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Manifold pressure blues

Ed_Wischmeyer

Well Known Member
In the five years I've had the RV-9A (third owner, O-320, carbureted), the manifold pressure readout (Garmin G3X Touch) has had an annoying tendency -- it lags the the throttle position. For example, reducing manifold pressure from cruise to 18", pull the throttle back to see 18", increase the friction so that the throttle doesn't creep. A few seconds later, it will read 16" or 16.5" or something.

We thought it was the restrictor at the manifold, or maybe the line between the manifold and the sensor was clogged, or... At the last annual, all of the old rubber engine tubes were replaced, so we did the manifold pressure line, too. When we took off the old line, we discovered there was NO restrictor. We put one in and put in the new line. No change.

So maybe it's the sensor. Stein had one, mercifully, and I swapped it in this morning. No change. (The old sensor has maybe 600 hours on it, apparently works as well as the new one, so if you need one, let me know.)

Another trait when I got the airplane, and I guess I just ignore it these days -- not sure if it's still there or not -- is the the manifold pressure wanders by +/- an inch in cruise. It's pretty obvious in the digital data.

The only other thing I can think of is that there is heavy filtering in the G3X Touch to reduce sensor noise...

Anybody else see this phenomenon? My old Cessna with 180 HP and analog gauge didn't do this...
 
Do you know if there is a restrictor in the fitting feeding the MP line? It is supposed to be there to flatten out fluctuations in the indications.
 
Ed,
No expert here.......
But MP is a measurement of intake suction or pressure.
So a fluctuation might be a leak. or intermittent leak.
Intake leaks affect engine performance/show up more at lower power settings.

You may have a cracked hose or fitting or even a loose fitting etc. that is hard to see or find. Could be anywhere in the MP system.
Best of luck in your search.
 
There is now a restrictor in the #3 cylinder. Bobs was one of the last packages I personally helped with before my repsiratory disaster.
Tom
 
In my experience, the G3X uses an excessive amount of sensor data "smoothing" in it's software. Many times a developer codes in smoothing algorithms to avoid presenting jerkiness in sensor based readings. IMHO Garmin over did it. I see this on my fuel flow readings, but never really noticed lagging in the MAP, as I don't look at it that much, but I'm sure they did the same.

This was very annoying when doing testing for injector sizing. It took forever for the EGT and GPH readouts to stabilize. I prefer to see the bouncing around, as it tells me something about what is going on. Unfortunately the G3X keeps all that in the background and presents a very stable reading that doesn't truly reflect what is happening at the sensor, at least not in real time.

Larry
 
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In my experience, the G3X uses an excessive amount of sensor data "smoothing" in it's software.

Larry, I agree that excessive filtering is the likely problem. Can't tell but I think that there may be a slew rate limiter in there as well as an inadequately damped second order filter. Hard to do parameter estimation with noisy data...

The whole rest of the system is new, the restrictor is there, etc. as I stated in the original post.

We shall see.

Garmin, any comments? I'm over $100 in new parts for what appears not to be a hardware problem...

Ed
 
Excessive system filtering sounds reasonable in the first place, but i think we would hear more about it as a root-cause for issues………..but it is not impossible to be.

Upon power reduction when you manifold blues happens, how does your RPM (i suppose a fixed pitch?) and FF behave?
Is it just the MP that changes or are there more values that change?

I would make a test run on the ground “in the chocks” , to eliminate airflow induced parameters whilst in flight, and write down what happens (or load down the file from the G3X) afterwards.

What MP do you see at full throttle? Is it ambient pressure (or almost) or are there a couple of inches missing as well?

If it is solely the MP i would vote for sensor issues, soft- or/and hardware related.
2” MP change should have impact on RPM (on a fixed pitch prop) and FF.

If more than one value changes it should/must be in the respiratory system of you engine……

Good luck and post results if you have.
 
Could you do a ground test using a mechanical gauge. It should be possible to tee into the existing line, or just use a mechanical gauge in place of the Garmin. If your figures are steady rather than changing with time, then the Garmin is the problem.
 
Another trait when I got the airplane, and I guess I just ignore it these days -- not sure if it's still there or not -- is the the manifold pressure wanders by +/- an inch in cruise. It's pretty obvious in the digital data.

I had this wandering MP issue with an MGL XTreme and solved it by adding more air volume in the MP line to dampen the fluctuations. Initial testing was temporarily installing an extra about 8 ft ?? of 1/4” ID plastic tube in the MP line aft of the firewall which stopped the fluctuations. Permanent solution was installing a pulsation damper from my junk bin in the MP line but anything that increases the air volume of the MP line would work.

Fin 9A.
 

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Thanks, all.

Constant speed prop, forgot to say that. But on my old RV-4, fixed pitch wood prop, you could change the manifold pressure a few inches with no change in RPM.

Don't have a mechanical MP gauge and that would be a wildly inconvenient experiment to try, and could only try it on the ground.

Adding more volume to the line to the sensor would help faster fluctuations that I don't have but would make the lag worse.

Probably won't get to flight test till Tuesday, but... at one point, had fuel pressure problems and changed the pump. Didn't help, changed the sensor. Still have flakey fuel pressure readouts that I ignore, but I am careful to check the electric fuel pump before engine start.

But the engine runs fine...
 
I had this wandering MP issue with an MGL XTreme and solved it by adding more air volume in the MP line to dampen the fluctuations. Initial testing was temporarily installing an extra about 8 ft ?? of 1/4” ID plastic tube in the MP line aft of the firewall which stopped the fluctuations. Permanent solution was installing a pulsation damper from my junk bin in the MP line but anything that increases the air volume of the MP line would work.

Fin 9A.

Interesting. I also have the XTreme and connected it to #3 with a brass barb and the line supplied with the unit. The line goes from #3 up to the motor mount where I installed a restrictor. The line then goes down to the RDAC and is a push on connection that is mounted on the face of the battery. No fancy fittings, no fancy teflon lines, just the same vacuum line used on cars for almost 100 years..... Display is solid and stable.

Back to Ed's issue.... With both manifold and fuel level issues, I would look for a bad ground.
 
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