Stump the Chumps... (MP Gauge Leak)

[vontresc]

So I was tooling along in my friend's 182 (o-470-r), and felt that we were going too slow for the power setting. So after a bit of troubleshooting we found that there seems to be a leak in the MP gauge/G4 engine monitor system.

Here are the symptoms.

• Idle MP is at the bottom of the green arc (16-17"). RPMs are correct for this power setting.
• Cessna gauge matches insight G4
• Takeoff power setting MP reading looked ok (makes sense since we don't have much vacuum being pulled)
• at about 2500 msl we had to run 2-3" higher MP to get speeds and fuel burns to match the book numbers

This all makes sense to me, but here's the part that stumped me. After takeoff I set the power to the top of the green (23-24") and climbed out at 5-600 fpm. from about 1500-2000' MSL it stayed at the set MP. In the next 500' to 2500 MSL the MP went up almost 3". That is the part that really doesn't make sense to me.

Can someone explain the rise in MP as we climbed and give me an idea of where the leak may be?

 

[bradg33]

So I was tooling along in my friend's 182 (o-470-r), and felt that we were going too slow for the power setting. So after a bit of troubleshooting we found that there seems to be a leak in the MP gauge/G4 engine monitor system.

Here are the symptoms.

• Idle MP is at the bottom of the green arc (16-17"). RPMs are correct for this power setting.
• Cessna gauge matches insight G4
• Takeoff power setting MP reading looked ok (makes sense since we don't have much vacuum being pulled)
• at about 2500 msl we had to run 2-3" higher MP to get speeds and fuel burns to match the book numbers

This all makes sense to me, but here's the part that stumped me. After takeoff I set the power to the top of the green (23-24") and climbed out at 5-600 fpm. from about 1500-2000' MSL it stayed at the set MP. In the next 500' to 2500 MSL the MP went up almost 3". That is the part that really doesn't make sense to me.

Can someone explain the rise in MP as we climbed and give me an idea of where the leak may be?

My Viking had a MP gauge leak not long after we bought it. If it's anything like my setup, you've got a vacuum line coming off the intake, going to a fitting on the engine side of the firewall. Then, another line on the cockpit side of the firewall (other side of the same fitting) to the back of the MP gauge. I started by getting a basic hand-pump vacuum brake bleeder from Harbor Freight and hooking a line from the inlet side of that directly to the firewall-side fitting. Pumped, checked the MP gauge to see if it moved and stayed. It didn't, it went down a little and then came back to atmospheric, which told me there was a leak on that side of the fitting (wasn't holding pressure at all). As a test, I took some of the rubber hose that I had and managed to temporarily swap it in for the cockpit side MP line (just kind of shoved it over the nipple-style fittings; it didn't need to be a permanent seal, just one long enough to check. Left the pump hooked to the engine side, pumped, pressure changed and held. Voila! Problem was with the line from the cockpit-side firewall to the gauge. looked closely at that line (rigid aluminium line), and found it was cracked where it flares to meet the fitting. Had a new flexible line fabricated by a local aircraft hose shop, swapped it in, and we were back in the air.

http://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-69328.html