'99 Vortec 350, Code P1509 (High Idle)

[ASJT3]

Hey Guys,

Got my first truck with a 350 Vortec engine, 244K miles. I've got a SES light with code P1509,

"DTC P1509 Idle Speed High - Idle Air Control (IAC) System Not Responding"

That's the only code that's thrown. The engine idles around 1000 RPM and will sometimes surge while driving between 1200-2000 RPM. I googled this code but not much came up. I've seen that a multitude of things can cause high idle, but since I'm getting a code with the symptoms, is this a pretty clear cut case of needing to replace the IAC valve?


Thanks,

Alex

[ASJT3]

So I've been dealing with this issue for about 7 months now, thought it was most likely the intake manifold gaskets. Well...this morning I replace the lower intake manifold, intake plenum, and throttle body gaskets, and not a dang thing changed. Still have the high idle.

I've replaced the distributor cap/rotor, IAC valve, IAT sensor, ECT sensor, and PCV valve. I've sprayed down the top side of the engine with carb cleaner looking for vacuum leaks, haven't found any.

I no longer get 1509 since I replaced the IAC, but now the only code I get is 0507 (Idle higher than expected).

Any ideas? This is driving me crazy and can't get the car emissions inspected til I fix it.

[EDIT] I should add, the problem gets much worse as the car warms up. Makes me think it's not getting ECT (even though i changed the sensor). Truck gets lousy gas mileage.

[777stickman]

So many things to consider. New IAC not adjusted properly prior to install, IAC relearn not performed, EGR bad, PCV bad, Spider assy bad, pressure regulator bad, map bad, TPS bad, someone prior to you messed with the idle stop screw, etc,etc.

You might consider an Alldata subscription for this issue.

http://alldatadiy.com/

[ASJT3]

Originally Posted by 777stickman

So many things to consider. New IAC not adjusted properly prior to install, IAC relearn not performed, EGR bad, PCV bad, Spider assy bad, pressure regulator bad, map bad, TPS bad, someone prior to you messed with the idle stop screw, etc,etc.

You might consider an Alldata subscription for this issue.

http://alldatadiy.com/

OK, I hooked up an ELM 327 obdii to usb scanner, using free scanTool.net.

Here are my specs at IDLE:

RPM: ~1300 (damn!)
Calculated load: 5.1%
Timing advance (Cyl 1): ~17.0 deg
Intake Manifold Pressure: 8.6 inHg
MAF flow rate: 1.6 lb/min

Engine coolant and air temp readings seem to be normal (192 deg, 50 deg).

Anything jump out at anybody?

Thanks,

Alex

[777stickman]

Anything jump out at anybody?

Intake Manifold Pressure: 8.6 inHg

Should be around 15-20"!!

[ASJT3]

Originally Posted by 777stickman

Anything jump out at anybody?

Intake Manifold Pressure: 8.6 inHg

Should be around 15-20"!!

Hmm.. Now this is the absolute manifold pressure correct? I measured ~20 inches vacuum using a gauge, so 29in atm minus 20 inches vacuum ~8.6. Seems right?

Now, i forgot to say something else, the truck came with a K&N filter. I checked and cleaned the MAF sensor, looked fine. Not possible that the filter is too high flow is it? When I tried to drive with the filter (accidentally) knocked off the intake tube, truck would sputter when I gave it gas. When I "choked" the flow with my hand, idle lowered considerably, to a reasonable level.

[maxwedge]

Right about the vacuum readings that is how the pcm calculates it. Possible the iac itself is bad or carboned up.

[ASJT3]

Originally Posted by maxwedge

Right about the vacuum readings that is how the pcm calculates it. Possible the iac itself is bad or carboned up.

As i said initially, I replaced the IAC to try and fix this problem, no change.

[maxwedge]

Should have seen that, anyway a highline bi-directional scanner can command the pcm to change idle speeds by taking over the iac, then you can see what may be happening, remember a dirty throttle plate can cause this also.

[ASJT3]

Originally Posted by maxwedge

Should have seen that, anyway a highline bi-directional scanner can command the pcm to change idle speeds by taking over the iac, then you can see what may be happening, remember a dirty throttle plate can cause this also.

Interesting, I would like to be able to do that, I'll look into it.

Originally Posted by maxwedge

remember a dirty throttle plate can cause this also.

When I changed the intake manifold/plenum gaskets last week I cleaned the throttle plate pretty well, doesn't seem to be letting any air bypass it. This evening I might take off the throttle body, remove the IAC and TPS and clean their bores out really well.

[ASJT3]

Wow. Problem solved.

I took off the throttle body for cleaning and noticed that the throttle plate didn't close all the way. Looks like the previous owner had adjusted the throttle plate stop screw to leave it open somewhat. Loosened it up until the throttle plate closed all the way, now idles like a charm!

However, this makes me curious...why would anyone do such a thing?

[maxwedge]

Because they didn't know what they were doing this also skews the tps setting.

[ASJT3]

Originally Posted by maxwedge

Because they didn't know what they were doing this also skews the tps setting.

Strange. Anyways, thank you and everyone else for help in solving this issue!