Low idle when ambient temp is cold

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G

gyates

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OK, I have posted on here before about idle issues with my '05 ST. Now I can maybe help narrow it down and maybe one of you has had the same issue. The ambient temperature the past several mornings in my area has been in the 20's in the morning. I park in a garage at home, and though it is not heated it is insulated, so always warmer than outside temp. After initial engine warm-up I am having issues with the idle being too low. Basically it is ilding around 500-600(indicated rpm's on tach), and stays there until the ambient temp warms up into the 30's and the a/c compressor begins to cycle regularly. I have replaced/cleaned several items including the IAC, throttle body, MAF, and a new fuel filter. But again, problem only happens when ambient temp is cold enough that the a/c compressor is not cycling (a normal feature for our trucks). I have no CEL showing. When the idle is in the 500-600 range the idle is a little rough, but when it hits the 750 range it really smooths out. Has anyone experienced this issue and figured a way to resolve it? Is there a temp sensor for the engine computer that may not be reading correctly? Overhead console and the temp gauge in dash appear to be accurate. I am not a mechanic by any means, but if a simple solution exists, well, I might be wiling to try it. And if it helps, my commute is a combo of highway and typical city stop & go traffic. Thanks.
 
Assume you've checked the PCV elbow?



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I suspect the temp sensor on the T-stat housing, is out of range. But not enough to set a code, yet.
 
Thanks so far. On the elbow suggestion, yes, I replaced it, even though mine was not cracked. Eddie, I do wonder about the temp sensor as a possible problem. All ideas appreciated. :) And I did not say in my original post, but truck has 77k miles. All maintenance is up to date as well.
 
Pulling the negative lead on the battery does not always reset the KAM (keep alive memory) in the pcm. Pull both battery cables and short them together for a few minutes (I use a jumper wire with alligator clips on it if my scanner isn't handy). When you "relearn" the idle speed after hooking the battery back up put the HVAC in the floor position so the ac compressor won't kick on and see if that helps out.



Do you have any way of looking at fuel pressure? Slightly low fuel pressure could cause your issue and with the fuel delivery setup that these have on them, they control the fuel pump speed to acheive proper pressure, it could be an issue.



How did you clean the throttle body? If you just sprayed it then chances are that you didn't accomplish much. They need to be opened up and scrubbed, I use a toothbrush and usually remove the throttle body.



Is your battery in good condition? What is the resting voltage after the vehicle has sat all night? What does the voltage pull down to while you crank it?



Have you considered the possibility of bad engine mounts? Mounts that have gotten hard over time will transfer more vibration throughout the vehicle, especially when outside ambient temperatures are lower. I had an ST that I purcheased a while ago that had a rough idle that I couldn't figure out. While the transmisison mount didn't appear to be bad it didn't look right (if that makes sense) I replaced the mount and viola...problem fixed.
 
l1tech. On the throttle body yes I removed it and scrubbed it, including both sides of the plate. And I will add that the lower 2 bolts were a bit of a pain to get to!! Also battery is only a few months old and have no other indications of a weak battery. When previous battery was on the way out I was getting the sometimes odd things we experienace, like random odd warning ights on dash, slow starts, etcc., so I replaced the battery before it went totally dead and no issues since. Battery was OE and over 6 years old. I had it tested before replacing and it was reading very low.



On the coolant temp sensor idea, I can buy that as a possible problem. Now that it has been mentioned I have been very closely watching my temp gauge in the cab. I have noticed that it never quite makes it to the half-way point in the gauge, always staying a little bit below that. In my wife's '04 Explorer the temp gauge stays dead in the middle once warmed up to operating temp, regardless of ambient temp. I know the truck is not running cool though as I am getting nice and hot heat in the cab. I had an older Explorer where the thermostat stuck open and not only was the gauge staying way low but there was minimal heat in the cab as well. Right now I am leaning toward replacing the sensor and going from there. They are not expensive and relatively easy to replace (though it looks like I will have to remove the throttle body again).



And related to the temp sensor, does anyone else think it odd that we have 2 sensors in place, but only one is connected, and there is only 1 connector? Seems an odd design to me. My older Explorer had 2 connected sensors, but just seems odd to have 2 in place but only 1 working. Hmm...



I bought the truck about 2 years ago with 55k miles, so I have very little info on previous maintenance. I know the fuel pump was replaced around 50k, but that is about the only repair I could find any info on. I had to replace the fuel inertia switch a couple of months ago. Other than these 2 items and replacing the IAC, the truck is basically still as it came off the line in late 2004. Maintenance I have done since purchasing includes tranmission flush (immediately after I bought it), rear diff. flush, regular oil changes, power steering fluid flush, replaced sway bar end links and bushings. and replace shocks. Coolant flush was done by dealer I bought it from. Overall I am very happy with the truck, I just want to resolve the slightly low idle when temps are cold. :)



Thanks to all and thanks for this sight!!
 
I don't think you are going to have much luck in changing the temp sensor. You state that the temp gauge seems to read lower than another vehicle, even though it is a Ford that is not a good comparison, but lets just go with it. The gauge reads lower which you interpret as the gauge not reading hot enough, this would in turn cause the idle speed to increase as the pcm increases idle speed to compensate for a cold engine. Lets assume though that it is the case and the pcm is casuing a rich condition by thinking the engine is colder than it should be, typically a rich condition won't cause a rough idle UNLESS it is grossly rich. A lean condition on the other hand will casue a rough idle.



