Well Gents (& Gals), I'm going to take my truck in to get it diagnosed by the professionals at the dealer..

Truck: 99 F250 5.4l 4x4, 4R100 trans.
Problem: Truck is performing poorly, running kind of rough, noticeable under a load, towing a fishing boat <2000lbs, going up hills when not giving it throttle, and sometimes just backing off the pedal in town.

The issue began about 6 weeks ago when pulling my boat back from the lake on a hot day. Truck was really running rough, bucking, and shifting erratically. I was thankful to make it home with the wife and kids that day. Scanned for codes and got P1125 Throttle Position Sensor A intermittent. For the next 4 weeks I drove it and no issues at all, then one day out of the blue it does it a little bit, but no where near as bad. Its progressively run worse ever since, but still very much driveable. Its mainly been intermittent, but has me worried it will fail when I really need it to.

What I've done to it: Cleaned Throttlebody, Replaced TPS sensor, cleaned MAF, new MC fuel filter, Oil (MC) and Air (WIX) filter, checked fuel pressure (35psi off/28 on), cleaned all my chassis grounds with wire brush and sprayed corrosion inhibiter on all contact areas, new battery (as of yesterday as 22mth old was shot amps-wise), flushed coolant and new thermostat, ran injector cleaner in tank, had it smoke tested for vac leaks (none found). Tranny was serviced 2 years ago, level, color, and smell are all good and regularly checked. Also had a shop hook there Snap-on scanner to it, but they didn't do any live tests, and no codes for them.

In the past 2 years: New MC plugs and boots, Transmission serviced, Fuel filter, IAC, trsf case and diffs serviced, oil changes etc, etc..

No codes since the TPS, it did come back once, but was cleared when I had my batt disconnected for some maintenance I was doing....I'm stumped thinking it might be a bad COP or possible injector. Anyhow I should have asked this sooner, before I gave up and took it in, but any ideas?

Sorry for the long post. Thanks for any advice.

 

Thanks for all the information thats is very helpful when diagnosing.

When you replaced the TPS, was it calibrated at all? Typically there is a voltage range with these.

Have you replaced your PCV valve? Right valve cover into a hose leading towards the throttle body.

 

I did not have the TPS calibrated (wasn't aware of the need). Motorcraft PCV valve was changed within the past year. One thing that I find odd, regarding TPS, which may be normal, is my scanner will say that the TPS is at like 15% or 18% (old one did as well) even when I am not touching the pedal. Don't think that is related, but my brain tells me its not helping fuel economy.

 

Sounds to me like you may have a partially clogged catalytic converter.

 

TPS does not need to be adjusted, i replaced mine and asked two Ford techs when i was at the dealer and both said no just bolt it up....

 

I was wondering that myself after reading some posts about folks who had similar issues.

Superduty Scaler, thanks for the info on the TPS. I asked the guy at the parts store the question, but I'm not real familiar with him and didn't know if that was the right answer or not.

Truck is at Ford dealership now. Paying for Diagnosis only. I'll post back what they say. Thanks for the help so far Gents.

 

Well this is odd... Dealer could find nothing wrong, no miss, fuel issues or codes pending/stored. I did replace the battery the day before, so obviously codes were cleared. He said he would keep my diagnostic ticket open, told me to drive it a bit and call him next week... No charge

Great news I suppose, except i want to hook up the camper and boat and go down steelheading this weekend... I think I'm going to press on and hope for the best.

 

When I had a truck acting like this, I had it to the dealership multiple times before a technician was able to correctly diagnose the issue as a bad catalytic converter. It was very frustrating. I replaced everything I could think of, but still experienced a loss of power when under load. It took almost a year to figure it out.

The way the technician verified it was to disconnect the exhaust ahead of the converter (while he was seeing the symptoms) and go for a drive. It was dramatically obvious at that point that the lost power had been restored.

 

I would think about the carbon build up. Lucas makes a deep clean which seems to work the best in my experience. one double treatment, and 2 normal treatments, on a long ride seems to blow all the carbon out. The last time I ran it I also use 0W20 oil to clean out the valve and ring area. Oil came oit pitch black like tar after 500 miles. looks like honey now 1000 mile later after I changed it.

some general info. your 5.4 has a 5W30 label on the oil cap as did my 2000 expo. later ford change to 5W20 to qualify higher on epa ratings across the board. my current vehicle is an 05 and runs very responsive with 5W20, but since I have tow paranoia, I'm running 5W40, and the thicker oil slows my engine down quite a bit.

 

Thanks Redford, I will look into doing this myself. It is just a couple of bolts ahead of the Cat correct, and secure the cat/exhaust and drive it?

I'm afraid I might like it too much... I'll give it a shot though. Most likely won't be for a bit (hopefully) next week or so.

 

I am and have used 5W20 for sometime now after reading a tech bulletin on it.
So is the lucas stuff for the fuel or oil? I've been using a techron additive (fuel system/injector cleaner) with little to no result that I can see. I did in the past to the Seafoam thing as well that claimed to do what you suppose as well.

 

The Lucas deep clean is for the fuel. Tried quite a few and this worked very well. The 0w20 caught a lot of the grub.

 

Are the cats in the y-pipe?

 

I believe the cat is behind the y-pipe. Unless there are some special little ones that I don't know of.

 

Ok, well when I was still running a gasser truck, I had a dodge so the cat was actually the y-pipe now looking back, I had to disconnect right off the manifolds. Knew immediately I had a plugged cat. Do you off-road your truck?