Topic Author
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Joined: Sun Jun 24, 2001 4:44 am

Major Fuel Flow Fluctuations With IO-470

Sat Oct 04, 2003 5:08 pm

Hey all,

Jhooper's posts about power loss reminded me of an incident I had about a month or so ago.

I was flying a Cessna 182P with a Continental IO-470 (yeah, IO-470). I had a really hot pax (temperature wise  Big grin), and, being a blistering hot day, climbed up to 9,500' to get her some cold air. It was a normal climb all the way, and I leaned the engine as recommended--50 degrees rich of peak EGT, cowl flaps up, EGT's on all cylinders steadily around 1370 and stable CHT's, OAT of 43 degrees F and I think the oil temp was around 170. Fuel flow was 13gph. I was using 2400 RPM on the prop, and I think I had something on the order of 22" MP...standard cruise settings for the engine. I have used it all the time, engine's made it through an overhaul and 2 annuals with me as the primary pilot with no major problems.

All of a sudden after being in cruise for about 15 minutes, I noticed the fuel flow gauge start to fluctuate wildly...back and forth from about 18gph down to about 4 or 5. I could percieve no engine roughness, nor did I notice any appreciable EGT/CHT change. Manifold pressure and prop RPM were steady, as were oil temperature and pressure. I immediately prepared for engine failure (of course thankfully I had altitude and an airport with 6,000' runway right below me) and then began tinkering with the mixture and throttle. Nothing I did seemed to change it much, but after about thirty seconds had passed everything returned to normal for some odd reason. I wrote a note when I landed to my A&P but unfortunately I had to leave town the next day and haven't been back to hear an explanation. From what I've heard, the airplane's been flown since over a long distance and is running fine, but I still haven't talked to the mechanic.

Any ideas?

Here's the airplane (and hey...that's me!) Pardon the weird expression...I was looking directly into the sun  Smile

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I got my head checked--by a jumbo jet
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RE: Major Fuel Flow Fluctuations With IO-470

Sun Oct 05, 2003 2:20 am

Because of the lack of other symptoms, I'd say it was a faulty gauge.

I flew a Seminole on which the left tach would constantly swing back and forth 1000RPMs (no typo, one-thousand). The engine was obviously not fluctuating like that.

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RE: Major Fuel Flow Fluctuations With IO-470

Sun Oct 05, 2003 3:04 am

I'd make a point to talk to the mechanic and see what they found. I've never flown a 182 with an IO-470, but with a fluctuation like that I would probably have to suspect something to do with the fuel pressure sending unit or perhaps, more serious, a fuel pump. Could it be a faulty gauge? Sure, but like Ralpha's tachometer, faulty gauges "constantly swing back and forth". From my experience, they continue their erratic indications. I have now told you everything I know about this kind of thing. I'm sure that there must be some experienced light aircraft mechanics on this forum.

Topic Author
Posts: 3454
Joined: Sun Jun 24, 2001 4:44 am

RE: Major Fuel Flow Fluctuations With IO-470

Sun Oct 05, 2003 4:03 am

Thanks for your replies! I've been trying to get hold of the A&P but haven't been able to...I'll be going home soon though.

One more thing to mention--Sometimes I notice when I give it full throttle (I mean ABSOLUTELY up against the stop) I'll get erratic fuel flow readings (and I think sometimes the manifold pressure gauge fluctuates too) but when I pull it off the stop just barely, it goes away and stabilizes.

Malfunctioning gauge or not, it's really a disheartening feeling to watch your fuel flow gauge suddenly plummet out of the green arc in cruise  Wow!
I got my head checked--by a jumbo jet
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Joined: Mon Nov 12, 2001 10:34 am

RE: Major Fuel Flow Fluctuations With IO-470

Sun Oct 05, 2003 12:01 pm

The engine driven fuel pump on injected TCM engines supply more fuel to the 'spyder' (fuel divider on top of the engine) than is required for operation.
This is for fuel vapour suppression. The additional fuel is returned to the tank...which tank however, depends on the aircraft model.
At higher ambient conditions at higher altitudes (depending on the engine model), some fuel vapour will start to form anyway, and as your fuel 'flow' gauge is actually a fuel pressure gauge (marked in gal/hr), the needle will fluctuate.
The usual procedure is to switch the electric fuel pump to the 'low' position.

This is especially noticable with turbocharged engines, but can happen with non-turbo engines as well.

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