cause of white smoke?
 

Hi, i have a 1990 300E with the 3.0 straight 6 sohc M103. The car has about 109500 miles on it so far. Since i bought this car it has always had a puff of white smoke pouring out of the tail pipe, with that said it has passed emissions check Twice with very good numbers. when i first got it it had issues with idling to the point where it almost dies out on me in a stop light, i used two bottles of techron fuel system cleaner so it no longer has this problem, but the white smoke continues. Once my dad had an oil change using synthetic oil and the oil slowly seeped out of it somewhere, so i thought that it might have been burning the oil through valve guides or valve stem seals but when i put in regular oil (to spec) i have not had any oil loss. with that said the fairly new spark plugs (platinum or some other) are covered in carbon (completely black). I also feel that the car is a bit held back when it comes to acceleration (i got beat by a neon lol...). gas mileage is terrible i barely get 10-15 mpg. there is also slimy black stuff in the exhaust (probably carbon?). I did not note any loss of coolant or mixing of coolant in oil or vice versa so no head gasket issues. Another thing i noticed is that the transmission box will some time clunk (like a piece of metal keeps hitting a rotating object) when i first shift into drive or reverse but will stop after the car starts picking up speed, it will also make that noise when turning left or right at 15-20 mph.

sorry for the long read but, i honestly cannot pin point the problem(s) and i really dont want to start throwing new parts at it. What i have come to as a conclusion is that it might be the following parts hopefully you guys can help me weed some out->

-O-2 sensor
-catalytic converter
-air/fuel mixture ratio
-ICV sensor
-Fuel Injectors
-Vacuum leak
-piston rings?

Parts that have been changed so far
-Throttle body
-air & oil filter
-tires

I would really like to have this car in perfect running condition, As a mechanical engineering major (2nd year) i dont mind rebuilding the whole engine as a project with fellow ASE Mechanical Engineers in my college.

Thanks in advance for your reply!

The tranny thunk could also be coming from the flex discs on the driveshaft. Also, make sure you have good vacuum going to the modulator on the side of the transmission - that regulates how hard the shifts are. The transmission control cable should from the throttle to the tranny should also be attached and adjusted correctly. You need good downshifting to get good performance out of the M104 (er, uh M103), and you should be able to humble a Neon any day of the week.

Given your wide swath of issues, a visit to a decent shop could help set your priorities. You can always say no to the work and fix on your own, but get some good diagnoses.

If those plugs really are Plats, I would start by changing them to standard copper core....... [ NGK are great on 103 engine]

Smell the white smoke, or have someone with experience sniff the smoke. Oil smells like, well burned oil. Antifreeze will have a sweet smell.
On the presumption its oil, then there is a good possibility you have stuck piston rings. You can have stuck pistons and not use enough oil in 100 miles to see the level drop on the dipstick but still be burning oil and making white smoke. I've seen this problem.
I think this occurs when owners use cheap (non synthetic) oil and don't drive very often or do regular oil changes. Using a good Synthetic oil will almost guarantee this won't happen.
Headgaskets on this engine are sometimes a cause of concern but to fix the rings you need to pull the head anyway so a new gasket wil be in order when you do the following:
Remove the head. Soak the cylinders in a few drabs of witches brew consisting of 50% Marvel Mystery Oil mixed w/ 25% diesel fuel and 25% GumOut. Turn the engine over by hand to allow the mixture to go into open cylinders, do this several times each for 3 or 4 days, rotating the engine until you think its done its job. Finally remove excess mixture with blue paper towels or clean cloth rags and reassemble. Drain the oil and replace with Synthetic. See if that fixes it.

Before you pull the head, pull the brake booster line and look for evidence of brake fluid...often overlooked ...

Your plugs are telling you something, but only if they're the correct ones. Black & oily = oil = valve guides or rings. Black & dry = carbon = rich = fuel/air problem. What state are you in (because of the emissions test)?