Hey folks,

I have a nailing/knocking issues in my 95 W124, the cause of which has been eluding me. Let me start with the problem and then the maintenance and diagnostics I have performed thus far.

Problem: injector in cylinder 3 nails/knocks at idle. Cruising down the freeway (~3000 rpm) it nails constantly (unsure if this is cylinder 3, but I suspect that to be the case). When I'm on throttle/under load (like going uphill) the nailing goes away, but occasionally I will get a one off knock if I do a WOT (wide open throttle) pull.

Diagnostics and maintenance:

Injectors rebuilt by greezer with bosio nozzles.

Compression test in PSI (okay at best)

1- 400
2- 345
3- 350
4- 360
5- 390
6- 400

Fuel lines replaced and fuel thermostat bypassed was bypassed to eliminate air in clear lines.

Delivery valve o-rings and crush washers replaced and retorqued to spec. I did repolish the delivery valve seats as I've read of others doing using a fine lapping compound on just the valve seat.

In tank filter replaced.

Diesel purged.

I switched injector 3 and 4 to see if the at idle nailing was the injector but the nailing stayed on cylinder 3. When I did this, I did notice more carbon on the cylinder 3 crush washer than on cylinder 4, which leads me to believe that cylinder 3 has retarded timing. Is this a correct assumption? If that is the case, could a poorly sealing delivery valve be causing the retarded timing?

The thing that bothers me is if it were poor compression causing this, wouldn't it be happening on cylinder 2 as well? I determined it was cylinder 3 nailing by cracking fuel lines at idle, and when I crack 3 it goes away. No other cylinder does this when I crack the fuel lines.

I do have air getting into the fuel somewhere, my guess is the hard lines, because it lopes at idle for a minute until it gets evacuated and then it idles mostly normally, but the nailing doesn't go away. I tested whether this is a specific line by cracking lines at cold start to purge the air. This works, but it's always a random cylinder. I believe this to be unrelated to the nailing on cylinder 3 but I thought I'd include it just in case.

At this point I'm suspecting the cylinder 3 delivery valve, or the injection pump. Any assistance or direction would be greatly appreciated.

I would like to get back to using this car as a daily driver... Literally the most comfortable car I've ever been in, especially on the Southern California freeways haha.

Likely is the injector can you move #3 to say #1 if the nailing follows you have your answer.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

I switched 3 and 4 and the nailing stayed on the cylinder. Didn't follow the injector.

Could be a delivery valve I think Diseal300 did a write up about it. If you need one I have a few good ones.

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92 e300d2.5t
01 e320
05 cdi
85 chev c10

Not sure of this at all. To me that cylinder sounds to be advanced in timing.
i do not suspect a delivery valve can do this. Speculation, Did you check the visual condition of the pre chamber when you had the injector out? Not that I really suspect it as well.

Now I may get beat up once again. I really do not like that you are hearing this at high RPMS.I suspect that cylinder is getting really hot. If you have a cheap digital meter with a two hunded milli volt scale. Most digital meters do.

I would read each cylinders milli volt reading from the disconected glow plug harness to engine ground. Record each voltage and mention it on site. Depending on the numbers you see. Will decide to move forward or not.

The glow plug element is made of a mixture of dissimilar metals. It acts as a poor uncalibrated thermocoupleThat generates a voltage from the temperature it is subject to. I have used them to indicate the differeance in running temperature of each cylinder in comparison to each other.

Using this method injection pump elements out of calibration can be calibrated back to a functional state. There is no risk involved if instructions are followed. As before any correction is started. You have a refferance point recorded to return to.

Let the engine warm up to normal operating temperature before taking the readings. I only get into this once again as you had the injectors rebuilt and calibrated. Plus you had the same problem before having that done. So a faulty injector is not the problem.

It is well known delivery valves can create issues. Without putting a lot of thought into it. A faulty one to me is more likely to delay timing than advance it. Yet there are strange effects in this world that do escape me from time to time. It to me is a simple dynamic tool to help find what is going on. In many cases it can also be used to correct the problem. It is not a new approach. I was totally unaware that it had been done over the years by others, Plus even had a patent registeresd by the actual manufacturer of the system we can use it on. So I did it the hard way at first. I suspect you may have a number three injection pump element out of calibration. Before going there we have to prove that cylinder is running hotter. A higher milli volt reading than the other cylinders will prove it is.

You have to follow every step though. I watched a few people go off on a tangent of their own when doing this. There is no normal. Your first set of reading should be around 10 milli volts. I expect the number three cylinder to be a higher voltage than whatever the others are. It is called what it is. Milli volt testing. I actually started ouit with the intent to design a good primary injection pump timing system. Only to prove with others involved it was impossible.

