Causes for slight hesitation on acceleration

[rgnjc]

My 2005 Vibe AWD (210k) has slight intermittent and slight power loss when accelerating from a stop or low speeds. It is noticeable when the engine is cold, though there are no OBD codes, and no loss in fuel economy.

I've had this issue since last year so its not so bad that I need to fix it, and its generally very mild but when it does happen it feels like the power completely cuts out for about 1-2 seconds before resuming. Last summer a mechanic said the MAF was the problem but I replaced the MAF as well as my rear O2 sensor (my diagnostic tool showed some strange results) but neither fix addressed the issue.

I'm at a loss (literally and metaphorically). I took it to a mechanic this week and they could not reproduce or identify the issue, so nothing got solved. It typically happens most when the car is just turned on and happens less after warming up, and its more dramatic in cold weather if the car isn't warmed up.

Do you have any ideas of how to diagnose the performance issue?

[SSizler]

Possibly the intake manifold gasket.
Have you replaced the original gasket yet?
They are a known / common failure point.
I know my 2006 1zz needs the gasket replaced, hope to do it this fall.
I get rough idle and a stumble sometimes when it is driven before warming up in the winter.

[SSizler]

You might want to check the IAC gasket also.
Sounds like something in the intake tract.

[rgnjc]

Yes good idea thank you - I ordered gaskets for the air intake manifold, the IAC valve, and the throttle body. All 3 were like $20 for the toyota parts from toyotapartsdeal.com.

I did replace the air intake manifold about 4 years ago with a fel-pro aftermarket gasket because the original was leaky in 2019. I've never replaced the gaskets for the other 2 but I do faintly remember looking at the IAC and throttle body gaskets when I did that job so its possible that in the process of doing that I unseated those gaskets and they didn't reform a good seal since they were original and old.

Will report back after I get those 3 gaskets in next week and replace them and see what happens.

[rgnjc]

Following up on this after doing some minor repairs. Although I addressed some other issues that clearly needed attention, I continue to have the slight hesitation and performance issues. I now suspect something is going on with the transmission and would appreciate any advice.

1. I replaced all three gaskets for the throttle body (Toyota Part 22271-0D030), IACV (22215-7A680), and air intake manifold (17171-22060). The old gaskets were definitely old and flattened (see picture). Toyota updated the throttle body and manifold gaskets and they are definitely "thicker" than the originals and now form a proper seal.

2. I replaced the 1st and 2nd exhaust flange gasket/spring bolts. Mine were original and basically crumbling/non-existent (see picture of each). I replaced: flange1 gasket (17451-0D060), flange2 gasket (17451-0D070), and I used the same bolts and springs for both (90109-A0013 and 90501-A0006). FYI removing the front O2 sensor and manifold heat shield allows you to easily clean the exhaust bolt threads with a wire brush and soak in penetrant. If you need to use a replacement nut for any of these bolts, the size is M10x1.25.

3. I now suspect a transmission issue is causing the poor performance. My fluid was extremely brown and looked like motor oil (see picture). My transmission was replaced 40k miles ago and was filled with Castrol transmax. I now flushed it with Toyota T-IV and might have to do a second flush since I only got 3.5Q out of 7Q total on the first drain/refill. No performance improvement after first flush though. I recently started to notice that sometimes the transmission has a hard shift from park to reverse/drive, although it doesn't happen all the time. Restarting the car seems to make the problem disappear until I restart it. I suspect a solenoid or sensor is acting up. No OBD codes. Not sure if I can address this one myself - I may need to take to a mechanic to diagnose. I'll follow up on any leads here and will report back if I end up having a mechanic diagnose it.