Injector knock, is it something I’m doing?? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 06-08-2024, 10:24 AM

I have a 1991 300D and 350SD.

The 300D has injector knock on injector/cylinder #1.

The injectors were refurbished. No change.

I swapped an injector from my quiet and smooth 350SD, and the 300D worked fine. So then I bought a “good used” injector. It knocked.

So then I took a different injector from my 350SD, installed it, and it too knocked. It doesn’t knock on my 350SD, but it is knocking in the 300D.

I always torque to 70nm. I bleed air. It’s plain to see when I crack the injector feed line that the knocking stops.

First I thought it was bad injectors. There could be two bad ones.

But now I have to wonder if somehow it’s me. It’s not like I haven’t swapped a lot of 60x and 61x injectors before. I have.

But could there be something I’m doing, or something with the hard line or return lines, that maybe I’m missing?

I do notice in the clear viton that there is some air in the return line. But it was also there with the quiet 603 injector I swapped….

Thanks for any guidance.

Diseasel300 · #2 06-08-2024, 11:20 AM

Given that you've tried knockless injectors and it still knocks, it's more than likely a poorly seated crush washer on the delivery valve for that cylinder, or a worn out delivery valve that no longer seals properly on that cylinder. Air will be present in the return line until the car is driven long enough to burp it out. The return lines are "leak lines" and very little fuel flows through them, it takes a while to get the air out when you've had them off.

It's also worth noting that brand new or refurbished injectors can knock or nail for up to 1000 miles after installation as they break in.

JHZR2 · #3 06-08-2024, 12:29 PM

Quote:

Originally Posted by Diseasel300

Given that you've tried knockless injectors and it still knocks, it's more than likely a poorly seated crush washer on the delivery valve for that cylinder, or a worn out delivery valve that no longer seals properly on that cylinder. Air will be present in the return line until the car is driven long enough to burp it out. The return lines are "leak lines" and very little fuel flows through them, it takes a while to get the air out when you've had them off.

It's also worth noting that brand new or refurbished injectors can knock or nail for up to 1000 miles after installation as they break in.

One knock less injector from my 350SD was perfect.

https://youtu.be/KNyLq5EADHc?si=LKmkbpE-WaFq0KYM

The “good used” and another injector subsequently knocked.

The one that didn’t knock was immediately silent. Even with the air still in the return line as can be seen in the video.

The refurbished ones were in for thousands of miles. There is no settling in or silencing.

Certainly a DV or something could be involved, but the noise is at the injector, not the IP. I can tell that definitely.

That’s why I’m kind of curious if it’s something else I’m doing when I install…

Diseasel300 · #4 06-08-2024, 08:21 PM

Quote:

Originally Posted by JHZR2

Certainly a DV or something could be involved, but the noise is at the injector, not the IP. I can tell that definitely.

That’s why I’m kind of curious if it’s something else I’m doing when I install…

The DV's are there to do more than just act as a check valve. Their design is also supposed to displace some fluid in the delivery pipe to create a sharp and abrupt end to injection when they close.

If the DV leaks, has a worn seat, or the copper crush washer is weeping by, it will cause a timing problem in that cylinder (late injection) and can cause a dribble at the end of the injection event, causing a knock or nailing. The noise is always at the injector end, not at the IP itself.

Having been through DV issues with my first 300SDL, I'd be looking at at least redoing the crush washers and O-rings to see if it helps before diving deeper. Otherwise, the noise is more or less harmless, just annoying.

My '85 524TD has very loud injector knock when cold on Cylinder 4 with the lift sensor injector. Nothing I have done will silence it, it's just the nature of the beast with this particular engine. Once it warms up (maybe a minute or two of running), it's as quiet as the rest. Whatever, car runs fine and does 31+mpg when scooting 85+ on the highway, so I'm not losing sleep over it.

JHZR2 · #5 06-08-2024, 09:58 PM

Quote:

Originally Posted by Diseasel300

The DV's are there to do more than just act as a check valve. Their design is also supposed to displace some fluid in the delivery pipe to create a sharp and abrupt end to injection when they close.

If the DV leaks, has a worn seat, or the copper crush washer is weeping by, it will cause a timing problem in that cylinder (late injection) and can cause a dribble at the end of the injection event, causing a knock or nailing. The noise is always at the injector end, not at the IP itself.

