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Hmmm... good points. My understanding of IAC is that it is only relevant after warmup (that is, it's not supposed to do anything until a fully warmed stte is achieved, then prevents stalling at a stop). Have tried listening for vacuum leaks with hose in my ear - nothign found yet, but will try again. Here's another interesting observation - was about 48 deg this AM when I started it - ran a little rouhg wfter high idle, but tolerable. Yesterday, after the long drive, let it sit (off) for 10 minutes, then restarted it - very bad (rough) for a minute. Something here is temp dependent. Going back to recheck the ACT (IAT) tonight - that's all I know of that reads ambient temp.
And your method for finding vacuum leaks is not my first choice. I use a hand held vacuum pump/gauge to test each vacuum circuit. Then you have to use a different methodology to find leaks at the intake manifold such as spraying the gasket area with carb. cleaner.
And your method for finding vacuum leaks is not my first choice. I use a hand held vacuum pump/gauge to test each vacuum circuit. Then you have to use a different methodology to find leaks at the intake manifold such as spraying the gasket area with carb. cleaner.
rla2005: Okay, thanks. Will perform functional check on IAB - new part, no codes, but still worth a try. In the link, there is no resistance or voltage check specified (and can't find any in Haynes) - the AutoZone site says a good IAB should read 7-13 ohms - any idea if this is valid?
My trouble shooting techniques leave something to be desired - still learning. Will get busy with vacuum pump. About the intake manifold - if it were a temperature-dependent leak at the manifold, it should not occur with warm engine after just sitting for 10 minutes, right? Will be checking anyway...
7-13 Ohms sounds about right. The IAC receives battery voltage on one side. The return side is controlled through the PCM. The PCM turns the IAC on by momentarily completing the return path several times a second. i.e modulates it on/off.
Your issue does not necessarily sounds like it is caused by the IAC, but it is something to check. I would also caution you to pull the plugs and inspect them for signs of misfiring. A faulty plug or wire can cause drivability issues that may not necessarily show up all the time such as a warm versus cold engine.
7-13 Ohms sounds about right. The IAC receives battery voltage on one side. The return side is controlled through the PCM. The PCM turns the IAC on by momentarily completing the return path several times a second. i.e modulates it on/off.
Your issue does not necessarily sounds like it is caused by the IAC, but it is something to check. I would also caution you to pull the plugs and inspect them for signs of misfiring. A faulty plug or wire can cause drivability issues that may not necessarily show up all the time such as a warm versus cold engine.
rla2005: Haynes manual verified 7-13 ohms for IAC - checked mine last night at 10.5 ohms. Haynes says voltage (probing, KOEO) should be 10.5 - I read 12.5V (error in the manual?). With eng running, I get 14.5V on one side (basically alternator output), around 14.2 on the other. Sound right? The ford fuel injection site said pull the connector while running and idle should stumble - it does not, but I am uncertain that this is a valid check (Haynes does not mention it).
Also checked ACT/IAT sensor against table on fuel injection site. Voltage out corresponds to to table on the site. Check after hot soak (engine off) and then ran engine and watched voltage increase as sensor cooled with air flow. Looks good to me.
Plugs, wires, cap, rotor were new July 2010. Only driven about 2500 miles since. Symptoms only started in the last 3-4 months. Will check anyway.
Your idle RPM should drop when you unplug the IAC, it is a valid test. If it does not appreciably drop one of two things is going on. First would be a vacuum leak. Second would be someone has jacked with the idle stop screw. Usually you can tell if the second has happened by looking for signs of the threadlocker being disturbed.
Your voltage and resistance readings appear fine to me. The IAC and other engine related actuators are fed battery voltage. I have never understood why Haynes insists the voltage is something else lower...
Okay, thanks. Thread lock looks undisturbed. Need to get busy looking for vacuum leaks. Can you tell me what inputs to the EEC affect positioning of the IAC?
Intake air tempuratur sensor and fluid temperature sensor. Cleared me up. Apparently the truck didn't know what temperature the air was so it wasn't metering the fuel correctly
I believe both the ACT/IAT and ECT sensors are good. IAT voltage is varying with temperature, as is the resistance of the ECT. Reference signal to the ECT read a little low - Haynes says around 5.0V, got 4.4V with ground to battery terminal. ECT voltage out versus temperature table on fordduelinjection.com appears to be same set of values as those for the ACT/IAT - I will verify proper voltage out versus temperature.
I believe both the ACT/IAT and ECT sensors are good. IAT voltage is varying with temperature, as is the resistance of the ECT. Reference signal to the ECT read a little low - Haynes says around 5.0V, got 4.4V with ground to battery terminal. ECT voltage out versus temperature table on fordduelinjection.com appears to be same set of values as those for the ACT/IAT - I will verify proper voltage out versus temperature.
Might be a good idea to pull the intake air sensor for a look see, they sure can get ugly looking from soot.
When I replaced the 3 capacitors in the pcm I noticed that the one by the big Z diode had a loose terminal that went inside the cap ...it was black on the end (See pic)... either open circuit or high resistance.
After replacing all 3 caps and reinstalling the PCM or ECM it now purrs like a kitten with no more stumble or popping out the exhaust. I did recheck the timing and it still right where I left it.
I prayed for wisdom from above... and my friend's here too
