Teknovyking

I just bought a 2003 GT V8 Convertible, it runs great but I have some issues that are completely baffled. It started with a series of Check Engine codes that I have cleared with replacing the TPS, Air Control Valve and Mass Air flow sensor.

Where I'm left baffled is I have to leave the Air Control Valve unplugged because when I do, the idle will rise to almost 3000 RPM and hold there... I've found that pulling the air feed line to the air control valve decreases the idle to just above normal.

So, with that I'm getting a P1504 code that I can kind of tell that its from the Air Control Valve being unplugged.

I heard it might be the PCM needing to be replaced and I would like to get some second opinions if possible.

Also would all of these affect my fuel MPG because it also seems I'm going through way more fuel that I know a V8 is supposed to consume... HELP!

cliffyk

When you say "unplugged" do you mean electrically disconnected? If so the idle speed should fall, perhaps even stall out, with the IAC disconnected.

When you pull this line are you also plugging it so that unmetered air will not be drawn into the intake? If not then the AFR is going very lean which could account for the drop in idle speed.

Yes, having the IAC unplugged will throw DTC P1504--here's more about the possible causes and diagnostic tips..

That is a possibility, however check some other things first.

The IAC's solenoid coil should have a resistance of 6Ω to 13Ω (between the two terminal pins on the IAC), and resistance of > 10kΩ between either pin and the IAC body--if these specs are not met the IAC is bad, replace it.

With the key on and engine off (KOEO) disconnect the IAC connector, there should be 10.5V or better between the red wire and ground--this is known a Vpwr. If not then there may be a wiring problem. Vpwr to the IAC is supplied via the same fuse (fuse #2, 20A, under the dash) as Vpwr to the injectors, so it's unlikely to be an issue if you are not getting 10.5V or better at the IAC.

If you do get the 10.5V at the IAC, you will need an oscilloscope to monitor the signal the PCM is applying to the IAC. At idle it should be a PWM signal looking like this:



Make sure the throttle stop screw is properly adjusted. There are those who erroneously believe it to be an idle adjustment screw, and treating it as such can really mess up the PCMs attempts to control the idle via the IAC.

How many miles per gallon are you getting? What sort of driving do you do?

With short trips around town 16 to 18 mpg is not unusual, with a 40/60 mix of in-town and highway 20 to 22 is common, pure highway can hit 24-25...