2003 Dakota 4.7L - Strange Symptom

[tony1679]

I am unsure what is going on. This is definitely more of an annoyance than a real problem, but I'd like to fix whatever it is.

My problem is noticed when the truck is first started, only when cold, and in park. If I apply throttle STEADILY at say 1,200 rpms, the rpms will actually dip, then rise, and repeat. So if I hold perfectly steady at 1200, it will dip to about 1000, and rev to 1400 and just bounce back and forth. The interval is about once per second, meaning 1-1000, 2-1400, 3-1000...

No sudden surging, no misfire, nothing "wrong." Just a varying engine speed. The only thing I rarely notice is a slight throttle hesitation. But again, rarely. When it's fully warmed up, it runs like a dream.

To paint a better picture, here is what I have done to the truck already while trying to cure a rough idle / misfire issue (fixed):

Cleaned throttle body, thoroughly.
Replaced air filter
Replaced IAC valve
Replaced TPS
Replaced spark plugs
Have used several bottles of Techron (not too frequently)

Thoughts?

 

[Chuckinator]

The first thing I would do is try different fuels

 

[tony1679]

^Well, I have done this. But why would that matter? Like I said, this is not a misfire issue. It seems to be something mechanically/electronically controlling the engine that is out of whack when cold.

 

[zrxkawboy]

You have tried everything that I would suggest - not sure what to say. I guess my real question is, on an EFI vehicle, why do you want to apply throttle in Park when cold?

 

[tony1679]

^Well that is a legitimate question. It is most noticeable when in park, but will also do the same while driving, until halfway warm. Sorry for that confusing tidbit, I should have wrote only when cold, noticeable in park.

The reason: I am cleaning out a slightly sludged engine. When cold, it isn't real quick to get oil pressure. I never touch the throttle before it gets initial pressure. But once it does, it takes another ~45 seconds to gain full pressure. By giving it a tiny amount of throttle, it builds pressure faster. That's how I discovered this problem.

 

[zrxkawboy]

I wish I had a suggestion. The only other things I would do would be to test fuel pressure (probably not the cause, but wouldn't hurt to test), and see if you can find the specs for the coolant temp and intake air temp sensors, and then test those (since it seems to be temp-related). I'm assuming no CEL.

 

[HangFire]

If I had the slightest suspicion of a sludged engine and low oil pressure, I'd drop the pan and clean the intake screen. I know it's a lot of work, but so is replacing the engine.

Cold engine weirdness can be a lot of things, some already mentioned. Off the top of my head, a bad or flaky heater in the oxygen sensor could be one of them.