in a tank that never gets ran dry for some time. With a tank over 30 gallons that is always getting sloshed around and only partially drained before fill well do the math. But hey throw parts at it and don't spend a few bucks for dry gas that don't hurt a thing. I could care less. Rule out the simple cheap stuff first if you are stuck.

Lets be clear about "throwing parts at the problem"

The plugs were done for the first time at 160K miles, It had a definate secondary ignition misfire.
Plug boots were done at the same time.
Truck ran great for about 5k miles then same secondary misfire which we found 3 COPs were tested and bad and changed. Changed fuel filter at this time just because I thought it would be a good idea.

Truck has NEVER thrown a code and it passed California smog test with flying colors...........

I am now driving the truck with this intermitent problem that is worse with a low tank of gas and when the ambient temp is higher.
I have not thrown any parts at this problem. That is why I am here on this site.
Mechanic said there is never a consistent miss on any one cylinder. He ran it in his shop logging data for 2 days. He did not charge me since he could not find the problem.

These trucks don't throw misfire codes very easy, thus I am not surprised you did not get any.

Continue with the pressure gauge until you experience a misfire scenario. It's the only way you're going to be able to know for sure if it is a fuel delivery issue.

will do, forcast is for warmer weather the rest of the week, maybe I can get some consistent results