I have been having an issue at very low temperatures. When the temp gets below 20 degrees I would occasionally (but more regularly the colder it is) have a VERY high hanging idle and the trans wouldn't shift out of first. If I let the truck warm up while stationary it would resolve itself until I drove it for a while and then come back.
I finally had a CEL come on yesterday when the above happened. I went by Advance Auto and used their code reader and got a P1121 code. I then checked what that code meant on this forum and found:
P1121 Throttle Position Sensor Inconsistent with Mass Air Flow Sensor
I checked the TPS and found the voltages at idle and wide open throttle to be quite off. I was getting very little change in voltage and the voltage was high at closed position. But, what I also found while doing this test was that when I released the throttle about a second to five seconds later I would hear a click. I did this several times and always got it. I removed the TPS and used a large screw driver to take the place of the throttle shaft and saw that the TPS was hanging at about 1/4 to 1/3 open. Sometimes it would click closed after a few seconds and sometimes it wouldn't.
I replaced the TPS and the voltages are correct now and that along with the smooth movement of the sensor the above problem seems to have been corrected. We'll see after driving for a while in the Missouri winter.
Thanks to all who contribute to the forum. I'm in your debt.
ETA: This is a 2002 Expedition XLT with about 120,000 miles and the 5.4L motor.
Last edited by ResJudicata; 01-01-2010 at 02:03 PM.
Reason: Additional info
FYI, I have been driving my truck since Friday. The idle is smoother than before. It used to idle about 650 rpm according to the dash tach. Now it is at about 750 rpm and smoother feeling and sounding. Acceleration is about the same.
But, the most interesting thing is that the transmission is shifting better. It used to feel like it was searching at times and the overdrive shuddered when it engaged. Now, I haven't noticed any searching, it shifts nice a smooth into each gear and even into overdrive.
The computer must use input from the TPS for regulating the shifting of the transmission in addition to just the mixture. I'm surprised by this but I guess I shouldn't be.
Well, knock on wood but I think this has been a very lucky and cheap fix. The TPS was $39.99 from O'reilly Auto and very easy to install. Apparently it has been a problem for at least a year but has never shown up as a CEL until last week.
ETA: And, no reoccurance of the earlier problems in the very cold weather. It has been in the teens or below since last week and no issues.
Last edited by ResJudicata; 01-05-2010 at 03:09 PM.
Reason: Additional info
Actually the computer uses the TPS, MAP and VSS as it's primary inputs for determining shift points. The atmospheric pressure, throttle position and vehicle speed are all major factors.
Thanks for the info. It's good to know that I'm not crazy. I haven't done much diagnosis on modern fuel injection systems. In the past it was with older systems that didn't cover so many parts of the vehicle.
Chris - your post here is outstanding... I have been experiencing very similar issues to what you describe - High Idle, can't shift out of 1st gear... I never received a CEL on mine and was getting nervous about what my issue might be.. I used a code reader from my friend and checked it anyway and low and behold the P1121 was there. That is what helped me find your post.
I also have an '02 Expedition with the 5.4L. I will do further checks and hope this turns out to be the same easy fix you experienced!
It's good to know that I posted something that is actually helpful for folks.
Unfortunately, (or fortunately depending how you look at it) I just traded my Expedition in on a new Focus. The truck was still in very nice shape so I got good money out of it. But, it was time to do some expensive repairs like ball joints and the directional valve in the heating system. Plus the fuel bill was killing me with the amount of miles I drive now.
So, I picked up a Focus Titanium that I can fill up for $30.
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