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Hard to start

  • rtc183
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1 week 5 days ago #21048 by rtc183
Hard to start was created by rtc183
Flew out for a hundred dollar hamburger yesterday for the first time with my 10hour engine while still testing and I went to lunch and came back an hour later and did not hand prop to check the oil. The engine turned over and started, barely, then shut back down after one second. The Dynon said to land right away. I cycled the power and after several more attempts it started and ran well after a few seconds. I think it was because I had too much oil in the crankcase and I should have propped it and checked the oil after the long sit in 45 degree weather. Any ideas?

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1 week 5 days ago #21050 by Bill Hertzel
Replied by Bill Hertzel on topic Hard to start
Please confirm we are discussing a 912iS fuel injected engine.

The Oil pooling should not make any difference.
It will clear in a few seconds.
An hour or so is Not considered a long time between starts.
Checking the Oil level is normally only done on the first start of the day unless you noticed a puddle under the engine.

Were both fuel pumps energized before the start attempt?
Did you have nominal fuel pressure before the start attempt?
It may help to get the fuel pumps running early in the checklist.
This gives time for the pumps to clear any vapor in the lines that may have formed shortly after shutdown when the engine was still hot.

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1 week 5 days ago #21051 by rtc183
Replied by rtc183 on topic Hard to start
912is one pump. Didn't wait long after pump turned on before hitting the starter. Pressure looked good by the time I looked at it after a couple of start attempts. Will turn the pump on earlier next time. I will also look at the Dynon download in a couple of days when I am back home.

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1 week 5 days ago #21058 by GlennM
Replied by GlennM on topic Hard to start
What size battery do you have ? The IS engine needs a bigger battery than a carbed 100. Rotax recommend a minimum of 16AH. Often people/manufacturers put in an IS with too small a battery and on cold days that can be a problem. If you have a lot of avionics you may need even bigger.

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1 week 4 days ago - 1 week 4 days ago #21065 by Flocker
Replied by Flocker on topic Hard to start

Bill Hertzel wrote: Were both fuel pumps energized before the start attempt?


What are your thoughts behind this question? Doesn't the OM say to start using only one pump?

Should both be energized first then use one to start?
Last edit: 1 week 4 days ago by Flocker.

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1 week 4 days ago #21072 by Bill Hertzel
Replied by Bill Hertzel on topic Hard to start
My thoughts are this...

The two pumps are Positive Displacement Pumps plumbed in series.
With both pumps operating, the same amount of fuel is going to be moved as a single pump and at the same pressure.
The normal difference would be that the two pumps would each do approximately half the work.

If when the engine was shut down, the heat within the engine compartment, in addition to the heat generated by the electric motors within the pumps themselves, raised the temperature of the fuel above its vapor point, the fuel would "Boil" locally and generate a vapor bubble in the fuel lines or the pump bodies themselves.

When restarting, this vapor bubble will need to be purged from the fuel lines for the engine to operate reliably.
The fuel pumps are capable of pumping liquid at nearly 100psi but can only pump vapor at ≈2psi (A confirmed value).
Until the pump(s) can get the fuel rail up to 45 psi, the only exit path for the vapor is through the relief orifice and the fuel return line.
Operating with both pumps can only accelerate the vapor pumping.
It may take 10-20 seconds for the pumps to move enough vapor that they regain their liquid prime and are able to pump the vapor bubble rapidly through the fuel regulator.
Additionally, if you start on a single pump and then energize the second pump sometime later, it is possible that you might introduce the vapor bubble that was within the second pump body into the fuel rail and experience the mystery stumble reported by some operators when activating the second pump.

Once full fuel pressure is established, the pumps will move ≈30 gallons per hour (≈1 liter every 30 seconds) through the fuel system.
This will be relatively cool fuel coming from the tanks and prevent any additional vapor formation.

Running a single pump prolongs the time it would take to clear the lines.
Operating both pumps simultaneously does Not draw twice the electrical power. Each pump works half as hard.

Whether you start in single or dual pump mode is fine.
You could also use Both Pumps initially to freshen the entire fuel rail and then revert to a single pump for the start.
In any case, it would be wise to confirm that your fuel pressure is nominal before the start attempt.
This may require waiting a few seconds for the pressure to stabilize, or modifying your checklist to energize the fuel pumps a few checklist items earlier.
The following user(s) said Thank You: Flocker

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1 week 4 days ago #21081 by rtc183
Replied by rtc183 on topic Hard to start
The battery is an Earth X and it cranked it fine. Still haven't looked at the downloaded data but I know I didn't have the pump on for long before cranking. I like the idea of running both pumps and checking pressure before going to one for start. Will post the data log findings in a couple of days.

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1 week 4 days ago #21085 by GlennM
Replied by GlennM on topic Hard to start
Battery should be OK then..
My POH says turn on the Aux pump for five seconds. With this I hear the pump stop cavitating and see pressure in the green. Then I turn off Aux and start the engine. Over 500 hr so far and no starting issues.

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1 week 3 days ago #21089 by rtc183
Replied by rtc183 on topic Hard to start
Mine will say that too. Thanks.

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1 week 2 days ago #21098 by rtc183
Replied by rtc183 on topic Hard to start
The Dynon download confirmed that the fuel pressure was marginal for the start attempts and then went down into the 30s or much less on one.
The final attempt had the pressure staying fairly high and the engine barely running until the pressure went back to normal and the rest of the flight was normal. Thanks for the help here.

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