What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Manifold pressure sensor problem

charrois

Member
Hi everyone. I have a strange manifold pressure sensor indication that's just started in the last couple of flights.

After takeoff, I reduce my manifold pressure to 25", which I then maintain as long as possible during climb by adding throttle (if I don't, of course, the pressure drops as I gain altitude). However, lately, I see my manifold pressure *rising* as I climb, even without adding throttle, which makes no sense if things were working as they should. On my latest flight, right after reaching 10000', wide open throttle was showing around 26" initially, though in my experience, it should be closer to around 20".

Over the course of 10 or so minutes, the indicated pressure slowly reduces to what it should be reading.

When I change throttle, the indicated manifold pressure changes quickly, and as expected, so my first thought isn't necessarily blockage in the tube running from the engine to the sensor. I've just seen (so far) that during a climb (and presumably during a descent, though I haven't tested this), the indication is unreliable until levelled off for several minutes. Is there a vent on the sensor to outside air that may be partially blocked? Or equivalently, is there a fixed pressure chamber in the sensor that should be holding a constant pressure but is leaking to ambient?

Obviously, I can just replace the sensor and I expect the problem would go away. But I'm trying to understand what might cause this (and if it's just possibly a partially blocked vent hole, it may be easy enough to just clean out).

Thanks for any suggestions!

Dan
 
On the increasing/decreasing MAP indication, I would inspect the plumbing from the primer port, hose/tube, AN fittings, sensor orifice for blockage or other contamination.

Is there a reason you reduce MAP on climb out? Noise abatement perhaps? If not, do yourself a favor and leave the throttle in 'til top of climb, then use the Propeller and Mixture to adjust for desired power & fuel flow.
 
Last edited:
I could possibly be that the o ring on the connector are sealing the vent hole. If you look inside the connector shell on the sensor, you will see the pressure vent hole. I had a similar issue with the fuel pressure sensor. Remove the o ring and see if that corrects the issue. Normally the sensor is able to "breath" through the wires.
 
Most likely you have a blocked hose to the sensor. They tend to fill with fuel and oil, especially if they are not routed "UP" after leaving the cylinder.
disconnect the hose at the cylinder and to the firewall, and clean it. See if any junk comes out when using a solvent like Brakleen.

Vic
 
No vent on a map sensor. Blockage with a liquid (you can get thick carbon based liquid from the gas in the intake manifold) is my best guess. That would explain why it eventually equalizes.
 
Thanks for the responses, everyone! I'll pull apart the cowl in the next couple of days to see about cleaning out the hose going to the engine and generally looking for crud anywhere, and will let you know how it goes.
 
Unfortunately, still no obvious culprits as to the cause of the problem. I removed the MAP sensor, and inspected the hose leading to it and it seemed clear and dry. I tried blowing into the hose (towards the engine) and there didn't seem to be any restrictions at all. The sensor itself seems clean, with no obvious contamination. I'm including a MAP and altitude plot from the EFIS logs during a climb out to 10000'. It started out at 25" and then dropped a bit (I can't recall if I may have brought the throttle back trying to see its reaction), but then climbed to a high of 26.7" by the time I reached 10,000' (and no, I don't have a turbo, so that should be impossible). Obviously, during a climb with no throttle changes, it should steadily decline, not increase.. and that's what it used to do until a couple of flights ago. Then, with no power setting changes, over the course of 8-10 minutes, it settled down to around 20.6", which is where it should have been in the first place.

Another reason I don't suspect a blockage between the engine and sensor is that it does react instantly to throttle changes - for example, when I advanced the throttle for takeoff, it jumped right away from 13" to 25", which it shouldn't have been able to do if the hose was blocked.

The exponential decay on the MAP after reaching altitude still leads me to suspect a blockage or leak somewhere, somehow. But I'd don't know anything about the internals of a MAP sensor to develop a theory on exactly what is causing this.

