RPM bounce when letting off gas.

Okay so there’s a post from 2018 where someone has had the same exact issue I’m having. Nobody commented on it or anything. So I’m going to post a link to it. ( https://www.tacomaworld.com/threads/2001-tacoma-rpm-bounce-when-letting-off-gas.540628/ ) He also shows a video, the video is just like mine. You let off the gas and boom the rpm’s jump down and back up. Also does it when I touch the brake. Will really only do it when I’m at 50-55+ mph. I’ve kinda blew it off for awhile because people was telling me it was just the TC unlocking. It’s definitely not that.. the only thing I could possibly think of is the downstream o2 is aftermarket (WALKER). Ever since I replaced that o2 sensor I’ve noticed the rpm bounce. I just don’t think the o2 sensor would cause that issue but maybe I’m wrong. I’ve cleaned my throttle body Maf and IAC. I have a bunch of evap codes and the beautiful p0420 code. Other than that my truck runs fine. Any help would really be appreciated!

 

You're tellin me that you have evap codes, a cat code, and you consider that running fine?

If you have any check engine codes, the ECU runs differently and you're likely getting bad MPG.

Why don't you try fixing some of those issues? Those things are directly related to what the ECU is trying to do with the RPMs. The off brand o2 sensor could be causing the P0420.

 

Those EVAP codes have been on my truck since I’ve had it and it has ran fine. The guy I bought it off of had the gas tank replaced so I’m assuming that has something to with that. The p0420 is more of where I’m leaning towards with that o2 sensor I bought. And when I say my trucks running fine I mean by it has the power and all it just has that one flaw. Thanks.

 

When your check engine light is on, the ECU is running in a constant Open Loop state (ignoring input from the sensors) because it thinks the data from the sensors is bad.

Evap codes in general usually have to do with bad valves or leaks within that system. Leaks mean fuel vapor or vacuum leaks. Neither of those are "fine".

A bad (or inferior) O2 sensor or evap issue can easily cause fluctuations in rpm as the ECU tries to compensate.

Continuing to drive around with codes (especially codes for systems designed not to pollute the environment) is just irresponsible.

 

I asked for help, not a lecture. But thanks for the helpful knowledge! I’ll buy another o2 sensor, then I’ll get back with you.

 

My 02 just turned 500K and the check engine light has been on for about 400K still runs fine, does it pollute? don't know but i know i drive it to work everyday in the coal mine. Ya'll would love me in cali......

 

I provided help- both of those things can cause your problem. You are just choosing to focus in on the sensor. The whole system needs to be working correctly for the ECU to do its thing- part of working correctly is not throwing evap codes.

 

I’m focused on the o2 sensor bc it didn’t do the rpm bounce before it. So I’m assuming that’s my problem. I know the EVAP has a leak, I finally started hearing a leak. I just didn’t think a EVAP leak would cause engine performance issues. I cut myself a week ago so I’m waiting til I get my stitches out to find the leak. But I appreciate your help!

 

Funny, my 3.4 swapped 4runner does this too, however it has no cat, rear o2 sensor or evap system in it yet. I do know that it has a leaking fuel injector that causes it to run rich, perhaps since these are waste spark ignition systems and with a leaking fuel injector it could cause the rpms to fluctuate, but thats my theory so far on mine and I havent had time to figure it out yet.