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Engine runs rough after start with cold outside air temperatures.

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This is the first winter with my 1980 231K. My last few starts were the first cold outside air temperature starts. I preheated the motor so that Oil Temp and CHT’s were well above 80 degrees, the outside air temperatures were in the 30 degree range +/-.

 

After a five second fuel prime it fires up every time on the first start attempt, but it does not want to stay running with cold outside air temperatures. It wants to die and I have to milk the fuel prime to keep it running or turn on the Low Boost pump. I can keep it running with the prime or boost, but it does not run smooth and after turning off the Low Boost or Fuel Prime it still wants to die. With all pumps off, I jockey the throttle to try and keep it alive and with throttle application it wants to sputter and die.

 

After a few consecutive engine starts, and 10 to 15 minutes of this, I finally get it running smooth enough to taxi to the Mooney Service Center at my home field. For the taxi over I have to keep the RPM’s in the sweet spot of 1200 RPM and ride the brakes, milking the prime from time to time to keep er alive, (This is after a call to this MSC two days prior about these starts and they just said the K’s don’t like cold weather starts).

 

The start is not the problem, it is the rough running engine after engine start.

 

So when I get over to the MSC, I start it up to show them the finicky cold outside air temperature starts and it fires right up with nothing abnormal about the start, “Could Not Duplicate”.

 

The last few times that I went out to the airport for these cold outside air temperature starts, I would normally be able to get it running smooth after the 3rd, 4th, or 5th start, it would start every time, but it would not run smooth even milking the prime until it would just decide to start normal on later start attempts.

 

The MSC removed the upper and lower cowls and checked everything. The idle was set at 800 RPM and RPM did not rise when placing the Mixture control to Idle Cutoff. So, they set the idle RPM to 730 RPM and now the RPM will rise to 760 RPM as the Mixture is pulled toward Idle Cutoff. The engine driven fuel pump is a fresh overhauled pump with only 15 hours on it, replaced it two months ago due to a fuel drip issue.

 

Of course, it started perfectly at the MSC before and after they changed the idle setting, which it tends to do once it gets it’s first good start for the day with cold outside air temperatures. The MSC said they have a few 231K customers and they all have similar issues.

 

I am heading back out to the airport tonight after letting it sit for about 8 hours to see if it still has the finicky starts, or if the re-setting of the RPM helped.

 

Has anyone ever experienced this, and/or have any suggestions to prevent these finicky starts? Is a solid 5 second prime too little and the cause of this, should I go for 10 seconds, should I try something else?

 

Thanks!

 

Ken

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Posted · Report post

Ken I have a 1981 231 and mine is the same as your 231.  In the summer it does the same thing but after startup you may have to hit the prime for another couple of seconds to get it running smooth.  Last week, its in a heated hangar, and it was -10c outside and I was on the prime on and off for a good 15 seconds, it would die if I wasn't.  I think this is normal for the plane, away starts on the first crank and farts around after that.

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Posted · Report post

Do you have an engine monitor so you can see the EGT of all 4 cylinders?

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It does sound like the idle mixture is lean, and on really cold days (desne air), it can't deliver enough fuel without some extra from the fuel pump. Running it at 1200 RPM gives it fuel through the main fuel meterng circuit.  The Lycoming's RSA-5 fuel injection mixture is not adjustable, but on a 231 it is.

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Posted · Report post

Its my understanding that most 231 are setup very rich, I would be surprised if to lean was the issue.

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Thanks for the replies. I do have an engine monitor, MVP-50P and all EGT's were pretty much in line. I have mag-out indications on the engine monitor and the mag-out never annunciated. I am starting to think that it really is finicky to cold outside air temperature starts.

 

FBCK, how long do you prime and where is your throttle set when priming and when cranking? I prime and crank with the throttle 1" open, it seems to like it there except for these cold temperature starts that I have not figured out yet.

 

Thanks,

 

Ken

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all internal combustion engines need a very rich mixture on startup with cold o/s air temps
some of you with birthdays close to mine will remember a knob on vehicles called a choke. This provided a rich mix untill internal engine temps would support combustion. Then along came automatic chokes and now computors adjust mix according to temp. Aircraft don't have this system. Carburated engines have a primer, which when very cold required more pups and if i remember correctly required more prime after startup on cold days to keep eng running untill it warmed a bit. Our fuel injected engines use the prime pump and in my case the low boost pump (I0550) to do this. After initial start I usually hit the pump intermittantly to keep running. I believe the procedure is in POH under cold starting.

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Posted · Report post

Do you have an engine monitor so you can see the EGT of all 4 cylinders?

What about the other two??

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I am having the same issue in my '80 231. I took it over to the shop on the field to have them look at it. This time she was being very weird. I too have a recently overhauled fuel pump and when the MSC set it up they did not adjust the idle pressures low enough (8.5 psi vs the 6.5 psi it should be). The shop was making the appropriate adjustments when all the sudden on the 3rd iteration it ceased to run off the engine driven pump. OAT is in the 80's. they are calling Continental in the AM. They are a Cirrus authorized shop, so they do have a clue about what they are doing.

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