Idle Problems

I have a 2004 SR5 Tacoma, 2RZ-FE 2.4L engine, 5-speed manual, 172,000 miles, with a K&N air filter/intake system. It was bought in 2008 and normally idles around 700 rpm after the engine warms up.
Initial problem: when the engine was started, it would not idle and to keep it from turning off, I had to keep the revs up to about 1500 rpm. I used both the (official) Toyota Repair Manual (two volumes) and an OBD II scanner to diagnose the problem. All testing and taking readings, it pointed to the Idle Air Control (IAC) valve being defective, and needed to be replaced. I bought a reman IAC valve and installed it only to find out when I started the engine one of the coolant connections on the IAC valve had an internal defect, and I obtained a replacement under warranty. This unit was installed, the throttle body (TB) was given a rudimentary cleaning, and the idle at start up was about 2000 rpm, which slowly dropped to about 900-1000 rpm when the engine reached operating temperature.
Second problem: I tried to adjust the throttle position with the throttle valve set screw and that dropped the rpm to 800-900 rpm. (The operation of the throttle valve was normal when I tested it.) After driving it for about a month, the idle started to wildly fluctuate between 2000 rpm and 1000 rpm in about 3 seconds. Ran the tests another time, and again the problem pointed to the IAC valve. I installed a new IAC, but also took the following additional steps prior to starting the engine: did a more thorough cleaning of the TB, used compressed air to clean the Mass Air Flow (MAF) sensor and checked the electrical aspects of the MAF, checked the throttle position sensor, checked the Engine Coolant Temperature (ECT) sensor, and disconnected the negative battery terminal.
Third problem: reconnected the battery, started up the engine, and the rpm started about 1500 rpm and dropped to 900-1000 rpm when the engine reached operating temperature. However, there is now a sound emanating from the K&N filter that sounds like someone beating a bass drum every 2-3 seconds, which doesn't change with revving up the engine. I have inspected all my vacuum hoses and connections, and nothing appears to be wrong. I pulled the Positive Crankcase Ventilation (PCV) valve, cleaned it and checked it's operation with compressed air, and everything is normal.
I believe my next step is to replace the MAF sensor, but I wanted to reach out to see if anyone can shed light on the situation or has had similar experiences, and how the problem(s) was corrected. During this process, I have periodically used the OBD scanner and received various error codes at times (such as P0505 - idle air control system, P0300 - multiple cylinder misfire, etc.), but when I clear the codes and run the check again, no codes are indicated. In addition, I have never had the engine check light come on. My other idea is whether or not the Engine Control Module (ECM) may need to be replaced. Before I throw in the towel and take the Tacoma to a local garage, HELP!

 

I have received no replies to my post. Has no one ever had this problem or similar problems?

 

Looks like it got buried before anyone with ideas saw it.

So MAF sounds suspicious to me, especially since it was cleaned with compressed air. Thats not a good way to clean them (they're delicate).


Its been a month, what is it doing now? If you drive it for a while, what codes show up?

 

Thanks for the response. I haven't been driving the truck as I wasn't sure if there might be a more serious problem (such as a broken valve spring, bent valve, etc.) and it's been sitting in my garage. Started it this morning and got a check engine light, still had the "bass drum" beating - this time about once per second, and my scanner gave me codes P0100 and P0101 which are MAF problems. I checked the MAF per the repair manual and some, but not all, of the checks indicated the MAF needs to be replaced. On my way to pick up a new MAF and hopefully that will solve the issues.

 

High idle can be a tough one to track down. Easiest way to test the MAF is to unplug it with the engine running and see if the idle returns to normal. The computer should revert back to factory base mapping and the idle would return to normal if the MAF is your issue. Cleaning it never hurts.

But, with the age of the vehicle, have you considered vacuum leaks?

 

Tried unplugging the MAF connector twice, and both times the engine died. The scanner is now showing three codes, P0100, P0101 and P0110. As far as I can tell, there are no vacuum leaks, but I will check again before replacing the MAF.

 

Did you try restarting it with it still unplugged? The PCM can take a few seconds to adjust and may result in a stall.

 

Unplugged the MAF and after several tries was able to keep the engine running by increasing the rpm to about 1100. Allowed the engine to get up to operating temperature, and the idle dropped to about 700 rpm, albeit a bit rough. However, the "bass drum" is still beating (regardless of engine speed). I plugged the MAF back in and idle went back up to about 1000 rpm, and still have the "bass drum".

 

I'd say try cleaning the MAF or replacing it. Things do point toward that being an issue.

Also, if you hit "quote" in the bottom right, the person you are responding to will get a notification.

 

To all respondents: Thanks for your input. I thought about replacing the MAF now (about $100), but what concerns me is still the "bass drum" sound. I would like to have some better idea as to what is causing that noise before I start part swapping. Otherwise, I could be going down a rabbit hole or several rabbit holes. I feel like the sound is akin to a pump with a faulty check valve on the discharge side. That would lead me to believe the sound is being caused by something mechanical as I referred to previously. I also thought about replacing the PCV valve, but it appears to be working fine as well. My next step is probably going to be to remove the valve cover and see if anything is out of order.

 

Keep us updated on what you find

 

Ended up taking it to a local garage. They checked it out and replaced the aftermarket IAC valve with a Toyota factory IAC valve, and that solved the problem (no more "bass drum" sound). The garage told me that is not uncommon for aftermarket products to be incompatible, especially when it comes to electronics and electrical items. The garage tried to get the idle adjusted down to spec, but were not able to. However, after I drove it 3 or 4 times, the idle is exactly where it's been for years, 700 to 750 rpm. YEA!

 

Glad you got it fixed ..