rough idle on 1st cold-start of the day

[koda2000]

This one's got be stumped. I have a 2001 Explorer with the 4.0 SOHC V6 with 128K. It runs great except for one annoying thing. The first time you start the engine in the morning (or whenever) if runs super rough for about 5 seconds then smoothes out and it's fine. Any other tme you start the engine that day (5 seconds later, 10 minutes later or 8 hours later) it starts and idles just fine. I have no CEL and no codes stored so I'm not thinking intake manifold gaskets. The air and fuel filters are new, plugs are new, ignition wires are new, MAF and IAC have been cleaned, IAC tests as OK and idle is steady 950-1,000 RPM once it smoothes out. Ive tried leaving the key on for a few seconds before starting the engine and switching out the fuel pump relay - made no difference. One thing I tried this morning was to press the gas pedal once before turning the key and it started up and ran smoother but at higher than normal RPM for a few seconds before returning to a normal RPM. That makes no sense to me. Why should cycling the TPS make any difference?

 

[yavapaires]

I don't know about cycling the TPS, but are you using/loosing any coolant? You could be getting a little bit of coolant in one cylinder, and the plug could be fouled upon startup. That happened on my 96 4.0 OHV. Just a thought.

 

[koda2000]

No. Not losing any coolant and no smoke of any kind when it starts. It acts like there's not enough fuel when it's started for that first time, but leaving the key on for 15 seconds before cranking and swapping the fuel pump relay doesn't make any difference. It's almost like moving the TPS is telling the computer something to the injectors, but why just that frist time of the day. It won't do it again, even if you immediately turn it of before it has a chance to build any temp, until many hours later.

 

[ranger7ltr]

Sounds like the ECT sensor is not working properly...The ECT feed info back to the PCM to determine how much fuel the engine needs especially when the engine is going through the first cold start of the day...

An ECT can be checked to see what the resistance it is outputting and that can be measured against a chart for the sensor... Check this link for more information...http://www.fordfuelinjection.com/?p=28

 

[koda2000]

I'm liking the ECT sensor theory and here's why; w/in the past couple of days the outside temp has dropped from 92+ degrees to 69. This morning I went to start the Explorer and it started instantly and idled perfectly (w/out touching the gas pedal). First time ever since I bought the truck 3 weeks ago and first time the outside temp as gone down.

Thanks for your input - Koda

 

[LONO100]

this might sound too obvious, but have you checked your oil level? maybe it is idling rough when you first start it when the oil pressure is still low, but gets better when the oil pressure builds up. it could mean you have low oil, the wrong recommended oil, an oil leak, a valve cover leak (where pressure can be escaping), or even a timing chain issue.

 

[koda2000]

oil level and oil pressure are fine.

 

[LONO100]

no valve cover leakage? or oil in the spark plug tubes? i only ask because i had a similar problem recently with my other truck, and once i replaced the valve cover gaskets and tube seals, problem went away. also try to test your plug cables ohm readings if you can to see if there is a short.

 

[alohamonte]

I chased a rough idle for a long time, replacing many parts, but it wasnt until I replaced the ignition coil it finally went smooth. So if you've covered all your bases and are scratching your head like me, check for weak spark.

 

[yavapaires]

Valve cover leaking WILL NOT cause a drop in oil pressure, unless you run it OUT of oil.