Engine Roughness

My Viking engine has always run as smooth as glass (well, other than the first time I fired it up with the prop on it, and didn’t have the blades all quite at the same angle – but that was my own fault).  Two days ago, I ran the engine for the first time in several weeks.  It was really difficult to get started, but I finally got it going, and once running, it ran quite well.  I fired it up again last night, and again, it was very difficult to start.  This time, though, once it started, it ran like it was running on three cylinders.  I shut it down quickly, and started checking everything.  I wiggled and tightened every connector, then started it again.  Same thing.  My son and I drained the header tank into a gas can, since it’s been so wet and humid lately.  Figured it might be water in the gas.  It was dark, freezing, and wet, so I left the rest of problem for today.  Today, I bought brand new gas, filled the header tank, and fired it up again.  Eventually.  Managed to kill the battery trying to crank it, and had to jump start it from my truck.  It finally ran, but with no difference from yesterday.  Ran it like it was only running on three cylinders.  I tried the alternate fuel pump and the alternate ignition, but that changed nothing.  Shut it down, and started from the top.

Checked every bolt in the PSRU.  Every one.  Tight as could be.  Checked all the prop hub bolts.  Tight as could be.  Checked the prop pitch on all three blades.  One was a bit off, so I re-adjusted all three, and again tightened down all 18 prop hub bolts.  Checked the rubber vibration dampers.  Checked every inch of wiring on the fuel injectors and plugs.  All look good.  Pulled all four coils and all four spark plugs.  All four plugs were scary loose (could almost remove them by hand without the ratchet handle – something I should have checked upon delivery before I ever ran the engine the first time).  All four plugs look good.  Three of them have that nice grey color they’re supposed to have.  The back one was a bit blacker than the other three, and had just a bit of gas on it.  Not bad, but enough to notice.  Reassembled everything with reasonable torque on the plugs.  Checked the PSRU oil level, and added a bit (the gears were dry, figured that’s not a good sign).  Checked the engine oil and coolant.  Checked the air filter.  Put everything all back together and started it (easily this time).  It ran better, but it’s still not right.  The idle is far lower than it was, and it doesn’t have that glass smoothness it had before.  You’ve got to give it a quarter inch of throttle to get the idle up to where it should be now.  I checked the throttle cable on the intake manifold, and it’s tight as can be.  Nothing slipped there.  It vibrates enough that it makes the SkyView hard to read, and the horizon and G-meter are bouncing all over the place.  It’s not rough enough that it scares me now, but it’s rough enough I wouldn’t fly it (were it ready to fly).

When I pull the prop through by hand, it’s smooth as can be.  Any thoughts?  I don’t know where to go next.

UPDATE:  Pulled the prop and ran it again.  Most of the shaking is gone, but the engine is still just not quite right.  It just sounds “off”.  It’s a little rough, and extremely hard to start (though it cranks over strongly).  It’s not like it’s missing, it’s just down on power, and not near as smooth as it should be.

UPDATE:  Got two phone calls and four e-mails already, all chock full of great suggestions.  Can’t tell you how much I appreciate it, especially the two phone calls.  The very simplest suggestion I received (thanks, Jan!) was to remove the wire from either the fuel injector or the plug on the back cylinder (the one where the spark plug was wet and black).  I did exactly that.  And guess what?  The engine runs precisely the same (well, after a great deal of cranking – it’s nearly impossible to start).  So it’s correlation, though not quite causation.  It’s enough of a smoking gun to focus my time and energy on that cylinder until I prove otherwise (great suggestion, Elmer!)  I’ve got a quick business trip, but when I return, I’ll stop by the auto parts and pick up both a set of spark plugs and a spare ignition coil.  I’m suspecting either the ignition coil or the wiring between the ECU and the coil.  But my money is on the coil.  More when I return…



4 thoughts on “Engine Roughness

    • Not impossible, but the ECU is pretty well sealed (and well away from everything except damp air), and it just doesn’t seem likely that enough water condensed inside of anything to cause problems. Certainly could be, though. I could take a hair dryer out there and thoroughly warm everything, I guess.

      • Just wondering if you got the problem solved and what you found. I have another question about the resistor in your blog for the viking/Gen-1Dynon, I know it said 100K resistor but when I look it up on the web the only thing I find is 100Kohm 1/2W 5% carbon film resistor, is that right or do you possibly have a part #? thanks for your help


      • Hey, Randy. Afraid you have me confused with somebody else. I’ve got a Dynon SkyView in my plane, not one of the older Dynon panels. I do know there’s a trick people have used with a resistor on one of the fuel injectors, but Jan strongly recommends against it.

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