Performance issues with the 4.2-liter turbo diesel engine

Hello everyone,

My question doesn't concern a VAG TDI engine, but rather a 4.2-liter inline-six cylinder engine from VM (Italy) - a very large diesel engine manufacturer. More information can be found below. The engine is turbocharged and equipped with a distributor injection pump, and it's actually installed twice in my boat. Since there's a lot of expertise here, I'm posting my question.

'After a period of inactivity or after driving for a while, when I give full throttle at approximately 1200 RPM to get the boat up on plane, both engines initially rev up to about 3000 RPM and the boat lifts out of the water. Then, the right engine loses power, dropping to around 2500 RPM for about three to five seconds before increasing again. At this point, both engines fully lift the boat out of the water, and I reach planing speed with a full load at 3600 RPM.'

'This only happens after a long period of idling or being stationary. Once I'm cruising and then reduce the throttle to idle before accelerating again, there's no loss of power – both engines have full power.'

Ich habe schon die Tankentlüftungen beider Tanks kontrolliert und dem Vorfilter / Wasserabscheider neue Dichtungen gegönnt - das scheint es aber nicht zu sein. Ich bin etwas ratlos - Luft in einem Filter die sich bei starkem Sog durch die Leitungen zieht und kurz Leistung kostet? Es sind aber keine harten Aussetzer vorhanden sondern einfach Kraftmangel. Ladedruckregelung spinnt? Warum dann aber immer nur einmal und dann nicht mehr? Sie ist übrigens rein mechanisch gesteuert. Einspritzzeitpunkt verstellt sich? Warum nur einmal kurz und dann maximale Leistung? I'm completely stuck and hoping to find some tips here that will help me.

Best regards and thank you in advance.

Björn.

Here is information about the engine: http://www.sealinkmarine.com.au/products-page/marine-parts/mercruiser-4-2l-diesel-engine/

It could be air. To test this, you would need to use a piece of transparent fuel hose.

I think there might be occasional issues with the ESP that cause it to malfunction slightly.

Hello and thank you for the reply. I've also been thinking about using a transparent hose, but it's not possible to drive at full throttle while having the engine compartment open 3 meters behind and observing the lines. I need a second person who can work with me to precisely identify the moment when the problem occurs and carefully monitor the hose.

Regarding the ESP (Electronic Stability Program): I can't quite imagine how it works. It seems to be controlled by a Bowden cable and is stuck in full throttle. Initially, it takes fuel as if it were accelerating, then briefly loses power, and then returns to full throttle. Is there anything else within the ESP that might be diesel pressure or intake manifold pressure-controlled, which could temporarily reduce power?

I'm leaning towards gasoline engines emotionally, but I know that gasoline engines can sometimes have hard misfires and jerking. Does this happen with diesel engines too, if they suck in air bubbles or diesel foam?

Greetings and thank you.

Björn.

Quote:

It is controlled by a cable and is permanently set to full throttle.

Well, it's not quite that simple. I don't know the motor or the pump, but "in principle," it works like this:
You select a speed or load using the throttle, which in turn generates a larger fuel injection amount.
The speed control unit (also known as a universal speed regulator) prevents the engine from deviating from the desired speed by reducing the injection amount and adjusting it as the engine approaches the target speed, thereby maintaining a constant speed.
If wear and tear or dirt accumulate, the regulating mechanism can become stuck. Proper adjustment is no longer possible.

It's pure speculation. So, first make sure there are no air bubbles in the system.
Also, check the turbocharger boost pressure as well. Then proceed.

How is the boost pressure measured? How do the fuel injection pumps receive the diesel? Is there an electric feed pump involved?
The good thing is that you have a working part with which all the measured values can be compared!

bdsf2003 wrote:

I've also thought about using a transparent hose, but it's not possible to drive at full throttle while having the engine compartment open and observing the lines 3 meters behind.

Besides the purchase price, there's nothing to prevent installing a 6-meter long transparent hose from the engine to the driver's compartment and back, allowing you to directly observe air bubbles during acceleration (which will naturally reach the ESP system with some delay).

