2002 S80 2.9 Rough idle when warm

puntmeister · Post by **puntmeister** » 14 Dec 2013, 21:52

Issue: Car idles rough after engine fully warmed up. Runs smooth when cold. No other drivability issues - accelerates fine. By "rough", I mean there is a bit of a rumbling (but no misfiring). I can also hear what I first thought was a bad serpentine belt - sounds a bit like a squeaky belt - in actuality, I'm thinking it is likelier some sort of vacuum leak.

Check Engine Light: When car is idling rough, it flashes. When idling smooth, or at higher rpm's, it is solid on.

OBD Code: P0132 (Front passenger oxygen sensor).

Attempts to repair thusfar: I figured the sensor was bad - I first removed the wrong sensor (the front driver-side sensor). I tested it - both for voltage swing with torch applied, and for ohm resistance for the heater. Checked out fine. Went ahead and torched it for a bit to clean off the carbon buildup (which was fairly minimal).

I then removed the front passenger sensor. It seemed to have a bit more carbon buildup than the driver side, but still nothing outrageous - no white buildup like I've seen in photos. I checked voltage swing with torch applied, and ohm resistance for the heater. Again, checked out fine. I figured the issue might be the build-up of carbon, so I torched it, put it back again, and hoped the problem would be solved.

Result: Nope. Still get rough idle once engine is warmed up, along with squeaking/whistling sound - only at idle (well, maybe a slight whistle at speed, hard to tell).

Question: What is the likeliest source of the problem? A vacuum leak? Which vacuum is most likely to be leaking? Best way to check?

What else could it be?

puntmeister · Post by **puntmeister** » 14 Dec 2013, 21:59

Further questions: If there is something wrong other than the sensor, is it normal that I get the P0132 code? Would a vacuum leak manifest as one oxygen sensor reading high, but not the other? Or what could cause one sensor to read high, but not the other? (The OBD error says P0132 1 circuit high voltage).

It appears this sensor with the code, the passenger sensor, has been replaced before (I just bought the car, so don't really know) - The wire to the sensor has been spliced - the sensor is the correct one, however - it has the same part# as OEM. The driver side sensor is OEM (but still could have been replaced, who knows).

I am new to this kind of car - I can't even find the spark plugs! (I am used to working on my Jetta Diesel...).

puntmeister · Post by **puntmeister** » 01 Jan 2014, 19:40

After lots of testing, and googling, I managed to get to figure out the source of my rough idle:

It was a bad PCV. There was a small rupture in the diaphragm, resulting in the rough idle.

I read a lot about the PCV failure (very common), wherein the PCV clogs with oil sludge, resulting in blow-by building up pressure in the crankcase. The easiest way to verify? - pull the oil dipstick out, and idle returns to normal (no whistle/whining). Additionally, there will be air blowing out the dipstick.

When I removed the dipstick - sure enough, idle returned to normal. However, there was no air blowing out - instead, there was heavy suction.

Took me a lot of googling, but I found someone with the same problem - the cause: there is a diaphragm inside the PCV (it is in the round part on top). It opens to allow blow-by to recycle into the intake during times when intake suction is low (during acceleration and high constant speeds). It closes during times of high intake suction (idle) - the reason it is designed to close during times of high intake suction is that the high suction results in too much negative pressure in the crankcase, and an ensuing rough idle.

Long story short - if (when) the diaphragm inside the PCV ruptures, the high suction during idle pulls from the crankcase, resulting in the whistling/whining (this sound is outside air being pulled through seals).

There are many posts online suggesting replacing the PCV is a very tedious process, wherein the intake and/or accessory bracket (along with A/C compressor, alternator, and steering pump) have to be removed.

In my 2002 2.9 non-turbo, neither the intake nor the bracket had to be removed - there was an access channel built into the bracket to allow for removal of the PCV bolts.

$64 for the PCV (from the dealer), and about 2 hours of fiddling to install.

garyhall5760 · Post by **garyhall5760** » 20 Mar 2014, 19:11

I am having just about the exact same issues. (Will pull the oil stick tomorrow to see if it smooths out the idle.) Did replacing the PCV also resolve your O2 sensor code?