Erratic Idle

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G

gyates

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I am looking for ideas on this issue. I am having some erratic idle issues with my 4.0 SOHC. So far I have replaced the IAC valve and its gasket. At time it idles perfectly at 750, but at times it idles lower. Ac on or off does not impact this. I can not find any pattern to when it idles good and when it idles low. I do not have a CEL so no diagnostic help from the computer. Could this be the beginning of the famous PCV elbow failure?
 
The PCV elbow should be considered preventative maintenance on the Job2 4.0 SOHC.

Easy to check, remove the throttle body cover and inspect the hose bottom for leaks.
 
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Thanks for responses so far. Rodger I have actually replaced the IAC and its gasket. It helped but has not resolved the idle issue. Also today I pulled the PCV elbow that is a known problem and mine is good, no cracks and no soft spots to indicate possible collapse.



I will add problem is much more noticeable under load, such as ac is on and wheels are turned in anticipation of a turn. But again I can't say this happens every time. Ambient temp does not matter either, nor does engine cold or engine warm.



Otherwise truck runs fine. under normal driving no issues at all. No weird hesitations. No loss of power. No misses.



I am stumped!! My best hope may be for whatever this is to finally trip a CEL code.
 
Wouldn't hurt to clean the throttle body, MAF sensor, and check the PCV valve.

All have been known to cause idle issues. CRC makes specific cleaners for them.
 
I had a similar problem with my '05 when I first got it. Had the IAC replaced; no help. It gradually cleared itself, probable a bad tank of gas !!
 
Year and miles of your truck? My son had a similar issue with my former Gen1 V6 at 90K+ miles. Low idle rpm, engine would die at stop light, etc. No CEL present. We replaced the air filter, spark plugs and put in a bottle of AMSOIL P.i. Performance Improver gas additive. Problem solved.
 
I agree with swshawaii.

you might have a dirty throttle body if the IAC was dirty and you replaced it. MAF and PVC would be the next items to check and clean as well as the infamous cracked elbow.



Also, don't rule out fuel issues like a dirty fuel filter, or dirty injectors...buth you might want to try some Seafoam to clean out the entire intake system.



...Rich
 
I had a wicked rough idle once and hard starting. Checked everything you did and couldn't figure it out. Turned out I had overlooked one thing. I was fiddling with the intake recently and I had managed to knock out the breather tube for the oil. The intake getting that unregulated air made for a rough idle and the starting issue. Put it back and viola! All worked well again.
 
Thanks for tips so far!! Truck is an '05 with 69k miles. And to Mrlunchbox I also found that hose off a few weeks ago. Put it back in place, but did not totally clear it up.



I have opened the throttle body and found it actually surprisingly clean.



Also battery is new and charging system checked out good.
 
Have you tried, disconect the battery for 15minutes. Do this when the motor is cold from over night. Or all day...

Hook it back up and start it and let it idle until the motor is normal temp. Do not touch the throttle until then. It might relearn its Idle perameters.
 
Thanks Eddie. Battery was recently replaced actually but disconect did not resolve the issue. Argh!!



I will add that the problem I am having happens after the engine is up to operating temp. On a cold start idle is perfect, around 1200 or so. Then once it runs a few seconds it drops to 750. But the erratic idle happens after truck engine is at operating temp, and again, this is random!! This morning on the drive to work it only idled a little low once while I was sitting in line to get on the interstate (indicated just about the 500 RPM mark and I could feel the truck struggling a bit as it was idling at this time, tough it neven stalled or really acted like it wanted to). But while idling in school traffic earlier today to drop off my son idle was perfect.



Again, thanks for ALL suggestions!! :)
 
I cleaned the MAF sensor. Sprayed it well with MAF cleaner, then let dry at least an hour. Took truck out for a ride and idle seems much better. Only time will tell if this solved the erratic idle but it does seem to have helped it.
 
Another updated. I have driven the truck about 3 days since cleaning the MAF Sensor. I can say the idle is dramatically better. I still seem to see a bit of a drop in rpm's when steering, especially at very low speeds, but the drop is not as dramatic as it was. If cleaning the MAF Sensor really did resolve this it will be the first time this has ever actually worked for me. lol!! Usually I have to replace things like MAF and IAC valves.
 
Hey everyone,



I've been a member of MyST since 2004 but it's been a long time since I've posted anything although I still visit the site pretty regularly.



I just wanted to say thanks for this thread and to Paul McMullen (if he's still around) for the throttle body cleaning project.



I was having a terrible idle problem and was able to fix it myself for $3 thanks to the project (I also had the spark plugs replaced but that needed to be done anyway).



Thanks again!
 

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