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  #1  
Old 12-10-2009, 06:43 AM
xdoomx xdoomx is offline
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Default Dead spot in acceleration from idle?

When I try and accelerate lightly from a stop, the rpm jumps straight to around 1800rpm. I have tried pushing down slower and softer, but no dice.
I've gone into the engine and tried pushing the throttle by hand, same result.
There is like 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!!?
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Old 12-10-2009, 08:59 AM
Johnbaum13 Johnbaum13 is offline
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Automatic transmission. The stall in the torque converter is set at 1800 rpm. The engine has to have a certain rpm before it makes any power, and if the stall was any lower, the truck would be almost undrivable. This is the way automatics are designed, and it accomplishes the same task as slipping the clutch in a manual.
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Old 12-10-2009, 09:05 AM
xdoomx xdoomx is offline
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I've got a manual transmission tho. Makes sense what you said but.
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Old 12-12-2009, 07:42 PM
Johnbaum13 Johnbaum13 is offline
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Well, then you must be more specific. With a stick, and the clutch engaged, rolling in first at 850rpm, if you touch the gas it will raise the rpm with the vehicle speed, no matter how hard you push it. So it jumping straight to 1800rpm as you start accelerate won't happen.

Now, if you mean with the clutch pushed in, it won't register any throttle movement till you push the throttle enough for it to jump to 1800 rpm, that makes more sense. On my truck, the revs will jump to about 1200rpm as I slowly press in the pedal. What this is, is the idle air control valve. The valve is set up to open fully when the accelerator is pressed to allow max air into the engine, as well as lower the wear on the IAC. So as you press in the gas pedal, the IAC opens as soon as it sees signal from the TPS, and so the rpms jump up when you barely touch the pedal, then return to idle after you let off the pedal, as the valve closes to maintain the preset idle rpm. You can actually hear the valve close on mine. However, jumping to 1800 rpm seems a bit excessive. I'm not sure why it would jump so high, but it may be an indicator of the valve reaching the end of it's life span.

Hope this helps.
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Old 12-13-2009, 01:58 PM
xdoomx xdoomx is offline
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Well Im operating the throttle by hand in the engine bay, to eliminate any possibility of the cable being stuck.

1200 would be ok, but Im basically waking the neighbourhood up everytime I want to 'edge' forward a few centimetres...

What the flip is causing this dead spot? Its most evident when warm by the way, and I have even just replaced the TPS (no change at all)
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Old 12-14-2009, 07:25 AM
xdoomx xdoomx is offline
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- 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)
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Old 12-14-2009, 10:35 AM
Johnbaum13 Johnbaum13 is offline
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Your tps is fine.

Your maf is fine.

What you hear is the IAC opening, as stated above. Clean the IAC and see if it helps.
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Old 12-14-2009, 10:38 AM
xdoomx xdoomx is offline
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Cleaned the IAC out, bit of black soot but not too bad.
Then I 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?
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Old 12-14-2009, 10:59 AM
Johnbaum13 Johnbaum13 is offline
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The IAC is operating "correctly".

500rpm is the minimum idle, so with the IAC unplugged, thats where it will idle.

Is it still jumping all the way to 1800 with it plugged back in?
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Old 12-14-2009, 11:03 AM
xdoomx xdoomx is offline
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Yes, when I plug it back in idle goes back to 750.
Apply throttle slowly, slowly, slowly, click, and im at 1800rpm.
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Old 12-14-2009, 01:02 PM
rdnekgt rdnekgt is offline
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when you watched the iac valve did it move slowely or jump cause it sounds to me that the iac is jumping open when you throttle up
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Old 12-14-2009, 01:05 PM
rdnekgt rdnekgt is offline
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with the iac on, truck off not running take of the air intake tube and work the throtte by hand and watch to see what the butterfly in the TB is doing is it opening smooth or jumpes open
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Old 12-14-2009, 01:24 PM
xdoomx xdoomx is offline
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- without starting the engine, moving ignition off to on, the iac jumped. it seemed to only have 2 positions. It went fully open for 2 seconds, then fully closed. Im not sure if this is right for this kind of vehicle? its similar to this type:
http://more.take2.co.za/moreusa-stan...000c7wqvm.html
not this type:
http://more.take2.co.za/moreusa-stan...000c7um0o.html
With the engine running, the IAC definitely seems to be 'popping' open, maybe its supposed to open a lot earlier, like as soon as I hit the throttle, but its only opening at 1800rpm?

- cleaned out the throttle body and around the butterfly early on. Thats all moving smoothly. Checked again now as per your advice: throttle and butterfly are solidly linked and move in tandem

- what else is there that would stop the rpm from rising? I move the throttle cam (& butterfly) at least a few mm by hand, and there is NO CHANGE in rpm until some kind of threshold is reached and it clicks over to 1800

stuck my finger quite far into the TB, seems to be a bit grimey down there, but for 100,000km thats normal I think.
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Last edited by xdoomx; 12-14-2009 at 01:28 PM.
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  #14  
Old 12-14-2010, 04:11 PM
ecogenie ecogenie is offline
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Default Re: Dead spot in acceleration from idle?

Okay but has anyone figured out why the acelleration problem? Mine is doint the same thing and just can not figure it out. How about some help somebody!!! This is really bad. Ford should put this on a recall or tell us how to correct it if it can be corrected.
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Old 12-14-2010, 05:07 PM
wvgranger wvgranger is offline
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Default Re: Dead spot in acceleration from idle?

Dirty mass air ?

----------

Is this 2005 4.0 a cable t-body or drive by wire ?
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Last edited by wvgranger; 12-14-2010 at 05:05 PM.
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