Just recently acquired a 1992 300D OM602.962. It has 208k miles on the odometer and a lot of engine performance issues. I am a diesel technician, but mechanical diesel engines and vacuum operated engine management is well before my time. I spend my work day diagnosing and repairing Ford Powerstroke and Cummins engines using a laptop. I understand the concept of this platform, but I have no idea where to begin sorting. So, Ill lay out the problems, what I have done or found so far, and hopefully I can be pointed in the right direction.

I have rough idle, excessive black smoke, and what I believe to be loss of power. I don't have huge power expectations for this engine, but it doesn't seem to be doing what it's supposed to.

• I pulled back the plastic cover behind the battery tray, and the overload relay appears to be original. I ordered a replacement.
• The EGR tube, valve, and port were completely clogged and the valve was frozen shut. I cleaned them out and freed up the valve. The valve seems to take excessive vacuum to open, but I do not have any value for how much it should take. In any event, the amount of vacuum it's receiving is not enough to open it at idle when it should be open. The soot that clogged everything up is very wet and oily.
• Blow-by. This engine, to me, has excessive blow-by. However, I work on very modern clean diesels so what I consider excessive and what may be completely normal for this engine are probably 2 different things.
• Hard shifting. It seems to slam into gear when shifting to D or R. Sometimes it does shift hard, but not always.

I don't see any obvious vacuum leaks. The previous owner was a hobbyist who replaced a lot of vacuum lines and fittings but didn't really seem to understand that the engine wasn't running correctly and so they didn't address that. Given this information, where would be the best place for me to begin diagnosing the engine on this car?

 

Did a bunch of research, and while there may be vacuum leaks I think a lot of the rough idle may be related to worn out injectors. Those appear to be original to the car. I went ahead and ordered 5 new ones.

 

I am a very experienced diesel technician asking for data and information relevant to diagnosing this engine. That way I can use my tools and instrumentation to determine what functions are not working properly. Unless you actually know what the necessary vacuum range is to activate things like the EGR and wastegate, your response isn't very helpful. Right now I have an EGR valve that seems to be taking excessive vacuum to open, but I do not have the information to confirm that. I have vacuum at the valve that is much lower than the vacuum going into the solenoid despite the fact that the solenoid is being fed full voltage by the PCM. Is it the valve failed or is it the solenoid? Data will answer that question. A diagnostic scanner will not. The same applies to the wastegate on the turbocharger. From what I have read, it should be producing ~10PSI at 4000RPM but I would like to confirm that information before I just throw a turbocharger at it for no reason.

As for the injectors, everything is pointing to them leaking or firing early. I flushed the pump and injectors out thoroughly with little to no change. It had very dirty fuel in it when I got it, and the tank wasn't much better. At 210k miles, if I pull them out to test them they're getting replaced or rebuilt whether they need it or not. It's going to get new glow plugs while I'm in there as well because they're cheap.

 

Is MB XDOS applicable to a model year 1992? According to your sig line it is not...

 

Came back to update this.

I replaced the injectors with Bosch reman units. I did not spray test them before install, as unfortunately we tossed out our test equipment a few years ago. We never see mechanical injection anymore. The pre-ignition I was experiencing before is nearly gone. I located 2 places where I was getting fuel aeration. One at the pre-filter/screen before the pump and one at the joint on the 30 year old OE fuel line coming from the strainer in the tank. I cleaned out the tank completely and replaced the strainer and line with new OE parts. I corrected the issue at the pre-filter. All rubber fuel lines are now new.

I set into the vacuum system. All of the degraded OE rubber vacuum lines are now replaced with much more durable silicone line. This significantly improved shifting, and while I haven't confirmed it yet, also charge air pressure. It drives like a completely different car. Most of the time. I now have an issue where after some driving, the system will cut charge air pressure completely. If I turn the car off and restart it, the problem resolves for a while. I assume this is a fail-safe built into the computer. My guess is that now that the computer is seeing what it's supposed to see, it sees something wrong. Looking around, I see schematics to build a diagnostic tester. One that incorporates resistors, and one that doesn't. I've also not found any information as to which pins are utilized on the DLC for the diesel engine, if that is relevant. If anyone has that information, that would be most helpful.

If anyone knows of a common point of failure causing this condition, that would also be very helpful as well. Thanks.

 

Sticky wastegate, or a problem with the wastegate actuator? I recall a similar problem with GM Diesels of the same era. They used a vacuum valve that operated on a duty cycle. The valve would get sluggish and allow it to overboost, so the ECU cut power to the valve and the wastegate was allowed to remain open until the ignition was cycled off.

 

Maybe.

Now knowing everything is hooked up correctly vacuum wise, I made a down and dirty code retriever out of a TruckLites marker lamp, two male pins, and a stripped piece of wire. Works well.

