unknown plastic burning smell under the hood

[akinaboy]

Hi AZ,
On my B5 A4 2.8 V6, it had burning oil smell under the hood.
After checking, I found that the valve cover gaskets were leaking oil.
I replaced the gaskets and oil stopped leaking.

Now, I have some sort of plastic burning smell after driving 5+ miles and I have no idea where this is coming from.
I took the plastic engine covers off to observe where it's possibly coming from but unfortunately there was no clear sign where it's from or what's causing the smell.

I wanted to see if anyone had similar issue on their B5 A4 2.8 before and what the fix was.
Also, is there some hose that gets clogged up commonly that I should check for potential problem?
Since I only get around 200 miles from full tank (50% city/ 50% hwy driving), I cleaned MAF sensor but it wasn't that dirty and it didn't make any difference.
I love the car and engine is very smooth and strong. The only complaint is the plastic burning smell.
BTW, the RPM idle is not solid stable either. It is at 750 RPM but moves up and down by very little amount, which I cannot feel but if that's some sign?

It's sunny day on Sunday and time to work on the car. I would appreciate for any help that would save my time.
Thank you

[Jimmy G.]

Bump for 2.8

[Lornnn]

plenty of plastic in the 2.8 engine bay, did you check breather hoses and vacuum lines? maybe you left one disconnected and its catching heat.. could also explain your rough idle
i might also check for blown fuses

[akinaboy]

plenty of plastic in the 2.8 engine bay, did you check breather hoses and vacuum lines? maybe you left one disconnected and its catching heat.. could also explain your rough idle
i might also check for blown fuses

Finally some helpful input posted.
I will check where the breather hose and vacuum lines are and check.
So, I guess that the car can still run although those hoses are not connected properly?
Thanks!

[Lornnn]

yeah, it'll run but not-so-hot or poorly
i imagine you'd be able to hear a hissing sound with the hood open and engine running, though

[jcheney]

I had a plastic burning smell once. It was the wires from the oxygen sensors. The heat shield got pulled back after 150,000 miles and exposed them. They ended up touching the cat.

[akinaboy]

yeah, it'll run but not-so-hot or poorly
i imagine you'd be able to hear a hissing sound with the hood open and engine running, though

hmm... no such hissing sound was detected. engine runs fine but with rpm being unstable (not roughly but slightly)

[akinaboy]

I had a plastic burning smell once. It was the wires from the oxygen sensors. The heat shield got pulled back after 150,000 miles and exposed them. They ended up touching the cat.

That's good point to check.
Do you know oxygen sensors were visible from underside of the car by jacking up the front wheels?
It feels like I spent past two weeks on working on this car. It's like endless work needed. lol

[BlackLock_A4]

If you spilled some coolant on the engine, it might smell like burning plastic. It will go away in a couple days if that's what it is.

[akinaboy]

If you spilled some coolant on the engine, it might smell like burning plastic. It will go away in a couple days if that's what it is.

There is no sign of decrease on coolant level in the reservoir and I didn't spill any coolant by myself so it's probably not the coolant but it smells pretty bad.
Sometimes, I can see slight smoke coming up .
All the gauges are showing good oil temp, water temp, etc... so I really have no idea where this burning smell is coming from.

One weird thing is that I found the vacuum breather hose with some cracks (this thing is really brittle) and with my slight touch, some portion of it got broken into pieces.
Inside the breather hose, it's very nasty with half the hose clogged up.
I wonder if this was the source of smell.
I used electrical tape to temporarily stop the leak.
I will monitor if the smell goes away after I replace the breather hose.