Dead spot in acceleration from idle?

[xdoomx]

When I operate the throttle by hand , the rpm jumps straight from 750 to around 1800rpm.
I have tried pushing down slower and softer, but that doesn't help, its like the only available revs are 750, and then 1800 +. nothing in between!?
There is a 'dead spot' ?

I've measured the voltage on the TPS and that appears even throughout the range. Haven't measured the resistance tho.

Any ideas at all, this is driving me up the pole!!?

 

[2813josh]

voltage will always be 5v i thnk. And youll need an oscope or mayb a little lu
ck and a good meter to check for a dead spot. sounds like tps to me

 

[AgPete139]

Have you tried to clean your Idle Air Control (IAC) valve located by the TB? I don't see why an '05 would be so gummed up, but it is worth the $0.50 of carb cleaner.

After that, take out your MAF sensor by your air intake (by filter) and then spray that with carburetor cleaner. Let it dry, and try starting it again. These get filthy very quickly (especially when people install oil-based K&N filters) and can yield poor readings by your ECU.

I have not heard of any TPS suddenly going out on any Ford.


1.) Clean IAC valve.
2.) Clean MAF sensor filament.

Takes 10 minutes, and both get dirty. I would highly suggest that first before you spend a lot of money replacing parts...


Pete

 

[xdoomx]

Ok I have started by replacing the TPS. No change at all.

I cleaned out the IAC valve and passages. No change at all.

Next stop the MAF sensor?? Bit scared to fiddle with an apparently very sensitive piece of hardware tho..

How about EGR valve? Remember this is MOST evident when engine is warmed up..

 

[Ranger44]

My guess would have been TPS too. And yes that's right, it is very hard to check a TPS with just a DMM, a scope is much better and can pick up dead spots. Good luck with just a meter.

A MAF will cause a messed up condition ALL the time. It doesn't move or change with the throttle, it measure the same thing, over and over.

The IAC could be scrubbing improperly and not buffering the acceleration from idle.

I'd have to ask a few more questions.

Is the truck manual or auto?
Does your speed change in relation with the RPM change?
Does the engine idle high or seem sluggish, run rough?
Any other previous work done to it?

I'd stop just replacing parts, and give us some info, and we'll help you find a practical guess. I say guess, because I'm not able to be there and see and feel it for myself. I'm here to help.

 

[xdoomx]

Thanks SO much I really appreciate it.
Its a stock standard 4.0 V6, 2005, manual.

- Idle at 750rpm, seems pretty solid, I can't pickup any missing or roughness at. Maybe a VERY slight shake but this is a big motor ater all?
- Runs super smooth and quiet
- Speed does change in relation to RPM change (?), if Im in gear of course - Im doing my testing in neutral (the symptoms also happen in gear tho)

- I have just replaced the TPS, didn't change the symptoms
- I tried disconnecting the MAF, didn't change the symptoms
- going to try disconnecting the IAC and will report back

By the way, if I put a listening device (spanner in my ear) on the air filter box, and start moving the throttle, I hear a very VERY light 'click' just before the rpm hits its next available range of 1800rpm. Maybe this is normal operation of the IAC or EGR though? (hope this isn't confusing the issue)

 

[xdoomx]

I have just unplugged the IAC control wires, and the throttle response is much better/perfect without it connected - totally smooth from 500-2000rpm. Idle speed dropped to 500rpm (I expected it to NOT idle at all?)
I didnt drive it, but the response of the throttle was even.

Now I went on to test the IAC operation: turned engine off, plugged the wires back in and removed the IAC unit exposing the valve. Turned the ignition on and observed the IAC valve opening for a few seconds, then closing.
Does this mean its A-OK?

I decided to also test the EGR valve. While idling I sucked on its vacuum port almost bringing the engine to a stall. Right, so that means the EGR isnt jammed.

Any advice of where to go from here?

 

[Ranger44]

I'd believe that since with the IAC unplugged, that it doesn't do the crazy jumping, replace it. It obviously is opening too quick right of idle or staying open after the throttle plate is open.

Only other things I'd suspect is anything that would submit feedback to the PCM and cause the PCM to change IAC function.

 

[xdoomx]

Ok sounds good.

I dont quite understand the function of the IAC tho. Is it like the automatic choke of carburettors?

Also, is it normally closed, or open at idle when warmed up?

 

[Big Jim M]

So my question to you is: What does it do while driving? All I have seen here is parked testing..maybe I missed something..
At idle the engine retards the timing to meet smog regulations. As the pedal is pressed the timing advances quickly, with the advance of the timing the engine normally gains quite a few rpm just from the change in timing...without any regrd to the amount of fuel delivered.
I have seen guys work on their carbs for days trying to get a steady advance in rpm from idle...all without consideration to the timing advance that comes quickly off idle.
If you don't "see" the problem while driving...it ain't a problem! Leave it alone.
As for the IAC. That is the Idle Air Control. It's job is to allow the amount of air into the engine at idle that causes the engine to keep running at the desired rpm.
Big JIm

 

[xdoomx]

Ok Im coming from a Weber 32/36 background, Im new to this EFI stuff.
With this carb, I knew if I put my foot on the throttle more fuel + air mixture was delivered and my revs increased. If I wanted to increase the idle, I turned a little screw to set the point at which it was allowed to drop (like holding the throttle at a certain point).. The choke was a flap which closed to block air supply off at cold and allow a richer mixture of fuel in (increased idle)

Now, with EFI, I put it into 1st gear, put my foot on the accelerator it JUMPS from idle to 1800rpm. To prevent my head going through the back windscreen I have to burn up way more clutch than I should need to than if I could attain lets say 1300rpm and pull off SLOWLY. So basically I need to ride the clutch for longer until rolling speed matches the high engine rpm.
This 'stickiness' makes it really hard to drive the truck 'sedately'