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

'First of all, thank you for the advice! No, there isn't an electric priming pump available. I will actually try testing it with a second person and the engine compartment open - since the boat is 250 km away, I can only do this on the weekend, and the observation needs to be correct during the first start-up, because after that, the error doesn't seem to occur anymore.'

Nochmal meine Frage: Führt Luft im System nicht zu ruckartigen Aussetzern? Oder ist beim Diesel (Schaum?) dann schlicht Leistungsmangel vorhanden? Bzgl des Reglers in der ESP bin ich gerade am recherchieren. Es gibt wohl wirklich diesen Alldrehzahlregler oder Fliehkraftregler in der verbauten Pumpe, evtl. sollte ich den auch mal im Auge haben. Greetings and thank you.

Björn.

bdsf2003 wrote:

My question again: Doesn't air in the system lead to jerky interruptions? Or is there simply a lack of power in diesel engines (due to foam?),

It depends heavily on where and how the air is present. If a large amount of air is suddenly drawn in without diesel fuel, it can indeed cause complete engine failure, including a sudden stall.

Often, a larger air bubble becomes visible when air is drawn in (as opposed to being pushed in), appearing as many small air bubbles in the supply line, which then results in reduced performance or uneven operation.

If your boat is located far away, you'll need two weekends: one to install the clear hoses (including a test run until all the air is removed), and another weekend to re-test whether any air has leaked in again.

PS: If you're already installing hoses, you might as well install some on the return line too. You wouldn't be the first person to have the ESP itself draw in air. (Although, given the delayed onset of the power loss, I'm currently less inclined to believe it's the ESP).

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

Ich habe ja zum Glück keine Eile und das Boot ist ja voll fahrbereit trotz des kleinen Fehlers. Ich lese mich gerade durch allerlei Bootsforen und Mercruiser-Diesel-Erfahrungen. Da die Drehzahl sonst 100% passt und auch ausgekuppelt ohne Fehler exakt auf jede vorgegebene Drehzahl eingeregelt wird soll der Alldrehzahlregler eher unwahrscheinlicher sein als Luft bzw. Schaum im System. Furthermore, it is likely not something that can be easily repaired by the user themselves.

On the boat, the line runs from the tank to the pre-filter/water separator. I replaced the seals on the lid and cap of this unit last weekend – hopefully that has already made a difference. From there, it goes to another filter/separator directly on the engine, where a hand pump is mounted on the flange for priming after filter changes. A leak could also be present here. The line then continues to the pump, and from there, the return line runs back to the tank.
Ich stelle mir das ganze u.U. so vor: Bei langsamer Drehzahl und weniger Dieselfluss bildet sich nach und nach ein grösseres Luftpolster in einem Filter und bleibt oben in diesem stehen. Sobald ich richtig Gas gebe und der Durchfluss massiv verstärkt wird vermischt sich dieses Luftpolster durch den starken Strudel mit dem Diesel und wird angesaugt. Ich habe also erst kurz Leistung durch den Diesel in den Leitungen bis das Diesel / Luftgemisch an der Pumpe ankommt. Due to this mixture, the engine's performance decreases until only diesel fuel is being used, at which point full power is restored.

Die Undichtigkeit wäre zwar kontinuierlich vorhanden, bei voller Last ist der angesaugte Luftanteil aber so gering dass ich ihn nicht merke. Nur bei Standgas sammelt sich diese Luft nach und nach in einem Filter wegen des geringen Durchflusses um dann bei erstem Vollgas wieder mit in die Leitungen gesogen zu werden und den kurzzeitigen Leistungsmangel zu bewirken. Is that conceivable?

Regarding boost pressure: I've also considered installing a Lambda-controlled turbocharger (LDA) on both machines (regardless of the problem). However, this could lead to misinterpretations. Of course, the boost pressure will likely decrease during moments of power loss. But whether this is the CAUSE or merely the CONSEQUENCE of diesel shortage/exhaust backpressure remains unclear.

Actually, only the clear hose will provide absolute certainty, but of course I'm still thinking about it. Thanks again, and best regards.

Björn.