Help, my 4.6 is sick.

[4.6andSlicks]

'01 Cobra w/ 50k on the clock. Had it 2 years.

All of a sudden it ldles a bit rough and stumbles at cruise speed. Runs great when you stomp on it but when cruising, you can feel it missing a few beats regularly. It's not throwing any codes but here's a clue.

When I reset the computer, it runs good for a couple of hours and then right back to stumbling.

Replaced the plugs (copper to iridium) and it seemed to get slightly worse.
I checked for vacuum leaks today but didn't find anything obvious.
Clean the MAF sensor just because.

What should I do next ??

 

[Dragstr05]

Seems to me your PCM is learning a poor strategy somehow. I would lean towards the IAC, but that wouldnt cause an issue while cruising.

Need more info if available. How long have you had the issue? And what has been done maintainance wise?

 

[BurningRubber]

Coil pack(s) possibly?

 

[4.6andSlicks]

It happened about 2 weeks ago after a spirited evening drive
Don't know if that had anything to do with it.

I don't beat it. Never over rev. A couple of 5 grand pulls maybe.

It's not a dead miss or anything but when your rolling along at a constant speed, it almost feels like you drop a cylinder for tenth of a second or so, once every 3 or 4 seconds.

Nothing done to the car engine wise. It has a JLT CAI, O/R H pipe with Flowmasters. Other than that, a stock DOHC.

All plugs were an even tan color when I changed them.

 

[04YELLOWGT]

Check your plugs. I run race gas in my car on occasion and it eventually fouls out the plugs. I always know its time to change them because when cruising a 60 the car will start to miss.

 

[4.6andSlicks]

Check your plugs. I run race gas in my car on occasion and it eventually fouls out the plugs. I always know its time to change them because when cruising a 60 the car will start to miss.

That was my first thought so I swapped out the NGK coppers with NGK Iridium. Problem got slightly worse, not better.

 

[wohorsy2002]

pull coil packs and check for moisture.... many miss problems solved here... all it takes isa tiny bit of moisture.. i saw coil pack or bad injector just maybe

 

[4.6andSlicks]

pull coil packs and check for moisture.... many miss problems solved here... all it takes isa tiny bit of moisture.. i saw coil pack or bad injector just maybe

When I changed to plugs, I didn't see any moisture and the car never goes out in the rain. Could be injectors.

What do you all think about that injector cleaning service the local oil change places offer?

 

[ThunderHorse]

Could be injectors.

What do you all think about that injector cleaning service the local oil change places offer?

I wouldn't do it unless you can get them to do it for 20 bucks or less. You can do the SAME thing most of those shops do just by going to any parts store and picking up a can a seafoam and doing like half a can through your brake booster line and the other half of the can in the gas tank. The reason why I say 20 bucks or less is because I know places like jiffy lube charge 60 dollars around here for that when you could just buy a can of seafoam for like 6 bucks and do it yourself.

 

[deftsound]

i would say maybe a bad circuit in the wiring harness or an 02 sensor thats giving a funky reading