fuel starvation w123

[whyameye]

1985 300D w123 running SVO and diesel since 2008.

In the last several months I have had a fuel starvation issue. The problem is apparent on both diesel and veggie but much more extreme on the veggie side.

Symptoms: runs OK but coughs occasionally on idle. After about 5-15 seconds under load the engine starts to lag. The problem is most apparent on the highway. I feel it hesitating especially going up a hill. The hesitation isn't consistent. Rather it sputters along with bursts of full power. The problem seems to get better and worse. Sometimes it completely disappears. Then a day later it won't run on veggie barely at all. It think it's likely air is getting in the line based on the symptoms.

I have new filters on both veggie and diesel. I changed the hand primer pump. I put the fuel line under vacuum before the lift pump. It held 20 inches no problem. I have replaced the diesel injector return lines. Years ago I removed the ALDA.

I also have put a fuel vacuum gauge inside the car to measure how much vacuum the lift pump is creating. It goes up to only an inch or 2 on the highway on diesel and 5-8 inches under severe continuous load on veggie. That seems reasonable so I don't think a constriction in the line?

I've read about the fuel relief valve spring on the back side of the injection pump. What are the symptoms of a compressed spring?

I'm also wondering if there is any way the lift pump could be leaking air other than at the hand primer and if there is a way to test if it is able to create enough vacuum consistently. (I suppose I could I purposely restrict the fuel intake and watch the vacuum gauge.)