Engine missing a beat at low RPM - How to fault find

Hi all

I posted a question about the torque on intake gaskets earlier. I found that the bolts were not very tight as part of an effort to find out why my engine runs a bit rough (intermittently) on low RPM. It feels like it misses a beat every few seconds are so, when the RPM sits between 610 - upper 700. If I increase the RPM a bit it starts to run smooth.

How do I approach trying to locate what might be causing the fault? Few things about my set-up:

Mattituck IO-360, with dual PMags and vertical induction. Pmags connected to an EI Commander. Car plug conversion, and I have installed eight new spark plugs a few weeks ago. I did an Ohm check on the HT leads and one lead has a slightly higher resistance than advised, and I have a new one on order. I have a Dynon D700 EFIS with EMS. App. 560 hrs total.

Initially I suspected an issue with my PMags but having spoken with Brad at EMag, I am less and less sure it is the mags. I have been testing them fairly extensively and they seem to produce their own power when expected, and when the engine runs a bit rough in the lower range, I get the impression they are receiving battery power, or at least to not kill the engine. At lower RPM I am below the RPM that the mags produce their own power (from app. 950 on my set-up,) so they need battery power to operate.

There is nothing on the engine monitor (I have individual CHT and EGT readings for each cylinder) that indicates that one cylinder is not 'firing' or at least all the temps give me a fairly consistent reading across all four cylinders. The roughness feels the same when running on either mag exclusively.

I checked the inlet manifold to see if there were air leaks, hence my post earlier, but again, if there was I would expect it to show on the affected cylinder. I did tighten the bolts a bit and the hose clamps to see if that made a difference. It didn't.

What else could I do to try and find the problem? I made a little video that shows the slight 'surging' when running idle and you see the RPM fluctuate by 40 rpm or so, when this happens. I don't think I can post the video here. I would like to add that when I increase the RPM the engine runs smooth and I have had no issues in flight with the engine sounding great and all the readings looking correct.

Thanks

Roger

Auto fuel or 100 LL? Does it run smooth after a cold start and only surge when hot?

Did it idle smoother before?

Here's a video of another engine idling a bit ratty, but working great at the RPMs it was designed for:

https://youtu.be/LhtX4Vy_YRI?t=43

I was chasing a smoother idle a while back as well. I've been told that at very low fuel flows, mechanical FI will probably be a bit rich, and cause a less than perfect idle. This might be what you are seeing.

What prop? Don at AFP says that a light prop will contribute to idle problems due to the lack of flywheel effect from the prop.

-Marc

Suspect fuel 'boiling' in lines

Since you noted it is fuel injected... I assume with a Bendix type? The fuel pressure in the lines after the servo at low power settings is very low and hence the fuel can 'boil' / vaporize in the lines on its way to the injectors. Insulating those lines (servo to spider) can help. Insulating lines from the firewall to the servo can help too by keeping the fuel temperature down.

Are yours insulated?

with a rough idle, the best place to start after induction leaks is idle mixture. Surging at idle is typically a sign of a lean condition. Induction leaks (most pronounced symptoms at idle) and idle mixture are top suspects. Surging is rarely an ignition issue.

If your intake gaskets got chewed up while loose, simply re-torquing them may not resolve the leak. Probably best to replace them if they were more than slightly loose.

As mentioned above, light props don't like lower RPMs. On my 6 with an FP prop, my engine is not happy at 650. I need to run 750+ on the ground for smooth running. I leave the RPM set low to help me slow down in the air, however, once on the ground and warm I need to bump the throttle a bit to give me 750. Mine is just rough; No surging.

Larry

blaplante said:

Since you noted it is fuel injected... I assume with a Bendix type? The fuel pressure in the lines after the servo at low power settings is very low and hence the fuel can 'boil' / vaporize in the lines on its way to the injectors. Insulating those lines (servo to spider) can help. Insulating lines from the firewall to the servo can help too by keeping the fuel temperature down.

Are yours insulated?

Click to expand...

I agree, sounds like the engine fuel is getting hot. Pretty normal for many RV’s.

Thanks for the answers and ideas.

I'd like to add that the roughness, seemed to have started recently. So if the issue is fuel vapor in the lines because of the temperature, why would that happen now and not previously? Although it seems to happen after the engine has run, and the weather is indeed warmer now (not quite as bad as in the western part of the US thought. The engine has app. 560 hours on it.

Someone suggested the fuel servo might be due an overhaul?

Not to put a damper on your quest for perfection, but I suspect that just about every IO 360 run a bit rough at low idle, particularly when warm/ heat soaked.

I can recall the first time I went to Oshkosh. One of the tips I read in preparation for the sometimes long idle/taxis that occur was to keep your idle RPM up a bit and use your boost pump on the ground when hot

I know mine idles like **** when hot. May be exasperated by my tight plenum. Have had the fuel control overhauled recently, no real difference. Nature of the beast I suspect.

I would think that vapor bubbles are being produced in the fuel line from the Bendix to the spider and from the spider to the fuel injector nozzles out at the cylinder heads.

Some other thoughts:

Do the Savvy intake manifold leak test.

https://resources.savvyaviation.com/in-flight-diagnostics


Have you done an automotive type compression test and have also been getting you oil sample tested by Blackstone labs? I ask because a rough running at low to mid RPM engine "could" (dont...PANIC...COULD) be a sign of a worn camshaft...if the camshaft lobe is worn down then at idle a hydraulic lifter would be somewhat collapsed at idle due to the lower oil pressure and therefore it would not pump up all the way against the rocker spring pressure. So...knowing that the camshaft lobes are not directly lubricated...even though the camshaft lobes are the highest pressure item on the engine there is no direct pressurized “hydrodynamic lubrication”. The lifters in most engines do not have a roller foot (roller lifters are available but rare) but instead the lifter foot just slides directly on the camshaft machined surface the entire time the engine is running. This is really significant because the design of the engine is such that each camshaft exhaust valve lobe drives one exhaust valve however the camshaft intake valve lobe is shared by two opposing cylinders (a 4 banger has 4 exhaust valve lobes and only 2 intake valve lobes) so those two (or three in the case of a 6 banger) camshaft lobes have two lifter feet sliding on them the whole time the engine is running and both lifter feet are constantly whipping of the engine oil that just happened to splash or drip oil on to the camshaft…so the intake lobe of the camshaft can be worn out faster. If you do a regular airplane type differential compression test it will not show a collapsed cam lobe because the test is done at TDC when the valves are supposed to be closed anyway...so do an automotive style compression check because that would show if every cylinder is sucking and blowing the same amount of air...if you do this compression check and two opposing cylinders are low compression...AND...the oil sample shows metal...now you can PANIC!!!
:

MAP line

Don't forget to include the MAP sensor line in your induction leak testing, it too can create enough of a leak to cause issues only at idle.
I also would suggest replacing the intake gaskets. I have not had any luck trying to stop a leak by simply tightening the bolts!