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Totally frustrated

gmohr

Well Known Member
Well after replacing the manifold pressure gauge and sender, checking for leaks
and verifying none I am about at wits end. The new gauge and sensor work
but now I am showing 3.5" low at 1500rpm and never reach 20" at max
throttle.

The only thing I have not checked at this point is the fitting on the number 3
cylinder for a blockage. I didn't because the gauge was reading fine until the
sensor went bad. Hopefully will get good results cleaning the fitting.
 
an MP gauge defaults to 29" and it requires a vacuum to get below that. Any significant restriction should have you reading high, not low, unless it is acting like a check valve and holding vacuum between the check valve and instrument.

If your running WOT and high RPM with no noticeable power loss, your not at 20" of MAP (assuming a lower altitude). Can't find a reason for a low MAP reading at this condition, except for something acting as a check valve (i.e. high vacuum at idle pulls the flap/valve allowing instrument to read the vacuum and when pressure rises, the flap closes, trapping the vacuum in the line.

Larry
 
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an MP gauge defaults to 29" and it requires a vacuum to get below that. Any significant restriction should have you reading high, not low, unless it is acting like a check valve and holding vacuum between the check valve and instrument.

If your running WOT and high RPM with no noticeable power loss, your not at 20" of MAP (assuming a lower altitude). Can't find a reason for a low MAP reading at this condition, except for something acting as a check valve (i.e. high vacuum at idle pulls the flap/valve allowing instrument to read the vacuum and when pressure rises, the flap closes, trapping the vacuum in the line.

Larry

Larry, thanks for the reply. I have checked and the instrument reads 29+"
at rest(ie engine not running.) With the engine running at 1500rpm
(CSProp) I get about 12.5". At 2000rpm I get 15" and that is about as high
as it will go. At full throttle, RPM around 2700rpm and MP is still at best 17".

I have checked everything I can think of. I could use any guidance on what
to look for. Someone else said they fixed their low MP by adding a resistor.
No clue where I would do this.

For the record this is one of the Van's electric MP gauges.
 
Since it?s a vans sold gauge have you called their tech support?[/QUOTE

I have been trying to get Technical Support during their assigned hours. Will
try again in about an hour. I am on the east coast they are on the west.
 
IE VMP35E and the Mothership

Spoke to the Mothership yesterday evening and learned something interesting.
According to Eric(at least that is what I think his name was, Van's is no longer
selling the IE VMP35E due a manufacturer issue with the microprocessor.
Could not offer any advice that had not been tried other than performing
a leak down check on cylinder 3.
.
So it is back to the drawing board trying to debug the low manifold pressure.
 
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Larry, thanks for the reply. I have checked and the instrument reads 29+"
at rest(ie engine not running.) With the engine running at 1500rpm
(CSProp) I get about 12.5". At 2000rpm I get 15" and that is about as high
as it will go. At full throttle, RPM around 2700rpm and MP is still at best 17".

I have checked everything I can think of. I could use any guidance on what
to look for. Someone else said they fixed their low MP by adding a resistor.
No clue where I would do this.

For the record this is one of the Van's electric MP gauges.

Only thing that makes sense is that vacuum is getting trapped or the processor is bad. I would put a long, cheap vynal hose on the sensor and, using your mouth, suck and release through numerous cycles and observe the readout. You should be able to confirm the reliability of your instrument this way. Be careful not to blow in the hose, as the sensor is likely not rated for pressure above atmospheric. While we call it a pressure gauge, it is really a vacuum gauge that indicates in pressure relative to barometric.

If you don't like the sucking part, autozone will rent you a hand pump vacuum tool, used for testing vacuum devices and bleeding brakes.

Larry
 
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Hi Gene,

Just to clarify - It?s not that we?re not supposed to sell it. It?s that the units are no longer being manufactured. The supplier of those instruments had an IC obsoleted out from under them. They no longer have the required components available to continue manufacturing them.
 
Hi Gene,

Just to clarify - It?s not that we?re not supposed to sell it. It?s that the units are no longer being manufactured. The supplier of those instruments had an IC obsoleted out from under them. They no longer have the required components available to continue manufacturing them.

Thanks Eric. I have corrected the above and in no way did I mean to say that
Van's was at fault. Can you tell me what Van's is recommending for MP gauges
now? I remember you mentioning Dynon/Garmin? Was that right?

Thanks again for your time yesterday.
 
For new builds, the major EFIS manufacturers have good solutions. Unfortunately, with the inexpensive gauges now becoming unavailable, owners flying older panels are being orphaned. We haven't found a good replacement for the gauges. There are alternatives but none as inexpensive as the "Van's branded" gauges:

http://www.vansaircraft.com/cgi-bin/store.cgi?ident=1536423476-332-315&browse=ei

And I didn't take it as you blaming Van's! I just wanted to make sure that everyone understood that this is an availability problem.
 
Since it seems to work engine off, I wonder if it is somehow over sensitive to buss voltage? Or if alternator return current is running thru the instrument? Did you try it engine on, alternator off?
 
Since it seems to work engine off, I wonder if it is somehow over sensitive to buss voltage? Or if alternator return current is running thru the instrument? Did you try it engine on, alternator off?

Bob,
That is something I have not tried. I am also going to recheck the ground wire when I go up to the plane.
 
Well the issue has been resolved. Seems I was expecting the gauge to register
too quickly. After performing a compression check and verifying that all hoses
and nipples were clear we hooked up a hand operated vacuum pump to the line
and the gauge was spot on. Put everything back together and took the plane
out of the hangar, heated up the oil and then ran up the engine. There is about
a 5 or so second delay with the new gauge and sensor. All in all it was me not
realizing how this gauge actually works. I have now been educated and look
forward to being back in the air tomorrow.
 
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