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Intermittent, short-duration, smooth power losses

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Disclaimer: These IAC pages are developed by individual IAC members and do not represent official IAC policy or opinion.

Collected by: Dave Hirschman


I've been having a phantom engine problem in my Pitts S1S and would appreciate some guidance.

Recently, the 180-horsepower IO-360 engine with a throttle body has had intermittent, short-duration, smooth power losses. Switching mags, adjusting the mixture, using the wobble pump seem to make no difference. All engine gauges remain steady and temperatures are in the green. Usually, the problem comes about after the engine is hot from 20 minutes or more of aerobatic flying.

At first, I suspected the ignition system. But I've installed two new mags, a new harness and eight new sparkplugs to no avail. I've also checked all fuel screens, cleaned the injectors and flow tested the fuel injection system. I have done a compression test, tightened the air intakes and searched for sticking valves, but I haven't found the culprit and I'm running out of ideas.

From: Dave Hirschman

Summary of Solutions:

Thanks for the many helpful responses to my question regarding the "Mysterious Engine Problem" that had been afflicting my Pitts S1S. Don Peterson and Mike Davis, go to the head of the class! You correctly diagnosed the symptoms (momentary power losses after 20 minutes or more of akro), and came up with the right fix.

It turns out that my tightly cowled IO-360 was getting hot enough to partially vaporize unburned fuel in some of the lines. Bubbles formed in a low-pressure area and were carried to the cylinders via the fuel injection system.

Simply insulating the lines appears to have cured the problem. I flew twice today during the hottest part of the afternoon, and the engine ran like a top. The previous owner of this particular aircraft lived in the upper Midwest and didn't have to deal with the heat and humidity of the Mississippi Delta . . .

From: Dave Hirschman


You don't say what kind of throttle body. Ellison? Bendix? Other?

You might rig in an exhaust gas temp gauge and track the trends. If you are getting escessive fuel line heat and fuel bubbling/vapor lock (as I suspect), you might see a rise in exh gas temp as a result of a lean mixture condition.

If this is it, various blast tube set-ups to cool things might work.

From: Don Peterson

Vapor lock? Maybe heat shield the fuel lines.

From: Mike Davis

For what it's worth: I had a similar "smooth" power loss in my Decathlon (AEIO320, CS). It was not intermitant but could be similar.

Check you muffler/exhaust system for loose baffles or other restrictions. On the Decathlon the power was down about 5% but it ran great, very smooth except it would go a bit rich, ie I could get some of the power back by leaning.

I went through similar evolutions before finding the restriction, a collapsed flame tube in the muffler. I'm wondering if you may have a loose baffle of something that partially blocks the exhaust after a bit of acro shakes it around.

From: Scott McMaster scott@mcmail.CIS.McMaster.CA

I'm not an a&p, however, you might look at your key switch. I have had a problem in the past with a "dead spot" in the middle of the L and R positions.

From: "Matt Chapman"

On other possible area might be a bad fuel hose. I have read of several accounts where a piece of the inner linner would fold up and restrict fuel flow. This can be caused by a tear of the liner when the AN fitting was installed on the hose. It could also be caused by deteriating fuel line. I like the steel braided teflon fuel lines better because of this problem.

Another area is to make sure the air vent into the tank is clear. No kinks in the line, no bugs in it.

Does this happen when the main fuel tank is full? If it runs OK with a full tank but not so well when the fuel tank is below 1/2 full it could be a crack in the flop tube in the tank that could be sucking air instead of fuel.

The last area would be someting is getting too hot. Make sure the lines have the fire sleve installed and that the exhaust stacks are not too close to any fuel lines.

From: Herman Dierks

© Dr. Günther Eichhorn
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