• 5 - EGR valve vacuum transducer or fault in exhaust gas recirculation control circuit
• 7 - Starter ring gear speed sensor
• 11 - Electronic Idle Speed Control Actuator or EGR valve vacuum transducer or Boost Pressure Cutout Switchover Valve

I ran the car with a vacuum/boost gauge T'd in between the manifold and the ALDA. The gauge reads up to 10PSI boost, and 30" of vacuum. As I suspected was happening, charge air is being limited. At first the car is capable of producing over 10PSI of boost pressure even before 2500RPM. If I punch it hard, it will actually peg the gauge past 10PSI. Again, before it reaches 3000RPM. According to the shop manual, it should produce no less than 10PSI at 4000RPM under load, so this is adequate boost pressure. If not too much. When cruising at 55MPH, it maintains a steady 10" of vacuum on the intake manifold. I assume this is the flap shutting to draw exhaust gas through the EGR valve. I cleaned out the EGR valve again and soaked it more thoroughly and completely cleaned the passages, and now the EGR valve is tested and working according to the shop manual.

However, I am not seeing any vacuum at the valve when the flap is closed. Since there are at least 2 codes for EGR transducer, I believe this may be the culprit. My logic tells me that since code 11 includes the EGR vacuum transducer as well as the Boost Pressure Cutout Switchover Valve, a fault in either of these components would cause the derated power condition. Since this car IS NOT equipped with the switchover valve (1992 Model year eliminated it in favor of a manifold pressure sensor), this narrows it down to the transducer even further. I'm going to look into the shop manual and see if it has a test procedure for the transducer. I see power to it but dont see vacuum so I feel like that's enough but I want to make sure.

If anyone has anything to add, that would be extremely helpful.

EDIT: I was looking at the wrong code list! I have more accurate info now.

 

From what I remember, the engine is about 140 HP rated.
That was adequate at the time, but shy comparing to new generation engines.
The vacuum systems on them changed over the years, but it was a MAZE.
I did own such, that I bought for my son's college and took me some time to restore messed up vacuum system. Benzword W124 section had excellent library of WIS and other help.
If I am not mistaken, MB of the time did have 38-pin plug for troubleshooting?
Anyway, from what I remember it wasn't the vacuum lines, that were main problem, but deteriorating rubber couplings and tees. Still remember the transmission vacuum coupling giving me good headaches.
Other common troubles were HVAC elements and even 20 years ago lot of owners had the system capped off.
Point is that once you loose vacuum from the system, wastegate no longer operates.

 

This car uses the 8 pin diagnostic connector, at least for US Spec. I got codes 5, 7, and 11. 5 is for EGR transducer, 7 is for the crankshaft position sensor, and 11 is for a short circuit in the wiring harness that is for the EGR transducer and the idle control on the rear of the fuel pump. The service manual states that code 7 will always be displayed if codes are retrieved with the engine off, which makes sense.

I checked the entire harness today and couldnt find any areas that were chaffed or damaged. I did find a bulge in the harness near the OVP, but cutting it open revealed nothing amiss. I also checked the connector at the EGR transducer as well as the pins for short to ground and continuity back to the DCU and the OVP and everything seemed alright. The connector at the idle control on the fuel pump was oily, nasty, and cracked. I cleaned it completely and checked it. Nothing amiss other than the crack. In retrospect I did not check the idle control itself for an internal short. It does seem to be working, as idle does change when it is unplugged and plugged back in.

I've got some more diag tomorrow to do and testing to see if cleaning up the plug at the idle control corrected the condition. If it doesnt, Im not going to waste a ton of time going through the wiring harness. Ill just convert it to a pressure mechanical wastegate and call it a day. With the EGR and Wastegate controls removed, the manifold pressure sensor will not be able to limit charge air and fuel just because it doesnt like what it sees.

 

I do/did own all MB diesels available in US since MY 1985. The only exception was 1990's 350, who had bad reviews.
Being active on MB forums in those years, I never hear positive experience about modifications on those engines.
So good luck with your ideas.

 

I plan to leave most of the electronic controls in place. The system will still regulate fueling through the ALDA and idle. Because this model year does not introduce atmospheric pressure into the ALDA to cut fuel, the ALDA will still regulate fuel based on manifold pressure as it is designed to do. The DCU will still regulate the idle speed. The only systems eliminated will be emissions control/EGR and it's ability to limit power when it sees a fault in the emissions system.

That's only if I discover that it can't be economically repaired because parts are NLA. If I can fix it correctly, I will.

 

When parts might be NLA., those cars are still daily drivers in California.
I used to live i SF Bay Area and could find any parts I wanted. The only new parts I had to buy was brake hose, as aged are not good for reuse.
So try to find fellow owner in CA and see if he can help you.

 

Ghost threads with no resolution shared are a huge pet peeve of mine, so I'm going to wrap this one up.

Did a little more poking around today under the hood, and put my hands on a few more things. I went through the entire harness end to end.
I did not find anything weird or rubbed through. I had a code 11 for a short on a specific circuit that is fed by the OVP. I rolled the dice and ordered a Uro OVP off of Amazon last night for delivery this morning and threw it in. I drove the car about 200 miles today and never lost boost pressure.

I believe the short is in the OEM OVP relay, but doesn't present itself all of the time or I disturbed the harness in some way to stop whatever shorting was taking place. Either way, the issue is resolved and the car idles much better as well. I'm not going to dig any further into this. Sometimes things are best left alone on old machines. Now to move on to addressing more of the deferred maintenance like flushing out the ancient coolant, replacing the thermostat and housing gasket that are starting to leak, and servicing the transmission. Those flex discs are looking a little questionable, too.