Bosch VEP: 3.5-Cylinder Idle, No Electronics

Dear diesel enthusiasts,
For a few years now, I've been enjoying reading and learning a lot in this forum. My name is Florian (55), and I'm an occasional mechanic with a focus on engines, starting with my first Rex. Now, I'd like to troubleshoot a problem again.
It features a 2.5-liter Mitsubishi turbocharged direct injection diesel engine, designated as the 4d56 by the manufacturer. In my case, it's a version with a turbocharger and a purely mechanically-pneumatically controlled injection pump. It's a Zexel distributor injection pump, with a completely identical design to a corresponding Bosch unit.
The compression is approximately 23 bar in all four cylinders, the valve clearances are adjusted, and the valve timing has been checked. The engine runs smoothly, has full power/torque, consumes as little fuel as it should, and does not consume any oil between the prescribed intervals (7500 km!). No water is being lost either.
However, once it's properly warm (in summer or after a highway drive), it runs rough at idle, almost like it's only firing on three and a half cylinders. The cooling system is fine; it never overheats. If I adjust the idle speed with the manual throttle cable by only about 50 RPM, it runs smoothly again, firing on all four cylinders.
I proactively replaced the fuel injectors – no change. Before the inspection, I slightly reduced the fuel flow, and later I advanced the fuel injection start by 2.5 degrees of the crankshaft. This improved performance, and reduced fuel consumption and soot. However, the behavior at idle has NOT changed at all. Does anyone have any ideas about this bizarre behavior?
Best regards from Munich.
from Florian

Hello Florian,

Could you tell us a little more about your vehicle?

"Helpful information would include the type, serial numbers, any error codes, the model and year of manufacture, or even the VIN."

I'd like to help you, but since the 4D56 engine has been produced since at least 1993, I'm not even sure which emission standard we're talking about.

Based on the throttle cable you have, I'd guess the manufacturing year is around 1996.




Best regards,

Michael.

Dear Michael,

Sure! This is a Mitsubishi L 300 4WD, manufactured in 1993, with 87 horsepower and 240 Nm of torque (VIN: 7107/ TSn 440). The VIN: JMBLNP25WNA000XXX. The "P" indicates the model series, "25" refers to the diesel engine, and "W" signifies the configuration as a minibus with a short wheelbase. I've replaced the last few digits of the VIN with "X" for privacy.

It doesn't display any error codes. You can completely remove the electronics if you take out the car radio. So, not quite, because there's still the glow plug control unit with some capacitors and resistors inside.

The vehicle's emission standard was originally Euro 1, but it was upgraded to Euro 2 thanks to the addition of an Oxikat catalytic converter. However, this converter was simply attached to the exhaust pipe without any modifications to the fuel injection system or turbocharger. Regardless, the catalytic converter did not resolve the described issue, so the problem likely does not stem from excessive back pressure.

Best regards, sincerely.
Florian.

Hello.

Quote:

I proactively replaced the injector valves - without any change.

What is the opening pressure? What are the target and actual values? Have all nozzle holder combinations been compared?
Is the ESP system purely mechanical?

Best regards, Rainer.

Hi,
The VEP (likely referring to a specific type of engine) has a centrifugal governor with a lever system and springs. It's possible that the mechanics are somewhat worn out, and no longer provide clean regulation at the mechanical idle point. The engine has likely accumulated a significant number of kilometers.
As a simple solution, I would adjust the idle speed slightly differently. It might also help to carefully adjust the residual amount screw. These kinds of things have been repeatedly discussed in the T4 forums.
hg
Herbert.

Dear Rainer,

With the new injectors installed, the original factory condition has been restored. The old injectors had already degraded after 260,000 kilometers, with the pressure being about 10% lower than normal. However, the spray pattern was still good. They could have been used for a while longer.

And yes, in the entire car, only the glow plug control unit and the tachometer are electronic. There are no electronic control or regulation components related to the fuel injection system! In the VEP (presumably referring to a specific engine component), the only electrical element is the electromagnetic shut-off valve.

Thank you for your inquiry.
from Florian

Dear Herbert,

The car has now reached approximately 290,000 km (around 179,000 miles) on the engine, and I consider some minor signs of wear to be normal.

After slightly adjusting the start of the injection timing (based on my feeling), I had to make some corrections to the idle setting. Additionally, I was able to increase the fuel quantity slightly without any visible soot. The power increased, while fuel consumption and soot emissions decreased.

I have already significantly adjusted several parameters. However, the phenomenon remains completely unchanged. The pump doesn't have a KSB (flow rate sensor), so it probably also has a fixed spring.

Thank you for your reply.
Best regards,
Florian

Hello,

"As Herbert already suspected, the mechanics are probably a bit sloppy. The fuel injectors were completely worn out after just 250,000 km in every case. I've replaced a lot of them."
If the injectors are already being removed, I would definitely install new ones given the mileage. Possibly, the idle could be improved by increasing the opening pressure of the first stage by 10 bar. This might reduce smoke and, at best, maintain the same performance.

https://shop.dieselschrauber.org/en/duesen_neu.html

I think the difference is obvious.

Best regards, Rainer.

