"Trailer-hitching" bucking problem, Rapid detrioration from running perfectly Options

[boxster914]

Hi everyone

My perfectly running 71 914 1.7 D-jet has deteriorated rapidly this past week.

Early in the week I noticed what felt like a power drop out during a drive. It was very slight and only happened once and I wasn't sure I didn't cause it by unconciously lifting off the gas.

The next drive it happened a few times and was more pronounced. It was then that I knew this wasn't an imagined problem.

On the third drive it was like a bucking bronco, surging and stopping and the engine cut out a few times. It was undriveable.

Today it will start but won't run. The engine cuts out when I try to rev it and the red light comes on.

I've done some searching here but haven't been able to draw any conclusions or come up with an actionable strategy to diagnose the problem. I'm seeing people trace it to spark plug wires, air fuel ratios and bad distributors.

Is there a step-by-step process that you can recommend for me to get to the cause of this? (I'm a newb so I probably will need help with abbreviations like AFR, TPS, etc.)

Here is a video of it's rough idle yesterday when it was still running.

https://youtu.be/n61CP6O8UgM

[boxster914]

OK so I have some results to report on.

I changed the fuel filter.

After the filter change the car had 3 instances of minor bucking (much less than the original problem) early on in a 20 minute drive and then it ran fine for the rest of the time with no bucking.

Although after I got home there was some idling dropouts as captured on this video:

https://youtu.be/u-ttZXnvmOk

Notice the drop out at 0:02 and 0:10.

I figured the problem was not solved so I moved on to thinking it is caused by a vacuum leak.

I consulted the excellent vacuum hose diagram at bowlsby.net

https://bowlsby.net/914/Classic/zTN_FI_Hose....7L_1970-71.jpg

I changed the 4mm line (yellow in the diagram) after the fuel filter change and took the car out for a 10 minite run and there was no bucking at all.

Here is a picture of the new line:

At the throttle body


At the vacuum can


I welcome your feedback.

What is the function of the 4mm vacuum line that I changed? I don't know what it does. Could this have been the problem?

Thank you for all of your help.