Having been through DV issues with my first 300SDL, I'd be looking at at least redoing the crush washers and O-rings to see if it helps before diving deeper. Otherwise, the noise is more or less harmless, just annoying.

My '85 524TD has very loud injector knock when cold on Cylinder 4 with the lift sensor injector. Nothing I have done will silence it, it's just the nature of the beast with this particular engine. Once it warms up (maybe a minute or two of running), it's as quiet as the rest. Whatever, car runs fine and does 31+mpg when scooting 85+ on the highway, so I'm not losing sleep over it.

Thanks. That’s helpful. And with 23x k miles on the car, the DVs, and an overall IP removal and re-seal is probably prudent. I’ve done all the seals on my 603, so I know how to do the mildly annoying job.

But here’s why I’m still doubting myself. I went and swapped a few injectors back in. I took the injector I used last time when it was smooth and silent, and I got a knock. Then I swapped a few others and had varying levels of success. Amongst them, I then put that same injector that was quiet before, back in, and it was quiet again.

Air, I get… but the car starts perfect and smooth. And I bleed the lines for a bit before revving the engine.

So while I can see that DVs could be a culprit, what I don’t get is why/how the noise differs with different injectors, or the same injector installed multiple times. I guess maybe different injectors could have slightly different pressures that interact with a faulty DV in different ways… but it still seems strange…

Anyway, worth planning to do a DV job regardless, I guess, though it’s a balance between aint broke don’t fix it, and good pm practice, and I’m not sure which it is just yet.

But I’d like to be sound with knowing that I’ve exhausted all other options. When I get eight injectors back from Greazzer, then I guess I can make that call 100%.

Thanks as always for your insight. This board has really slowed down in the last few years, and your posts, and your history of posts, really is some of the best stuff on here…

Frank Reiner · #6 06-08-2024, 11:36 PM

A generous dose of a cetane improver may give some indication of whether there is a mechanical problem with the injection components, or a fuel problem.

Diesel911 · #7 06-09-2024, 12:55 AM

Quote:

Originally Posted by Diseasel300

The DV's are there to do more than just act as a check valve. Their design is also supposed to displace some fluid in the delivery pipe to create a sharp and abrupt end to injection when they close.

If the DV leaks, has a worn seat, or the copper crush washer is weeping by, it will cause a timing problem in that cylinder (late injection) and can cause a dribble at the end of the injection event, causing a knock or nailing. The noise is always at the injector end, not at the IP itself.

Having been through DV issues with my first 300SDL, I'd be looking at at least redoing the crush washers and O-rings to see if it helps before diving deeper. Otherwise, the noise is more or less harmless, just annoying.

My '85 524TD has very loud injector knock when cold on Cylinder 4 with the lift sensor injector. Nothing I have done will silence it, it's just the nature of the beast with this particular engine. Once it warms up (maybe a minute or two of running), it's as quiet as the rest. Whatever, car runs fine and does 31+mpg when scooting 85+ on the highway, so I'm not losing sleep over it.

Yes. I don't know what Mercedes called it but in US fuel injection it is called fuel retraction.

JHZR2 · #8 06-23-2024, 12:08 AM

Ok I’m still at my wits end over this.

Had greazzer look at the original injector, and rebuild an additional one with a matching nozzle.

Installed them both, got noise. I think the replacement was marginally better so I left that.

Kept investigating. I’m thinking more and more that #4 is also contributing to the noise.

Swapped #4, no dice. Swapped another newly refurbished injector from one of my 693 engines. More harshly metallic knocking sound!

There isn’t enough fuel return to move the air out of the lines. I’m not sure if this is normal or not.

I can definitely hear the knock stop on #4 when I loosen the fuel line. Not sure I noticed that when troubleshooting before. I do think #4 knocks more since I’ve started to mess with it.

