No Manifold Pressure at idle

B

BigTex

Filing Flight Plan
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Oct 18, 2012
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Ponder, Texas
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Display name:
Gary
I have a 1966 Mooney M20C and was out flying Saturday and noticed that my manifold gauge showed no manifold pressure at idle. In fact even at run-up it showed very little until I cycled the prop. Not until I with WOT on takeoff did it spring to life and seem to operate normally. The engine also seemed to be working correctly.

When I shut off the engine, the manifold gauge did read correctly based on atmospheric pressure.

I'm thinking there's might be something to do with the plumbing to the manifold pressure gauge. Has anyone else seen this type of behavior before?
 
Since it should read atmospheric pressure (basically) with the engine off, to get all the way to zero requires a big problem.
 
BigTex said:
Yes... It reads zero manifold pressure.
Click to expand...

The engine is pulling enough vacuum at idle with the throttle plates closed it is burying the MP needle. The gauge is probably getting weak.
 
Let me clarify. It shows zero at idle and around 29" when then engine is off.
 
Yeah, that's correct although the needle has pulled too far into vacuum if it is all the way to 0.
 
Check your air filter, do you have full power?
 
I'll check the air filter... It does have full power on takeoff. I'm seeing around 27 squared on takeoff.
 
BigTex said:
I'll check the air filter... It does have full power on takeoff. I'm seeing around 27 squared on takeoff.
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Take off is a low vacuum condition that is why the MP is closer to atmospheric pressure. Idle is a high vacuum condition but your engine shouldn't pull 28" of vacuum lol. I suspect you have an instrument going weak on you.
 
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Since this gauge is a dual MP/FP gauge, does anyone have any thoughts on a replacement gauge? Or should I look at finding some place to have it serviced?
 
BigTex said:
Since this gauge is a dual MP/FP gauge, does anyone have any thoughts on a replacement gauge? Or should I look at finding some place to have it serviced?
Click to expand...

May not be the gauge. But service of some sort does seem in order. ;)
 
Assuming it turns out to be the MP/FP gauge, any recommendations on services centers that could IRAN this gauge?

Also, does anyone have any experience with the Aerospace Logic FM200K manifold pressure/Fuel pressure gauge? It looks like it might be a good replacement.
 
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BigTex said:
Since this gauge is a dual MP/FP gauge, Or should I look at finding some place to have it serviced?
Click to expand...

When I was having TIT gauge issues Dan at Lasar * (800) 954-5619 * told me he could get most of my original gauges repaired if necessary. in my case it was just the probe, which he had in stock.

Jim
 
Diagnosis:

a. Does it change when prop is cycled during run up?
b. do you have an engine egt gauge for all cylinders?
c. Is it higher on 1 cylinder?
d. what happens when you pull ALT air?

induction leak or clogged air filter - you been flying a lot? See if you have a mouse nest just before the air filter; . .
 
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