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Induction leak or MAP sensor leak?

J

jwilbur

Well Known Member
I've got two flights done in my RV10 and working out some potential bugs. I've isolated some data of the run-up on flight #2 (flight #1 run-up looks the same).

Is this normal? Shouldn't MAP stay constant during the mag check and prop cycles? And then stay low around 10" during my engine idle check? The plots below show MAP jumping to around 21" while the engine idles at around ~550 RMP? I'm wondering if this could be a sensor leak possibly?

But then if it were just at the sensor, wouldn't cylinder #5 (where the sensor is connected) run just a little bit leaner than the rest and therefor have the highest EGT? In this case EGT #5 is among the lowest.

runup-flight_2-768x385.jpg
 
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No so I think your data is consistent to what you should be seeing--MP should increase as RPM decreases. In simple terms as the RPM decreases the engine is drawing less air through the induction system and the sensor shows that as an increase in MP. During the mag check you have a ignition driven RPM drop so again there should be a corresponding MP increase.

BTW, that the inverse relationship between MP and RPM is one of the reasons why we're taught to reduce power from the left (throttle reduction first, Prop Second) and add from the right (Prop first , throttle second) so that you don't potentially exceed any MP limits on the engine.
 
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Auburntsts said:
No. MP should increase as RPM decreases. In simple terms as the RPM decreases the engine is drawing less air through the induction system and the sensor shows that as an increase in MP.
Click to expand...

That makes some sense. So you're saying these plots do look normal to you?
 
jwilbur said:
That makes some sense. So you're saying these plots do look normal to you?
Click to expand...

Can't speak to the magnitude of your MP increases as a result of the RPM changes, just that there should be a change. Someone smarter than me will have to weigh in on that qualitative aspect.
 
Your plots all look normal. You mention idle at around 550 RPM with MP at 21". I don't see that on your plots. On an IO-540, mag checks should be around 2200.

Vic
 
vic syracuse said:
Your plots all look normal. You mention idle at around 550 RPM with MP at 21". I don't see that on your plots. On an IO-540, mag checks should be around 2200.
Vic
Click to expand...

I was running at 1800 RPM for the mag check. The scale on the left is for MAP. The scale on the right is for RPM. Above where I annotated "IDLE CHECK" is where you can see the MAP go down to around 10" and then jump back up as the RPM falls to around 550.
 
My real mystery

So here's the real mystery that caused me to look at the run-up. On my first flight, when I reduced power and began my descent, I see this in the data. EGT #1 falls ~600 degrees.

Power up and EGT climbs back up with everybody else. On flight #1, I see this 3 times. Once during slow flight at altitude, once here, and once after the go-around. This is the most pronounced.

Thought it might be an induction problem, so tightened everything up (turns out worm clamps were under-torqued) and flew again. Flight #2 shows the same thing when descending, though only a drop of about 300 degrees and no dip in #2 or #4 (as is visible here).

flight_1_egt_drop_annotated-768x385.jpg
 
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If there's no other symptoms and the mag checks have been normal here's a couple of easy troubleshooting steps that might point to the smoking gun:

-- swap the EGT probe with another cylinder and see if the problem follows the probe

-- Pull the plugs on #1 and see if there's any fouling/damage, check the gap, and check the resistance.
 
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Auburntsts said:
-- swap the EGT probe with another cylinder and if the problem follows the probe

-- Pull the plugs on #1 and see if there's any fouling and check the resistance.
Click to expand...

Thank you. I'll do these things. ... As additional info which might be related:

1.... when pulling power while landing on flight #2 there was a "pop-pop-pop-pop" sound that came from the front right side.

2... And in flight #1 There were two instances where there was a sort of a bang. Almost like something tapped firmly on the lower right fuse. In the excitement of the first-flight I can't quite nail what else was happening at the time and afterward I dismissed it as a non-issue. I had taped down some ballast in the baggage area and reasoned it was some duct-tape pulling away in one of my sloppy turns.

I appreciate the help.
 
Well, if the symptom persists and the probe and plugs aren't the problem, then maybe do a compression check on #1 followed by a borescope to see if you can spot anything in the cylinder out of the ordinary.
 
Well, the "popping" can't be normal.....

However, I find that my N01 EGT falls rapidly after landing - much quicker than the others. I have just assumed that it is due the heat dissipating quicker due to the intake/cooling configuration.
 
Auburntsts said:
No so I think your data is consistent to what you should be seeing--MP should increase as RPM decreases. In simple terms as the RPM decreases the engine is drawing less air through the induction system and the sensor shows that as an increase in MP. During the mag check you have a ignition driven RPM drop so again there should be a corresponding MP increase.

BTW, that the inverse relationship between MP and RPM is one of the reasons why we're taught to reduce power from the left (throttle reduction first, Prop Second) and add from the right (Prop first , throttle second) so that you don't potentially exceed any MP limits on the engine.
Click to expand...

Let's get back to basics: the engine is an air pump. RPM is not the prime variable here. The throttle restricts that air. Open the throttle and more air goes through and results in more pressure in the manifold. Opening the throttle tries to equalize the MP with ambient. The MP gage is measuring in absolute pressure.

I don't think you meant "MP should increase as RPM decreases". That would only happen with a constant speed prop at WOT, as the air pump RPM is not pumping as hard against the fixed restriction of the intake system and fully open throttle plate.

To the OP - do you have a restrictor orifice in the MP line?
 
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