After hot starts engine isn't smooth for a few mins.... New to Fuel Injection...

[Huckster79]

Im still getting used to Fuel Injection from flying a carb for a dozen years... 

I notice after a hot start, which I think I've got down now pretty good, that the engine just doesn't run perfectly even for several minutes afterwards, not rough in general but just a periodic rpm drop then back to normal so to try to describe it visually:

1100rmp...................................................................................1050rpm.......1100rmp....................................................................1025rmp.......1100rpm.......................................................... etc... Probably lasts 1-3 minutes.... 

Is that just the vapor lock clearing out and its process of getting cool fuel flowing continuously again?  Or is that a sign of something?  Never happens on a cold start.  

 

 

[ArtVandelay]

1. Run RPMs 1200+
2. Turn on boost pump.
3. As a preventative measure, open cowl flaps on approach to cool engine down.

[AndreiC]

I have the same issue, and haven't figured out what it is. It smoothes out above 1400 RPM, but is rough below. You can also occasionally hear unburned fuel occasionally making a mini-explosion inside the muffler. My mechanic said not to worry about it for now, and to get the idle mixture adjusted at the annual. Still, not very confidence inspiring. Would like to find out what it is too.

Best,

Andrei.

[Hank]

3 hours ago, Andrei Caldararu said:

I have the same issue, and haven't figured out what it is. It smoothes out above 1400 RPM, but is rough below. You can also occasionally hear unburned fuel occasionally making a mini-explosion inside the muffler. My mechanic said not to worry about it for now, and to get the idle mixture adjusted at the annual. Still, not very confidence inspiring. Would like to find out what it is too.

Best,

Andrei.

If this happens on the ground, you can fix it by leaning the mixture.

[A64Pilot]

The after firing, burbling in the muffler is from mix being too rich. If you lean lean aggressively, so much that you can’t attempt a takeoff, or I promise you one day you eventually will takeoff leaned out and that’s killed people in the past. You can’t hurt an engine at all at taxi power with mixture, many even promote it to keep plugs clean.

The roughness after starting is most likely vapor lock, it’s hot now so of course it’s worse, depending on where the vapor lock is occurring boost may help, pressure raises the boiled point of a liquid and vapor lock is of course the liquid fuel boiling.

[AndreiC]

22 hours ago, Hank said:

If this happens on the ground, you can fix it by leaning the mixture.

Nope -- tried that and it did not do anything. I suspect vapor lock as mentioned above makes some cylinders not fire occasionally, and the unburnt fuel from those cylinders then catches fire in the muffler.

[EricJ]

22 minutes ago, Andrei Caldararu said:

Nope -- tried that and it did not do anything. I suspect vapor lock as mentioned above makes some cylinders not fire occasionally, and the unburnt fuel from those cylinders then catches fire in the muffler.

Does your airplane have a sniffle valve?    If so, it might be leaking a bit more when hot than cold for some reason.    It just makes a little vacuum leak that will be more pronounced at low throttle, especially idle.   It's not a big deal.    My airplane doesn't have a sniffle valve and it still does that little lopey idle waver sometimes.

[kortopates]

Does your airplane have a sniffle valve?    If so, it might be leaking a bit more when hot than cold for some reason.    It just makes a little vacuum leak that will be more pronounced at low throttle, especially idle.   It's not a big deal.    My airplane doesn't have a sniffle valve and it still does that little lopey idle waver sometimes.

A J without a sniffle valve- didn’t know that was possible? Interesting, Is it original that way or did the cylinder drains get plugged?


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[EricJ]

37 minutes ago, kortopates said:

A J without a sniffle valve- didn’t know that was possible? Interesting, Is it original that way or did the cylinder drains get plugged?


Sent from my iPhone using Tapatalk

Mine's a 77, and as far as I know it never had one.   It did get an engine change about thirty years ago, though.

[AndreiC]

How do you check if your plane has this sniffle valve (first I've heard of it) and if it is a source of a vacuum leak? I can wait until the annual, and will mention it to the mechanic, but I am also curious.

[PT20J]

44 minutes ago, Andrei Caldararu said:

How do you check if your plane has this sniffle valve (first I've heard of it) and if it is a source of a vacuum leak? I can wait until the annual, and will mention it to the mechanic, but I am also curious.

There will be a drain hose connected to the bottom of the sump. If you run the boost pump for 30 seconds or so with mixture rich (engine stopped) you should see fuel draining from it. The sniffle has a check ball. If you blow on the drain tube it should close and if you suck it should open. (If it drains, you don't really need to do the suck test unless you really like the taste of gasoline ). This thing is a homemade Mooney part. When I put in the new engine I cleaned it but it never sealed perfectly. I doubt they seal perfectly when new.