black smoke normal? - PeachParts Mercedes-Benz Forum

jimandsuzy · #1 07-03-2007, 08:49 AM

I have an 85 300sd and it trails noticeable black smoke when I take off (full throttle). I thought this was normal but today I was behind a 300d turbo and it took off from stop at what appeared to be full throttle and did not smoke any. Do I need some adjustment? I'm new to diesels. Thanks Jim

winmutt · #2 07-03-2007, 09:30 AM

Quote:

Originally Posted by jimandsuzy

I have an 85 300sd and it trails noticeable black smoke when I take off (full throttle). I thought this was normal but today I was behind a 300d turbo and it took off from stop at what appeared to be full throttle and did not smoke any. Do I need some adjustment? I'm new to diesels. Thanks Jim

Have you services your ALDA? How many miles? Poor idle after start? Details man details!

Bio240D · #3 07-03-2007, 09:40 AM

Hi Jim,
A little smoke at full throttle is usually considered normal, but Black smoke is a sign of over fuelling. This means there is not enough air to burn all of the fuel being injected, and the "left over" fuel is only partially combusted producing black smoke.

Things that can go wrong on the "air" side of the equation are :
A dirty air filter will restrict the amount of air entering the engine and cause lots of black smoke.

A non-functioning turbo ( I've seen some that were literally siezed) will restrict airflow into the engine causing black smoke.

A crushed exhaust pipe will restrict the exhaust preventing turbo boost and restricting the amount of air the engine can ingest, you guessed it, Black Smoke! Check this stuff first because unless someone has messed with your ALDA (more on that in a minute) lack of airflow is where your problem lies.

Now for the Fuel side of the equation. There is a square diaphragm on top of the injection pump called the ALDA. It senses Intake Manifold pressure (boost), and limits the amount of fuel the IP can supply untill the turbo has created enough boost for the engine to be able to use more fuel. Its very purpose is to Prevent overfuelling. Many people adjust or "turn up" the ALDA trying to get more power. This causes over fuelling untill the turbo can supply enough air for the amount of fuel being injected (black smoke), but it does make the car feel a bit quicker probably due to high Exhaust Gas Temperatures getting the turbo spooled up faster. The small screw with a locknut on it in the center of the ALDA is the adjustment. If the small screw is turned fully Counterclockwise, or all the way out, the PO or their mechanic has "turned up" the ALDA.

I hope that helps.
Cheers, Chris

vstech · #4 07-03-2007, 09:40 AM

black smoke as I have learned here is Horsepower that you cannot use... resulting from too much fuel being dumped into the engine with inadequate air. check your air filter.

jimandsuzy · #5 07-03-2007, 09:45 AM

I haven't serviced the ALDA. Don't know how yet. The idle is fine, everything seems OK. It smokes when cold or warmed up. I see other diesel trucks and busses black smoke on acceleration so I thought mine is normal until I saw this 300d today.

KarTek · #6 07-03-2007, 09:46 AM

Since you're new to Diesels, most (if not all) Diesel engines have a mechanism in the injection pump or programming if it's electronic to control "initial smoke". This is the rich mixture condition when you get on it hard and results in a puff of smoke until either the turbo spools up and balances the mixture or the engine ingests enough air on NA models.

On mechanically injected MB's initial smoke is controlled I think as stated above by a device called the ALDA. Some owners adjust theirs to extract a little more performance but the price is more smoke on acceleration. A trail of smoke may also indicate someone has monkeyed with the total fuel delivered for more performance. Generally, if you have proper fuel atomization, more fuel = more performance until the air ingested can't keep up and EGT's rise beyond acceptable limits.

My knowledge is more truck related but the general principal is the same. MB just does some things differently.

On edit: sorry for repeating info. It looks like we were all posting at the same time!

Bio240D · #7 07-03-2007, 09:56 AM

Yes, a veritable fount of helpfulness has gushed forth!!:

CHris

jimandsuzy · #8 07-03-2007, 10:00 AM

The turbo works, I feel it come on at about 2200 rpm, the exhaust looks ok, and the air filter is clean so I'll look at the alda adjustment. Thanks for the replys.

KarTek · #9 07-03-2007, 10:04 AM

And we are so eager to welcome Diesel converts into our midst...

winmutt · #10 07-03-2007, 10:45 AM

Everyone is WAAAY to eager to adjust the ALDA. That should be the LAST resort before tinkering with the IP. Let us know if yours has been tampered with. ALDA adjustment is usually a reactionary fix and does not fix the underlying cause.

ConnClark · #11 07-03-2007, 11:24 AM

Quote:

Originally Posted by jimandsuzy

I haven't serviced the ALDA. Don't know how yet. The idle is fine, everything seems OK. It smokes when cold or warmed up. I see other diesel trucks and busses black smoke on acceleration so I thought mine is normal until I saw this 300d today.

A small amount of black smoke is normal under hard acceleration or heavy load, or at least until the turbo kicks in. You have to remember that the 300d is a lighter vehicle than your 300sd so it should smoke just a tad less smoke. There are things that contribute to smoke other than lack of air that also could be causing the smoke. Fouled injectors, poor compression, valves need adjusting, injector timing, valve timing, fouled prechamber holes, and a bad EGR valve or EGR throttle switch all can contribute to black smoke.

EGR contributes to smoke by, for lack of a better term, retarding combustion which leads to some incomplete combustion. Some drivers have come to the realization that EGR is retarded and taken measures to deal with it. If the 300d driver did this he might make less black smoke than you. When you push the pedal all the way down there is a switch on your valve cover that should electrically shut off the EGR so this is something to check. Also sometimes an EGR valve will stick open.

A diesel purge treatment may help. It cleans the injectors. Clean injectors shoot a better stream of fuel at the impingement ball in the prechamber that in turn leads to better combustion. Adding power service to your fuel may also reduce smoke by cleaning the injectors and starting combustion just a bit sooner.

To clean the soot out of your prechambers you need to remove and clean them manually or give the car an Italian tune up. The Italian tune up is more fun as you just find an open stretch of road and drive it like you stole it.

You may not feel comfortable checking all these things but doing some may fix the problem. If the ones you can do don't fix it a mechanic can

Edit: also you have an 85. Your rear end is a 2.88 where the other car probably had a 3.07 rear end, so your engine may have to work harder to get your car going. Also since you have an 85 it has a nasty ARV valve on the turbo that limits boost to increase EGR flow, this too can be dealt with.