Panrdino

If this is not the right forum for this, please let me know.

I have an LS3 430 installed in a Titan T-51 airplane. It has an intermittent rough running problem whereby it will run fine for 40 minutes or so then start to run rough and the plane has to be landed. The next day it is fine at startup but the same thing happens when it is flown. It uses a Link Xtreme G4+ ECU and is not overheating. The ECU does not have closed loop lambda sensing and does not report any problems. The engine is rev limited to 300hp, with about 4000 rpm or less being used at takeoff and 3000 or less at cruise

They have replaced the ECU, plugs, wiring harness, coils, and injectors without change, so they are thinking it could be a mechanical problem with the engine. But I’m not aware of any mechanical problem the that could present this way.

Is is there something about the mechanical makeup the LS3 engine that could cause this?

i think it has to be an electrical problem, either via spark or fuel.

thank you.

Jim

LLLosingit

iTrader: (4)

Does the ecu data log? A log should help narrow it down? 40 minutes seems like a long time for a problem to show up, Heat soak would normally show up sooner if that was the issue.

Panrdino

It does data log but apparently nothing that helps them figure out the problem. It does not sense or record misfires

Che70velle

iTrader: (6)

40 minutes...maybe a fuel issue from altitude or temp? Perhaps pump is overheating and cooling off when plane is grounded? I just don’t see this being a mechanical issue.

G Atsma

^^^^^^^^^^ I'm thinking this too.
Just thought of something else- fuel tank vent clogged ?

LLLosingit

iTrader: (4)

I was hoping he was going to say he logged fuel pressure but I would think an airplane would have a specific requirement for it's fuel cell. I see some parts changed but not Cam/Cranks sensor, If it's capable of logging it should show a fuel pressure or sensor issue. I can't think of any mechanical issue that would only show up after a long period of time and clear up on it own once it sat for a length of time.

Panrdino

That’s an idea — I don’t think they have a fuel pressure gauge on the instrument panel, and it sounds like the ECU doesn’t record much. I’ll forward the idea to the mechanics.

Thank you.

Jim

G Atsma

Yeah good point on the fuel cell. Looks like most of us are agreeing it could easily be fuel pressure related. It's just about the only thing that would be time related like this is.

Panrdino

Mechanic states they do not log fuel pressure, but the plane has 2 pumps and the pilot is a very experienced test pilot. I’m sure he would look at that gauge if misfiring occurred. But I’ll text him anyway.

Yeah i cant think of a mechanical problem that would do this. Replacing the engine is not going to help.

if, say, a valve stopped opening or stayed open, it would either bend the pushrod and/ or destroy the rocker arm / assembly or contact the piston and be bent. ( I assume this is an interference engine.)

BTW, my mechanic says the cam sensor was replaced

Che70velle

iTrader: (6)

I hoped it would have two pumps in an aircraft application. I know I’d sure want a backup pump, as these pumps aren’t the most reliable thing ever built. Now knowing that a gauge is in place, I’m sure the pilot observed all readings. Usually when logging, fuel pressure isn’t recorded (usually) and it’s the first go-to when a misfire is detected, yet not recorded in the logs. Going to be something simple here. It is an interference engine indeed, and something like a valve or pushrod issue would mean rough running, or non-running all the time. Intermittent rough running is generally an electrical or fuel issue. Have them double check all grounds, both with a wrench and an ohm meter, and make sure the ECM is securely mounted, yet mounted in such a way to dampen vibrations. Harness for ECM to engine, same thing...mounted securely to reduce vibrations. I’m sure it is, but have them check it out.
Was the cam sensor replaced due to a reading/logging issue, or simply a shot in the dark type of deal?

Panrdino

Sorry it was the crank sensor that was replaced, and it was just a shot in the dark.
I thought these engines didn’t have cam sensors, since they don't have VVT. No?

LLLosingit

iTrader: (4)

They do have a cam sensor, It's located on the timing cover on an LS3.

Joystick

This Link Extreme ECU, is it a MAF or Speed density system? If it is SD and not correcting for pressure altitude correctly that might be your problem. It would then get worse with increasing altitude. No problem with MAF.

Che70velle

iTrader: (6)

I thought about that being an issue, but the pilot would see it clear up as altitude decreased upon descent to land.

Panrdino

That was their first thought - inadequate fuel tank venting / flow capacity, so they enlarged the fuel hose diameter and checked the venting. That was a YEAR ago. Then winter came to Ohio and nothing happened for 10 months.

Panrdino

Could a bad hydraulic lifter do this? Stick and cause the misfire then unstick and the misfire goes away?

cookseyb

Knock sensors.Noise levels different at altitude. Don't set a code other than random missfire

randeez

could be fouling the plugs at altitude and thats why it doesnt clear up on descend? really doesnt sound like a mechanical issue, agree it would just run rough all the time

first post says its rpm limited to 300hp/4k rpm at take off and 3k rpm at cruise, fueling will be drastically different at 7500'+ didnt notice if he mentioned that ecu uses a map sensor, said no closed loop so i imagine someone has tuned it for sd?

installing a wideband o2, even temporarily, would atleast point you towards mechanical or fuel/spark issue

Panrdino

More info: there are actually 2 levels of misfire. One will go away if you back off the throttle but the other only goes away if you shut the engine down. If you then restart sometimes it’s there and sometimes it isn’t. And neither appears until you have been flying for about 30 min. Varies between 27 and 33 min.

G Atsma

I wonder if the vent is somehow getting pinched somewhere after a certain amount of time