RPM fluctuations

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btah

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Hello, I'm new to the forum. Hopefully I can get some help and also help others. I posted the following on a different forum but it must be unique (or boring) enough to not generate a lot of input.

It is rather long so hopefully I don't lose you from boredom.

The problem:
My truck will sometimes feel like it is going to die and I have to pump the gas (or hold the pedal to the floor) for a few seconds to get it back to normal, usually when just pulling away, say once every 15 trips around town. If I am in park and try to keep the idle at 1000RPM I will get hesitation and surging between 900 to 1100 RPM. Also, if I try to keep it at 1000RPM and turn the wheel to add some load, the idle will drop to @600RPM and surge to@1200 RPM when I let go of the wheel. IF I am in drive and try to keep the idle at 1000 RPM, the RPM will fluctuate but not as much as if it is in park. No codes (using the key on-off 3 times and also the trip button while turning the key on methods) or from the Advanced Auto Parts scanner.

Here is what I have done to try to solve the problem (truck has 66,000 miles):
plugs
oxygen sensors
PCV
removed and cleans TB and IAC, neither of which struck me as being very dirty.

I have tried an Autozone TPS, didn't make a difference so I put back on the factory TPS.
I have tried an Autozone IAC but the truck wouldn't maintain an idle. I returned it as defective and put the factory IAC bac in.

Most of the posts I read say "replace the TPS" or "replace the IAC" without a whole lot of explanation behind the suggestion. To help with the investigation I did the following tests:
TPS unhooked
IAC unhooked
TPS and IAC unhooked
TPS and IAC both hooked up

Results:
TPS unhooked and IAC hooked up
Idles a little rough at 600 RPM
If I keep it at 1000RPM it stays pretty steady. If I turn the wheel it will drop to about 900 RPM and surge to @1100 RPM when I let go of the wheel.

TPS unhooked and IAC unhooked
Idles rough at about 400RPM
harder to start
If I keep it at 1000RPM is stays pretty steady . If I turn the wheel it will drop to about 900 RPM and surge to @1100 RPM when I let go of the wheel.

I just realized the results of the above two are pretty much the same except for the initial idle.

TPS hooked up and IAC unhooked
much harder to start, need to give some gas to keep it going initially.
If I keep the idle at 1000RPM it hesitates and surges between 900RPM and 1100RPM. If I turn the wheel it will drop to @600RPM and when I let the wheel go it surges back to @1200RPM and then back to 1000RPM.

Both hooked up
Starts fine and idles smooth at 600RPM.
If I try to keep it at 1000RM it fluctuates between @900 to @1050 RPM. If I keep it at 1000RPM and turn the wheel it drops to @600RPM and then surges back to @1200 RPM before returning to 1000RPM.

I just realized the results of the above two are pretty much the same except for the initial idle.

So, I am not sure what to make of all that. Anyone have any thoughts? And, thanks in advance

If you have gotten this far, thanks...
 

Redtruck-VA

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Welcome to the forum. It sounds like you are trying to force an 1000 rpm idle. The PCM target idle is around 550rpm and it will try to maintain that idle. The power steering/AC if it is like the hemi, when turned on will raise the idle to 650rpm. The TPS simply outputs a signal of .05 to 4.5vdc to the PCM and should be calibrated when reset or PCM is reset. The IAC is controlled by the PCM to maintain the target idle. There is a throttle blade adjustment screw (I think) and if adjusted the IAC will probably compensate to maintain target idle. Is there something specific you are trying to achieve? Hopefully someone familiar with the 4.7 will post. Good luck..
 
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btah

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Thanks for the response.

My main concern is that it will sometimes want to die (and has in fact died a few times) and I have to push the pedal to the floor to get it to come back to life. It is annoying and not normal. The wife won't drive the truck (which is not necessarily a bad thing) because she is afraid it is going to die on her in traffic. She also doesn't want the kids to drive it (which definitely isn't a bad thing) for the same reason.

Holding it at 1000RPM was just a symptom that I noticed. Every other care I have tried that on it holds steady. I just leads me to think that at some point I am going to be stuck on the side of the road in the middle of nowhere. I would rather avoid that.
 
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btah

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And just to make sure I am responding to your post. I am not trying to force the truck to keep a 1000RPM idle all the time. The truck actually idles very nicely at @600RPM. I simply notice by playing around trying to resolve the dieing issue that if the accelerator is held at @1000 I get the odd fluctuations.
 

