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The other day I started the engine of a PA28 and put it at 1200 RPM as per the checklist but the sound didn't seem right - so I tried each magneto and the power dropped a lot on one of them. I then pulled the throttle to idle and the engine stopped. It was on for maybe 30 seconds so it was still cold.

I restarted the engine, went through the standard checklist and everything was fine the second time (I ran the magneto and idle checks once the engine was warm).

Is this behaviour expected from a cold engine (large power drop on single magneto and engine stop at idle)?

For reference, the aircraft is a PA28-180, I believe the engine is a LYCOMING O-360-A4A with ca. 500 hours. The outside temperature was 15C.

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    $\begingroup$ I started a cold Cessna 152 yesterday following the POH priming instructions and it started right up, then about 10 seconds later before I did anything, the RPM went up to about 1200 then slowly died down and engine shut off. I then restarted the engine (without priming) and it started right up and the after taxing the run-up before takeoff was normal and I flew for 1.1 hrs without issue. I don't usually hold the behavior of a cold engine against it, but I do listen carefully to what it tells me during the run-up procedure when its warm and immediately before take-off. $\endgroup$
    – Devil07
    Nov 20, 2017 at 14:41

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What you experienced is common, but it would serve you well to better understand your engine.

There are quite a few engine options for the PA28, so without more specifics I cannot give you specific guidance, but I will address this in a general form.

Spark plugs foul from several different sources. The leading sources are from excessive unburnt fuel, and from oil. The unburnt fuel can cause carbon and fuel based buildup on the plugs, or it can come in the form of lead fouling, where lead deposits bridge the spark gap or insulator in the spark plug. Oil fouling tends to happen in higher time engines, but can happen in new engines during their break-in period. Normally, running at cruise power keeps the plugs clean by scavenging these deposits off.

Again, engines vary, and manufacturer's recommendations vary, but in general, I run with the engine lean during taxi. This reduces buildups and will clear some fouling. Because the engine running is impacted by engine temperature, I generally do not do a run-up until after taxi, when the engine is warmer.

If an engine is running rough, or the mag check fails, or the engine repeatedly stalls, I will do a clearing run-up, where the engine is run at a higher power setting, typically 1800 to 2000RPM and leaned until it will just continue running. This creates higher temperatures, and tends to burn off oil or other fouling. This is done for anywhere from 1 minute to about 3 minutes, and then the mixture is enrichened and a normal run-up and mag check are done.

Magnetos do not foul, rather spark plugs foul, and typically the R mag will run the top plugs on one side of the engine, and the L mag will run the top plugs on the other side. Bottom plugs tend to fould more, especially if there is oil fouling. But the point is that whether the L or R mag fails, or runs rough, doesn't tell you exactly what spark plugs it is. A cylinder temp gauge or other instrumentation may, but usually it is determined by disassembling and inspecting the spark plugs.

So to get to the OP question: Large drops at runup are a sign of either a condition which may resolve itself, or conditions which are causing the engine to run less effectively. If they can't be respoved the plane needs to be seen by a mechanic, or someone more familiar with it. If the mag is rough, and the engine has just been started, then it is premature to perform a normal mag check. Let the engine ward up more, during taxi, and then check the mags. The engine should not die at idle. If that happens repeatedly after the engine is warmer (not even up to operating temps), then there is a problem, and the engine should be checked out by a mechanic or someone with more experience.

In addition to not clue as to what engine you are dealing with, we also don't know the local meteorological conditions. If you are having problems at 0F it is more typical than having problems at 35F. But in general, engines should not stall at idle even when cold at 40F temperatures.

A final comment. I have trained many CFIs, and I spend quite a bit of time on engines and their operation and work-arounds. Much of this is not in FAA publications, or even training publications. I find that many CFIs are not a knowledgeable about engine operations and related methods. Being engine smart will help you in your flying, and it will reduce your risk.

Addendum:

As a general rule: Lycoming: R Mag top left plugs, cyl 2, 4 and bottom right plugs, cyl 1, 3. Continentals: R Mag fires the top plugs, L Mag fires the bottom. The impulse is the R Mag, as the top plugs tend to get less oil fouling.

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  • $\begingroup$ Thanks for your answer - additional info: PA28-180, I believe the engine is a LYCOMING O-360-A4A - ca. 500 hours. Outside temp was 15C. $\endgroup$
    – assylias
    Nov 20, 2017 at 14:13
  • $\begingroup$ If you have this problem with that engine at similar temps, there is a problem. It could be that the last person flew it full rich, or that it is leaking a little oil. An OAT of 15C should not cause a rough run or stall after start. When you preflight, look at the oil add history. With 500 hours the engine is at it's prime, assuming that it was broken in properly, and has not experienced any problems. $\endgroup$
    – mongo
    Nov 20, 2017 at 17:58
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    $\begingroup$ One more thing, just to soapbox, when starting that engine, you should use about 1 or 2 shots of prime, at 15C. At -20C you may need 4 shots. The throttle should be 6 to 12mm open, and should NOT be pumped. If it has to be pumped the primers are clogged, and by pumping the updraft carb, you are introducing the #1 cause of engine start fires. Depending upon where the plane was initially delivered, it will have either a single primer or four primers. If you have the time, offer to help a mechanic with oil changes or an annual. Even if you are not mechanical, do grunt work. Great education. $\endgroup$
    – mongo
    Nov 20, 2017 at 18:03
  • $\begingroup$ You probably want to add the OAT was 15c and Engine type to the original post to cut down on poor answers. Any properly operating normally aspirated engine should have no trouble at 15c. It's plenty warm enough to maintain atomized fuel. $\endgroup$
    – Rowan Hawkins
    Nov 20, 2017 at 20:52
  • $\begingroup$ @mongo I'm guessing here based on my general engine experience... The engine start fire would be caused by flashback of atomized fuel in the throttle body because of the throttle action causing a backfire start condition. $\endgroup$
    – Rowan Hawkins
    Nov 20, 2017 at 21:10
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Modern computerized engines have multiple sensors for air temperature and flow, engine temperature, exhaust temperature and oxygen content that allow the computer to select the optimal fuel-air balance for the engine when started. When you start one of these from cold it's rare for it to run rough. Most airplane engines are manually controlled and carbureted, and it's very normal for them to be a bit lumpy when cold in cold weather, in my personal experience Continental engines are more prone to this than Lycoming. It's less common for a mag to have a big drop, but it does happen when a mag gets fouled and it usually clears itself up. As long as the engine is smooth once heats up you're fine. If you still have a mag drop you can run up the engine and then lean it aggressively to burn off the gunk from the mags.

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