4.0L OHV Rough Idle Problem

[tcbaklash]

When my truck is left sitting while idling, the idle will begin to lope and is powerful enough to shake the whole truck when the engine gets warm.

I don't have a tachometer connected to the truck right at the moment, so I have no clue what my RPM is when this is happening. I cleaned out my IAC, but the plastic inside is rotten. I cleaned out my MAF sensor (which is from a 2.9L, comes stock with a smaller intake hose). I also replaced my throttle positioning sensor that solved a different problem. I have not yet replaced O2 sensors.

 

[RonD]

O2 sensors are not generally used at idle, computer always runs engine richer at idle, so you could just have worn spark plugs that are fouling on richer mix when it is longer term.

If you plan to leave engine idling long term then gap the spark plugs at .059 instead of .054, this gives a stronger spark for cold starts(also rich mix) and idling, you can go as wide as .064 if you want.
Although the wider the gap, the less recovery time, for the Coil, so wide gaps can effect high RPMs, say above 3,500
Racers use narrower gaps, they don't spend alot of time idling, lol.

If IAC valve is falling apart then that would be the problem.

2.9l MAF should be OK to use although I would look for a 4.0l MAF

 

[jeremysdad]

Yep. I read this 2 hours ago. 'Plastic inside iac is falling apart'.

Can't clean it if it isn't there. Buy a new one, report back.

 

[tcbaklash]

O2 sensors are not generally used at idle, computer always runs engine richer at idle, so you could just have worn spark plugs that are fouling on richer mix when it is longer term.

If you plan to leave engine idling long term then gap the spark plugs at .059 instead of .054, this gives a stronger spark for cold starts(also rich mix) and idling, you can go as wide as .064 if you want.
Although the wider the gap, the less recovery time, for the Coil, so wide gaps can effect high RPMs, say above 3,500
Racers use narrower gaps, they don't spend alot of time idling, lol.

If IAC valve is falling apart then that would be the problem.

2.9l MAF should be OK to use although I would look for a 4.0l MAF

Plugs are 2k miles brand new, could it be the brand? Some guys swear by Motocraft, I slapped in NGKs because they're inexpensive, but I don't think it would really make a difference. I'll be sure to re-check the gap. However, summer is coming, and it gets HOT here, I don't think I'm going to widen the gap for normal driving's sake, but I will consider it for when wheeling. I'm going to grab a new IAC today for sure though.

As for the MAF sensor, I checked part numbers when searching for one on RockAuto, and the sensor for the 4.0 was just one number off in some instances, and one manufacturer had the same part number for 2.9L and 4.0L, but only time and effort will tell if this is a contributing factor.

 

[RonD]

4.0l OHV needs Motorcraft or Autolite spark plugs, eats other brands, can't say why?

Tried NGK once, like you they were less money, ending up changing them in a few months, my 2.9l didn't mind NGK, 4.0l OHV seems to only run well on Motorcraft/Autolite, and I just use regular copper.

My original spark plug wires lasted 18 years, well probably 15 years, lol, but changed them at 18 years when local Ford dealer had a sale on Motorcraft tune-up parts, made a big difference at idle and accelerating.

 

[tcbaklash]

4.0l OHV needs Motorcraft or Autolite spark plugs, eats other brands, can't say why?

Tried NGK once, like you they were less money, ending up changing them in a few months, my 2.9l didn't mind NGK, 4.0l OHV seems to only run well on Motorcraft/Autolite, and I just use regular copper.

My original spark plug wires lasted 18 years, well probably 15 years, lol, but changed them at 18 years when local Ford dealer had a sale on Motorcraft tune-up parts, made a big difference at idle and accelerating.

Wires were changed when I swapped the engine, I wanna say they are MasterPro brand. Not the best, I know, but I don't want to shell out big money on this old truck. I installed new Motocraft Platinum plugs and a Duralast IAC today and the idle problem was not fixed. Been thinking about boxing up the IAC and returning it because I'm really poor right now.



In the video check for the idle after the engine is revved. It appears normal for a few seconds and returns to a rough idle. You can see the truck rocking and the engine even more. I honestly don't know much about engines and such, so that's basically why I'm here.


EDIT:
So I took the truck out for a spin. I THINK the Motocraft plugs made the engine quieter and smoother while driving, but it could also be the placebo effect...

 

[tcbaklash]

I installed a MAF sensor from a 4.0L and it didn't fix anything

Could it be my exhaust leak causing this?

 

[RonD]

Just for the sake of "taking it off the table"

4.0l Coil pack wiring is like this:
3 4
2 6
1 5
front

1 2 3 side is pretty easy to get right, but check the wires, 1 and 2 or 2 and 3 can get mixed up.

Now the "I know it is right side"
5 6 4, this side gets people that do spark plug wiring all the time, just humor me

Follow each wire to its spark plug, ONE AT A TIME, 5 6 4, front to back

 

[tcbaklash]

Wires were installed correctly during the swap. I made absolute certain. I got the exhaust leak taken care of (and a new Flowmaster 40 ), problem still persists.

An acquaintance of mine said that it may be EGR, since mine is disabled... Any thoughts?

 

[RonD]

At idle computer runs engine Richer.

This can cause misfires
Spark plug gap spec is .054, try .060

 

[tcbaklash]

At idle computer runs engine Richer.

This can cause misfires
Spark plug gap spec is .054, try .060

I gapped to .060 when I installed the new plugs.

Oh well, I'll figure it out eventually. Thanks for the help!

 

[tcbaklash]

I got my brakes serviced today and my idle problem is gone...

I can't even comprehend this.