Suction at tail pipe - PeachParts Mercedes-Benz Forum

BigRatFink · #1 04-27-2009, 09:56 PM

My 78 300D won't start. Everyone and their cousin has looked at it, but no one seems to know what the problem is.
During one of the tests, my neighbor suggested putting my hand over the exhaust pipe while he cranked the engine.
I felt suction, alternately with exhaust.
Anyone have an idea as to why that would be? Timing belt issue?
The engine won't even fire with WD-40.
Any suggestions will be greatly appreciated.

#2 04-27-2009, 10:42 PM

Have you adjusted the valves or checked for a broken camshaft?

Simpler=Better · #3 04-27-2009, 10:56 PM

Is the starter polarity wrong? Are you spinning the engine backwards?

leathermang · #4 04-27-2009, 11:00 PM

Burned or dropped exhaust valve perhaps....

JonL · #5 04-28-2009, 11:47 AM

A little suction like that probably isn't abnormal. The exhaust valves stay open a little bit past top dead center so that when the engine is running the inertia of the exhaust gas leaving the cylinder continues to scavenge the residual exhaust even as the piston starts its down stroke. At cranking speed the gas velocity isn't high enough to create that scavenging effect, so there will be some slight suction in the exhaust system.

A highly restricted intake could make the suction you felt more pronounced and could also keep the engine from starting. Check your air filter and the air inlet duct. Maybe you sucked in a plastic bag or something.

Other than that, there could be a valve issue, but you'd probably hear a lot of ticking.

F18 · #6 04-28-2009, 04:47 PM

I agree with JonL. Check the air filter housing and intake manifold for blockage. Was this car coming out of winter storage?......then there might be some critter nests in there.

I think we all assume that you are getting fuel delivery and the glow system is functioning and the valves are adjusted correctly.........

carnut · #7 04-28-2009, 04:51 PM

And the engine is spinning fast enough? Diesels require a very strong battery to spin the engine. Too slow turning over will keep the diesel from exploding to start the car. Did it start and run good yesterday? or are you trying to start a car that has sat along time??

leathermang · #8 04-28-2009, 10:59 PM

Quote:

Originally Posted by JonL

A little suction like that probably isn't abnormal. The exhaust valves stay open a little bit past top dead center so that when the engine is running the inertia of the exhaust gas leaving the cylinder continues to scavenge the residual exhaust even as the piston starts its down stroke. At cranking speed the gas velocity isn't high enough to create that scavenging effect, so there will be some slight suction in the exhaust system.

That is a very imaginative description... but I just can't go with it.
The idea that the slowing down of the exhaust gas due to the cranking speed being slow making a vacuum effect as far back as the end of the exhaust pipe just does not fit my intuition of the physics.
First, those two , or even just one, mufflers would sure mix up the pulses.... a dampening effect which would preclude an actual backwards movement being available at the end of the exhaust pipe... I can see a lessening of the exhaust speed...but not a reversal needed to give this effect... remember that other pistons are sending gas into that confined stream also... even if one could do that the combo of several feeding that tube would surely.
Also, this supposed valve opening which occurs after top dead center... ignores the fact that we are dealing with the two strokes of the crank in that bore which have both valves closed ... Compression AND Power.. so that does not fit either..
I agree with the concept of inertia creating a scavenging effect... but given the whole situation I think slow cranking or idle would only take that away.... as compared to going into the other side of the realm... (of vacuum. )
I think your mental concept of the valve overlap comes from gas engines....where a hot cam can cause that kind of effect at certain rpms...
Nice try though....

CarlG · #9 04-28-2009, 11:11 PM

Perhaps some more information would be helpful. When was the last time the car did run? Was it in storage? If so, how long? Has anything major happened to the car since the last time it ran? Any other information you have would be helpful.

leathermang · #10 04-28-2009, 11:18 PM

CarlG, WHAT are you trying to do ? Make this into a logical investigation of the physics and mechanics involved instead of an exercise in creative writing ? What does this place look like to you.. some kind of DIY Diesel Forum ?

JonL · #11 04-28-2009, 11:54 PM

Quote:

Originally Posted by leathermang

That is a very imaginative description... but I just can't go with it.
The idea that the slowing down of the exhaust gas due to the cranking speed being slow making a vacuum effect as far back as the end of the exhaust pipe just does not fit my intuition of the physics.
First, those two , or even just one, mufflers would sure mix up the pulses.... a dampening effect which would preclude an actual backwards movement being available at the end of the exhaust pipe... I can see a lessening of the exhaust speed...but not a reversal needed to give this effect... remember that other pistons are sending gas into that confined stream also... even if one could do that the combo of several feeding that tube would surely.
Also, this supposed valve opening which occurs after top dead center... ignores the fact that we are dealing with the two strokes of the crank in that bore which have both valves closed ... Compression AND Power.. so that does not fit either..
I agree with the concept of inertia creating a scavenging effect... but given the whole situation I think slow cranking or idle would only take that away.... as compared to going into the other side of the realm... (of vacuum. )
I think your mental concept of the valve overlap comes from gas engines....where a hot cam can cause that kind of effect at certain rpms...
Nice try though....

