Manifold pressure sensor problem

[larryMar]

https://www.avweb.com/features_old/p...ressure-sucks/

[BillL]

Originally Posted by vic syracuse

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.

[charrois]

Originally Posted by lr172

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.

[lr172]

Originally Posted by charrois

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.

[charrois]

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

[Bill.Peyton]

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.