Massive engine problem

Jeffry Stetson wrote:

"I fitted a new Pierburg pump today." I hope you don't mean the old gold fuel pumps. Those have been grounded years ago after numerous failures. The newest version Rotax ones will have a Rotax part number embossed on them (not Pierberg!). Also, they have a provision to attach a hose to the drain hole for leading fuel safely away in the event of a diaphragm rupture.

Have you done a differential compression check, both hot and cold?

Have you done a borescope inspection of the cylinders?

Some of your issues remind me of a stuck valve on Continentals & Lycomings, usually from lead buildup. A valve problem here could be from air in the lifters from an incomplete initial purging of the oil system. Since the engine and oil system started dry, I'd do another, very patient and thorough purge of the oil system. Before purging, I'd also pull off the valve covers and check for mechanical damage, like for example broken springs, missing spacers and such.

Yes sure the old golden one. It was a test, no more...

Have you done a differential compression check, both hot and cold?

Have you done a borescope inspection of the cylinders?

No and no.

Chris

Jeff Blakeslee wrote:

I’ve been following along with this and some very good suggestions have been made. Looking at the video the manifold pressure never exceeds 26 even at full throttle, so it appears the engine is down on power.  I was on board with the possibility of an exhaust blockage from a loose baffle, or a stuck valve, or a bad lifter. But those don’t explain the one instance when the engine stopped completely and forced an off field landing. That one instance points me toward a fuel or spark problem. 

The video was taken during flight at about 4000 ft. 

In general: Sorry for the late replys. Tomorrow I'll go to the plane again and will perform some checks/tests and will come back here with the results.

Thanks to everyone who tried to help me! :) :)

Chris

Chris wrote:

Kevin Stewart wrote:

Assuming that you can create the symptoms on the ground, could you just stop the engine and inspect the fuel level in the float bowls? That might show if you have insufficient fuel flow.

Are the fuel tank vents operational, i.e. not creating a vacuum under full load?

Here's a question for some of the other forumites: what is the effect on the EGTs if the mixture becomes lean?

How I could do this? If there is a low fuel level during the vibrations and I stop the engine, the fuel will fill the chambers immediately. Or what you mean?

Chris

The 912 stops very quickly so I don't imagine that the float chambers fill that quickly and certainly not if there is a fuel supply problem. If you leave the throttle open, then fuel will still be drawn into the engine even if there is no ignition. If the fuel is clearly not at the correct level then that might give rise to a sufficiently lean mixture to cause rough running.

It's a difficult problem and all you can do it keep thinking up different tests to produce different evidence until one of us correctly identifies the root cause.

Have you tried putting the symptoms into Chat GPT or Deepseek? They may just suggest something we haven't thought of.

"Yes the prop wasn't dynamically balanced ever, but it's the same prop like before the engine was overhauled and everything was fine."

Damage from an unbalanced prop may take many hours to become evident .

My CS prop manufacturer recommends dynamic balancing every 100 hrs😈

Holy moly, my whole posting was not uploaded... :(( So let's do it again:

Sooo, back on the plane today after a very nice forum colleague gave me real hope with a tip regarding the original Rotax drip cups and that they were not flat on his flange. As they were new during the overhaul (my old ones were in bad shape), this was of course extremely plausible! So I threw out the cups, checked the flange again, the O-rings were properly in place and in perfect condition, the surfaces were also perfect. So no bad outside air is able to get in at this point, the rubbers now sit directly on the intake manifolds.

Unfortunately, no improvement... :(

Here are 3 pictures from today and the current situation. All normal stock I would say, HACman is not in the aircraft:

I also recorded 3 videos. At the moment when the vibrations started, I raised my finger:

Vibration 1

Vibration 2

Vibration 3

I blocked the return fuel line as a test, no change.

It's unbelievable, it was 0 degrees today, it occurred immediately with every run. And in the summer I've been jetting around for hours without the slightest problem. I haven't changed anything since then. It's amazing to me that the temperature has such THIS influence.

Chris