I'm not totally convinced that you have a low idle problem. Obviously idle speeds will very with engine load such as the ac being on or turning the wheel with the vehicle stopped or just by putting the vehicle in gear. In theory the pcm should set a a code for lower idle than expected if it though there was an issue.



Does the idle feeling change between reverse, neutral/park and drive? In reverse and drive try holding the brake and goosing the throttle a little and then let the engine return to idle, does the feeling change at all?

 
l1tech, to answer your question, when ambient temp. is low (generally below 40f), this low idle appears. Does not matter if hvac is on or off. I have effectively tried what you mentioned about hitting the gas a little harder and letting it settle while in gear, and it always settles around 550-600 (indicated on tach). and it is not a smooth idle at that level. Once ambient warms up, idle improves to 750 (indicated on tach) and is much smoother. Other than the low idle I see no real performance problem wit the truck. Has plenty of power even with the low idle, and gas mileage is decent (though it has dropped a little from 15 to 14.5).



This issue may or may not be the coolant temp sensor, but I did order one from our Site sponsor fastpartsnetwork. I had to order something else anyway from him (my auto dim mirror has died), so I threw in a sensor. On my mirror I can atually see that one of the connection prongs in the mirror has broken loose from the board it mounts to. I bought a replacement from a wrecked truck, and it has a short in it and only works about half the time (I did get a refund from the seller).



Again, all ideas apperciated. :) As I said, I am not a mechanic, I do not have nice scan tools and such, so for me some of this is a shot in the dark. I am willing to put a $20 part in and see if it helps, but if much more than that it goes to a mechanic. :)

 
to l!tech. Tonight as i was arriving home, with ambient temp in the 30's, I was getting the low idle condition. After I pulled in my driveway I did exactly what you suggested. With left foot on brake I hit the throttle, revving it up to 1200 or so, and leaving the transmission in drive. After revving the truck settled at 750 idle. I did the same thing with truck in reverse and got the same result. What does this suggest to you?
 
Not really the result I was looking for. I wanted you to pay attention to how the truck felt. By giving the vehicle gas while in drive or reverse we casue the engine to rock left or right, this unloads the motor mounts and if there is a noticeable difference in idle feel while idle speed remains the same then we know we have to look at engine mounts as a possible cause of the rough idle "feeling".



WIth the way your idle comes and goes I'm starting to think that you may have an intake manifold that is leaking vacuum but On ewould need to see fuel trim data with a scanner while the vehicle is started cold and warms up.
 
l1tech. I now understand what you were asking. As far as the feeling there really was no difference in how the truck felt when I gave it gas and let it settle. I have thought about the possibility of a vacuum leak as well. I know I can spend lots of cash at any mechanic chasing this down, but I am trying to do what I can with my very limited mechanical abilities to figure out what the issue is. I guess the reason this is driving me crazy is that my wife's '04 Explorer with the V6 does not have this issue. Idle in it is perfect regardless of ambient temp, etc.. In theory these 2 vehicles have the same engine, but something is different in my Trac!!



Again, all ideas appreciated!! I am still hopeful someone else in Trac land has had this issue and has actually resolved it!
 
Follow-up, of sorts. In my area of the country the ambient temp has been really warm the past week or so. I am still having idle issues, though not near as prounced as it is when ambient temp is below 40f. Today I had a little time to poke around under the hood. I do not find and loose hoses or unconnected wiring harnesses. I did discover, somewhat by accident, that the 4 bolts holding the throttle body in place were a little loose. One of the bottom bolts I only had to barely turn the socket to move it. I have tightened the bolts up and driven the truck. It is hard to say for sure if this is my issue as the ambient temp was 62f today, but truck was idling much smoother. As some background, I took the throttle body off a few months ago to do a complete cleaning, so maybe when I was tightening the bolts I did not get them as tight as they should be and natural vibration loosened them a bit. Time will tell if this makes the difference, but so far, I can tell it is idling steady at an indicated 750 on the tach. I did not switch out the coolant temp sensor yet as it is a bit of a challenge to get to and I only have one arm that is currently functioning (I had surgery on the other one recently, shattered the radius head at my elbow). Thanks for tips so far. :)
 
So today I finally switched out the coolant temp sensor. Since I began this thread I have noticed that at somewhat higher ambient temps in the 50's my idle was low, but before the ambient had to be below 40 for the low idle to occur. This afternoon our ambient temps were in the upper 50's and idle was running around 600. So this evening I finally broke down and replaced the sensor. It is a PITA to replace!! Basically I took off the air inlet tube, then the throttle body. Taking the retaining clip off to remove the old sensor is easy. What is a pain is putting the retaining clip back in after the new sensor is in!! it was pure luck that eventually I got the clip in! I also pulled negative battery cable to help reset the computer. I have driven it about 12 miles where I purposely stopped at every traffic light possible. I also pulled into a parking lot and did about 3 stop/restarts of the engine before driving home. Once home I let it idle about 10 minutes before I pulled it into the garage to park. At time of test drive ambient temp was 45. What I noticed is that the idle is holding right at 750 now. I also noticed that the temp gauge in the cab is now dead in the middle once the truck warmed up to operating temp. Before it was always a little below the half-way mark no matter what ambient temp was. Also I had begun to notice a slight "rotten egg" smell from the exhaust but tonight I got none of that, even after sitting and idling for 10 mins. So maybe I am a little too early tp say this, but it appears I may have solved the low idle on my truck!
 
I hope you fixed it. What I like to do is not clear the pcm after any repairs as this can give you a false sense of the repair being effective as the pcm relearns fuel trims and idle adaptations. If you see improvements after the repair then clear the pcm and start fresh. Keep us posted on the outcome.....
 

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