Hello all:

I will do my best to keep this short as possible -- but I am at my wit's end. I have a 2003 Explorer Sport Trac XLT 4x4 with a V6 SOHC and approx. 67K on the odometer. For the past several months, I've experienced some issues at idle, posted a few times in the Explorer forum, and searched this site exhaustively, but still haven't identified the cause of the problem.

When the truck it started cold, it idles around 900 RPM without problem. As soon as it warms up, the idle falls to around 600 RPM and the engine begins to stumble during complete stops like it want to stall -- but never does. And I mean that it literally changes a soon as the engine reaches normal operating temperature. I was staring at my temperature guge last week at a stop light, and when the temperature needle reached its normal spot (just under halfway), the RPMs fell from 900 to 600. There is no CEL and no stored codes.

Further, I have noticed that the engnine shudder is a little worse when the the weather is cold, and a little better when the compressor is running in AC and defrost modes.

About the only thing I've done so far is replace the IAC in November, but there were no changes. Other possible causes that I have read in these forums are fuel pressure regulator, fuel pump, thermostat, MAF, EGR, throttle body, O2 sensors, and vaccum leaks (i think this is all of them). My question for the engine gurus is are all of the above-listed possible causes? And if so, where should I begin? I would like to exhaust all possible solutions before taking it to the dealer since it is out of warranty. Any advice/direction you can provide will be greatly appreciated.

 

Given the suggestions you posted, I'd check the fuel pressure. Check it when the engine is cold and running well, and then again after it reaches normal temperature and begins to stumble. A FP check will point to or eliminate a problem in the feul delivery system (fuel pump, FP regulator) as the culprit. A quicky check on the FP regulator is to unplg the vacuum line from it to check for the presence of fuel. If there's fuel in the vacuum line then the regulator is bad. But if there's no fuel, it doesn't neccessarily mean the regulator is good either...

I have to say however that the first thing that came to mind while reading your post is the IAC. You say you replaced it with no improvement, but did you check and clean all passages as well as the throttle body? Also worth checking/cleaning is the Mass air meter/filament.

 

a failed FPR would show up first as fuel starvation at warm cruise and WOT

pull IAC electrical plug while in warm idle, engine should die
indicates PCM is sending warm idle IAC control voltage and wiring is ok

look for a sticking EGR that hangs open when warm, causes mix to go lean thus low warm idle
EGR should be closed during cold start/idle

also check condition of plugs and ign wires, may have heat cracked plug or wire that droops and shorts out with heat

how is warm cruise, WOT operation and fuel mileage?

A/C on kicks in idle boost to about 1k rpm to carry a/c comp. load and increase fan speed cooling

 

Thanks for the advice and sorry I didn't respond sooner. My wife gave birth to our first child on Thursday, so it has been a little hectic.

TigerDan -- I have never had the fuel pressure checked on any of my vehicles. Is that something I can do (other than "quick check" of the FP regulator as you described), or should it be performed by a mechanic. Also, I am like you when it comes to the IAC. My problem sounded like a classic IAC issue, but it didn't change a thing. Now, I didn't do the other measures you suggested like cleaning the passages, TB and MAF. I have read alot on here about cleaning the MAF and TB, but how do you clean the IAC "passages." Thanks for the help.

96_4wdr -- I am a moron when it comes to this stuff. Is WOT "wide open throttle," and by warm cruise, do you mean cruise control or just regular driving when the engine is warm?

I did an electrical test on the old IAC based on detailed post in the Explorer forum, and it check out well within normal limits. I still went ahead and replaced it, just to find that the engine still acts the same with the new IAC. And is there any simple explanation as to how to see if the EGR is sticking?

I can check the plug and ignition wires for cracks, and I should also mention that my gas mileage is really good. I would guess that I am getting approx 22 miles per gallon highway.

 

I had an air leak in the intake gasket that would cause my 93 Explorer to behave the same way. When the engine is cold, the fuel system is running richer so that the effects of the leak are masked. Once warmed up, the system leans out the mixture and the effects of the air leak are evident. You may want to investigate the possiblility of an air leak somewhere in the system.

 

Hi Robert, welcome to the site! Thanks for adding your advice to the thread, excellent first post!

Aballard, when I say "clean the IAC passages" I'm referring to the passages in the plenum/throttle body where the IAC bolts up. You have to remove the IAC and look into the passages for any carbon build-up that could restrict airflow, and clean them accordingly.

WOT is indeed Wide Open Throttle, and warm cruise just means steady driving at highway speeds with the engine warm, either with or without Cruise Control, it doesn't really matter.

As for the EGR, you can remove the vacuum hose from it and apply a vacuum, usually with a hand vacuum pump (at idle there should be no vacuum to the EGR, it's controlled by a solenoid valve that turns it on and off at appropriate times). With the engine idling at normal operating temp it should really fall on its' face when open the EGR with vacuum. It's also not a bad idea to pull the EGR off and clean its' passages just like the IAC. The carbon build-up in there can hold the EGR open and cause some idle issues.

 

someone had mentioned to me that it could be a problem with the upper and/or lower air intake gaskets. Do you all think that is a possibility? If so, is it something I can diagnose myself?