Strange Idle Issue

Hi guys,

New to the first gen world , just picked up a 2001 4cyl 5 speed manual with 138k miles on it. I love the truck and I really want to get it up and running perfect. I plan on having this truck for a while!

I'm having a strange issue that I need help diagnosing. The guy I bought it from had just replaced the back half of the exhaust and claimed that this caused the o2 sensor to "get out of whack". If you unplug the battery an plug it back in, no check engine for 5-10 miles, after that it starts to idle high (1800 rpm) and holds that idle. Other than the idle the car runs perfect, Acclerates great, good power etc.

To go along with this high idle, sometimes after driving steady (say highway driving 60 mph) for a sustained amount of time, when you begin to brake and slow down, the idle begins to dip down to 1100 and then bounce back up to 1800 then dip back down to 1100 and bounce back up. It will do this a few times and then eventually just stay at 1800 and have the high idle I just described.

Any ideas? All the vacuums seem to be in their place. The engine runs great otherwise, no visible leaks, everything looks real clean. I would love some help or possibly just pointing me in the right direction! Thanks!

 

If you plan on keeping this truck 'for a while' get a manual (Haynes to start with, then eventually FSM).

Is the CEL on? If so, get the code, and post it.
What EXACTLY is the 'back half of the exhaust' ? Does it include the converter and sensors?

 

Sorry, Kind of a rookie when it comes to these things. Yes the converter and sensor were replaced. I'll try and get a code and post it soon! Thanks for the response

 

Hi Guys, I got my codes today and also have another observation: when the car is cold (first start of the day) the idle seems perfect, right at 1100 or so and sounding nice. It seems as the car warms up the high idle starts to creep up. Then every now and then it will do the jump back down to 1100 then back to 1800.

My codes are :

P0141 HO2S-12 (Bank 1 sensor 2) Heater Circuit Malfunction
P0505 Idle Control System Malfunction

Any more ideas?

 

Take a look underneath and see if you, in fact, even HAVE 2 sensors (one before and one after the cat) or if they are properly mounted and plugged in to the harness. Who knows what the previous owner (PO) did. Sensor 2 is AFTER.

The P0505 points to the Idle Air Control (IAC) valve, which is attached to the bottom of the throttle body.

By the way, is your truck a 5-lug or 6-lug ?

 

Would i need to replace the IAC valve or just clean? It's a six lug

 

I've yet to have any idle problems, so no actual experience here. I'm disappointed that no one else has chimed in .
This has been discussed many times on TW, so do a search and start reading.
The best way to do a search on TW is to use Google, this was the result :

https://www.tacomaworld.com/customsearch/?q=P0505

 

Try cleaning the IAC.. you might as well clean the throttle body and mass air flow sensor as well while you're in there. I just went through some weird idling problems. The code I had was originally for the Throttle Position Sensor. I cleaned the MAF, TB, and IAC. Then afterwards I forgot to reconnect a tiny vacuum hose. It kept a high idle for a while, and 1,000 miles later it gave me the same TPS code. I ordered a new TPS, and on my way to installing it I found the vacuum hose that wasn't connected off the TB. Probably didn't need to replace the TPS, but I went ahead anyway. The truck runs great now.