low/rolling idle when cold outside

[gijoecam]

I've got an interesting idle problem that's been getting progressively worse for a while. First, a little background.

'98 Ex Sport, 4.0 SOHC, 100,047 miles on it (as of this morning). I was having some low idle issues an intermittent failure to idle and rough/low idle in the late summertime. I had it happen about a year ago, at which time I cleaned the IAC and it fixed it. I figured this was the same problem, but instead of wasting my time again, I replaced the IAC with a new Ford part. That helped, as did the tune-up (i.e. plugs and wires) last month, but it still seemed to be idling a little on the low side (just over 500 rpm). I wasn't too concerned as it was smooth.

Now, since the weather turned very cold here in the last week, if I let the truck sit outside overnight, when I fire it up, the idle is quite erratic. It slowly creeps up to around 1700, then slowly rolls back down to around 300, sputtering and spitting once it's below 500, then it smooths out and rolls back up to 1700 again, repeating the cycle. It's not a fast swing in RPMs either; if you graphed the RPMs over time, it would be about a 10-15 second frequency. It will do this for a few minutes if I let it, but it has yet to stall (although it sounds like it wants to, for sure!!) If I touch the gas one time, or drop it in gear, everything is back to "normal" (i.e. slightly low idle).

I don't even know where to start looking. No codes (yet) so that doesn't make it any easier. Sooooo......

Could this be a sign of the lower intake manifold gasket being bad? It would certainly explain the low idle when warm, but I'm still stumped on the rolling idle and sputtering?!!?

What do y'all think?

-Joe

 

[Runnin'OnEmpty]

Joe, since you've already replaced the IAC, and that made it a little better, I'd try cleaning the throttle body. Carbon builds up at the spot where the throttle blade meets the housing, and closes off the idle airflow. Then the IAC tries to compensate.

It's possible that a failed intake gasket is causing the rolling idle, but I'd clean the throttle body first, and see if that solves it. An intake leak will usually set a lean code.....

Good luck,
ROE

 

[gijoecam]

Hmmm... I hadn't thought about the TB.... but, if the TB was dirty, wouldn't it have idle problems all the time?

I forgot to mention it, but it only does it the first time on a cold morning.... If it's been in the garage overnight, it's fine, save for the slightly low idle.

I'll still take a look at the TB later this week.

Any other idears?

-Joe