In cruise at 210, 2 hrs into a 3.5 hr flight. Right engine faltered, for maybe ¼ of a second but enough that a 6 year old wondered what it was. Half hour later, same thing again. I was thinking a slug of water in the fuel but I’d sumped the tanks and saw nothing. Then again, ½ second this time, MP dropped an inch but came right back. Not enough time for yaw but disconcerting.
At the beginning of descent it completely quit for 2 seconds, yaw, rudder, etc. Fiddled with the mixture, came back smooth. But now it needed 2 gph more to keep TIT normal. If I dropped the RPM to 1700 it happened less frequently. Played with mixture for a while, tried to go LOP, which I’ve been experimenting with, but it wouldn’t go LOP, normal peak TIT is 1615 & 21 gph on that engine, this time it climbed to 1700 at 17 gph before the engine ran out of fuel. All EGT’s followed the TIT. Did a mag check (ROP), all good. Fuel flow and pressure were normal and didn’t change when it faltered. EGT’s and CHT’s were all normal throughout. For a while I thought it was the engine driven fuel pump but when I shut the right pump off nothing changed. Switched it to high boost (which is only for engine driven pump failure), pressure and flow shot up and the engine nearly quit. Tried this several times while it was faltering. The only thing that seemed to affect it was the mixture, usually richer helped but leaner did the trick once.
This continued to happen throughout the descent, in both freezing and above freezing temps, until MP was below 25” with RPM at 1700. The approach and landing were normal and it idled normally.
We recently (10 hrs ago) had heated fuel manifolds installed, which circulates engine oil around the fuel spider to eliminate the ice problem in the manifold caused by water in the fuel freezing inside the manifold. The engine is at 200 hrs or so, only recent work is the heated manifolds.
Hope to pull the cowls tomorrow and look for something obvious.
Ideas?

I would go right to those spiders....

Any precip on or before departure?

No precip or IMC entire flight. Switched the fuel vent heat on for a while also.

If nothing is obvious, swap them to the other engine. See if it follows.

I had a 200 hour Ram engine in a straight 58 do something similar to this. I never did figure it out until the engine started making metal soon thereafter. We replaced both motors with IO-550s. My current 58P was doing something similar albeit not as bad. I fixed it by lengthening the idle mixture link to match the idle fuel flow on the other engine (4 GPH). I was ready to burn the airplane. My issue was at idle, yours is at cruise.

The fact that the boost pump changes nothing tells me its not the engine driven fuel pump. But, the fact that it is happening in cruise tells me your fuel flow divider might have a crunchy diaphragm and it could be sticking. Pull the flow divider and if the rubber inside is not supple I would replace it.

Your mags could also be arching so check the ignition system likewise.

Turbines don't do this sort of thing BTW.!

Is this is a RAM engine, if so make it their problem. Same deal with my old IO-520 Baron, it would stumble and there were no changes in temps or flows. Something could be coming apart inside.

Check the timing on that engine. Sounds like a mag problem. How many hours on the mag since last inspection?

Jack, be sure to advise on your findings. Agree that the FD diaphrms should be checked.

Ha! I confess, I spent 30 minutes walking around a PC12 and B200 when I went back to the airport tonight.

Regarding a suspect mag, I've done many mag checks in flight on these engines...this was much worse than killing one mag while the other is doing its job. If it's mag or ignition related, they're both going out at the same time.

If something is coming apart inside the engine how does it run smooth 95% of the time?

It is a RAM engine, and the spiders are RAM.

Ha! I confess, I spent 30 minutes walking around a PC12 and B200 when I went back to the airport tonight.

Regarding a suspect mag, I've done many mag checks in flight on these engines...this was much worse than killing one mag while the other is doing its job. If it's mag or ignition related, they're both going out at the same time.

If something is coming apart inside the engine how does it run smooth 95% of the time?

It is a RAM engine, and the spiders are RAM.

I'd suspect fuel with that intermittent nature. It doesn't matter that you don't get any water when sumping. I'd also pay particular attention to the fuel spider and the oil heating considering that was just done. Do you have an engine monitor you can down load data?

Andrew

This could be caused by several things. For my own peace of mind, I would check all of the valves and check the magnetos for corrosion and arcing.

It isn't spark, or compression, or air.

It is fuel delivery. Has to be.

Not in the flow divider, IMO, since all cylinders go out and you don't see CHT/EGT wandering between cylinders. That is, the fire goes out, comes back, the entire engine at once.

Water in fuel. Airplane outside in any recent rain/snow? Just because you sump doesn't mean there isn't water. The fact is happened most on a nose over is consistent with that, changing attitude of the airplane can make water flow around in the tank.

Air in fuel. Had this problem on a Comanche. Electric fuel pump leaked in just a way to let air into the fuel system, it would build up in some high spot, then a big bubble of it would suddenly enter the fuel line and the engine stumbled. Overhaul of boost pump fixed it. Another symptom was boost pump overflow line dripped fuel. Now that was a Lycoming, way different than a GTSIO-520, but perhaps there is something similar.

Ice in fuel. If flying in really cold, water crystals can form in fuel and build up. They can then momentarily block something and then get pushed through. Rare in piston aircraft, but can happen.

If you have a C model, don't they have float valves in the tanks? Maybe those aren't working right? This would be indicated if it doesn't happen with full tanks, but starts at, say, about half full.

Next thing to try might be crossfeed the misbehaving engine and see if the problem remains even though the tank changed.

I do not believe this is an internal issue with the engine itself. It is hard to come up with a mechanical problem which can be so on and off with engine operation.

Mike C.

I agree...I think...it's the only thing that makes sense.
But I don't understand why less fuel didn't reduce TIT eventually. It continued to climb until the engine stopped (about 02:49, it's only a blip on the data, I should have kept it there longer). It also needed more fuel or less power to keep temps "normal"...which I'm having a hard time finding in the data.
Those things make me question what seems to be an obvious fuel issue.