idle rpm varying

[c paine]

2004 4.0 L SOHC V-6 MT. When I'm idling to a stoplight in neutral, for example, rpm stays at 2500 until I get below 10 mph; it then drops to 1500 until I'm below 5 mph, when it goes to 700 (normal idle). But as soon as speed drops to zero, rpm goes up to 2500 and stays there. Doesn't matter how long I was rolling; could be 15--20 seconds if its downhill.
CEL code P0171 sets occasionally (lean bank 1 sensor 1); but there is no vacuum leak detectable w/propane. I cleaned MAF sensor, then replaced it; no change. This just doesn't seem like an O2 sensor failure to me. Any advice?

 

[Dirtman]

The RPMs staying high before coming to a stop is normal. It's an emissions thing, but obviously it shouldn't shoot back up once completely stopped. Remove and clean the IAC valve.

 

[RonD]

+1 ^^^^

After engine is warmed up and doing its 0MPH high idle, unplug the 2 wires on the IAC Valve, and see if the idle drops down to 500rpms or so
If so that means the computer(PCM) was setting the high idle for some reason, it was holding IAC Valve open on purpose

If RPMs do NOT drop with IAC Valve unplugged, then I would check PCV Valve hose and replace the PCV valve
PCV Valve should be closed at idle, vacuum holds it close, if its "weighted valve" is dropping open then RPMs would go high

 

[c paine]

I tried what RonD suggests: Stopped and idling at 2500; unplugged the IAC valve, the rpm does drop to about 700 (normal idle).
Any thoughts on why the PCM might be driving it open? If the idle is going that high, it must be commanding more fuel to the injectors, right?
I unplugged the output speed sensor (which drives the speedometer): no change in idle performance, still jumps up when I come to a complete stop (but not before); so I don't think its the speed sensor since the speedometer works fine.
Could the extra air be what is causing the P0171 code (lean Bank 1 Sensor 1)? Why would it be on only one side?
And I also cleaned the IAC valve; it wasn't very dirty, and didn't make any change.

 

[c paine]

Actually I misspoke in the last post, sorry: With the output speed sensor unplugged, the rpm STAYS AT 2500 as I slow to a stop; it never drops down.

 

[RonD]

Well if the computer is setting the high idle via IAC Valve then something is wrong with the computer, I know, DUH, but not much you can do about it yourself
Its most likely a circuit issue in the computer, or you would get a code telling you the computer can't set correct idle, so computer doesn't "know" there is a problem

You could try getting a Bluetooth OBD2 reader, they are cheap
Then you can watch Live Data that the computer is "seeing", maybe find something amiss, I wouldn't know what to look for
Maybe TPS % and RPM reading when its idling high

 

[c paine]

Yeah, I have an OBD2 scanner, was looking at the fuel trims because of the lean code, but not seeing anything normally considered indicative of much of anything, certainly nothing consistent from occurrence to occurrence. OK, thanks for suggestions.

 

[RonD]

TPS should show 15-19% at idle, foot off the gas pedal

Coolant temp, warmed up, 180-190degF

MAF should show 4.5-5.0 g/s at 700 rpm idle, with 4 liter engine

 

[c paine]

That's just about what I'm seeing. TPS dead stable at 18.4% with foot off gas, goes up as I accelerate. MAF ~4--5 when it idles down to 700, MAF ~11--12 when 2500 rpm.

 

[Orca]

If possible, you should use your scantool to monitor IAC duty cycle, in percent. It would tell you what the PCM is commanding and you could see if it correlates to the actual engine RPM you're seeing, to see if the IAC valve's solenoid is misbehaving or not. Even better would be a simple graph of IAC with engine RPM and TPS (and maybe even VSS [Vehicle Speed Sensor] and ECT [Engine Coolant Temperature]). Something is clearly flaky, but with proper monitoring, it should be possible to pin down exactly what.

P.S. My typical TPS is 19.6% with foot off pedal.

 

[c paine]

I was suspicious enough of the IAC valve that I replaced it; no change. Unfortunately my scan tool doesn't report IAC duty cycle; maybe I can stick some probes in the connector, its a square-wave drive so duty cycle should equate to average DC voltage, pretty close. (I don't have an electronics diagnostics lab available right now; damn virus.) But I'm confident now that the problem originates somewhere else.
And the way the rpms behave is not what they used to do; they would stay high for a few seconds, but if I was rolling to a stop for more than 5 seconds, they would be down to 700. Now they do not drop that low until speed is below 5 mph; and as soon as it hits zero, they jump up. Something is sensing that speed change, but I'm running out of culprits.
I checked RPM and VSS; all match to gauges and seem reasonable.

 

[RonD]

TPS uses a 5 volt scale, 0v to 5v
Throttle closed voltage should be .69 to .99 so under 1volt, that's 15-19% of 5 volts, thats what the % represents
Wide open throttle is 4.5 to 4.8volts, so 90% to 95%
Now anything under 1 volt is idle, and anything over 4.49v is wide open, you don't get a lower idle with .69v vs .99v
And you don't get more fuel at 4.8v vs 4.5v
Doesn't work that way


Just a heads up on IAC Valve replacement, must be Motorcraft or Hitachi brand, other brands don't work well with Ford computers

 

[Orca]

It's a long-shot, but I haven't heard that you've checked the Engine Coolant Temperature (ECT) sensor output for sane values when the idle is abnormally high. The PCM undoubtedly uses it somewhere in the algorithm for the IAC valve command. Maybe it's a larger factor at 0 MPH (VSS) because that's when you'd be warming up the engine?

If you've already done that, then just ignore that bit.

Also, in case you want to check your old and new IAC valves, the resistance between the 2 terminals of the connector on the IAC should be between 6 and 15 ohms.

And, of course, the (key-on) voltage at the IAC valve connector must be adequate (10.5 volts or higher per my reference).