The motor on my '04 with about 52k on it has recently started acting strange, particularly when cold and at lower rpms.

When I try to accelerate the motor hesitates and/or gives variable response. Past 3-4'000 rpm the motor starts sounding and pulling better, but I haven't want to push it too hard.

I was wondering if it might be one of the fuel injectors and ran some techron gas system cleaner through, but this unfortunately didn't improve the situation at all.

Any suggestions on what the problem might be, and the next steps for dealing with it?

Thanks!

 

There are multiple issues that it could be, but it is most likely your O2 sensor. They need to warm up to be effective, and if it is hovering on the edge of failing, it could easily be causing this. The issue mostly when cold is one of the bigger pointers to the O2 sensor. (Cold is of course, relative. They need to be at least ~300+ degrees F I think). Sending the wrong info to the ECU could make it not have enough fuel or too much, and you have power loss, burbling, lugging, etc...

Front O2 sensor when you go shopping. The rear is just emissions control in the Cat.

 

Dude, I had the same issue, took it to the shop, I needed a new CAT, hopefully you don't but sucks if you do

 

Your failing or failed cat likely damaged the O2 sensor then. Mine is doing the same thing and I am back to getting a new O2 sensor after my cat failed and apparently damaged it. It was fine for a few months after the new cat, but it slowly got worse and worse and worse.

 

How many miles on the coils & plugs?

 

Above 3250rpm the SSV begins to open (also depends on engine load).
You could just be feeling the transition the car goes through as an additional port for airflow is opened.
If you look at a dyno sheet and see several "dips" along the power band, those are the various valves opening.
Your symptom could also indicate the SSV itself is beginning to stick and taking longer to fully open.

Just another idea to throw out there...

 

The coils and plugs should have about 20k on them. Not long after I purchased the car one or more of my coils failed, which fortunately got covered under warranty. While I didn't verify this by pulling them, they told me that they'd replaced the coils, plugs, and wires.

The luggy behavior is definitely a newer thing that started happening recently, so unfortunately there's clearly been some change from the "normal" situation, re SSV or other causes.

I can certainly imagine an O2 sensor causing this, and I sure hope it's not the cat!

 

Whether it is a O2 sensor or CAT they should both be covered under the emission warranty am I not correct?

I am having the same problem you are describing and would be interested in hearing how you resolved this.