How do I solve a leak at the injector?
 

1991 300d, 258k

I just finished up replacing the delivery valve O-rings and crush washers and I put all the fuel lines back on, when I saw a leak at the #1 injector. There were occasional air bubbles coming up at the base of the injector.

I took the car for a spin and 20 minutes later, a bit of smoke was coming out from under the hood from that area.

I made a video of it but I can't upload the video for some reason. I have it as an MWV file and even at 2 seconds, its 416kb size way exceeds the paltry 72 kb they allow us to upload. How can I upload a video? Here's one from YouTube that looks fairly similar; you see a leak at the base of the injector. https://www.youtube.com/shorts/Wlad5JyXEs4

But it looks like maybe an injector gasket or something needs to be replaced. What part number and/or how hard is it to do this? Seems like it'd be pretty straightfoward.

First; replace those leak back fuel lines ,is leaking.

As mentioned, change the leak back hoses.

This is something that happens that can freak people. You just were bleeding the injectors. Fuel or in fact if there was an oil leak gets down between where the injectors thread in and those bubbles can appear. In part they appear due to the vibration/shock of the injection and burning fuel.

If there is a real fuel leak if you let the car sit and idle say an hour you would see and feel an obvious leak running down wards or even on the floor. If that is not happening normally the bubbling goes away over time as you are diving the car due to the engine heat.

It is extremely rare, but it is possible for the lower injector body to develop a hair line crack. However, when that happens the fuel leak is going to get extremely obvious as you run the vehicle more.

No. It's not the fuel lines
 

No. The fuel lines are not leaking nor are the rubber hoses.

It's at the bottom, where the injector is, as shown on the photo.

I even used a paper towel to absorb the diesel fuel up in that area, but still more diesel and air bubbles come. Smoke came from that one area when the car got hot, but there's no diesel or smoke at any of the other injectors.

Trust us.
You have a liquid pooling around the injector body, and the bubbles are from heat.
Return lines usually cause it.
But as mentioned, oil seeping from the valve cover can cause the phenomenon.

Injector lines leak, and injector nuts crack allowing a leak...

Injectors and seats dont leak.

Get a can of brake cleaner, clean away all the oil and fuel, let it dry, then look to see where the fluid is from.

Then fix that.

I believe you were right!
 

The air bubbles is what got me. I thought air was escaping from a gasket somehow. I hadn't considered that it would be diesel fuel bubbling from heat, but I think you were right.

I hosed that area off and now I think that problem is all fixed.