bad mileage/high idle

Since I got my scan gauge, I've really been watching my mileage. Well, the other morning I was having a hard time getting good mileage. I had noticed that my rpms hadnt dropped as low as they do at other times. Sometimes it goes down to ~900rpms while warming up. Ocassionally it will stay closer to ~12-1300 and not come down all the way. That night on my way home, I restarted my truck a few times and the rpms eventually dropped to the lower range. I had no problems getting good mileage then. I was getting about 4-6 instant mpg cruising over what I had gotten that morning. I have been watching my rpms while warming up and noticed I have had several days where it wont drop down and the mileage is similar to what I got the first day. Temps were in the 30's and I always let my truck warm up to 75 degrees engine temp.

Anybody have any theories or knowledge related to this? I saw something on a scan guage thread about timing advance, but I didnt understand it fully and dont know if it is the same thing.

 

Your idle RPM's are controlled by ECT (Engine Coolant Temperature) sensor.
Based on the reading from it, ECU adjusts idle.
Since ECT is thermo-resistor it will go bad at some point of vehicle life. Also not enough coolant will make that resistor do crazy reading. Any bad ground will also make that thing read bad.

 

I have not paid attention to idle rpms once I get home and the engine is warmed up. I've only really looked at them during warm up before I leave. I'll see if it is still staying high when I get home. The engine coolant temp seems to be reading right from what my scan guage is telling me. I'll check on the coolant level as well.

Any idea where the ect is located?

 

Anybdoy else got any ideas?

 

I will look for pic of ect when i get back home. I am not sure from where scanguage takes info and how accurate is. but I am sure ECT is the one controlling idle and how much fuel gets dumped. Bad ECt will do exacly what you described to the point that when goes completely goes bad your RPMS at idle will always be at the max idle (I think 1800 or so) and never go down.

 

How cold was it? Was your defrost on? Defrost engages the A/C to dry the air, which idles up the engine a bit, and places more load on it. It all sounds pretty normal to me, unless its acting up when at normal operating temperature, with the defrost turned off. If it is, look for vacuum leaks.