W124 260E powerless after headgasket replacement

[H1lmar]

I just bought myself my first Mercedes, been wanting to own a w124 for some time now and got myself a cheap 1990 260E auto driven 330.000km with running problems. I've been trying to sort it out but I'm at a dead end right now. I'm trying to get the engine running before I try to find a replacement engine. The car starts and idles at first like a champ. After being running and warming up a little bit it loses almost all power and I can't even rev it. Does not choke, just no power to even rev the engine alone. And it is like it is idling to low, shaking and might be losing some cylinders, not sure. Has some power at first, not enough to pass 10-15mph, but that only last for like 20secs.

Bear with me please, english is not my primary language.

So, this is the story. This guy buys the car with a supposed failed headgasket. He replaces the headgasket and probably all the gaskets that are in the headgasket set. Intake and exhaust gaskets and so on. He gets the head checked and valves reseated. And assembles the engine. He has according to him been trying to fix this on and off for about three weeks before I buy the car. Replacing spark plugs and leads, bought a replacement fuel distributor but did not install and some basic checks.

He takes it to some shop to get the car checked after giving up, they say that the piston rings are the problem. They say did compression tests and got low compression, but similar compression on all six cylinders.

And that is where I buy it.

I've got some experience with repairs so I check the basic stuff. Did not see smoke from exhaust, the exhaust is broken in half somewhere under the front seats though. Rest of exhaust system not under. Don't notice air blowing from oilcap or dipstick. Recheck the compression. I did dry tests on cold and hot engine, getting from 170 to 180psi cold and 160 to 170 hot. So I thought it should compress enough to not cause this huge powerloss. Checked timing, spot on and valves seem to move freely. Checked spark plugs after reading about the resistor spark plugs problems, the car had resistor ones in it, replaced those with NGK non resistor ones that should be used with the M103. That changed nothing. I noticed that the middle springpin in the distributor cap was broken so I replaced the cap, still the same.



That is where I'm at now. Note that the alternator and fan belt is not installed doing those tests but I have a good battery so I guess it's not a problem, I think he had it installed while doing his own troubleshooting. Alternator is in the trunk so I can reinstall it.

So, in short, supposed headgasket problems, don't know the symptoms.
Headgasket replaced
Got some head work professionally
No power after assembly
Timing correct
Seems to compress
Ignition system seems to work and correct spark plugs
Problem seems to get worse as the engine warms up.

So, this leaves me with a bit of a headache, I suspect electrical problems or he screwed up the assembly of the engine.

What should I try next?

Compression testing again, hot/cold and wet/dry? Leak down?
What sensors or electrical stuff should I check?
Could this be a vacuum related problem?

Please help me so I can enjoy my first Mercedes