Engine overhaul time - Lycoming IO-360-M1A

[Steve]

Charles - I don't recall whether you mentioned what your EGTs were running. Any significant difference on #4? I see that your shop was smart enough to switch out all of the bits on that cylinder. That eliminates a lot of possibilities, but still leaves you without a definitive answer. Lycoming may say that the cylinder would be OK at 480 degrees, but I wouldn't be comfortable with that.

[Charles]

Hi Steve, I did a few lean tests (per the GAMI procedure) of the engine and the EGTs are all in the same absolute range and they all peak within 0.5 gph of fuel flow of one another. This is what ruled out induction leaks. In one test, the EGTs vary between 1500 (#4) and 1570 (#1) around peak EGT and the peak occurs between 8.6 gph (#3) and 9.1 gph (#1).

[MarkA]

Charles, if you haven't already check it, it's possible the fuel flow divider (spider) might be the source of some of the fuel flow imbalance. Take a look at the following thread for some details:

viewtopic.php?t=8598

GAMI indicates they'd like to see a .5 gph spread or less to effectively run lean of peak so it sounds like you are currently pretty much at that limit.

[mhoran]

Sigh, just picked up my plane after waiting three weeks for repair and my governor is still leaking. Sensenich found that an o-ring had been improperly installed and that also replaced the adapter plate. But it seems that still didn't resolve it. Hopefully I can convince someone to give me a "new" governor because this is getting ridiculous.

[mhoran]

Sensenich sent a replacement governor and I finally had it installed today. We had no issues during ground runs or in the pattern. After an hour flight I found it was leaking again.

Not sure what's going on at this point. We checked the case and studs and they look fine. But oil is definitely seeping out from around the governor.

[Steve]

Wow Matt, that really is frustrating! It looks like your governor is mounted exactly the same as mine (same length studs, same fasteners, gasket, etc.):

Prop governor

I am wondering if perhaps the mounting pad on the crankcase is warped. Did your mech check that? Could there be a small crack in the casting? Maybe do a dye penetrant inspection of that area of the case? One of your photos shows some scratches on the crankcase mounting face. They look small to me, but the oil pressure coming out of the governor is significantly higher than engine oil pressure. Even a tiny leak path could leak when the governor sends high pressure to the prop.

The other thought I had - could the leak possibly not be coming from the governor, but from the thru-bolt immediately behind it? Those bolts have O-rings on them inside the crankcase, which have been known to get pinched or torn during reassembly, leading to oil leaks.

[mhoran]

We did check the case with a straight edge and it looked good. There are a few scratches but they look superficial. No cracks that we could see. But I think the next step is to bring the plane back to the overhaul shop, as inconvenient as that will be, in the event the case needs to be sent back to DivCo.

We've pretty much ruled out a leak from the thru-bolts. Thru-bolt leaks would be much slower than what we're seeing. Also, it wouldn't make sense where the oil is if it were just a thru-bolt leak. James Watson of the overhaul shop also checked his assembly photos and saw nothing that would suggest a thru-bolt leak.

We did notice without torquing down the governor that it rocked ever slow slightly in place against the case. But once we torqued it down we could not get a pick in behind the gasket. So it's all quite odd... No smoking gun.

[Rich]

Any chance something about the drive splines are mismatching and preventing the proper compression of the gasket? You mention it would rock before torquing down. How about checking to see if there is any gap with the gasket not in place and comparing that to the gasket thickness?

[Steve]

I like Rich's idea of checking the fit of the governor without the gasket in place. There is no adjustment for protrusion of the prop governor drive gear from the case, the only adjustment in the gear train are different thickness thrust washers placed on the idler shaft to adjust the lash between the governor drive gear and idler gear. The protrusion on yours looks the same as on mine, though:

[mhoran]

I think you guys might be on to something here. We couldn't think of what might cause the rocking before we torqued down since the case edge is square, but this would explain it. Despite my photo looking nearly identical to Steve's, I'd imagine even the slightest excess protrusion could cause a leak.

The odd thing is that I don't remember the governor leaking right when I got it back. It started leaking a number of hours later, as I was on my cross country trip. But maybe it was leaking very little and I didn't notice. This is why we'd written off any case issues initially, but it's clearly not the governor at this point.

Regardless it seems a trip back to the overhaul shop is required, and fixing this may require invasive surgery.