2004 Tacoma Idle relearn.

I just replaced the fuel pump and fuel filter on my 2004 Tacoma 3.4L (disconnected the battery). Starts up fine but idles really weird. Will go from 800 to as low as 500 and bounce up and down. Does the ecu on these trucks just need to relearn idle or do I have a bigger issue? Thanks in advance.

 

Yes it will learn some over time, just drive it normally, it may also be working air out of the system and dealing with pressure fluctuations from such in the fuel rail.

 

Cool, thank you. Do you know if this happens anytime you disconnect the battery?

 

Every time you disconnect the battery for long enough (usually 1 hour or more) the ECU's learned and custom tuned fuel map will be reset and you will start from zero yes.

The re-learn curve won't be as drastic if you haven't changed anything but each time it will have to relearn some to get optimal economy and power from your trucks specific setup and quirks.

If you change something major to the combustion cycle (intake exhaust injection spark) it's a good idea to reset the ECU.

If you're experiencing some driveability issue one of the steps in diagnostics is to reset the ECU by disconnecting the battery terminals and touch the two terminals (obviously without the battery DO NOT SHORT YOUR BATTERY) together for 1 minute to drain and clear all learned behaviors before reconnecting to the battery and test driving again.

The newer your truck the more drastic the change, the more modifications you've done the harder and longer it will be for your computer to learn.

With my truck I'm modified so much when the truck first starts after an ECU clear it instantly revs to nearly 3k then flat out dies, second start it idles really low right in the edge of stalling 4-500rpms and slowly creeps up to 1200 over a minute or so and stays there until the truck warms up and then it will idle normally and smoothly at 750rpm. It also won't allow the air-conditioning to turn on until three successful drive cycles or 1 hour of running without issue.

Takes about 2 weeks of normal driving for my milage and power to normalize for me in my 2010. Your 1st gen is a bit less electronically complicated and shouldn't take as long, especially if you're well maintained and mostly stock.

 

Battery disconnect usually gives me a low (~500rpm) idle for a half day or so before it evens out