Hey guys, Got another question. This one has stumped me forever.
When my truck is cold (truck in sig) the idle is perfect. Once it warms up the idle drops pretty low and then starts to feel like a slight miss. Below is 2 pics, both in gear. The one in the day is when its cold. The one at night is once its warmed up. Ive cleaned everything (IAC, Throttle body, new air filter, plugs, wires, cap and rotor, fuel filter...
I checked the TPS voltage and it is 5V in and about .91 with key on engine off. Once its running it goes from .91 to really high and bouncing around erratically. Guessing that's not normal?

 

Then when warmed up idle is way lower and feels like it has a slight miss.




When first started and cold idles perfect. This is in gear.

 

Could be too lean at temp. Check your fuel pressure, O2, and coolant temp sensors.

 

Just to clarify..

Did you intend to say the TPS voltage goes really high, after warm-up, and bouncing around OR the idle does?

If your TPS voltage starts becoming erratic after warm-up, then you need to verify there are no dead spots in it from closed throttle to WOT. Do this once cold and then warm.

Check for codes. The TPS becoming erratic after warm-up should throw the CEL and store a code pertaining to it. If you check for codes and there are none, as in receiving a system pass code of 11 or 111, then you may have a PCM problem. The 5 volt reference voltage is 5 volts and failing Capacitors are the tell-tale sign if the 5 volt reference line is going haywire.

 

It has a new Coolant temp sensor. The 5 volts in remains, just the return is .91 with key ON engine OFF then when the engine is RUNNING the return voltage jumps and bounces around the 10-11.5 volt range. Too quick to really get a number. It drives great and downshifts correctly. No CEL either. No codes when scanned. Just a lower then I'd like idle.

 

Fuel pressure is good its just a lower idle. It idles great until about 10 mins of driving. It never stalls or feels like its going to it just idle lower and when it idles lower it feels like a slight miss. But cold idles great and smooth as a brand new truck.

 

Have you checked the pressure after it's warmed up? If the pressure is good warm, it could be O2 sensors. Any codes in memory?

 

Okay but what does the reference voltage do when it's running and the return is bouncing? Because there ain't any possible way for the signal to exceed the reference.

Since you're probably using a digital meter, go get you an analog meter and use that. They may be old school but they can respond faster than a digital one and sometimes you can catch subtle movements of the needle that a digi would never show you.

 

Ill try a analog. Im young but I have one lol. Im not to sure on all the voltages. I just saw that and thought it wasn't good lol. I'll try it when cold and warm and running and off and report it all back. Thanks guys.

 

Also, One thing I noticed between this and my buddies 93 F-150 5.0
When cold his idles around a 1000 -1200 for a min or 2. Mine when cold starts at 1,200 but drops to 750 or so within about 2-3 seconds. No high idle at all. Do 460's have a high idle?

 

Did you ever find out what the problem is? Mine is doing the same thing.

 

Still doing it. I'm just used to it.