[RedArrow]

Hi!
Welcome to the forum!
Got popcorns? here`s a long answer

IDk where to start my reply but I also have a few questions...

It is very nice that you want to save the car that your father was driving. I wish I could do that too... even better, to rebuild it together with him.

about injectors...
What do you mean by saying that the injectors were changed?
Did you install new injectors from another car or did you rebuild your injectors that are in the car now?
Probably you sent your set of injectors to someone to rebuild them. Would be importat to know what kind of nozzles they used inside those injectors and also important how (if) they set them right.
For now, that`s it about the injectors. Try to verify with the rebuilder and ask what they did and how. Did they know at the time of rebuild that it will go into a specific car a dirsel volvo. Some rebuilders just throw in new nozzles, sometimes cheap bad quality nozzles, and often they dont even test the injectors for spray pattern, opening pressures, etc.
Definitely verify who did the rebuild and how.

A very nice and professional injector job isnt even that easy but once you have a great set properly set, you can do 75000miles on them, some say maybe 100k.

About the lack of power...
Your car was very slow even brand new!! even in Europe LOL but those days that was the style of driving and nobody was bothered by it. They arent race cars for sure but they have very nice torque and it comes at low rpms too which is great. With the right owner, they ran 200-500-700 thousand kilometers easily with minor refurb jobs.
If you think that your car has no real power, which you already explained, try to think it through and try to find why.
It would be great if you had tools the proper factory tools that are somewhat necessary to do the timing , at least. You could use some of what the VW cars used and put together your own kit.

I would start with rechecking the timing bc sounds like you have multiple sign of a misadjusted engine, a tired one, or both. I would put my bets on the first.
What you are explaining is serious lack of power.
My dad had the same car in 1990 and it was a 1984 nonturbo d24 244 car and we still have it, but that car cruised at 135km/hr at any time and all day without no overheat or loss of power. Dont ask me about the 0-60 but it did have proper power and never ever let us down.
It would climb ANY hill, slowly maybe but would pull up a trailer uphill with another d24 car on it.
And it would pull it all day, i have proof bc I know someone who kept transporting cars for years and he used a nonturbo d24 245 to pull them home from Germany on a 2-axle trailer! He often brought the w123 MB cars which were really wanted that time in Hungary.

Anyway. So. Your car need to claim back its power. Recheck and set timing.
Dont worry about compression for now. even with low compression and a worn tired engine, you should have ample amount of power to climb hills and able to feel that car driving stable as it always should.
You really need to know how it got timed and by who and verify again IF they know about these cars at all. Maybe they timed it totally wrong...sounds like it.

My other question.
Do you notice coolant loss? Not just a leak what I mean but dont you have coolant `disappearing` time to time?
If yes, you gotta start with figuring out what`s wrong. Try to look and see if you have (exhaust gas) bubbles in your expansion tank surfacing with the coolant (warm engine or after longer driving with the thermostat already `opened`.
When my dad had a Volga wagon, we would go fishing for years with it with a broken headgasket and the only sign was that it would overheat on hot days and also it never could climb even slopes not just hills.
But it ran well on flat ground and grandpa style driving...and we never got it fixed actually.
Dont stress about your HG going bad now... It isnt the case but no one really knows.

You gotta go through other important details and try to set the car up properly.

Next topic:
You probably need a fuel filter if your tank is usually kept lower than full esp if it didnt run or was parked for a few years.
If you have bad fuel or rust in the tank, those particle will always reappear and screw you at the filter over and over again.
You could actually install a cheap inline filter between where the fuel comes in, before the factory filter; if you suspect that your fuel is /was always contaminated.
Might be the case.

Another ape-man method but it is a good test:
To close out injection pump being `weak` and to close out issues of contaminated fuel that is coming in from the bad fuel in tank etc etc;
you can always find a 15-20liter container and place it in front of the passenger seat inside and `wire up` some fuel hoses from right there and run the car from that contained fuel and see IF you can experience some evolution about how it runs. Many d24 people used that enourmous size washer fluid tank, modded to accept fuel and ran around town like that. Funny at the gas station though. But some pumps were too leaky to suck all the way from the rear main fuel tank and this solved their problem. Until you get the main tank removed and cleaned, it always worked.

Check fuel lines below the body all the way from the tank to the engine. I remember doing offroad fishing hunting and we had some troubles with air intrusion at the underbody a few times. Not much air is needed to make things complicated.
Also keep your engine as clean as possible to be able to spot just about anything. An oily leaky dirty mess is not your best friend for a successful diagnosys!!! IDK how your car looks so I`m just saying, dont take it to your heart. The fuel line between the filter and the inj pump should be transparent/clear, so you could see fuel going thru WITHOUT any bubbles of air. Make sure that the almost invisible clamps are NOT letting air in.
Often they dont at idle but they do when driving longer trips and fuel and tubing gets hot. IDK where you are at so I am trying to mention multiple things to look for as a first plan. To start somewhere...

My other question.
Is your injection pump visibly wet anywhere and does it leak anywhere?
if you have air intrusion just about anywhere between the fuel tank and the motor, it may not run perfectly and it can even run very poorly.
Here it would be nice to know IF it idles fine all the time, If it runs always fine all the time, etc. Power loss is one thing but does it stall does it cough does it do weird stuff as you drive it?

You should be able to wake that car up if the motor is healthy so dont give up.
There`s a lot for you to check but the forum can also help you all the time.
Take videos if you notice anything worth mentioning so we could see it, hear it.

Black smoke usually means too much fuel, white smoke definitely is not good unless it is the very first 20-30seconds at the coldstart.
White smoke to me indicates bad injectors. If the white smoke is smoke, that`s better than having water vapor as white smoke.
Blue smoke would mostly be issues with the piston rings being worn etc.
To me it sounds like someone turned the fuel up to compensate for poor running. There are other ways to adjust the fuel for the motor so you need to sort this out step by step.

About a possible head failure again:
Check your oil for murky milky substance. if water is present inside, you will immediately see at/in the valve cover.
Also check AND smell your coolant at the reservoir and see if you can notice something.

MY BEST ADVICE IS THAT YOU SHOULD FIND SOMEONE WHO HAS OR HAD THE SAME CAR AND TALK WITH HIM OR MEET UP FOR SOME TINKERING.

Forget the mechanics bc most of them cant and wont deal with such cars. And even more of them dont even know how to.

The ones who had proper training on these specific engines or atleast worked at their oldschool brick car dealerships in the early 80s, are now 60-120y old lol.