Was wondering if 4.9's are especially suceptible to carbon build up because of the mandatory use of 91+ octane!
the reason I ask is because i did a few fyull throttle runs in mine, two in the city and two on the freeway and I noticed that the first run which was in the city produced a big cloud of smoke out of the pipes but the other three there was no smoke at all.
I thought there was something wrong with my car until I saw that the other three runs didnt produce any smoke.
I tried to put 2 and 2 together and am trying to force myself to believe that the smoke coming out was carbon, thats what im hoping.
Prior to the first full throttle, I had gone probably a whole week without opening it up which is logical to assume that carbon could have been building up that whole week!
any input appreciated.
P.S, I do know for a fact my car needs a good tune up, not an oil change becuase I just changed it a few days ago. Im talking spark plugs, wires etc!
Thanks

You could have a lazy injector which is decreasing your mileage and causing your engine to burn too much fuel (or an old oxygen sensor). If the car runs rich, there will be leftover carbon from the burn process, and you'll experience what you just saw.

I know for a fact my o2 sensors need to be changed.
How do I test for a lazy injector?

go with the o2 sensors. they are lazy. 1 in each manifold

I would agree and change the o2 sensor first (I believe the 4.9 only has 1--at least my 91 Deville with the 4.9 only had 1).

Sometimes you can use the injector balance test in the manual to test whether or not the injectors are working properly.

You should have two on an 92' Eldorado. One is right under the front manifold. I really had no problem with this one. The other one is excessible from underneath the car. I jacked and removed the passanger wheel. Did not actually have to remove the wheel but did not know that going into it. It is a high reach. This one was hard to break off for me. I ran the car for a 2-3 minutes to loosen it. Wrapped a towel around my hand and arm. It then came off very easily. Watch your ass around hot stuff down there. You should have a code if one of these is bad. That has not always been the case for me. I have noticed MPG drops way before any code came up.

Is it shifting rough?
Missing, stumbling, surging at idle?
Any thing abnormal with drivability?

More then likely it is nothing. Mine smokes alot on cold days.

There are no codes set, and the driveability is fine.
No rough shifting or hesitation. Nothing abnormal about the way it drives.
Thinking about changing the spark plugs and wires anyway even though I have no reason to believe they are bad, havent took them out and looked at them yet.
the weird thing is that it smokes only under full throttle and after that it doesnt smoke anymore until I baby it for about a week thats why I say it could just be carbon buildup.
Im going to look into the O2 sensors though.
Thanks

You usually have 1 o2 sensor per exhaust on these engines IIRC, so the eldo may have 2 like mentioned.

You generally get codes if they are electronically dead and not giving a signal to the computer, mine was lazy but set no light, replacing it upped mileage 20-30%

Whats the mileage on it now?
I wouldn't just jump into replacing plugs and wires if its low mileage or they have been recently redone. If you want to look into it I would check the wires first since they are easily visible. I replaced the stock wires on my 4.9 at about 92k miles and it helped quite a bit, my performance and handling seemed good but my mileage was crap with the old stock wires. You may pull one or two plugs to check though. IF the wires still have the cylinder numbers printed on them I would bet they are original, unless someone replaced them with the oem part. Mine say bosch on them now and are not labled so anyone knows they are after market :)