Water Ingestion?

Last week in my RV-4 I was at 11,000 MSL, IMC and in moderate rain and light turbulence, and had been for 10 - 15 minutes. The OAT was about 44 degrees F.

Instantly my engine went from normal to EXTREMELY rough and it felt as though I lost about 50% power, though the RPM stayed the same - C/S prop.
I did not notice a manifold pressure change, but I'm guessing one occurred. No change in CHT or EGT or anything else I observed. Boost pump on, switch fuel tanks, mixture rich - no change. Carb heat on - at this point I got a slight change in engine sound and feel, but I can't really say it was any better. Checked Left Mag, then Right, then Both again, these of course caused a slight change in the engine, but no improvement.

Request vectors from ATC to VMC (my gyros are still vacuum, as in engine driven) and toward nearest airport over 3,000'
Still able to maintain altitude at this point.

Carb heat off, then on, still not much change and no improvement. Carb heat on and continue toward VMC. Gradually the engine smoothed out and also the rain ended.

The next 10 hours of flight were without anomaly, though I did re-enter a short period of rain. I have also flown in rain before, but never as heavy as on this occasion.

My best guess is I was ingesting water.

Setup: O-320 with Ellison EFS-4 Throttle Body Injection. The EFS-4 is mounted pointing forward and utilizes a 90 degree plenum box on the bottom of the engine. The EFS-4 has a typical carb heat selector box in front of it then about 3" of scat tube running directly forward to the cowl opening where the only filtration is a basic screen door type screen. No air filter.
Yes, it would seem that rain would proceed directly into the EFS-4 and then through the engine if the carb heat were not selected. I've flown in rain several times before however, and apparently the rain/water going through the engine causes no detectable effects. My thought is the rain collected in the carb heat box until it got "deep" enough to spill a considerable amount into the EFS-4 all at once. I've since drilled small drain holes in the low spot of the carb heat selector box to allow any pooling water to run out.

I don't think I had "carb ice" because the onset was not a gradual loss of MP but rather instantaneous extreme roughness. Also the EFS-4 is not prone to carb ice by its sort of lack of a venturi. I understand the 90 degree change plenum box does operate below freezing most all of the time and perhaps liquid rain froze in that box, but again, I'm not sure how this would have caused a sudden onset of roughness.

Any thoughts out there? Anyone experienced roughness before from rain ingestion?

I don't have a clear answer, but I am wondering if you checked the plugs to see if the rain super cleaned them as well as the combustion chamber. If you did collect water and then it was ingested, then it would be very clean inside. That would be one clue to this possible cause.

Furthering the question - you are right, typically rain is not "water dense" enough to degrade power. The only two other things that come to mind in addition to your scenario are an arcing plug wire, but that would not clear up so quickly. If it were a cooling cylinder that caused a sticking exhaust valve, it would be hard to imagine it would repair itself so quickly, but it is certainly worth checking to insure an issue near the edge was not triggered. If it was, look for bent push rods and a tight valve.

Did you leave the mixture full rich after trying it? My guess would have been a water soaked filter restricting air flow, so going leaner may have fixed it. Did you try that? Full rich at 11K may have made the engine run rough normally.

You describe a classic setup for carb ice, including the OAT. Do you have a throttle body temp sensor? If you use the vans stove pipe carb heater it may not raise the temps enough in cruise conditions to get rid of the ice. I had the vans set up and in cruise could only get about a 1-2 dergree rise. BTW, on the ground I got about 5-8 degrees. I replaced the vans setup with the Robbins muff and adapter to the FAB and now get a 10-15 degrees rise in cruise. This is enough to get the temp out of the freezing range if you were in icing conditions.

The way you describe the resolution is exactly how I might expect carb ice to resolve itself if you only had a small rise.

The only other thought is if you drilled a drain in the FAB box, outside the filter?

Can you post a picture of your induction setup?

No air filter.

Fastwind67 said:

The EFS-4 has a typical carb heat selector box in front of it then about 3" of scat tube running directly forward to the cowl opening where the only filtration is a basic screen door type screen. No air filter.

Click to expand...

No air filter.

BillL said:

No air filter.

Click to expand...

