'02 sport Trac idle problems

[skipatroler]

Alright - I've been searching for a while and cannot figure out my problem. It's a 2002 Explorer Sport Trac, 4.0 L V6, 4x4, it has 244k miles and up until a month or so ago it has run beautifully! It started with a small "surging" now it idles really low when I stop (500rpm - and bounces to 1000 then back down) acts like it wants to die. I've replaced the fuel filter, plugs, wires, and even found a vac hose line broken (fixed that too). Still have the low idle speed, and I noticed that in park or neutral when I rev it - it'll max at around 3500 and act like it isnt getting enough fuel - in drive I seem to have plenty of power and have seen 5000 rpm when trying to pass. I did run the codes and I had a cylinder 4 misfire - which I cleared and has not come back since the plugs and wires were changed. Now to add a final problem it has developed a bad knocking in the last week and I can see the tensioner bouncing - I checked and changed the oil since it was time for it. Any ideas? Thanks in advance!

 

[swshawaii]

Clean the IAC valve, throttle body, and MAF sensor in that order, and test drive after doing each one to isolate the issue. On the ST, there is a park and neutral rev limiter that kicks in around 3000 RPM that prevents damage when there is no load on the engine.
Also check the PCV elbow if you haven't yet. As for the "knocking", remove the belt and inspect ALL pulleys for binding. If no problem found, run the engine without the belt for less than one minute and listen closely for internal engine noises. See link below.

http://www.explorerforum.com/forums/showpost.php?p=3233677&postcount=2

 

[Kjhadfield]

I had a rough idle and did the combo air filter/maf/throttle body & iac cleaning. Although I can't narrow down what fixed the rough idle, I suspect (almost sure) the iac valve. During cleaning, this was the dirtiest. I guess if the are bad enough the valve sticks and can't get a correct reading and the engine is trying to find the balance? (Somebody want to clarify more technically)

Anyways after the clean, purred like a kitten. I can't remember what noise it sounded like but way back I had to change my belt tensioner. Maybe a squeak, don't think it was a knocking thou. The old one had excess play when the engine was on. Was told a little is normal

 

[skipatroler]

Update

Update on the idle problems - Cleaned the IAC and the surging went away, although it still idles really low and still acts like it wants to stall - but it's acceptable. I cleaned the MAF sensor - and no change, cleaned the throttle body - again no change. I'd still like to figure this one out once and for all.

Now for the knocking sound - which is more like a tapping noise than knocking - took the belt off, checked all pulleys and guides and all were normal. The tensioner seemed like it wasn't "springy" enough. I ran the engine briefly without the belt - and the noise is still there. I did put a new tensioner and belt on (since the belt was showing wear, and the tensioner wasn't that much)

It does sound like it's the valve lifters chattering, the sound is definitely coming from the front driver's side (i believe that's cylinder #4 - WEIRDEST cylinder numbering i've seen!) and it seems to lessen when i reach higher speeds also seems to almost go away if i drive it for a while - almost - but it's definitely still there.

What's funny is that it wasn't making any noise prior to me changing the spark plugs and wires (which one of the wires was found touching the exhaust manifold and grounded out)