I cannot remember that far back. Yet something led me to figure out it is well suited to what I am describing now. The trials back them were painful as simple as this is. I for example initially read from the ground post of the battery. A real complex not to do thing. Ground for the meter must use the engine block. You can still buy a digital meter if you do not own one or can borrow one for around ten dollars.

If you decide to do this post it so I can check your thread for future posts. If not is fine as well. I have to have a look at your first results. To decide the next step. Chances are very good you can correct this problem.

Thank you for the offer dieselbenz1! I may just take you up on that because I know they are NLA from the dealership.

I did have concerns that my injection pump was out on a cylinder, but from what I've read they are usually good for the life of the engine. I will read the glow plug voltage as you described and see if the cylinder is running hotter. That would be a good indication of advance or retard, and painfully simply to check. Thanks for passing on the knowledge. If it's a lower voltage than the others that might indicate a delivery valve and higher than the others it would indicate pump timing. I did look into the prechamber and saw the bead and the glow plug tip, looked to be normal.

A general comment but low compression on 2 accent cylinders can indicate a head gasket leak between the 2 cylinders.

Removing the injectors on the 2 cylinders and apply compressed air into one of the cylinders (make sure both valves are closed on that cylinder). When you do that if you hear air pressure hissing into the other cylinder you have a head gasket leak.

I don't know if faulty hydraulic valve lifter/ tappets can cause a similar symptom of low cylinder compression.

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84 300D, 82 Volvo 244Gl Diesel

Good you have some ideal of the concept. I assume that a lower voltage would indicate retarded timing by the way. Odd but all the early complains where always advance element issues. Retarded timing of an element would be silent and not attract any attention in my mind. I agree that superficially there would be more carbon present with a retarded element. As the burn is not as clean or hot.

I also agree that the injection pumps are a very durable design. When I first joined the site a substantial number of injection pumps where being changed or rebuilt with no change.

I thought I might contribute by attempting to describe the fuel suply system. As that is where the problem really usually was.

To me it was and still remains a function of these cars to develop some familiarity with. It makes them so much easier to repair. Plus many really need a tune up they may never have had.

There is still huge resistance to this. It is both cheap and pretty easy to do So I have a hard time with it not being mainstream.

It is really overall not that unusual to have a complaint like yours. One thing I have always wondered about is why a pump element drifting out of calibration occurs. In becoming advanced in comparison to the others. Metal sealing washers do compress with age is about as far as I got. I had to deal more with the effect over the actual cause. Copper never really stops compressing under pressure. It is of some concern in some applications.

What was done to correct this problem is perhaps not as accurate as a recalibration on a test stand. Yet for all practical purposes the timing of one element is not ultra critical on these engines. If a cylinder is running at the same temperature it is reasonable to assume it's contribution to the power balance is decent.

It probably does have a compromised head gasket. The previous owner never replaced the radiator cap and it was not holding pressure when I got it... Their solution? A lower temp thermostat 🤦 brilliant. I would love to determine if the headgasket is bad, but it's so much work to get there. To get compression numbers I had to pull the valve cover so I could fit my compression tester in there. I fear removing the glow plugs as it will likely lead to me having to pull the head, I can't afford to send the head to the machine shop anyway so it'll just have to do. I'm saving money for a motor swap as it is, I just want to be able to daily the car until I get everything on hand. That way I am not playing a game of spend 3 years with the car in pieces.

Afaik, the spill port cut off is set by the height of the rollers under the elements. I could see how poor oil change intervals could lead to the rollers and or shims becoming worn over time. But, I would think that would further retard the timing, so not really sure. I do wonder if I could just set one injector at a higher pop pressure to bandaid it... That would be easier than pulling the injection pump lol.

I held back on why perhaps this engine was overheating.Just less so with a lower temperature thermostat.

The radiator cap is off or loose. As even with the reduced temperature before the new thermostat opens. The third cylinder is possibly generating a boiling of the coolant in the head area. This inhibits coolant cirulation so the engine overheats.

With a lower temoperature thermostat just less so but that air would increase the pressure in the coolant system as well when generated. Same as a cracked head would basically. So it is going to leave one way or anotther. An open radiator cover also lowers the boiling point of the coolant.

I do not blame you for thinking you might need a new engine. I personally think there is a fair possibility that getting that advanced cylinder fuel feed fixed may cover your other concern. If you have a temperature reader. You might find also an increase in the exhaust manifold area that services that cylinder.

If I am right and the engine has not been damaged yet. It is time to solve the problem. A really hot spot in the head will not remain passive forever. Again hearing that cylinder at speed as well means it probably is the worse case I have ever seen on site so far. Of this type of issue.

