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Fuel issue at start

Hi there,
I've noticed the last 3-4 times I've started the engine (IO-520 BA), that there is a fuel (starvation?) issue maybe 5 to 10 seconds after the engine starts and stable at 1000-1100 RPM.
The first two times it caught me off guard and the engine shut down. After restarting, I kept the RPM a little higher and the engine behaved normally with no issues.
The last two times I was expecting it, so when the engine started to stumble, I turned on the electric fuel pump just long enough to keep the engine running smoothly and then I had no issues.
All run ups are normal after that.

The start up procedure has always been the same:
Throttle FULL, Mixture Rich, Battery ON, Fuel pump ON until the fuel flow stabilized (16-18GPH in 3-5 sec), Throttle back to idle and then add two turns, Ignition.
The engine starts after maybe one prop rotation and stabilizes at 1100 RPM.

The only difference lately is that the engine heater has not been on, but I don't think that that might be a factor.

Any suggestions/ideas of how to troubleshoot are welcome.
Thank you

Model: Bonanza V35A
Posted 3/3/2023 - 1 year ago
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Rodrigo;

We have had a winter like I don't recall much the same as the past up here in the PNW. Not as cold and as much snow as usual, but general damp cold. That translates into less vaporization of the fuel. Generally, a cold-soaked engine requires a little more prime to get to run successfully.

Or pre-heat.

You mentioned that hasn't been used as much lately, and your priming procedure is pretty much always the same; this could be a key. Naturally, there can be other contributing factors: Fuel system set up, ignition being top notch and the like. However, you indicated that after the initial stumbling is caught and aux pump helps, and after that operations are normal, it tells me that extra fuel is what she is asking for.

The fuel injection system pressures and flows are set to be accurate when the engine is up to operating temperature; there are no computer controls that compensate at lower temperatures. The next time after you fly, at shut down, check your idle mixture. Establish a smooth idle at 700 -800 RPM, and check your mixture by either of these methods. If your tach is stable, look for a RPM rise of about 50 before the engine quits on a slow mixture pull to idle cut-off (ICO). If your tach is not stable, look for about a half inch of MAP loss during the same time (3/4 of an inch if using digital equipment). As the engine starts to die off, return to full rich and try it again, look for an average. If you get less RPM rise, or less MAP drop, it might be time to have the fuel system looked at by your maintenance provider.

Mike
ABS Tech Advisor

Posted 3/3/2023 - 1 year ago
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Thanks Mike.
I was thinking I might be dealing with a leak introducing air into the system.
I’ll start by priming the engine a little longer and see if it stumbles. I will also do the shut down check.
Hopefully everything is working as it should. I’ll also get the pre heater working again and see if that makes a difference.
I’ll circle back with the findings.

Thanks again for your input,
Rodrigo

Posted 3/4/2023 - 1 year ago
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I do not think "priming it a little longer" is the answer. The engine eats up the prime fuel rather quickly and then if the OAT is very low or the engine was not pre heated it may or will quit unless you provide it with enough extra fuel for a few moments until there begins to be some available heat so the incoming fuel will vaporize enough to keep the engine running.

Regards, Lew Gage

Posted 3/4/2023 - 1 year ago
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In reply to: Lewis Gage | 3/4/2023 - 1 year ago

I do not think "priming it a little longer" is the answer. The engine eats up the prime fuel rather quickly and then if the OAT is very low or the engine was not pre heated it may or will quit unless you provide it with enough extra fuel for a few moments until there begins to be some available heat so the incoming fuel will vaporize enough to keep the engine running.

Regards, Lew Gage

Lewis,

You were right. I did require a lot more than a “a little bit longer” to get a normal and stable start with a cold engine.
I went back today after pre heating the engine through the night and it started up right away with the normal priming.

Cheers,
Rodrigo

Posted 3/5/2023 - 1 year ago
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In reply to: Michael E. Thompson | 3/3/2023 - 1 year ago

Rodrigo;

We have had a winter like I don't recall much the same as the past up here in the PNW. Not as cold and as much snow as usual, but general damp cold. That translates into less vaporization of the fuel. Generally, a cold-soaked engine requires a little more prime to get to run successfully.

Or pre-heat.

You mentioned that hasn't been used as much lately, and your priming procedure is pretty much always the same; this could be a key. Naturally, there can be other contributing factors: Fuel system set up, ignition being top notch and the like. However, you indicated that after the initial stumbling is caught and aux pump helps, and after that operations are normal, it tells me that extra fuel is what she is asking for.

The fuel injection system pressures and flows are set to be accurate when the engine is up to operating temperature; there are no computer controls that compensate at lower temperatures. The next time after you fly, at shut down, check your idle mixture. Establish a smooth idle at 700 -800 RPM, and check your mixture by either of these methods. If your tach is stable, look for a RPM rise of about 50 before the engine quits on a slow mixture pull to idle cut-off (ICO). If your tach is not stable, look for about a half inch of MAP loss during the same time (3/4 of an inch if using digital equipment). As the engine starts to die off, return to full rich and try it again, look for an average. If you get less RPM rise, or less MAP drop, it might be time to have the fuel system looked at by your maintenance provider.

Mike
ABS Tech Advisor

Hi Mike,

So, here is what I found out. With the engine cold, I primed it a lot longer than normal and it started and kept running without stumbling. I left the pre heater on through the night and this morning the engine started right up with normal priming. You were right, it was a temperature issue.
I also did the idle check multiple times but I got the same results every time, namely the complete opposite to what you mentioned. I got a rise in MP and a drop in RPM, whether I pulled the mixture slowly or fast, I got the same result. Do I need to call you and make an appointment?
(I hope the video attachment works showing the MP/RPM gauges)

Posted 3/5/2023 - 1 year ago
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A drop in RPM without the rise, or a rise in MAP without the drop indicates too lean; sounds like a fuel system pressure check and any adjustments are in order. My schedule is pretty tight right now, check with your regular maintenance provider if they can get you in.

Posted 3/5/2023 - 1 year ago
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Thanks Mike. My “regular maintenance provider” is a mobile A&P since the shops here in North Seattle area airport are booked way way in advance. Is this adjustment a somewhat easy procedure? For this kind of procedures I’d like someone with experience so as not to start chasing my tail and making things worse.
Do you have any recommendations in BFI/PAE/AWO area?

Thanks,
Rodrigo
rmulchi@gmail.com

Posted 3/5/2023 - 1 year ago
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Easy if you have the tools and documentation. Doing it on a regular basis is bonus points. Suggest you call Lyn Holderman; his company is Vertex Aviation (360-757-4876); I found out last weekend they are opening a branch at Arlington. They work on mostly Cessna (185's are a specialty, those have the injected Continentals), with some Beech clients. Tell him I suggested him.

Posted 3/5/2023 - 1 year ago
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Great! Thank you very much.

Rodrigo

Posted 3/5/2023 - 1 year ago
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Fuel issue at start