Injector knock, is it something I’m doing?? - Page 3 - PeachParts Mercedes-Benz Forum

JHZR2 · #31 09-16-2024, 12:06 AM

Quote:

Originally Posted by rrgrassi

When I had my '91, cylinder #2 had a nailing injector. Swapping injectors did nothing. Turned out, in the IP, the delivery valve was almost a 1/4 inch too high, causing over fueling. I bought a used injector pump, but never got to install it.

That’s interesting. How did thst happen? Was there a way to solve it through ip work?

May be worth swapping a good used IP at some point just for fun, and then I could re-seal mine.

JHZR2 · #32 09-21-2024, 01:43 AM

It gets more interesting.

I’m on a 1000 mile road trip, and I’ve been doing a lot of highway. I’ve been running at
high concentrate cetane additive (Archoil).

After high speed runs it idles beautifully and quiet. No tapping at all.

After idling too long or sitting off for some time, the tapping is back again. Short slow driving keeps it there, these trips don’t get rid of it the noise. Only a hundred miles at high speed does it. It’s not like I haven’t driven like this plenty before but I think the difference is that I’m checking idle very soon after coming down from high speed use.

dieselbenz1 · #33 09-21-2024, 12:14 PM

Maybe you should consider trying a water injection?

JHZR2 · #34 09-21-2024, 12:37 PM

Quote:

Originally Posted by dieselbenz1

Maybe you should consider trying a water injection?

Not sure what that would have to do with anything? What is the purpose? I’m not familiar.

JHZR2 · #35 09-21-2024, 12:44 PM

I tried another thing - foot off the interstate and didn’t turn off the engine. Perfect idle, quiet. Blipped the throttle, no knock.

Let it idle a bit, blipped the throttle. The knock came back around 1100rpm. Then it tapped as always.

The other thing that is odd is that the injectors won’t stay dry. The hard lines are torqued properly, and the return hoses were really hard to get on, but are indeed on.

I had removed the tygon lines because they seemed soft and easy to remove. But the wetness remains hundreds of miles later.

dieselbenz1 · #36 09-21-2024, 05:05 PM

Quote:

Originally Posted by JHZR2

Not sure what that would have to do with anything? What is the purpose? I’m not familiar.

Cleans out the cylinders, rings and any carbon on the valves

JHZR2 · #37 09-21-2024, 06:07 PM

Quote:

Originally Posted by dieselbenz1

Cleans out the cylinders, rings and any carbon on the valves

Gotcha. None of those things are relevant to this issue though.

This is an issue from the fuel system.

Alec300SD · #38 09-21-2024, 07:39 PM

Possible that the knocking is due to less than perfect mating of the sealing surfaces between the #1 injector and the #1 hard line (or a tiny hairline defect in the hard line itself).
The seal integrity could then vary with temperature and/or vibration.

Try swapping in a different hard line, or use a copper conical washer between the #1 injector and the current #1 hard line.

JHZR2 · #39 09-21-2024, 08:35 PM

Quote:

Originally Posted by Alec300SD

Possible that the knocking is due to less than perfect mating of the sealing surfaces between the #1 injector and the #1 hard line (or a tiny hairline defect in the hard line itself).
The seal integrity could then vary with temperature and/or vibration.

Try swapping in a different hard line, or use a copper conical washer between the #1 injector and the current #1 hard line.

I’m pretty sure it’s #4 injector.

That’s the one that has the most noticable change when cracking lines.

You may be on to something, but it’s weird that after hard driving it will be perfect until I give it some pedal while idling, then it will come back again….

That would seem to be a seal, but what does the throttle have to do with it?

Maybe it would just take longer to introduce air or some sort of change with the flow at idle vs unloaded higher rpms??

Alec300SD · #40 09-21-2024, 09:55 PM

The higher cylinder temperature when cruising at higher speeds allows for a more robust combustion event (as does higher cetane fuel).

Once the cylnder temperature comes down, like sitting overnight, the knock returns.

Blipping the throttle introduces more air incursion (think venturi effect).
This dampens the injection event and recreates the knock, even when the cylinder has not yet cooled.

JHZR2 · #41 09-26-2024, 12:46 PM

Quote:

Originally Posted by Alec300SD

The higher cylinder temperature when cruising at higher speeds allows for a more robust combustion event (as does higher cetane fuel).

Once the cylnder temperature comes down, like sitting overnight, the knock returns.

Blipping the throttle introduces more air incursion (think venturi effect).
This dampens the injection event and recreates the knock, even when the cylinder has not yet cooled.

That makes sense.

So the issue is air incursion then.

I had videos as case in point…

Sounding good after a long drive:

https://youtu.be/vu2ALi8Nve0?si=zHXafFU6nQ_zayRQ

And reversion not long after:

https://youtu.be/xrRdcK4W2D0?si=vih26sykG77psdRh

Thus is entirely repeatable.

rrgrassi · #42 09-29-2024, 03:20 PM

Quote:

Originally Posted by JHZR2

That’s interesting. How did that happen? Was there a way to solve it through ip work?

May be worth swapping a good used IP at some point just for fun, and then I could re-seal mine.

Someone had been in the IP before I got the car.

There are little notches in the delivery valve that are supposed to line up with brass fingerlings (lack of a better word) and drop into place. You can see them from the top view of the IP with delivery valve removed.

JHZR2 · #43 09-29-2024, 09:45 PM

Quote:

Originally Posted by rrgrassi

Someone had been in the IP before I got the car.

There are little notches in the delivery valve that are supposed to line up with brass fingerlings (lack of a better word) and drop into place. You can see them from the top view of the IP with delivery valve removed.

I think that’s a different situation than air ingress though… I don’t recall that there is a ridge or other alignment spot to force the valves to align to anything.

JHZR2 · #44 11-09-2025, 10:07 PM

I messed around more with the car today - I had a good used set of injectors. Swapped them in. One wasn’t so good, as I had a lot of white smoke.

But otherwise they sounded marginally better.

https://youtu.be/3_APp2krSA4?si=fXD-051SnKdSFNeZ

But the “full of rocks” type sound, like rocks are bouncing around in there, still occurs at around 900-1400.

I’m tempted to swap IPs, but that’s a pretty big pain…

jdulle · #45 11-13-2025, 04:44 PM

Are you sure it's not the lifters? Mine does a similar thing. After I've run it hard, or on the highway at high rpm it is much better. Then returns with slow speed driving.