Could you melt a hole in a piston eventually is also possible. I suspect you are generating more effects from the really hot cylinder than the cooling system can really handle.

You posted enough information to make me have a hard mental look at your problem I am just posting this information much earlier than I wanted to as I felt it was a shame if the earlier information was not motovating you enough,

Just to do some easy cheap checking. Again there is a good chance your secondary problem is concerning you enough. Perhaps you have not considered it may also be the source of that one as well.

Anyways either by my way or another I feel you really have to get going on this problem. Many problems can wait fairly safely. This one sounds not to be one of them.

So, it's been windy lately and I didn't want to run the engine with no manifold until it wasn't... Anyway, I ran the engine up to temp and measured the voltages. Cylinder 2, which is the lowest compression produced a voltage of about 9.5mV, cylinder three was about 7.3mV, and cylinder four was about 7.9mV. So, that tells me those cylinders are running colder than the others which would agree with the carbon buildup on the injector heat shield. I ordered more crush washers and O-rings so I could take apart the delivery valves for inspection. It's possible that I didn't properly set the crush washer, or lap the delivery valve. In my mind, that could cause the timing retard as the delivery valve is meant to keep line pressure up between injector events.

My one concern is, I don't know the quality of the glow plugs. I've never replaced them and have no history.

Can you post the other cylinders milli volt readings as well? Very strange that number three and four cylinders voltages seem the same. Sometimes you hae to swap glow plugs to verify they are producing the same voltage as each other.

Already something strange is in play. A two milli volt difference is massive. Could be old plugs that are outputting erratic voltages. Usually you just swap two glow plugs to check this. If you are willing..

I would like to see see two total sets of glow plug voltage readings posted. All voltages recorded with engine just after starting cold. And done again with engine at normal operating temperature. I have never seen a two milli volt spread before. Plus identical on two cylinders beside each other. I suspect the cylinder voltages on the cylinders not posted will be about 9 volts. Although in your case you have to know.

I think already those two cylinders are running colder than than the others, But not by retarded timing. Stay with the program. I never go for a problem before establishing what it is. It is so early but to have some form of pre ignition going on. Yet no temperture indication of too hot. Yes cylinder four does seem to light off just before in the 606 diesel engines firing order. So we are probably chasing a head gasket issue.

Identical on two side by side cylinders. To me does indicate the possibility of a head gasket breech. Between those two cylinders. They are both running weak. Anyways The fulll milli volts tests are needed.

There is no normal base line reading on the glow plugs. They are just cylinder comparison devices. Yet the 9.3 mv volts you found sounds about normal. It is better to stay with some testing at this time. Than guessing to narrow things down.

Basically some fuel escapes from the fourth cylinder and pre ignites in the third cylinder on compression before the third injects fuel. You add more fuel with the third cylinders injection. It gets noisy. Why it still runs cooler. Is there is not much oxygen present to get a strong burn. So you have knocking and a low temperature. I suspect this is where we are going.

I design tests on the fly. Lets see what is going on first with all the voltages. Luckily it should prove easy to establish if fuel is escaping from the fourth cylinder to the third. I think if the third cylinder fires after the fourth in the firing order. Was a substantial clue. The closer the better. Yes the fourth cylinder lights off just before the third on these engines. In the firing order it appears.

I am really old now and want people to consider milli volts as a useful tool before I depart. It is the only dynamic troubleshooting tool we have for these old engines. At least I do not see as many people making disparaging remarks about it anymore. Follow common sense and the rules it can do so many things well.

It was also mentioned by posters who did this job that it may take a few hundred miles after doing the job for the engine to quiet down and run smoothly. Good luck!!


OM606 engine (W210 E300D/TD) delivery valve seals

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96 E300d

You have come along very fast now. I do not want to see any confusion. Diesel early though that it could be the head gasket is right. So was yours as the probable cause of the overheating.

The head gasket has hopefully aged out. I suspect that engine might have been really overheated some time. So that head has to be checked really carefully once off for warpage. Allowance for material removal will be small. If any. There may be a thicker head gasket available for the 606 engine.

I do not know where you live. You may need a good automotive machine shop before this is over. Certain shops can straighten heads a lot before minimal machining.

Application of air to one cylinder to see if is present in another. May or may not be a reliable way to check this. As the compression remains not too bad. It can be checked out dynamically though. Dynamic tests are far superior to static tests in many cases.

The air ingress into the system is not really involved. A separate issue for later. You also know it is not the injection pump. There are a variety of ways to check for inter cylinder fuel leakage. I will try to think of the safest one that is also reliable. You could move on that right away.

Other than I want to see all the milli volt readings. Just to make certain there is not a third problem lurking. I do not think there is but the check is so easy. I do not like suprises.