Quote:

...After I adjusted the start time (based on my feeling), I had to make some corrections to the idle speed....

Assuming that the ESP here functions like the earlier mechanical Bosch distributor pumps, the control slide would have been at its limit.
hg
Herbert.

Good evening.

You've already received the key information from Herbert and Rainer.

During the start-up or warm-up phase, the engine is running. Once it's warm, you need to adjust the throttle both up and down. This already indicates the influence of centrifugal force.

If you haven't meticulously adjusted the pump using the measuring tool, and then manually adjusted it significantly, roughly 2.5 degrees retarded, and then tried to adjust the idle speed again... it reminds me of certain apprentice exams where, due to time constraints, the pump couldn't be removed again, so it was simply adjusted retarded, and then an attempt was made to adjust the idle speed. The result rarely achieved a full score.


In my documentation for this engine, it is noted that the initial adjustment of the pump must absolutely be performed with the engine warm.

Have you done this?


Best regards,

Michael.

Good morning!

Dear Michael,

Thank you for the explanation. And, yes, I took the measurement at approximately five degrees Celsius. The regulation about using a warm engine was new to me!

I didn't make the adjustment "to early," in order to address that phenomenon (okay - I had also hoped for that as a side effect ). Rather, it was intended solely to improve performance and fuel consumption. (To my knowledge, Mitsubishi reversed the fuel injection timing by this amount to adjust the nitrogen oxide emissions for Euro 1).

After that, I had to increase the idle speed by about 150 RPM, which I did by adjusting the limiting screw. In the end, I was able to increase the amount [of something].

What's bizarre is that even after these interventions, the strange idling behavior hasn't changed at all. To reiterate: the RPM doesn't fluctuate! However, you can clearly hear that the engine is only running on three and a half cylinders. If I then adjust the speed by just 50 RPM up or down, it immediately stops.

In the original configuration, the phenomenon remains exactly the same.

Let me be clear - I see this as a minor flaw that's easy to live with. The engine otherwise runs "like a dream," which is a real pleasure. However, I would certainly be happy if there were a simple solution...

Sincerely,
from Florian

Hi,
I asked for more information out of curiosity, as the 3.5-cylinder engine is a bit difficult to understand.
How does this manifest itself? Is it possible to associate it with a specific cylinder? Are the "misfires" synchronized with the firing order, i.e., every other revolution? Could it be a resonance with an auxiliary component (e.g., a faulty alternator with a broken freewheel)?
hg
Herbert.

Herbert,

It then runs like a gasoline engine with one of the spark plug wires disconnected. Just as quickly as when you reconnect the wire to a gasoline engine, the diesel engine runs smoothly again as soon as you touch the accelerator pedal. In other words, there is no noticeable transition.

My next step would be to disconnect each fuel injector line individually while the engine is running. This would likely allow me to identify which injector is causing the problem.

What else I was thinking about are the pressure-holding valves in the pump head, located before the injection lines. If one of those doesn't seal completely, could that also have an effect?

I would rule out the aggregate, as besides the water pump and power steering pump (which I don't suspect), there's only the alternator (including the vacuum pump), but that definitely doesn't have a freewheel.

Thank you for helping me think.
Best regards, Florian.

Hi,
"In principle, a faulty pressure equalization valve (located at the connection for the respective line to the ESD system) can also cause idling problems. However, a remote diagnosis is difficult in this case, and I personally don't necessarily see a defect in a single injector or in the pressure equalization valve in its supply line as the cause of the problem, which is limited to a specific engine speed."
Since the engine doesn't have individual cylinder idle control, you're essentially searching in the dark. You need to be able to narrow down the problem to a specific cylinder. Then, by swapping injectors and pressure regulators, you can get closer to identifying the cause. When installing the pressure regulators, a new seal is required for each one.
hg
Herbert.

Hello...

Equal pressure valves would have been my first approach.
I've replaced these quite a few times on 2.4D (AAB) T4 engines, due to that same "jerky" behavior.
The valve seals occasionally wear out on one side and then start to leak. Often, the problem is also due to the valve spring, which can also become old and lose its tension.
For some pump models, the springs are also adjusted using shims (similar to those used under the spring in a direct-acting hydraulic control).

Increasing the idle speed shortens the time between each stroke, which also reduces the time for fuel to flow back, and thus minimizes the "stuttering."

Replacing a single valve set is often counterproductive because it creates a discrepancy with the other used valves, and the idle problem doesn't improve.
Therefore, tighten it 4 times, with the correct torque, and it should be good.
Otherwise, if you live with it, nothing will happen, but it just doesn't sound good at a certain RPM.

Regards,
Mpire

Hello Mpire, and fellow participants,

It seems I can keep this topic open for now and address it later when I have more time. The problem with the bus is the location of the pump under the driver's seat. You have to spend at least 20 minutes just to get to it, and then another 20 minutes before the test drive. (And so on). You only tackle something like that when you're in a good mood and have plenty of free time.

I'll start by checking the pressure relief valves, as it seems like that's a relatively simple task. Thank you very much for this tip!

Please forgive my layman's approach to precision mechanics, as I am a trained humanities scholar. I will let you know as soon as I have any new information.

First of all, thank you for your help.
Best regards,
Florian