Hot Starts / Fuel Line Boil

E

ehaecker

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Location
Memphis, TN
Greetings all,

For you 7KCAB/8KCAB operators... My 7KCAB HATES hot starts and running at low power following a flight. Say, if I get fuel after a flight then start up to taxi to my hangar, it will start pretty easily, but it runs like absolute CRAP. It will sometimes give me a false start and quit a few seconds after starting...once I get it running, it will occasionally "miss" in a rhythmic fashion, burble and pop like an old radial. Am I the only one with this issue with an injected 320? Is there some sort of purging procedure that has proven successful to purge the vapor out of the lines? Once I increase power above about 1500 RPM, it smooths right out.

My friend with a PA-30 said his IO-320's did the same thing. Just wondering if there's a way to smooth it out a bit.

Thanks!
 
My 8KCAB does the same thing. I have tried the boost pump and playing with the mixture and running it up, all with no change.
 
Helo pilot said:
My 8KCAB does the same thing. I have tried the boost pump and playing with the mixture and running it up, all with no change.
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Good to know, I've tried the same. It's not ever a problem, just an annoyance when taxiing after a flight. Draws some funny looks from folks on the ground.
 
My current 8KCAB does not, but the previous one did. Turned out to be a fuel pump going bad.

There is also an idle mixture set screw on the fuel servo. Never monkeyed with it, but that seems like something that could cause such behavior, perhaps by being set too rich.
 
Big Ed said:
My current 8KCAB does not, but the previous one did. Turned out to be a fuel pump going bad.

There is also an idle mixture set screw on the fuel servo. Never monkeyed with it, but that seems like something that could cause such behavior, perhaps by being set too rich.
Click to expand...
Hmm, good point. It does sometimes "pop" and hesitate slightly when adding power after a touch and go or something. It's rare if it does, but I've noticed it. Perhaps a tad rich on the low end.
 
That rough running when hot seems to be common. Doesn't seem to affect anything.

Hot starts panic me. In 17 years I have never been stuck, but there have been times when I thought I was going to be.

I need a technique that doesn't take 12 blades. My current technique is:

Mixture lean, throttle cracked, boost pump on. Mixture slightly in for a heartbeat, then back to lean.
Throttle wide open, crank. Twelfth blade it catches, jam the mixture forward and pull the throttle back.

Again, I need a better technique.

Both my fuel pumps are relatively new.
 
I have no knowledge, just spit balling. But I do think the internet has a tendency to magnify anecdotes into a "known problem." IMO if hot idle was an inherent issue, most planes would experience it.

My aircraft always idled smoothly, hot or cold. It did have a hot start problem after fueling. For about a year I figured it was the "vapor in the lines" thing. Then I went back and read the factory hot start procedure and actually followed it. No problem starting.

A cheap and easy way to test: the next time your engine is idling rough, try leaning the engine and see if that smooths it out.
 
Bob Turner said:
That rough running when hot seems to be common. Doesn't seem to affect anything.

Hot starts panic me. In 17 years I have never been stuck, but there have been times when I thought I was going to be.

I need a technique that doesn't take 12 blades. My current technique is:

Mixture lean, throttle cracked, boost pump on. Mixture slightly in for a heartbeat, then back to lean.
Throttle wide open, crank. Twelfth blade it catches, jam the mixture forward and pull the throttle back.

Again, I need a better technique.

Both my fuel pumps are relatively new.
Click to expand...
Hot starts panic me too. But eventually I figured out I was flooding the engine. I did not believe it would start without a prime, so I primed it "a little bit". When I started following the factory procedure and starting with mixture cut off and no prime, it fired on the third blade.

I will say whoever adjusted my fuel servo did a hell of a job. That was one smooth running engine in all phases of operation. When I pulled it for overhaul, everyone at the airfield said I was crazy because it was so strong and smooth. Unfortunately, strong/smooth and making metal are not mutually exclusive, at least for a while.
 
I have tried that. My engine needs a bit of fuel to start - just a teeny bit. Then it needs a lot to keep running.
 
Bob Turner said:
I have tried that. My engine needs a bit of fuel to start - just a teeny bit. Then it needs a lot to keep running.
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I can get away without priming if its been less than about 15 minutes. After that, she likes a little boost. Once she starts, I have to pretend to be an amateur and rev it up to about 1500 RPM to keep it from quitting on a "false start". Once stabilized, it'll idle with the occasional "miss" and "pop" as fuel in the lines continues to vaporize due to heat and lack of pressure/flow. Mixture seems to have no affect. If anything, running lean makes it worse and almost quit, which I hate, because running full rich on the ground when its hot is no bueno. I typically ground lean until it's just barely running, but following a hot start, that is nearly impossible.
 
Big Ed said:
My current 8KCAB does not, but the previous one did. Turned out to be a fuel pump going bad.

There is also an idle mixture set screw on the fuel servo. Never monkeyed with it, but that seems like something that could cause such behavior, perhaps by being set too rich.
Click to expand...

Hey Ed could you expand more on how you determined it was the fuel pump, perhaps other symptoms? My Citabria has always had a hot idle stumble with the occasional backfire pop that I learned to tune out due to people on the web saying it was "common". The other day while climbing out on a cross country my engine started to miss and I initially dismissed it as an improperly set mixture so I adjusted it. That was when I noticed my fuel pressure was at the bottom of the green/lower red mark and the engine continued to burp. Turned on the boost pump and that brought me back up to a stable 30 pounds and I flew it back to base with my tail between my legs. Didn't do much in the way of trouble shooting unfortunately but I turned off the electric pump at one point and observed the fuel pressure drop again and the engine nearly quit on me again. Once back on the ground I did a full power static runup with the boost pump off and had no hesitation (damn intermittent issues....). Wondering if its indeed my fuel pump or if I experienced some form of vaporlock in the air (it was a hot Phoenix day afterall).
 
Did you try pressurizing on the ground with the electric pump? Overflow indicates a bad mechanical pump. It comes out a special line from pump to the lower cowl lip.
 
Bob Turner said:
Did you try pressurizing on the ground with the electric pump? Overflow indicates a bad mechanical pump. It comes out a special line from pump to the lower cowl lip.
Click to expand...

I haven't but I think I read about this on a mooney forum while researching the issue. Basically run the boost pump to build up pressure on the ground then see how long it holds with it off?
 
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