So I’m at a loss for what it is. If somehow I’m not purging enough air?

hercules · #9 06-23-2024, 01:18 AM

Mercedes had lots of problems with knock, in so much that in 84-85 they moved the
injector 1mm closer to the dispersion ball. Too large of fuel droplets was thought too be
the cause. I would agree with this idea.
When a injector mists but sprays enough the timing will be late and it will be much quieter.
When cleaning the delivery valves I have notice debris around the valve seat holder, I do
feel this does cause problems with seat sealing .

hercules · #10 06-23-2024, 01:28 AM

Do check fuel pressure, low pressure and lower fuel level might possibly allow any air
bubbles to move to one or more cylinders at idle.

JHZR2 · #11 06-23-2024, 07:34 AM

Quote:

Originally Posted by hercules

Do check fuel pressure, low pressure and lower fuel level might possibly allow any air
bubbles to move to one or more cylinders at idle.

One thing I’ve noticed is that the fuel in the return lines never fills them, there’s always some empty space. Not sure that means there is air in the system, but it is noticable.

Diesel911 · #12 06-23-2024, 08:22 AM

I was looking at some pics of the 1991 Mercedes 300D fuel injection pump. After the 3 flange nuts are loosened you can turn the adjustment to change your fuel injection pump timing.

If there is not some sort line already scribed from the block to the fuel injection pump flange (so you can return it back to where it was) you can make one. Simply try advancing your fuel injection pump timing a tiny bit and snug down the flange nut bolts and see what happens.

Fuel quality can also case nailing. I got an amount of bad fuel many years back that did that.

JHZR2 · #13 06-23-2024, 08:48 AM

Quote:

Originally Posted by Diesel911

I was looking at some pics of the 1991 Mercedes 300D fuel injection pump. After the 3 flange nuts are loosened you can turn the adjustment to change your fuel injection pump timing.

If there is not some sort line already scribed from the block to the fuel injection pump flange (so you can return it back to where it was) you can make one. Simply try advancing your fuel injection pump timing a tiny bit and snug down the flange nut bolts and see what happens.

Fuel quality can also case nailing. I got an amount of bad fuel many years back that did that.

It is on my to do list to verify timing. Thing is it could be so many things. What I know is that one time I put in a used injector and it was quiet. But yet when I did it again, it knocked. This is what spurred the question of “is it me?

It has also been thousands of miles with the knock since I owned it. PO said it was a lifter. On certain occasions, or maybe almost always, it has been next to impossible to trace to a specific injector. This why I thought it was lifters. Only after putting in another remanufactured injector have I really found the ability for knocking to effectively be on or off on #4. But with a different, quiet injector. Which again points to something in doing or something else.

IP seems quiet with stethoscope.

JHZR2 · #14 06-23-2024, 09:52 AM

And to follow up, here’s a video of what I’m seeing on injector #4. This is the second “quiet” injector from my 603, which were just refurbished.

When I first put the injector in, it was almost a metallic sound vs just a knock.

But I can crack the injector and hear a noticeable difference. This is consistent across at least four injectors I’ve tried:

https://youtu.be/IGO0fZeGzlQ?si=3lFjWiLK6H7q5PcX

The six 603 injectors I’ll hopefully get installed today. And re-validate their “quietness”.

Diseasel300 · #15 06-23-2024, 10:55 AM

Unless you have serious chain stretch from extreme lack of maintenance, IP timing and base engine timing are the least likely to be your issue here. 60x engines just don't eat chains like the 61x engines do and there's no offset keys to adjust cam timing either. I've serviced 603's with 300-400K on them and the original chain with 1-2˚ of stretch and left them well enough alone.

I would have hoped that my previous reply about the delivery valve leaking would have been heeded, but nobody ever listens to someone with experience, so why am I not surprised? There's a reason I rarely contribute to this forum anymore. The injector isn't the issue, it's delayed or improper combustion. I've dealt with many 60x engines with this noise and DV's generally improve or eliminate the issue. Newly rebuilt injectors shimmed to the correct pop pressure always seem to exacerbate a leaking DV compared to an older worn/weak injector. The older engines with the "vertical" injection prechambers are far noisier and always have a certain degree of a metallic "cackle" at idle. Some degree of knock is to be expected and has to be tolerated.

In this case, you have a combustion problem, likely a leaky DV or worn DV seat. Air in the return lines is normal. You don't normally see it with the black/grey rubber ones and it takes a LONG time for it to work its way out. The leakage through the return/leak ports in a well-lapped injector is very small, so the flow through those lines is minuscule.