Thank you George_San_Jose1 ! I really appreciate it.
I noticed today that short term fuel trim goes up when I press the brake pedal down at idle in park or drive. Tomorrow I am going to see how long that happens for. I think it’s supposed to happen when I release the pedal (or pump it repeatedly) but not when I press it down and hold it. I am also going to press it down, hold it, and then clamp off the vacuum hose connecting the booster to the intake manifold to see if the STFT and the idle improve.
I suspect the booster is likely normal because it passes the “pedal tests”, and because when I disconnected the booster and capped the manifold side, the idle still felt somewhat rough. But it seems like maybe there could be a small leak in the internal valve connecting the rear (atmospheric) chamber of the booster to the front (vacuum) chamber. Will report back. Thanks again.
I’ve never had to diy’er diagnose/repair a brake booster. But it would surprise me to learn it is normal for a properly functioning brake booster (with brakes not applied) not to hold vacuum.
You are correct, George_San_Jose1! It does hold vacuum with the car off and the brake pedal not pressed.
I learned a few things:
When I push the brake pedal down, I get a lean spike. Idle STFT go from -2.3 to + 2.5. It stays at that level for about 5 seconds and then comes back down.
When I release the brake pedal, it does the same thing again to the STFT on the way back up.
However, the car still vibrates in drive with the brake pedal applied even when the vacuum booster is disconnected and the manifold side capped.
The big new clue/development is that this vibration happens much less when the car is warmed up and I have been driving around for a while and then I come to a stop. In fact, if I’ve been driving without stopping (i.e. on the highway) for a while and then come to my first stop, it feels smooth without any vibration for the first 5-10 seconds and then it starts a bit.
By comparison, when the car is warmed up but I’m just driving in a parking lot or on city blocks, it still vibrates in drive with the brake on.
Sometimes I feel it start under my seat just before I come to a complete stop…like when I’m at 1mph and I’ve already been pressing the brake, but just before the vehicle actually completely stops.
You may need to take your truck to a Toyota specialty shop with access to Toyota’s pro-level scan tool. Subtle ignition system problems are hard to ID without. If you wanted to try something diy’er-ish, cylinder compression, pcv system, egr system.
I will do the things you mentioned. I think my engine does the EGR function with the valve timing but I will look into that.
Some new observations:
Could a voltage regulation issue be causing the idle roughness?
One thing that’s really weird is right BEFORE I come to a stop the control module voltage drops suddenly from 13.5v to 13 v. It is not caused by pushing the brake pedal (I’ll brake several seconds before this happens and hold the pedal down). It happens at like 2mph or 1mph. Right when it happens I can feel the truck start to shudder in my seat and it will then continue while I’m stopped. There is no other time when the control module voltage drops that low.
Welcome to the forum… Matt has not been on since their last post in March of 2023, so he may not see your question…
The best thing to do is to just start from the beginning, starting with your vehicle info, Is your Tacoma a 3.5L or the 2.7L above, Manual or automatic?? How many miles and what have you checked or done to find the issue… How long have you owned it…
The 3.5L Tacoma’s are known to have a rough ish idle, it is an emission thing dealing with the RPM’s at idle or something and not much can be done about it, my 2023 with <15K miles does it, it does it more with the A/C on… Spark plugs are due at 60K miles ONLY for the 3.5L and at 120K miles for the 2.7L…
Okay. I have 2017 Tacoma sr with the 2.7l. It didn’t always do this but just started a few weeks ago. Only when in drive or reverse at a red light with foot on brake, the whole truck vibrates really bad. I have changed spark plugs, cleaned throttle body, cleaned MAF and IAC sensors as well. I know somthing is not right because it didn’t always do this. Very similar to the issue he had as well. My ac doesn’t work so that has nothing to do with it. The rpms stay at around 650 or so during this. I don’t know what to check next.
It’s very unlikely it’s a misfire. It would run way worse if one out of four cylinders were not firing. There is no check engine light and only does this sitting there in gear with the brake on.
Thanks @davesmopar it only does it with the foot on brake when in gear. If I take the foot off the pedal and put the parking brake on it stops. As well if I’m in park and I push my foot on the break, it doesn’t do it either.
Same thought. I got a bit confused because I seemed to recall blocking off the vacuum source but that apparently was the original thread. I had a problem like this once and it was a bear to chase down because the leak quickly got bad enough for it to stumble all the time. It was the last place I looked
Depends on how bad the leak is, and how much it is affecting the fuel trim imho… The vehicle does not vibrate in park/neutral at idle with the brakes applied, so probably a small leak…
Maybe it is not turning on the CEL because it only shows up at idle, in gear with the brake pedal depressed, since most of the time is spent above idle and or without the brake pedal being depressed… Or something along those lines anyway, just thinking out loud…
Meaning it hasn’t had enough time to mess with the learned values…
