long time lurker, first time poster. i would like to start off by saying just browsing the forums has been a tremendous help, ive been able to double my meager knowledge. with that said, i come here with an issue. ive searched high and low, and its hard to tell what exactly my problem is. about a month and a half ago, the z started to "vibrate" underneath my feet while idling. nothing serious just now and then. gradually it seemed to get a bit worse, especially as it the tempature dropped. as time progressed, so did my problem. while under 1k rpm, the engine itself would rattle now and then, and i can feel it underneath my feet and in the car. it never sounds like it is going to sputter out, but i never really let it sit underneath 1k rpm for more than a couple minutes. i should mention that while it sputters/surges, the rpms remain constant. while at a stop light i noticed my exhaust was breathing real heavy, like milky steam. under load, i can not see anything like a trail behind me, and it only starts after the cars warmed up, and sitting under 1k rpm for more than 15 seconds. standing behind it, it would appear to be more steamy than smokey. taking off while its like this causes a bit of hesitation 1/2 times. once it gets going, i feel it runs fine, never feel vibration or engine surge or see my exhaust smoking like a turkish bath house. so with that, along with my oil seeming fine, i feel i can eliminate head gasket issues (hopefully). my car has the SES light on, but the previous owner said it was due to the cats. i can see them still attached, but i think they may be hollowed out. the stock exhaust manifold looks kind of old, as in it had headers then they were removed and stock was put back on. i broke down and bought a simple code reader, and was worried at this point it would throw a gajillion codes. nope, only P0420 and P0430 which is bank 1 and bank 2 catalyst system efficiency. these codes have been on before i had my issues, so i feel i can eliminate that as my issue? i have only basic mechanical skills, and im trying to work on my car as much as i can. i recently changed the PCV valve, and it seemed to help... a little. when parked its hard to hear what would be a definitive leak, especially with how loud these cars are naturally (plus hollowed cats). when it gets milky, i have looked in the engine bay, and underneath to see if i can see a leak or anything but nope. could a leak plus lower tempatures be throwing my o2 sensors off? it shows no code for o2, and my code reader only reads codes, doesnt show diagnostics. i heard if the afr is off it could cause the milky exhaust. im hesitant to take it to mechanic, as i want to do as much as i can. sorry im so long winded, just trying to put out as much info as i can, and im sure im missing some stuff. ANY input appreciated.

 

When the exhaust is 'milky', what does it smell like? Anti-freeze?
Is the exhaust ever black?
The simple fix could be ECT sensor. This is easy enough to prove.
And, yes, a leak will throw off your A/F in closed loop.

 

My sense of smell isn't that great, throw in the fact that allergies only leave me one good nostril and... You get the point. But it doesn't seem like it has any definitive smell. Nothing too overwhelming. The exhaust is never black. I swear it seems more steamy than smokey. How could I test the ect sensor? Wouldn't it throw a code?

 

I remember reading somewhere to check your oil cap, it should have some milky rings if its a blown head gasket... Just a thought

 

If no black exhaust, then you're not running rich.
To check ECT...ECT can be removed, placed in a pot of water on the stove, with a high temp thermometer (stove thermometer).
After you turn the burner on, the water temp rises. As the temp of the water rises, monitor the resistance reading from the ECT.
There's a chart somewhere, which shows what the resistance should be on the ECT, compared to the coolant (pot of water) temp.
As stated in the above comment, it is also possible that a gasket leak can add oil, or coolant into the exhaust.
You could try a compression test to each cylinder.

 

U need to find a scanner that will show real time data. So u can monitor alll systems in open and closed loop. Start with the Cat codes...they can effect alot of tune issues.

 

I was having almost the same issue. Exhaust leak from the manifold was the problem.