Sorry you don't like it!
I don't know the particular valve timing events for a MB diesel engine (I should look it up...) but there is most certainly an overlap period. In fact, a diesel could tolerate a much "hotter" cam than a gasoline engine for three reasons:
1. There is no throttle, so the scavenging effect will not be spoiled at light loads... the intake vacuum will not attempt to keep the exhaust in the cylinder.
2. There is no gasoline in the incoming fresh air charge, so there is no possibility of raw fuel going out the exhaust during the overlap ruining efficiency and raising emissions.
3. The exhaust has a lot of excess air (unlike a gasoline engine), so some exhaust dilution of the incoming air will not cause misfiring the way it does in gasoline engines.

V-6 gasoline engines are rarely used as marine engines anymore because they had chronic problems of aspirating sea water in through the exhaust exactly because of the phenomenon I described.

If there is any restriction in the intake, the negative pulses in the exhaust will be amplified.

At cranking speed, the air in the exhaust will be moving at low velocity and under low pressure. It will behave as a nearly incompressible fluid. Any suction generated at an exhaust port will be transmitted throughout the system.

If I can get two helpers tomorrow, I'll try an experiment. One to hold the "stop" lever, one to crank the engine, and I'll hold my hand on the tailpipe.

leathermang · #12 04-29-2009, 02:06 AM

Quote:

Originally Posted by JonL

Sorry you don't like it!
I don't know the particular valve timing events for a MB diesel engine (I should look it up...) but there is most certainly an overlap period. In fact, a diesel could tolerate a much "hotter" cam than a gasoline engine for three reasons:
1. There is no throttle, so the scavenging effect will not be spoiled at light loads... the intake vacuum will not attempt to keep the exhaust in the cylinder.
2. There is no gasoline in the incoming fresh air charge, so there is no possibility of raw fuel going out the exhaust during the overlap ruining efficiency and raising emissions.
3. The exhaust has a lot of excess air (unlike a gasoline engine), so some exhaust dilution of the incoming air will not cause misfiring the way it does in gasoline engines.

V-6 gasoline engines are rarely used as marine engines anymore because they had chronic problems of aspirating sea water in through the exhaust exactly because of the phenomenon I described.

If there is any restriction in the intake, the negative pulses in the exhaust will be amplified.

At cranking speed, the air in the exhaust will be moving at low velocity and under low pressure. It will behave as a nearly incompressible fluid. Any suction generated at an exhaust port will be transmitted throughout the system.

If I can get two helpers tomorrow, I'll try an experiment. One to hold the "stop" lever, one to crank the engine, and I'll hold my hand on the tailpipe.

OK, You have just proven that your concepts on this subject are taken from a gasoline four stroke engine. You do not need to know the valve timing for a MB diesel engine because all four stroke engines have both valves closed on the compression and power stroke. And those two strokes happen one after the other.
Concerning this " intake vacuum ".... did you ever wonder why our diesels have a vacuum pump instead of taking vacuum from the engine like a gas engine ? Think about that...
You are correct that the concept of a hot cam in a gas engine involves more overlap of the opening of the valves... but that is not possible with an engine which is depending on the compression of the air to ignite the mixture...
Negative pulses in an exhaust may not have the ability to make it past a good damper like a muffler....
So a V4 and V8 are just fine for a marine engine but a V6 inherently sucks water back up the exhaust ? That is your theory and you are going to stand by it ?

GregoryV022 · #13 04-29-2009, 02:17 AM

leathermag, chill, hes just trying to help you.

his explanation does make sense. the over lap in the valve timing after the failed combustion stroke would suck exhausted air back into the cylinder. and suction is suction and will go through both mufflers. this is because the air all has to move at the same time. also if a lower than normal amount of air was being sucked in on intake, it would amplify these suction pulses.

im not saying i would stand by this diagnosis, but it would work it the right conditions. maybe a very small chance, but hey you may want to think about buying lottery tickets.

also, you have made it really hard to diagnose your problem because we have very limited information. if you would listen to carlG, he asked for more info and i would do the same.

JonL · #14 04-29-2009, 09:08 AM

Leathermang,
First, if you read my original post on this, I said that a LITTLE suction is PROBABLY normal. I said this because a) I believe that to be true, and b) the OP should focus on normal troubleshooting procedures and not get caught up in figuring out why his exhaust is sucking BETWEEN EXHAUST PULSES when he covers it with his hand.

That said, I DO stand behind everything I've written. I am running late today (going to work on twin Detroit 2-strokes in a Hatteras), so I'll write more on this later.

leathermang · #15 04-29-2009, 11:10 AM

If we were dealing with individual exhausts like a one cylinder motorcycle it would be easy to go with that description ...but my car only has one exhaust at the rear and all four cylinders feed into that... and I think I have two mufflers... which by definition are dampers with regard to this type stuff....
Why do yall reject out of hand the idea of a bad, either burned or dropped, exhaust valve ? That would produce at least twice the sucking backwards of the exhaust stream flow that you are trying to describe as the physics of this situation.