Hi all!
I have small, but irritating problem with my 3.8L.
When car is idling (both cold and hot) I (think) I have slight misfire. It's really subtle, I can feel it on my back on the seat and on my arm when I put it on the door. No codes/check engine light, rpm needle is steady as a rock.
I already changed:
- all spark plugs to iridium ones
- all cables
- all ignition coils
- fuel filter
- cleaned air sensor
- unpluged MAP sensor (it didn't changed anything except error in ECU so I connected it back)
- I tried additve for cleaning the injectors (no luck)
I'm currently out of ideas what can cause it.
Car runs and drives perfectly fine, accelerates with no hesitation, does not throw any codes.
This behaviour happens ONLY on idle.
My last guess will be the engine mounts as sometimes I hear clunk when I start the engine (only when it's really hot after harder drive, so I suppose that rubber gets softer then).

Do you have any advice what can cause this? Any sensor?
I attach video with all sensors reading. In ECU1# and 2# it says "closed loop, using oxygen sensor feedback to determine fuel mixture"
(It's around 1 minute long)

 

Always start with the year.

There are so many things it could be. Back when I did warranty work these types of issues were the hardest to track down.

I do not think you are feeling a miss. What you are feeling is a slight variation in the power of the cylinders at idle. If the engine was missing it would set a code on the PCM. The PCM tracks the rate the crank turns. It will pick up up on the fact a cylinder does not fire because the crank will slow down slightly. Yes a single miss can be detected by the PCM. So it does not have to see coil 3 did not fire but it knows cylinder three did on work on that rotation. The PCM does not set the miss fire code as soon as it sees a miss fire. It starts counting. Once the misses reach a certain level within a defined time it sets a code. With a decent scan tool you can look at the miss fires counts. Almost all cars get some miss fires. There are also failures like low fuel pressure or a bad O2 that could cause misses across the board. The PCM will Code this as a Random miss fire. This is why I do not think you are feeling a miss.

If not a miss then what? At no load idle your motor needs very little air and fuel. I believe what you are feeling is small disturbance in the flow of the air or fuel. I use to work from the most common to the least common and this would fix 95% of the cars. I would guess at some point you would say its not worth it I will just live with. Not an option for a warranty tech. Each step can make small improvements.

I don't think you list O2 sensor as items you replaced. Those are maintenance items. You may want to start there. Just the two in the front effect the way the car runs. O2s do not effect the way the car runs when it first starts.

My step one:
Check vacuum with the vacuum gage. With luck the needle will be jumping around a small amount. (you can use this to tell if you make improvements)
Clean and test the EGR valve for leaks, EGR valve should be 100% closed at idle. Any leaking exhaust can cause idle issues.
Clean IAC and Port (if the car is drive by wire then clean the throttle plate.)
Retorque the throttle body and any intake bolts you can get too. Retorque NOT over tighten.
At that point with the engine running spray carb cleaner on every intake gasket seam you can see. Listen for a change in idle. It will be minor. Replace gaskets as needed.
Clean injectors with a pressurized cleaner on the car.
( I worked at one dealer that was very customer focused they made the above part of a standard tune up)

If you still have the issue.
Pull intake. Pull injectors.
Clean and visual inspect the intake. You can test the EGR valve and see if it seals. Send the injectors out to be flow tested. (it may be cheaper to replace them but the new ones could be as bad)
Clean all gasket areas so they a like new and verify they are flat. Carefully assemble.

As you can see in the above I am focused on micro vacuum leaks. This is not just a EFI thing. It will cause issues with standard carb small blocks. It was a huge issue with the TPI cars in the late 80\s

After that you are in the 5% area, now you are talking about a sensor that may have resistance in a wire or a connector that is too small to cause a code but enough to effect the idle. Now you are sitting in the drivers seat with a $2000 scan tool looking for something that shows different then what you are used to seeing on that engine. (this can take years of experience)

When I did this stuff I was working on cars that where less then 3 years old with less then 50k on them. I am sure there are a lot of things that come with higher miles and old age that could cause the same issues. Loose timing chain, a lob on a cam. Carbon buildup on the valves. Could all effect the small flow of air at idle.

 

Wow thank you very much for such an complex response!
Sorry for confusion - year is 1999, manual, around 83k miles.
I forget to mention but I already cleaned throttle plate, car is drive by wire.
I will take a look into things that you mentioned