I'm struggling with low power and poor mpg. Been through and replaced just about everything. Using my Scangage, I can see the LOD reading is abnormally high, hovering around 50% at idle and increases from there with any driving. Coasting downhill I might see a low 40% reading... The only thing I can determine that might be part of the issue is a pronounced hiss from the throttle body, like a strong vacuum leak. However, I have gone through multiple cans of starting fluid, carb cleaner, etc, and simply cannot find a leak. Futher, I have pulled each of the vacuum lines from the throttle body, and using my hand vacuum pump, have verified that none of the lines themselves leak. So, can anyone else with a V10 hear an audible hiss at their throttle body during idle? Is it possibly just air rushing past the throttle plates? I just cured some exhaust leaks so I've not had the benefit of a quiet engine to determine if this is new or not...

 

Are you talking about calc load? Mine hovers high also but no hiss and great power. Try disconnecting the cat at the three bolt fitting and see if your power is back. Yes. It will be louder. No. It won't hurt to try it. The hiss is strange to me. I will be at my brothers today and will listen to his. His is same year as yours.

 

Yes, calculated load from the Scangage. Already tried disconnecting the catalytic converter, no difference. Near as I can tell, the hiss is coming from the vent on the IAC, which is new, and makes the same noise as the old one did...

 

My IAC was making a terrible low hum when the truck was cold. I replaced with a brand new one from car quest and got the same noise only less often. The mileage was bad and power was intermittent. I got another new one and put it on. No change. Went to dealer and bought Ford part. No more problems now. That truck is the one in my sig and it's calc load is around 21 when idling. My 2000 4x4 is around 35 when idling. Hope something in this helps you. The IAC should not be venting like that. Maybe it's stuck open. Maybe you got a bad one like I did.

 

What do the long-term fuel trims look like?

Cleaned the MAF lately?

How's the fuel pressure and fuel filter?

My '01 hisses a lot at idle, and there's nothing wrong with it

 

Probably should have been more specific in my OP about what all I've done in an attempt to resolve the power and economy issue thus far:

* New MAF (O'reilly)
* New IAC (O'reilly)
* New TPS (O'reilly)
* New SP479 plugs @ 0.054"
* New COP's with boots (NAPA Premium line)
* New air filter (Wix)
* New fuel filter (NAPA Gold)
* Dropped the cat, no difference
* Fuel pressure is 30psi @ idle
* Pulled vacuum from FPR, pressure jumped to 40psi (simulates WOT)
* Verified fuel pressure at WOT with load (6000# trailer): 40psi
* Smoke tested intake and vacuum lines, no leakage anywhere EXCEPT the IAC vent which will billow smoke from either upstream or downstream smoke introduction...
* Fuel trims look fine, no abnormal percentages in either STFT or LTFT
* Calculated load is high regardless of conditions, almost always greater than 50% even coasting downhill with trailer on
* Inducing a vacuum leak (pulled hose from brake booster) jumps STFTs to +30% as you'd expect, but calculated load drops to ~40%, which I can't wrap my head around...

Long story short, EVERYTHING checks out except it's still low on power and getting about 4mpg less that it did a few weeks ago (before I put in the new plugs and cleaned the TB). Unfortunately, I never used the Scangage before so I'm unsure if my LOD calculation was high before either, but the power and economy has certainly changed due to SOMETHING... Over 2000 miles so I know it's not simply bad fuel...

 

Did you push the rubber plug back in, where the MAF wires pass into the air box, and is it still in place?

 

Yes, rubber plug is in place and stays put. Also confirmed no leakage there via the smoke test... Intake tube was FULL of smoke...

Is it possible for the fuel pump to deliver adequate PRESSURE but inadequate VOLUME? For pulling power, it simply seems to run out of steam... I'm guessing it's somehow related to the notion that it's running at 70% load just cruising, so there is only 30% more power to apply to hills and such, but could a profound lack of fuel be the root of that?