Does anyone ever fix their idle problems?

I see quite a few threads posted by people having idle issues, but not many with resolutions.

Here's mine that has been going on for a few years:
2001 2.7L 5 speed 4x4 with 200,000 miles
Used to get CEL and stalling at stop signs, cleaned throttle body, IAC, MAF and fixed the problem.
That fix stopped working a few years ago.
Now I have replaced:
IAC
MAF
Fuel Filter
Water Temperature sensor
spark plugs
PCV valve
rear O2 sensor

For the past year and a half I have had no CEL on but it idles too low when cold. Now that it's getting colder out, I have to keep my foot on the gas to keep it running. Runs great once warmed up.
My best ideas at this point are:
1. to buy an OEM IAC (bought an aftermarket part from China)
2. Replace TPS
3. Replace EGR Gas Temperature sensor

I am open to better ideas. To me the problem seems electronic and not fuel related. I think the computer doesn't know that it is cold

 

I would start with the TPS. You should be able to check it with a multimeter to see if it is good

 

Get a scan tool and just take a look around. It's really the only way to know and everyone's problem is going to be unique

When you say rear o2 do you mean upstream o2? Because that's the only one that matters. The rear is to test cat efficiency and won't affect engine performance. Sounds like it's too rich in open loop during warm up. Might be time for some new injectors, fuel regulator, pump.

 

Thanks Clay,
I have not replaced the upstream O2 sensor yet. I just replaced the rear because it was throwing a code a while back, not because I thought it would help the idle situation. I just wanted to list any clues that might be helpful. I was planning on replacing the upstream sensor and both the cats because that code recently came on. However, after I replaced the fuel filter and changed the oil, the CIL went off and hasn't come back on in 600 miles. That is about the only good luck I've had with this truck lately.

And regarding the scan tool, I took it to a shop that has the "$10,000 scan tool" and paid them something like $180 to diagnose the problem. They couldn't find anything and told me to change the plugs, pcv valve, fuel filter and get their special $400 fuel system cleaning. I did that all my self (minus the $400 cleaning) and it didn't make a difference. This was very disappointing because this shop is right across the street from my office and very convenient. Now I don't trust them at all.

Do you think that a non-mechanic like me would really be able to find something with a scan tool that they didn't? Or would it be worth finding a better shop?

 

You can find a good scanner for under $100. Admittedly it is hard to read them at first and you would have to teach yourself the basics of diagnosing etc but I think it's good knowledge to have for the long term, depending if you're a do-it-yourselfer or not.

Generally diagnosing a vehicle condition is a wild goose chase. It could be a number of things and without knowing how to read a scanner yourself you're at the mercy of Larry the Mechanic to check your truck after each new wave of replacement parts and be able to determine which did what. If you have a scanner and some free time you'll likely be able to hone in on your issue and solve it.

As of right now I'd look more towards your fuel system as the culprit. Have you tried injector cleaner?

 

Does anyone know how the computer controls the IAC? I mean doesn't the computer continuously try to open the IAC if the idle is low? So, every engine's IAC might be open a different amount at idle. The computer just learns the correct amount to open it.

If that's the case, then it would seem that the computer is opening the IAC as much as possible, but the air passages are clogged. Maybe there's a way to manually hook the IAC up to the battery while the engine is running to open the IAC as much as possible and see how high the idle is.

 

That is how I would think it works, but I'm not really sure. I think because I have a new temperature sensor that the computer knows when the engine is cold. And because I have no CEL on, the computer thinks the IAC is functioning the way it should. But, if something is clogged then perhaps when the IAC opens it wouldn't change the idle. I think that if it was a fuel problem or the TPS, I would have issues even when the engine is warm, which I don't. Before I throw more money at parts, maybe I should take it apart again and check for clogs and if the IAC is mechanically sound.

Thanks for the insight. I will keep you posted.

 

If your IAC is like the 96 IAC (that's I'm familiar with), you can check the resistance across the terminals, and there's a way to hook battery up to specific terminals and check that the valve is fully open and fully closed. Refer to the repair manual for your year.

 

So I have a 97 3.4l 4x4 taco that Loves so idle around 2k rpms, most of the time. But occasionally lowers back to normal. Any ideas why?

 

The IAC commonly builds up with intake carbon on the vane.
And is often culprit if the idle fluctuates or the engine stops running altogether.

An dirty IAC can net a false value reading due to internal build up/restrictions.

An IAC can be cleaned... and then re-tested for value
to be sure it is within factory specs.

The IAC is purdy expensive component to replace
if the cleaning doesn't do the trick
or the test value is not within spec.

The IAC will not become clean
even when the intake is chemically cleaned.

The IAC must be pulled and cleaned by hand.

Be sure to use sensor safe chemicals when cleaning.
You don't want to leave any chemical residual inside the IAC

I had to use a hardwood dowel on one IAC vane I cleaned.
I physically had to scrape the carbon build up
off the IAC's internal vane.

Cleaned up a few IAC's over the years...
and they worked just fine after a good cleaning.

Only one I attempted to clean up
was DOA and needed replacement.

 

Great information, Thanks. I will give it a shot.

 

I finally got around to checking my IAC valve and this is what it looked like. What do you think that pink stuff is? Anyway I ordered a new from Toyota. This is a $30 special, ordered on Alibaba and shipped from China. The valve wouldn't move at all even after I cleaned it out. It never worked right from day one. I guess you get what you pay for.