Rolling idle when VERY cold

[gijoecam]

OK, here are the symptoms and conditions:

98 Sport, 4.0 SOHC, bone stock save for a K&N drop-in.
Only when it's below 40 degrees outside.
First start, cold start when sitting overnight.
Cranks and fires just fine, idle holds at 1200-1300 for a few seconds, then slowly falls to around 850-900, then rolls back down to around 400 where it sputters for about 4 seconds, then it revs up to 1500-1600, then repeats the up/down cycle three or four more times. It never stalls when the idle drops, although it runs rough, and after four or five rolling cycles, it flattens out and idles at a smooth and stable 800-900 rpms. Once up to operating temperature, the idle in gear settles back down to around 650 rpms where it's always been.

If I park it in the garage (which stays at about 50) it's fine. Anytime the temps are over 40, it starts and idles fine. If I let it soak in freezing temps for 4 hours or so, it does it on the first start-up only. Subsequent re-starts result in a normal idle speed for the conditions.

I'm at a complete loss. The IAC is relatively new. It started doing it at the end of last winter, and by the time I changed the IAC, it was warm out and the problem only happened once or twice more before I could troubleshoot it. The problem has re-surfaced with the cold weather lately, and it's getting old.

Ideas? I'm clueless... this is a new one for me.

-Joe

 

[gijoecam]

Somebody's got to have some idears.... bump-itty-bump....

 

[NHale623]

Check the IAT and ECT sensors.

 

[churd69]

I had a problem like this in my dakota and it was a crankshaft position sensor, not sure if the explorers have them, but it maybe worth a shot?

-Rich

 

[gijoecam]

The ECT is up under the thermostat housing, isn't it? Never thought about that one....

IAT... Isn't that right on the intake tube? If it was the IAT, wouldn't it continue to roll the idle after the engine runs for a minute? Doesn't seem like it would go away... I could definately see the coolant temp causing it though... Is there an easy way to monitor the coolant temp signal?

Another question: Is the ECT sensor on the SOHC independent of the coolant temp sending unit for the gauge?

Thanks, guys!

 

[NHale623]

Not real sure where the ECT is on a SOHC engine, but it should be close to the sending unit for the gauge.

There's a chart floating around, I think BrooklynBay posts it alot, that shows the expected ohm and volt readings for a given temperature. They're the same for both the ECT and IAT.

I would think after the engine has a chance to warm up, the engine would go closed loop and the effects caused by a malfunctioning sender would be mitigated to an extent.

I know they're very easy to monitor with a scanner, if you have access to one.

 

[chamuko]

hello

I have similar issues with my explorer , changed the IAc MAF and still silmilar issue at hand so we attached the comp to the truck an dnow reads 2 codes
P0136 O2 sensor B1 S2 malfunction , bank 1 too rich & bank 2 too rich , both under P0175 & P0172.....so try your with the comp mine truck it needs these 2 sensors changed , good luck!!!