SaabCentral Forums banner

Black Smoke Coming Out of Tailpipe?

2K views 7 replies 5 participants last post by  Cosmicflash 
#1 ·
A friend was following me yesterday and said that some puffs of black smoke came out of the exhaust a couple of times accelerating down the expressway:(.
It was just after a cold start maybe two minutes into the trip. He followed me for 30 miles and no smoke was noticed after the initial puffs.

So, what might have caused that? I believe black smoke is unburned fuel?
Is that normal as I rarely never see any of my cars from the rear:p
 
#3 ·
Any hard acceleration (or deceleration) is an open-loop mode. That means the ECM is not adjusting the air/fuel ratio based on feedback from the O2 sensor. Instead it utilizes predefined fuel maps to accommodate your "torque request."

The fuel map will err on the side of richness to prevent excessive temperature in the combustion chamber and exhaust.

So the black smoke is normal. You may see more smoke if the car has not yet reached operating temperature, in which case all modes are open-loop.

Here's a list of some of the parameters monitored by OBDII. There's no more turning the idle screw on the carburetor:cheesy:
http://www.obddiagnostics.com/obdinfo/pids1-2.html
 
#5 ·
Rentokil,

It's a gasser as all of our US Saab are. Diesels are quite uncommon here. Less than 1% of our passenger cars are diesel--pretty pathetic compared to Europes 40%:roll:
Diesels around here do smoke quite a bit (they have been referred to as "smokers" in the past). The US is getting this Fall low sulfur or euro specd diesel so perhaps that will help our diesel market grow.

Ctrlz,

Thanks for that info. I didn't know they had the open-loop mode while cold. I guess that makes sense. I was guessing since we have one the most advanced engine mgmt. systems on the market why would it smoke? That explains it:D
 
#7 ·
i can answer that one:


if you gas it, the engine immediately gets the order:
more fuel (yes, without "asking" the lambda-sensor)
however, with a supercharged engine your T/C will need a second or so, to increase the revolutions and thus the air the T/C can deliver (which is a function of rpm, airflow and pressure), so in the first instant the engine simply does not get enough air for complete combustion of all the fuel.

thats all.

this is why in some modern engines (MTU2000, MTU6000)you have TWO T/C's in line (or three,not parallel!!!), for transient load situations, because two small T/C will increase revs faster than one big charger.
so only when really needed both (or all 3) are active.

for a suction engine it works quite similar (the engine needs to rev up), albeit not so pronounced.

and by the way:
ever looked at your torque curve and wondered why your T/Ced Saab has so much torque at low revs and stays that way, regardless of the revolutions, but a non T/Ced engine increases torque with revolutions?
THAT's why!
the T/C will deliver air dependent on the amount of fuel burned and exhaust gas produced.
for a suction engine the amount of air is always the same, one cylinder filling and thus dependent on the revolutions. plus you have the added problem: the faster it turns, the shorter the time the valves are open, so you need to install adjustable cams or similar to change the timing as revolutions increase (nice to have with a T/C engine, but not nearly as neccessary)
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top