[1fun281]

Every once in a while seems more like at a cold start up my 4.6 is running rough at an idle, and under acceleration. the rpm's drop real low at an idle and when cruising it stumbles, i want to say its a iac problem but not really sure any suggestions on what it could be or what i could do to possibly fix it?

[trinity_gt]

Possible EGR problem. If the EGR valve isn't closing properly -- as it should at idle and certain part throttle conditions -- it's basically a vacuum leak and can cause idle problems.

IAC wouldn't cause stumbling or missing at cruise. You may have ignition issues. You don't say what year you've got but if you're 99+, check the COPs. If your 96-98, check the wires and coils. In any event, check the plugs and plug-wells (for oil or coolant contamination.)

Any engine light/codes? If you've got a misfire I'd expect the PCM to log it. If the front O2 sensors are far-gone enough to cause driveability problems you should have seen a MIL too.

[1fun281]

99 gt offroad x, with check engine light assuming because of offroad midpipe, im going to pick up some mil eliminators from a friend, to see if that helps a little and i noticed it seems to be rough after i get on it a little and open it up but thanks i think im going to do all new plugs soon anyways see if that helps

[Nightfire]

Trinity knows what he's talking about. Ive been here for a while and this guy is like a walking Mustang encyclopedia. Listen to him

[wmburns]

How about the other usual stuff? Clean MAF and look for Vac leaks?

A dirty MAF will over state air flow at idle and under state at higher flow. This results in the engine going from rich at idle to lean at load. This will show up as neg values in long term fuel trim. Going to positive numbers under load.

As O2 sensors age, their response time decreases. This causes a delay as the motor goes from rich to lean. IMO, this can contribute to stumbling off of idle. Recommend replacement if O2 sensors > 100k miles.

OBTW; a vacuum leak will also show up in nearly the same way. Typically, at idle the percentage of unmetered air is greater than under load. This will show up as a change in the long term fuel trim numbers.

Even a very small vacuum leak can cause problems.

[joshjwc9]

You say you have a CEL but haven't pulled the codes? Take the car to the auto parts store and pull your codes. You might simply have had a coil pack or COP go bad depending on the car, start with the cheap or free fixes before you start putting money where you dont need to put it...

[1fun281]

thanks everyone i will try all of these im going to replace all the hoses, and i need a few grommets i have a friend who looks at my engine who is leaning towards a vacuum leak im going to pull the codes and see what it is too, im also going to change all the spark plugs see if that helps and clean the maf

[GDawg]

Check for vacuum leaks definitely.

[98COBRA281]

ryan(1fun281) just left my house, we replaced the iac hose, pcv hose, put a grommet around the AIT sensor, and cleaned the MAF and AIT, and seemed fix the problem so far...