2005 Colorado idles terrible but runs ok

I used a OBD scan tool and all it told me was my idle air control system rpm is lower than expected and my evaporative system has a high purge flow. I have no idea what this means, can I fix it or do I have to send it to a shop. Also when idleing the short term fuel trim 1 is at -3% whatever that means. Thank you for any assistance.

It started dying after fill ups but now it will eventually die after taking my foot off the gas.

“But runs OK”. ???

Get a can of Mass Airflow (MAF) sensor cleaner and use it to clean your MAF sensor. Then clean the throttle body per link below.


Cleaning both will likely recover your idle speed to normal range. Here’s more info if that doesn’t clear the code:

Then have your evaporative purge solenoid checked. It’s probably stuck open.

Rough idle but runs Ok is typical of vacuum leaks. Evap purge flow too high sounds like a sticking purge valve. The valve could be responsible for the idle symptoms. Check that first.


I hear what you’re saying, but I’ve worked on plenty of vehicles with vacuum leaks, that actually idled great

My point is simply that vacuum leaks don’t always equal rough idle

The short term fuel trim at idle doesn’t seem to point to a vacuum leak, in my opinion


please post your exact fault codes, not the interpretations

Example P0496

@db4690, I’ve had cars with vacuum leaks also idle and run fine. But idle rough and run fine is a symptom that has me look immediately for vacuum leaks. And 90% of the time I find it.

But if the vacuum leak is that massive to cause a rough idle, I’d normally expect the short term fuel trim at idle to be very high positive numbers

Thanks for the quick responses and links. I have cleaned the MAF but not the throttle body yet. Will try that today as well as checking the evap purge solenoid. These are the codes PO506 & PO496

Yes, the link I posted above to the coloradofans.com forum is for fixing the P0506 code. Hopefully cleaning the throttle body will do it.

My wife thinking I was dragging my feet (it’s her truck) took it to a well known shop today where they told her they could replace the alternator and that would solve everything…about $600.
Knowing I could change the alternator she declined. Is the alternator diagnosis possible?

No, all it would solve is a mechanic’s need for a boat payment.

You have plenty of good suggestions from this thread to check out: sticking purge valve, vacuum leak, dirty/faulty throttle body. A $600 alternator is not one of them.

Some kind of air/fuel mixture problem. When idling there’s very little gas and airflow, so even a small error in either will result in poor idle. The error – whatever it is – remains during high speed driving, but it becomes a smaller % of the total gas/airflow so doesn’t affect the engine performance.

Things that cause small errors in the mixture are

  1. air leaks that allow unmetered air into the engine,
  2. faulty fuel injectors, either clogged, or sticking open
  3. faulty MAF, MAP, and O2 sensors.

The purge code might mean there is something wrong with the evap system, and that can definitely affect the mixture too. If I had this problem on my Corolla I see if loosening the gas cap (temporarily) made any difference in the idle quality. Note that doing this may turn on the CEL.

The -3% fuel trim number, the following info might be useful to understand what that means. The engine computer calculates how much fuel to inject l based on several factors, the most important being the amount of airflow into the engine as measured by the airflow meter (MAF or MAP). Then the computer measures the response of injecting this amount of fuel by measuring the O2 sensor reading. If exactly the correct amount of fuel is being injected, the O2 reading will show all the fuel and air is being burned. If the O2 reading isn’t what it is expected to be (b/c maybe the fuel injectors are slightly clogged so less fuel is being injected than the computer thinks), a “fuel trim” is applied to correct the situation. It is how much extra fuel (or less) needs to be injected to produce the correct O2 sensor reading. -3% fuel-trim I think means the computer has determined it needs to inject 3% less fuel than its calculations warrant. So either the fuel injectors are injecting more than they should be, maybe one is slightly sticking or something, or there’s not as much air going into the engine as the MAF says. -3% to + 3% fuel trim though is close to what is normally observed on correctly running engines, so it’s difficult to know if that value is diagnostic in your case. I’d discount that as a cause of your poor for the time being.

My first suspects would be air leaks or dirty fuel injectors. The latter is pretty uncommon, so maybe focus on the air leaks theory first. Unless you have some experience working on modern day car engines, excepting something obvious like a vacuum hose that has fallen off or a rubber intake manifold boot that is cracked wide open, you’d probably need a shop to help you with that.

Liked jesmeds throttle body cleaning solution. Sea fome or techron might help also!

Yes, as Barky says, another cause of this symptom can be the throttle body getting clogged with gunk from the PCV and/or EGR systems. The air passages into the engine are very limited when the throttle valve is closed (idling), and even a small amount of gunk clogging those can make a big difference in idle quality. Cleaning the IAC system at the same time also makes sense.

Thank you all very much.