Alright, I've posted a lot lately about this Weird RPM, and Idle Problem that's been bothering me. And I can't figure out what it is, and I don't know if it's nothing. So it's driving me crazy.

I'll be driving, and it shifts fine, but when I go to stop. The RPM's drop to about 400, and you can't hear the truck at all. Then after you come to a complete stop, it kicks back up to about 700RPM's and you have that little vibration like I'm used to. I don't know WHY It's doing it, but it's driving me crazy.

I wouldn't think that the RPM's should ever be that low.

I replaced the IAC about 5,000 Miles ago.
Had A New Battery Put in about 2000 Miles ago.

Don't think the battery would have anything to do with it. But the whole truck has just been acting weird since I had it replaced. Like, fluctuating Idle RPM's from about 500, 800. The Engine KIND OF sounding like A Diesel, and now this.

If anyone could pleasee get back to me, that would be great.

Thanks in Advance. Sorry about the long post

 

Try cleaning the MAF.

JMC

 

I'm not going to look up your other posts so maybe you've done some of these already.
Clean the throttle body.
Check for rough TPS operation.
Mass Air meter.
Check for air getting in between the air meter and the throttle body on an intermittant engine movement basis.
Plugup connector corrosion.
Possible Ox sensor wire faults.
Possible VSS related falt.
Possible EGR related fault.
Check for codes set in the PCM that may point to the fault.
.
This is how the idle is controled in 'principle';
The PCM has the reference rpm value set in program. The input from the ignition system acts as an rpm tach source.
The IAC, Ox sensors, ACT and other sensors' inputs control the fuel, air and ignition timing and continiously adjust, especially the IAC, to hit the target idle rpm value in program within about 50 rpm, on an engine that has no faults. This operates as a closed loop system. You see here that there are two closed loop systems depending on each other. The idle loop and the OX sensor loop operations.
The IAC is the key controller of airflow around the throttle body plate and directly controls rpm just like you do when you operate the throttle.
This device (IAC) is pulse modulated in order to get the fine control of air needed to keep the engine within the small tolerence limits of the program value.
There are other control functions that go on involving the IAC, TPS, VSS when coming down to idle from higher road speeds. Some of which are fuel limiting, timeing changes, min. air control to prevent stalling and some program updating for it's memory tables.
These actions are done among other things to add a little fuel mileage, do PCM testing of certain functions and update running tables with information when the computer is doing the least control functions and there is more time available for these upkeep operations.
This is a big part of the needs during 'driving cycles' concerning a lot of PCM operations.

As you can see, any interference in these actions will cause the poor tracking/hunting you experience.
I'm sorry it isn't easy to understand and the fault easy to get hold of so you may need to get expert help with the problem from a shop that understands the functions.
Sort through each area one at a time and guard against introducing an extra problem along with the original fault.

Good luck.

 

also check the PCV on the valve cover

 

Thanks Guys so much, I got some electrical contact cleaner from Home Deopt. Going to clean the MAF tonight. Also going to check the PCV.

Thanks again,

 

I had the same problem with a different vehicle, and it was a leak in my throttle body, yours might not be in the same place, but you definitly have a vaccum leak some where.

 

check the vacuum line 90 degree elbow at the end of your pcv line in the back of your plenum, they tend to melt a hole in them from the heat.

 

haven't found any vaccume leaks. some days it fixes itself. Some days it has the whole weird RPM speel. Weird, I'll be replacing or cleaning the MAF soon...

 

Do you notice this happening on days when the humidity changes; if so, it could be a bad/loose ground. Just my $.02. Good Luck.

 

hmm possibly. Here in Florida, Monday And Tuesday had much less humidity for a change. Then right back to hot and humid. I notice the problem more than once a day, sometimes I start it up and it's fine. Other times it does it. Could happen if I just go in a grocery store and come back out. It's weird.... I'm open to ANY suggestions. Could you explain more on what you said before? Thanks

 

cobra909- what bad ground are you talking about. Im having the same problems on humid and rainy days????

 

Some additional thoughts.
When coasting down with closed throttle....detected by the TPS,......
the PCM does a lot of things.
It detects the speed the truck is traveling (VSS) in reference to the TPS and Mass Air readings among others.
This causes a reduction in fuel, changes ignition timing and closes down to a small airflow, the IAC. (attempts to save fuel and emissions)
Now if the IAC does not, or is not camanded to respond to the PCM attempts to return back to the programmed idle speed by allowing more air, and fuel is not raised, as the speed slows, you will have the problem you discribe.
In the presence of a fault, and the truck stationary, this routine is exited and the crank routine takes over and tries to recover the engine idle back to the original conditions for normal operation again and may overshoot the idle some amount while regaining the proper idle..
I would check the VSS connector seating/corrsion/wiring to be sure it is sending the speed reference info to the PCM for all these functions.
The VSS sends frequency signal back depending on rotation speed and you can see it's operation by meter also and even spinning it with a drill, off the truck, for output.
Computers are nice arn't they? You either lovem or hate-em.

 

I had the same problem: found that the elbow of the PCV was cracked!