My 2000, F150, 4X4, 4.6V8, five speed manual idles great and sounds great and throwing no codes. It doesn't have a lot of zip, but it really never has, at least since I bought it at almost 60K miles.

I've never towed much of anything with it until recently. I started towing a 6 1/2' X 10' trailer with a 1,600 pound side by side or a 1,300 pound mower with a large grass collector. Both of these loads have lots of wind resistance.

The lack of power is REALLY showing up pulling these loads at highway speed. Sometimes, even on pretty flat ground, the cruise control will hold the throttle on the floor at 60 or 65 MPH and it can't pick up speed.

I have maintained the truck well, but there is something amiss. It is due a fuel filter, so I will change that. I've never replaced the O2 sensors, so I'm considering replacing the front ones.

I am thinking about using a vacuum gauge to see if I can detect plugged cats. As I recall, there are two on each bank. I have a scanner, so I was thinking about monitoring front and rear O2 sensors and see if I can get the 100 degree difference that indicates a properly functioning converter.

I'm thinking of cleaning the throttle body.

Something else that is odd about this truck. Being a stick shift, it does not want to go all the way back down to idle speed unless the vehicle is stopped. For illustration of what I'm saying, you can kick the transmission in neutral and coast, and it will stay above idle speed until the vehicle stops, THEN it will idle down normally. I have always thought that this might be because so few manual transmission trucks were built that this is something in the chip for an automatic that they didn't take out for the manual. I also have wondered if the previous owner, chipped it and this is the result. I really don't want to spend $400 for a tuner in order to change this, but if I thought there is something in the firmware causing the trouble and I could correct it by purchasing one I would do so.

Any thoughts, comments or experience sharing will be appreciated.

 

My V6 4wd with a manual does the same not returning to idle until stopped thing, I've always thought it was odd myself. I am very curious now as to what may be the reasoning behind it.

 

before throwing parts at it like oxygen sensors that rarely ever go bad (not saying that they don't) change your fuel filter like you already want to do since it's a maintenance item. a restricted fuel filter and a dirty air filter can definitely change the way a vehicle performs. then if the problem still occurs after you verify these two necessary things plug in your scanner in and watch your live data for misfires, vacuum leaks, or if your oxygen sensors aren't switching correctly.

to check for plugged cats you would use an exhaust back pressure tester. I sure have never seen exhaust temperature sensors on a gasoline truck like a diesel.

good luck and let us know what you find

 

For those that remember the emission carbs of the 70's and 80's, the IAC acts like a dashpot when decelerating.

 

Yes, I understand that, but the old dashpot or the IAC itself don't know that you are decelerating unless the car is in gear providing engine braking. My idle speed remains above idle at any speed above 0MPH, EVEN IF the manual transmission is in neutral, coasting to a stop. If the IAC is involved, and it probably is, it is getting a signal from the ECM. There is no load vs. speed sensing capability of a dashpot. The effect from the IAC is because of signaling from the ECM.

The old dashpot was a dumb device that simply delayed full throttle closure for a few seconds after the accelerator pedal was fully released. Unless you are speaking about a later version in the emissions years of the seventies, when there might have been something used for this purpose with a vacuum signal as well as, or instead of the damping effect of the dashpots of the sixties.

I am not being argumentative. I am trying to think through what you are properly pointing out.

 

In the old days (yeah I'm plenty old enough to remember them) we used a vacuum gauge to check for a plugged exhaust. I don't know if this will work very well for a cat, but it's something I have to work with. I don't have an exhaust pressure gauge.

Also, although I plan on changing the fuel filter, I really don't think that is my problem. If a lean condition were the culprit, given how gutless it is at speed, I'm pretty darn certain that it would be throwing some lean condition codes. As you say though, the filter is due changing, and you never know.

I have a bunch of stuff going, so it will probably be a few weeks before I really roll up my sleeves and get into this issue, so I'm hoping for as many comments and suggestions as possible before then.

Thanks to everyone for the comments and suggestions. The more minds involved in the problem, the better chance of coming up with a solution, and I will indeed share it.

 

I would take the throttle body off and clean it. I know the 4.6 is notorious for the passages getting clogged up, and that could cause the slow to return to idle issue.

 

The ECM determines deceleration by the relationship of the VSS and the TPS.

 

Thanks for the further responses.

