IAC woes

[Pete99]

There are a lot of IAC threads out there, and I've read many, but I have a couple of specific questions.

My idle started surging a few weeks ago, and the guy who changed my tires disconnected the IAC valve and advised me to clean it. In the meantime, I idled at <500 rpm, and stalled a lot.

I took out the IAC, soaked it in throttle body cleaner a bunch of times, but it didn't come that clean. In a pinch, I grabbed a $40 Duralast IAC, figuring anything was better.

Since putting it in, my acceleration has turned sloppy and jumpy, the idle is all over the place, and my exhaust still stinks of unburned gas.

Is this because I bought a cheap IAC? Or is it likely something else? My CEL hasn't been on in a long time, oddly. So I haven't checked codes lately.

Also, after soaking the old IAC full of the acetone-based throttle cleaner again and again, it's still gunky. Should I be doing something differently?

 

[RonD]

The IAC valve is a controlled vacuum leak, pretty simple device.
When fuel injectors were first used the standard throttle plate idle screw just didn't work very well.
The IAC valve was invented, it opens a little to let more air in and raise the idle and closes a little to reduce the idle.
Control vacuum leak.

Test the voltage at the IAC connector, key on/engine off, should be about 10.5vDC
Switch to OHMs and test the two pins in the IAC, should be 7-13ohms
If IAC is removed connect it to the engine harness and watch valve move(key still on), then disconnect it and watch it close, repeat several times.
With engine off IAC opens almost all the way, that's why when you start the engine it goes up to 1,200rpm then drops down to 700rpm if engine is warm or 1,000 if its cold.

If IAC valve is sticky then computer can't set a stable idle.
If you have a vacuum leak elsewhere, the same thing happens, IAC is fine but computer can't stabilize the idle because of the other leak.

Dirty MAF sensor can also cause unstable idle.

 

[Pete99]

Well, funny you should say that. I was at AutoZone for something else, and had a sudden inkling to change my air filter. While I had the intake open, I hit the MAF sensor with cleaner, and now this problem seems to have gone away.

 

[RonD]

Good work

MAF(mass air flow) sensor, tells the computer how much fuel to mix with the air volume that is coming in.
When(not if) it gets dirty the computer see air volume going up and down, like your foot is feathering the gas pedal, so computer is adding more fuel then less, up and down just like it is being "told" to do.

 

[Pete99]

Well, it was good work while it worked. I drove home several miles and it was fine, and again the next morning, and then... it was back. I removed and replugged the crankcase vent and it seemed ok for a few minutes also, but not long. Repeated cleanings of the MAF sensor have not repeated the same result.

 

[RonD]

It certainly reads like you have an air leak somewhere, idle going up and down is the computer trying to stabilize the idle using the IAC.

Check the air plenum tube for cracks, MAF only works if all air coming in passes through it.
Also it can be a vacuum leak on a vacuum device, cracked vacuum hose as well, but the device that is using the vacuum needs to be air tight or it's a leak, so remove vacuum lines, one at a time, and plug it, see if idle stabilizes.
Example would be the EVAP system, it opens and closes via a solenoid, if there was a vacuum leak on the fuel tank side it might come and go with solenoid opening and closing.

Unhook the battery ground cable for 5 minutes, that will reset computer for cleaned MAF, sometimes old data can mess things up.

Also a scanner hooked up while engine is running can help ID issues

 

[Pete99]

I finally got around to hooking up a scanner. Guess the CEL is burnt out because it's throwing plenty of codes, as suspected.

P0102 Mass or Volume Air Flow Circuit Low Input
P0174 System too Lean (Bank 2)
P1443 Evaporative Emission Control System - Vacuum System - Purge Control Solenoid or Purge Control Valve fault
P1151 Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 2

Any guidance on where to start? Without anything else to go on, it seems like the PCV/solenoid are the cheapest things to replace here, followed by a remanufactured MAF sensor.

I get the idea from reading posts that I'll get some feedback one way or another by disconnecting the MAF sensor, if that's where the fault lies. Is this true? Haven't had a chance to try it yet.

Am I right in thinking the other two codes (0174, 1151) are likely consequential and not causative?

Is it silly for me to go trying fixes, or should I just go get the smoke test? I have other things to spend my truck money on, so I'd love to track this down myself if possible.

 

[basketranger]

IF easy access to scanner and truck is still drivable. I would clear all codes and check after roughly 50 miles of driving.
IF not I would go after the O2 sensor(s).

and evaporative I would start with fuel cap (cheapest possibility)

 

[Pete99]

Here is the order of latest efforts, and results:

Recleaned MAF, poured in cheap fuel treatment: no result
Added can of Seafoam to full tank of gas: no result for first 5-mile trip, then 100% better on next 5-mile trip!, then... back to usual.

Replaced PCV valve, sprayed throttle with throttle body cleaner: nothing evident

Added another can of Seafoam with a new tank: nothing on first few miles, then on second drive -- perfect! Then back to normal idle roughness, underburned gas, putt-putt noise.

Finally put about 5 oz Seafoam in oil because it was leftover. Results pending.

- - - - - - - - -

Question: why would idle be normal for a brief perod after Seafoam poured in tank, then go bad?

 

[RonD]

Not sure why the short reprieve from the seafoam.

Have you tested the coil?

This coming and going seems a bit odd
There are 4 pins on the coilpack connector
1 is for 12v power
3 are grounds for each of the 3 coils in the pack, each coil fires 2 spark plugs

Set OHM meter at 200
test 12v pin to Gnd pin should be 0.3-1.0 ohms, so very low
test all 3 ground pins to 12v pin.

Set OHM meter at highest setting
test gnd pin to spark plug hole then next spark plug hole for that pin
Should be 6,500-11,000 ohms.

Lay out is like this
gnd----spark--spark
gnd----spark--spark
gnd----spark--spark


This test will show if a coil is bad, but doesn't "prove" coil is good, they can have intermittent issues when warmed up.

Also what do the spark plugs tell you?
Do they show a rich condition?