Well it could easily be either then. Engines can swallow a fairly large amount of water (WA injection for recip engines is usually around 10% of the fuel mass flow) but heavy rain without a filter to block it might well extceed that. Hmmm.

May not drain with engine running

Fastwind67 said:

I've since drilled small drain holes in the low spot of the carb heat selector box to allow any pooling water to run out.

Click to expand...

The water probably will not run out a hole, differential pressure will be pulling air into the system as opposed to draining water out.

Instantly my engine went from normal to EXTREMELY rough and it felt as though I lost about 50% power, though the RPM stayed the same - C/S prop.
I did not notice a manifold pressure change, but I'm guessing one occurred. No change in CHT or EGT or anything else I observed.

Click to expand...

Do you have an engine monitor that records data? Producing less power when being at the same RPM, MAP, EGT, and CHT is basically impossible. One of those had to change and would give you a huge hint.

Both times I have encountered actual carb ice it has manifested itself in the same fashion you describe. Flying along FDH (Fat, Dumb and Happy) and it was like a light switch had been thrown. Engine went instant rough. Application of carb heat made it slightly worse before it got better a couple minutes later which is what you would expect.

One was a 152. There probably was a decrease in RPM before the rough but I did not detect it.

The other was a 182. Glanced at the MAP gage during the rough and it showed a slight decrease.

The conditions you describe are pretty much the text book conditions to get ice. Possibly not ice in your TBI but perhaps elsewhere. Ice blocking the flow of air anywhere could cause that issue. I suppose your theory of water pooling until it got deep enough is possible. Improbable but stranger things have happened.

My gut reaction is not an ingestion problem. I have operated all kinds of reciprocating engine airplanes in some pretty healthy rain and not encountered an ingestion problem. I have known some round engine folks encounter ignition issues during heavy rain.

Further

Thanks for all the insightful thoughts guys.
To address several things mentioned in the above threads:

I have not yet checked the plugs to see if they're super clean but I'm starting an annual inspection tomorrow and will then see, along with checking for a possible bent push rod. I was an hour into a 7 hour flight when the problem occurred, and then had to return 7 hours home, so the spark plugs are probably not going to still be super clean.

I agree, I could not have lost horsepower and not had any change in RPM, MP, CHT, EGT. I suspect I had a MP change, but did not note that reading, though it seems it would have been among the top things to check. Of course pulling carb heat on would have lowered it about an inch all on its own. The RPM did not change but I have a C/S prop. I believe my MP must have been affected but did not note that and do not have any data recording devices.

If I did get ice I suspect it was in the 90 degree plenum box behind the TBI. That was low on my list of suspects because of the rapid onset of roughness, but if others have experienced induction ice to behave this way, I suppose it is a very real candidate. I have the robbins heat muff, but it is only about 5 or 6" long. I've always wondered how much temp rise it gives me. I have no temp monitors in the induction system.
As I understand it the "typical" carb ice situation is gradually deteriorating MP from the restriction. When carb heat is added the melting ice causes a rougher/rougher engine for a while until the ice is gone.
I can't say it got any worse when I applied carb heat, but it did gradually get better.
Quite curious if others (in addition to xblueh2o) have experienced the rapid onset of roughness from carb ice, as opposed to the gradual loss of MP.

Yes, I did leave the mixture full rich once I first pushed it in, until the problems dissipated. In retrospect I thought of how that may have aggravated the problem. Once it smoothed out I then adjusted the mixture again. Later in the flight I went to full rich mixture at 11,000' just to see if on its own that would create a problem. There was a very slight change when I went from 75 degrees rich of peak to full rich. If I had not been paying attention to see if a change occurred I probably would not have even noticed it. Leaving the mixture full rich was probably not helping my situation, but I also don't think it was the reason the roughness persisted.

Good point about water not running out of my drain holes due to differential pressure in the air box. I wondered, and still do about that. Since I have no air filter and 165 kts ram air blowing straight into the airbox I wonder if I actually have positive pressure in the airbox which would force the water out. Again, I'm not sure about this, but the drain holes were cheap. I suspect with carb heat on the drain holes are less likely to drain because at this point I would not have ram air into the box and the situation would surely be as you described.