The idle speed issue I try to describe is something designed into it, not something like a sticky TB or anything. It has been like this since I bought the truck, and I'm quite sure if anyone were to drive it, they would believe that it is something programmed into the firmware as opposed to something not working right.

The ECM issuing commands based on VSS and TPS inputs makes a lot of sense. Problem is that it is a manual transmission which would make all that seem to be unnecessary. It seems to me that it is something designed and intended for automatic transmission models, but given the extremely low production number of manuals, it was not worth the R&D effort to change it for such a low number of manual transmission units sold. My manual transmission Mustang does not do this, but the manual transmission production numbers on the Mustang are much higher.

 

Actually, it is not because of the transmission, it works that way to minimize emissions. Maybe there is a TSB that would reprogram the ECM to not hold the IAC open as wide, which would lower the rpm when you are - what, coasting in neutral? (In NYS it is actually to coast out of gear.)

 

It is indeed plausible that the idle issue could be something for emissions reasons. Although I don't like the behavior, that would explain it.

BTW, the coasting in neutral was simply a way to explain the issue. The coasting in neutral laws that are in place in some states are quite antiquated. They were important in the early days when one or two short hauls from highway speed to 30MPH faded the brakes to ineffectiveness.

I don't know if there is a coast law in Texas or not. My written driving test was 51 years ago so I don't remember. I drive nothing but stick shift vehicles and in certain situations I will kick it in neutral and coast to a turn off, but all my stick vehicles have modern four wheel disc brakes in top shape.

 

The vacuum gauge will do it. The more restricted it is, the easier it is to recognize.

Separately, what axle ratio do you have? Biggest complaints at highway speeds always seemed to be with the 3.08 ratio. And the additional weight of 4WD just adds to it. Really needs 3.55 or greater, depending on actual wheel+tire diameter.

 

Axle ratios are 3.55.

 

The IAC operation on Deceleration normally closes down to a min opening, fuel injection is cut as well as ignition timing is often advanced as the PCM decides.
All this is normally transparent to the driver until there is a trouble, then it's noticed.
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This is also a fuel saving operation and what is known in computer parlance is the PCM does "house keeping' at that time when the PCM has the least to do, it updates variable tables with changing values as they just recently occurred.
'.
This deceleration is sensed by the TPS going to rest or a very low signal along with the vehicle speed dropping in it's output signal as road speed decreases.
As the motor approaches idle conditions for an automatic, the IAC is opened to recover normal idle and fuel is added again to a level dictated by the OX sensors and coolant system temperature.
The difference between a manual transmission as opposed to auto is the clutch is operated to signal the PCM to 'exit' that routine and recover idle conditions when it's time to open the drive train via the clutch operation.
When this routine goes faulty the idle often remains high and tends to push the truck while in gear or be bothersome for high idle until the PCM reduces it from a different routine.
I see this complaint a lot on manual transmission trucks especially the 4.2 V6.
A good thorough look at live data with a scanner should see this faulty operation such as the IAC opening is excessive when it should not be, the Mass Air signal is too high from passing more air when it should not etc which verifies the condition.
Basically what the system deals with is computer logic (Boolean logic) of many 'and', 'or', 'not' inputs to decide what line in the program to enter the desired operating routine.
It's much the same decision making on Acceleration for up shifting and down shifting the auto transmission by using the same sensors and type of decision making except the fuel is not cut and ignition timing changes as the load changes.
If it were not for fast computers (PCM) all this would not be possible to the extent it is and works.
Good luck.

 

I had a few minutes to kill this afternoon and took time to put on a fuel pressure gauge. I got a steady 30PSI, even when revving it up really good. The pressure showed minor fluctuation as would be expected from a properly working regulator.

30 is on the low side of the specifications of 30 to 45. If I understand correctly, at wide open throttle, it goes into open loop mode, this does not monitor the O2 sensor and regulate mixture. If I understand correctly, in open loop mode, injector pulse width is determined from a table. If this is true, it stands to reason that with pressure at the bottom limit, it would not be fed as much fuel as it would with higher fuel pressure.

I had low fuel pressure on my wife's similar truck and it was throwing lean bank codes. From that driving and troubleshooting experience, I got the impression that it requires a dangerously lean condition to cause it to throw the lean bank codes.

I will wait for the gurus to confirm or deny my theoretical understanding.

Thanks for your comments.