The MAP sensor I'm using was, if I recall, provided by Advanced Flight Systems for my EFIS 5600. I see no markings on mine at all, other than a Part number (12247561) and Lot number. I'd like to contact the manufacturer for their input and suggestions, but I'll also contact Advanced Flight Systems to see if they have any input.

Though I can just replace the sensor and see if that fixes the problem, I'd like to understand what could be causing it first.
 

Attachments

  • Screen Shot 2021-01-16 at 20.58.56.jpg
    Screen Shot 2021-01-16 at 20.58.56.jpg
    75.8 KB · Views: 230
Last edited:
Delphi MAP Sensor -- used by AFS for a looooooong time.

You could also check the wiring and signal levels (see attached schematic)
 

Attachments

  • Screen Shot 2021-01-17 at 11.45.08 AM.jpg
    Screen Shot 2021-01-17 at 11.45.08 AM.jpg
    68.5 KB · Views: 265
for example, when I advanced the throttle for takeoff, it jumped right away from 13" to 25", which it shouldn't have been able to do if the hose was blocked.

I think that is your clue. WOT at 1000 MSL should result in 29"+ of MAP. I would guess 3500 should result in 26 or 27". I would consider replacing the sender. Are you sure you don't have a snubber at the cylinder port. These can be as small as .004" and will easily block with carbon.

Larry
 
Last edited:
Most likely you have a blocked hose to the sensor. They tend to fill with fuel and oil, especially if they are not routed "UP" after leaving the cylinder.
disconnect the hose at the cylinder and to the firewall, and clean it. See if any junk comes out when using a solvent like Brakleen.

Vic

Yep, and the goo inside will become soft with heat and act normal. I don't have this issue, but a 10 friend had this and he cleaned some black goo out of the hose. He cleans it out now on alternate annuals. I think it is fuel residuals.
 
I think that is your clue. WOT at 1000 MSL should result in 29"+ of MAP. I would guess 3500 should result in 26 or 27". I would consider replacing the sender. Are you sure you don't have a snubber at the cylinder port. These can be as small as .004" and will easily block with carbon.

Larry

Altimeter setting at the time of the flight was 30.07. The airport was at about 4000' MSL, so I figure the rough expected MP with the engine off should have been around 26 (Unfortunately, I never checked). But with WOT, having a pressure of around 25" didn't seem that off to me, since I don't think it should actually ever be higher than it would be with the engine off.

In any case, I'm hoping to get to the hangar tomorrow to try replacing the sensor - in the process, I'll be sure to clean the hose and any connectors and orifices as well as I can. I'll report back on how things go.
 
Altimeter setting at the time of the flight was 30.07. The airport was at about 4000' MSL, so I figure the rough expected MP with the engine off should have been around 26 (Unfortunately, I never checked). But with WOT, having a pressure of around 25" didn't seem that off to me, since I don't think it should actually ever be higher than it would be with the engine off.

In any case, I'm hoping to get to the hangar tomorrow to try replacing the sensor - in the process, I'll be sure to clean the hose and any connectors and orifices as well as I can. I'll report back on how things go.

You're right. 4K' should be around 26", so within the margin or error.
 
Just a quick followup.

I pulled the hose going to the MAP sensor off and cleaned it - a couple of drops worth of oil in it, seemingly at the low point before it rose to climb into the sensor, but certainly nowhere near enough to plug the hose. Plus, the fittings to the engine as well as the MAP sensor were clear.

However, in looking into the sensor orifice, what I had thought was an internal membrane as part of the pressure sensor turned out to be a drop of oil/crud that somehow got into the orifice. I got suspicious when the replacement sensor I got (a Delphi PS10075 at my local auto parts store that seemed to be a cross reference match for the part number I was replacing) didn't look quite the same inside.

I have no idea how the drop of oil could have migrated up to the sensor from the low point in the hose, but somehow it did. In any case, I replaced the sensor, did a test flight, and the MAP readings seem rock solid again.

Thanks for the help, everyone!

Dan
 
Glad you found the issue. That is exactly the issue I was speaking of in my post 3, except your sensor vent orifice was blocked for a different reason than the gasket.
 
Back
Top