Is this Blow By?

[michael3006]

I have booked my 1985 SD23 for a compression test next week.
The recent cooler weather has resulted in hard start then no start.
I replaced the glow plugs and checked the circuit all good.
The only way to get it to run now is by towing it and slipping the clutch on 5th gear.
Once she runs it sounds smooth and drives well.
I am fairly sure I have lost compression on more than one cylinder.
Here's the question when I open the oil filler cap I have a lot of air blowing out.
I know this cause when it was running I tried to pour in some oil additive to increase compression. Fat chance I know, but I have to get to work somehow.
The thick oil addative was blown straight up out of the filler hole.
Does this mean I have compressed air/fuel blowing past the valves up into the valve cover?

[dn29626]

That much blow coming out is a bad sign.

[michael3006]

Yes indeed it does seem bad.
Do you think it is pressure coming past the valves?

[dn29626]

Stem or ring problem. You may be looking at an upper end rebuild, maybe full trebuild.