First post - hope this is the right place...

Truck is a 2002 4x2 with the 5.9 (The only year this motor was installed in the Gen III's - that's why I'm here). Purchased new by me. Has 75K miles. Has always been babied as a daily driver, with very infrequent light towing duty. Has been very trouble free, save for a rear end rebuild done under warranty.

A couple of years ago, I would notice on rare occasions the idle would hunt a bit or stumble - just enough to feel - but only after the engine was warmed up, idled fine when cold. Barely enough to move the tach needle. Also about the same time the truck would "lurch" once or twice as it tries to downshift - but only under a specific set of circumstances:
- Only when fully warmed up
- only when barely accelerating from highway cruising speed, such as maintaining speed while going up a hill, or when slightly increasing speed. If the throttle is opened more than slightly (harder acceleration - it doesn't occur).
Until the last couple of months it happened randomly - just enough to be an annoyance, and not enough to even see consistent indications. And never happened when cold. Oil and fuel consumption appear to be normal - it's never burned or leaked a drop of oil in it's life.
No MIL / CELs. I've checked codes with an OBDII reader - zero.

Now the stumble is worsening, and it happens when cold. I will be driving this truck from MD to CA the minute I get this solved - so I'm a little desperate.
Just replaced air filter and PCV. Nothing loose or unplugged under the hood. Taking it in to the dealer is my biggest fear and will try almost anything first...

I'm hoping the symptoms point at a known issue by someone here. I did Search, but came up empty. My thoughts as to possibilities:
CAT clogged. Would explain the idle, but would it cause other driveability probs that I'm not having..
MAF? O2 sensor? Please don't say plenum gasket issues...

TIA

 

You may or may not have a bad plenum, but I don't think it would cause those symptoms.

The 5.9L V8 doesn't have a MAF sensor, it uses the older MAP type system. The MAP could be one possible explanation, but I'd save it for one of the last things you look at... that little sucker is $90-$100.

When was the last time a tune-up was performed? New plugs, plug wires, distributor cap, and rotor? Those are the first thing I'd do if they haven't been done already.

If that doesn't solve the issue, test your TPS and IAC. The Haynes manual has a good test procedure for them. Use a multimeter for the test, and don't be "AHA!" if the IAC doesn't move when the Haynes says it should. 2001 model years, I know for sure, don't move the IAC when the key is turned to "ON", 2002s may be similar. Rely on the multimeter readings.

O2 sensor(s), the pre-cat ones, could be the issue. When was/were it/they last changed? If it/they is/are factory original(s), I'd replace it/them, regardless if they are causing the issue. 60,000-75,000 miles is about their life before they start dying or working inefficiently, it seems more often than not.

 

Update:

After reading up quite a bit, I decided to replace the TPS and ICV - hoping for quick and cheap solution... The TPS is very easy to access so I did that first. Problem remained.
Next was the ICV. Harder to get to but not too bad. Seemed to help but the problem was still there. BTW, the throttle body was clean as a whistle..

With a coast to coast drive a few days away and out of time, I did the unthinkable - I took it to Pep Boys. I told them to only replace the Dist cap, rotor, plugs and plug wires . Mine were all original with 75K miles. They recommended a a fuel inj cleaning (total BS) and I figured what the hell...
Problem solved.
The cap and rotor were badly pitted and I'm sure that was the biggest culprit. The mechanic - actually a really good guy - said the wires were in bad shape as well.
I went ahead and had the pre-cat O2 sensor replaced at the same time for piece of mind.

Just arrived in CA after a 2800 mile drive pulling a heavy U-haul. Ran great.
Hope this helps.

Mitch