Redtruck-VA

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I'm not a 4.7 guy, so bare with me. But this sounds to me to be a IAC hunting problem. Is the throttle plate closing properly? Dirty Throttle Body? Dirty or bad IAC? This is what I would look at. Good luck
 

chedched

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The RPM fluctuation is normal. The power steering is putting more load on the motor. The same is true with the AC but for the AC there is a idle up in the ECM so you don't really notice it.

The issue you are having is the truck is trying to die on you. What exactly does it do when the vehicle is trying to die? What are you doing when it happens? Is it always the same driving conditions or randomly?
 

truckin151

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When coming to a stop, does the idle bounce around a lot? Have you checked for any codes that might have popped up but not thrown a CEL (there are a few that will do this).
 
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btah

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Responding to the last 2 posts:

The RPM fluctuation is normal. The power steering is putting more load on the motor. The same is true with the AC but for the AC there is a idle up in the ECM so you don't really notice it.
-> I have heard that a few times. However, every other car I get in and hold the pedal so it runs at @1000 RPMs, it stays there. Next time you are in your vehicle, give it a try and let me know what yours does.

The issue you are having is the truck is trying to die on you. What exactly does it do when the vehicle is trying to die? What are you doing when it happens? Is it always the same driving conditions or randomly?
-> It always happens when I am going slow, lets say under 15 MPH. Almost always (if not always) it has happened withing a block of starting the truck and leaving. It sometimes will happen in a parking lot leaving a store. I have never had it happen at a stop light so it seems like if you pull away from a stop at a normal speed like on a busy street, it doesn't happen. So, I would say always after I pull out of the driveway or out of a parking spot. It dawns on me as I am writing this that I think it has always happened after going from reverse (either from driveway or parking spot) to drive and pulling away. Not sure how relevant that is, just adding it in.


When coming to a stop, does the idle bounce around a lot? Have you checked for any codes that might have popped up but not thrown a CEL (there are a few that will do this).
-> I have noticed a few times that when I am coming up to a red light I will see the idle start to drop down normally, then drop rather quickly, and then bounce back. Again, probably when I am at the 10 to 20 MPH mark. I wonder if there is a dead spot somewhere in a sensor that I keep hitting. I have the truck at a shop right now getting an airbag light issue resolved. They cleared all the codes and nothing has come back other than airbag related code.

Just to add (ignore if not relevant). The shop I have it at told me my clockspring was bad. They replaced it and that didn't resolve the issue. They now tell me the airbag control module is bad and are going to replace that tomorrow. The mechanic actually said that he was wrong about the clockspring and would only charge me his cost and not charge me labor. Nice huh... He scan was saying there were codes throw for the squib(sp?). He didn't tell me the exact codes.
 
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btah

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Oh, and as far as what it is actually doing when it wants to die: It will start to stall and stutter and I get through it by pushing the pedal to the floor and it figures itself out after a few seconds. Then it acts normal.
 

truckin151

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this probably doesn't seem relevant but does the cruise control work? if it doesn't I have a good idea what it could be, if it does it's out the window
 
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btah

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Cruise works find (so does the horn). However, I am interested in what you thought it might be.
 

truckin151

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mine did this for a while, surged at a stop and eventually would stall out. Ended up being the speed sensor/abs sensor around the wheels. it was over heading or gummed, when I did my brakes and cleaned everything up the surge and stalling stopped.
 
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btah

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And that sensor is part of the cruise control system?
 

truckin151

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it affects it I know that. as long as the code was present the cruise didn't engage
 
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btah

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Well, here's the story. One of the things I did a few months ago was to replace the O2 sensors. I used Bosch Premium. I couldn't remember if the stalling and wanting to stall happened before I replaced them or not. I put back in the original O2 sensors and the problem I had of wanting to die seems to have gone away. I post again if the problem comes back.

Also, and I have no idea why (any opinions would be appreciated), as soon as I put back the factory O2 sensors, by airbag light went out (has been on for a month or so solid). I think the airbag light originally came on some time after I replaced the sensors.

Thanks to those that took the time to respond to my posts. Hopefully this can help someone out.
 
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btah

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Airbag light has came on and went off a few times so I guess it makes sense that it didn't make sense...
 

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