ML320 please help

[LostKiwi]

When you block the pipe to the servo is the engine speed still the same?

 

[benzmaester]

Hi LK, when I unblock the pipe, the engine stumbles and then picks up on bad idle. When I block the pipe, the engine note returns to normal.

 

[Arudge]

In post no. 7 you stated that the idle speed was 800 rpm, and now your saying it's 550 rpm, which is more like it. Does this mean the idle changes? sometimes its good and other times its high?

 

[Arudge]

I need a bit of help here because i'm no entirely familiar with your engine.

We could do with testing the vacuum pump, is yours as the video below? ( I realise it's in a CLK)

What i suggest is removing the pump, putting power to it and again place your finger over the vacuum port that you have pulled off the hose. We should be looking for the same sort of vacuum that you have already felt.

 

[Arudge]

If it's any help, my vacuum leak wasn't consistent. Sometimes everything was ok, then it wasn't.

 

[benzmaester]

Hi Andy, I will check this soon as I can as my Mrs has decided to write off my other car and now I have to use the ml for work. I am on nights for next 3 days so will get down to business Tuesday. Thanks

 

[Arudge]

If you could post an image of your vacuum pump, hopefully at the front of the engine, just so we're talking about the same item.
If your so inclined, when your ready, you could take the front cover off the impeller housing and take some images of the impeller. Do not remove the impeller, if it's the same as mine it's made of graphite, very delicate.

 

[Arudge]

As you need the car, you could run without that pump in place, but you will need to stop off the hose that the pump is connected to.

 

[Uncle Benz]

I need a bit of help here because i'm no entirely familiar with your engine.

We could do with testing the vacuum pump, is yours as the video below? ( I realise it's in a CLK)

What i suggest is removing the pump, putting power to it and again place your finger over the vacuum port that you have pulled off the hose. We should be looking for the same sort of vacuum that you have already felt.

We are getting a bit confused here. The pump in that video is secondary air injection, not a vacuum pump. It pumps air into the exhaust to help prevent poisoning the cat. It won't be giving the kind of faults we are looking at here. A range of possibles exist, from water in the fuel rail, bad plug leads or blocked catalytic converters, which are very common in my experience on this model. A breakdown of which cylinders have recorded misfires would be very useful.

 

[Arudge]

We are getting a bit confused here.

Yes. When I googled vacuum pump I got a mechanical pump which I presumed was attached to the block. When I found the video it clearly showed an electric pump. Hmmmm.

Now, mine has an electric vacuum pump but it doesn't look like that. Mine sits under the inlet manifold and is piped directly into the Inlet manifold.
My air injection for the exhaust is naturally drawn from a spigot on plastic elbow on top of the throttle body, there is no pump, maybe the larger engine requires more air and needs to be pumped.

Thank you for the clarification Uncle Benz, I'm still confident that between us all we'll sort this fella out.

So the question Is, where is the vacuum pump?

 

[Arudge]

from water in the fuel rail, bad plug leads or blocked catalytic converters,

We can discount the water in the fuel rail because the fault is identicle on LPG, which has it's own fuel delivery system. Bad plug leads would show up far worse on LPG because LPG is a far more resistive atmosphere. Checking the cat for back pressure is simple enough, and you would have rather hoped that either the indie or the LPG guy would have tested that.

I've had chronic multiple mis fire and it was vacuum related. Sometimes it ran for weeks with little issue but out the blue the engine would misfire and shut down. Leave it for 10 mins and it would be fine.

A smashed vacuum pump impeller and debris getting trapped in the one way valve.

Stripped, cleaned, new pump and it's been perfect ever since.

 

[Arudge]

This was my story.


http://forums.mercedesclub.org.uk/index.php?threads/clk-200-kompressor-w208-vacuum-leak.162619/

 

[Uncle Benz]

The M112 is naturally aspirated, and has no vacuum pump. The brake servo is supplied with engine vacuum directly from the intake manifold. Sure, a vacuum leak could lead to some poor running, but I'd associate this mainly from idle where the throttle is nearly closed, and manifold vacuum is high. Under heavy load conditions the throttle is open, therefore manifold vacuum low. In this instance the fault occurs under load. Manifold vacuum will be low, so the effect of any vacuum leak will be negligible. I don't believe a vacuum leak is responsible for this running fault.

My money is still on a partially blocked or restricted catalyst.

 

[benzmaester]

The M112 is naturally aspirated, and has no vacuum pump. The brake servo is supplied with engine vacuum directly from the intake manifold. Sure, a vacuum leak could lead to some poor running, but I'd associate this mainly from idle where the throttle is nearly closed, and manifold vacuum is high. Under heavy load conditions the throttle is open, therefore manifold vacuum low. In this instance the fault occurs under load. Manifold vacuum will be low, so the effect of any vacuum leak will be negligible. I don't believe a vacuum leak is responsible for this running fault.

My money is still on a partially blocked or restricted catalyst.

Hi Uncle Benz,
Thanks for your reply I think and hope that you are right. I have just bought the cats and will replace them all tomorrow. I will let you know. Do I have to reprogramme the ecu after this or will it adjust to the new cats?
Thanks

 

[Uncle Benz]

No, it will be fine.

 

[benzmaester]

Ok thanks

 

[Arudge]

I wish you well and i'm keen to know if this resolves your issue. Uncle Benz has already corrected me regards the vacuum pump although i'm at a loss as to why mine uses one, as my brake servo also draws it's vacuum from the inlet manifold.

However...

I have to disagree about his understanding about the vacuum. The vacuum is, and has to be constant across the whole rev range. The only exception can be the overrun, where vacuum would further increase.
Back in the days of carbs you had to maintain a constant vacuum across the main jet in order to draw the fuel, if the vacuum falls away the fuel would stop flowing. This was the principle behind the SU C.V. (constant velocity) carburetor. A constant velocity of air across the bridge of the venturi. The only difference with fuel injection is the method of fuel delivery, the rest is the same.

But i have learned that, IF there is a drop of vacuum as the revs rise, it's a sure sign of a blocked exhaust or catalyst.


Always something to learn.

 

[LostKiwi]

Yours uses one because it has a supercharger so may have extended time with positive pressure in the inlet manifold. If someone left foot brakes several times the stored vacuum in the servo is quickly consumed do an alternative source of vacuum is required.

On a normally aspirated engine manifold vacuum is more likely to be present (gear changes, momentary lift etc) so a vacuum pump isn't required.

Vacuum in a normally aspirated engine can vary between 0 (full throttle) and 25 ins of mercury (overrun).

 

[Arudge]

Yours uses one because it has a supercharger so may have extended time with positive pressure in the inlet manifold. If someone left foot brakes several times the stored vacuum in the servo is quickly consumed do an alternative source of vacuum is required.

I realised it was to do with the supercharger but couldn't wrap my mind around it. If you left foot the brake whilst still under boost then the vacuum pump must be strong enough to remove the positive pressure and replace it with vacuum? I'll check the plumbing but i'm pretty sure the vacuum pump plumbs into the inlet manifold.

Vacuum in a normally aspirated engine can vary between 0 (full throttle) and 25 ins of mercury (overrun)

When i tested mine i had 34 ins at idle and it didn't change as i revved to 2500 rpm. I realise this was a static test and we're not under load, therefor not drawing anywhere near as much air, so i'll concede defeat for now.

Thinking about it, didn't BMW use a vacuum device as an eco driving device? Mmmm.

When my car is back to full strength i'll rig the vacuum gauge up and go for a drive, i need to prove this to myself more so than anybody else.

 

[LostKiwi]

When i tested mine i had 34 ins at idle

Don't think that can be right. Atmospheric pressure is 14.7psi which is roughly 30ins mercury so a perfect vacuum will be 30ins below atmospheric.