bonal Posted December 16, 2015 Report Posted December 16, 2015 Took our plane out today and noticed at under 1000rpm on the ground the suction gauge was close to zero as soon as I go to 1100 it jumps to 3.5 and then anything over 1300 it's in the middle of the green where it's always been. Honestly I can't say if it's always been this way but I kind of doubt it because I would have noticed the low reading before. Power off on final it's still dead center green. Wondering what others are getting below 1000rpm and if it's time to service my vacuum pump. Quote
aaronk25 Posted December 16, 2015 Report Posted December 16, 2015 I have about 1200 hours on mine and it has done the same thing for the past 300 hours but when checked for wear its within tolerance. My guess is the carbon veins get a few small insignificant grooves in the face of the vein with time so it requires just a bit more rpm to generate the same suction. I don't believe it means it's going to fail mid flight just not sealing as well as new. I'd have replaced it but the next one might malfunction sooner and I have a problem changing perfectly good working parts. Sent from my iPhone using Tapatalk Quote
DXB Posted December 16, 2015 Report Posted December 16, 2015 Mine's low before runup but never near 0. After runup it tends to stay 4.5-5.5 irrespective of rpm 1 Quote
ArtVandelay Posted December 16, 2015 Report Posted December 16, 2015 Never 0, 2.5-3.5 at low idle Quote
Hank Posted December 16, 2015 Report Posted December 16, 2015 Mine's pretty low at startup, too. Don't know if I ever noticed it before my pump died, but in the last couple hundred hours since then, it's really low when I crank up in the morning, does perfect at runup, then drops when I pull the throttle back to 1000, and is always fine in flight. Now I have to pay attention to it's behavior the next couple of flights. Thanks for doing that to me . . . 1 Quote
Raptor05121 Posted December 16, 2015 Report Posted December 16, 2015 Mine is low at startup. Maybe 2"? I flew a Cherokee 180 the other day, OAT was maybe 50*F (cold for Florida) and even though we had suction at idle, our AI was unresponsive. Being not a cloud in the sky I continued with the flight. During runup (maybe 10 minutes later), the AI unstuck and sprung into action. I think maybe the cold weather had something to do with it (tied down outside overnight). 1 Quote
Shadrach Posted December 16, 2015 Report Posted December 16, 2015 Mine is around 3" under 1000RPM. Lo Vac light comes on under 800rpm. The regulator becomes the limiting factor around 1400-1500 RPM. As a side note, I replaced my Vac Pump a few months ago because we thought our pump was getting soft (~800hrs SNEW). The new pump performed just like the old one, which is to say it topped out right around 4.2".  This is within spec per the MM, but a tad low.  It took three regulator adjustments to get it right. The Hi Vac annunciator light is very sensitive in my plane. Even a hair above 5.5" and the light comes on. Quote
bonal Posted December 16, 2015 Author Report Posted December 16, 2015 Will be keeping an eye on it just for the record it was under 40 degrees but was post flight so I don't think OAT was a factor since there was plenty of heat under the cowling Quote
JaredDavis Posted December 16, 2015 Report Posted December 16, 2015 Seems normal for My C which has about 20 hours on a a new pump. I had to replace my suction gauge since it was inaccurate. Might want to check the gauge. Quote
rbridges Posted December 16, 2015 Report Posted December 16, 2015 I've seen my "low vac" light come on when idling at a low speed. Â I guess it's similar to what you're seeing. Â Quote
aaronk25 Posted December 17, 2015 Report Posted December 17, 2015 I'm almost certain that there isn't a correlation between low vac at idle power and a upcoming failure. All it means is the blade face and wear ring arnt making a perfect seal so it needs a few more rpm. I think the failure comes when a blade breaks off and then causes failure of the pump by taking the other blades with it and/or wedging in the pump causing the pump to shear the coupling. Sent from my iPhone using Tapatalk Quote
cnoe Posted December 17, 2015 Report Posted December 17, 2015 I checked mine today for your reference and found that at ~1,000 rpm vacuum was decent but dropped off quickly at ~900 rpm (see photos). My vac pump is a Rapco showing good life on the vanes (thru the inspection port). Â 1 1 Quote
bonal Posted December 17, 2015 Author Report Posted December 17, 2015 Mooney spacers kick butt, thanks guys. I always keep an eye on the gauge in run up and as part of my routine scan but will be keeping a sharper eye going forward. Quote
aaronk25 Posted December 17, 2015 Report Posted December 17, 2015 Mooney spacers kick butt, thanks guys. I always keep an eye on the gauge in run up and as part of my routine scan but will be keeping a sharper eye going forward. Your comment also got me thinking about Low IFR take offs concerning the D.G. and A.I. After run up it may or may not make sense to keep the RPMs up enough to keep the vaccum in the green. I know the DG should be checked against runway heading before or during rollout but it probably isn't the best to sit and idle a low vac if your waiting on the tower to clear you for take take off into low overcast. I would imagine the vac powered instruments would slow down some and could cause some error in the DG especially after turning a 90d corner to the runway. Thoughts? Sent from my iPhone using Tapatalk Quote
bonal Posted January 2, 2016 Author Report Posted January 2, 2016 Well last couple of flights have been very cold and it starts out showing near zero under 1000rpm. Then in the middle of the green as soon as it is at 1300 and I noticed that at the end of the day it's at around 3 at 1000 rpm so seems like it might be effected by the cold. Any way like with all things to do with a vintage machine will keep a watchful eye. 1 Quote
Raptor05121 Posted January 2, 2016 Report Posted January 2, 2016 I went and started mine yesterday and she also does this. I want to say my pump is under 2 years old, still looks new under the cowling. At <1000rpm, its near 0 with the low suction light on, but my AI is erect. It takes 1200rpm to get it in the green. I've got a backup system that I can pull and it will instantly go to 5" Quote
ryoder Posted January 4, 2016 Report Posted January 4, 2016 On January 2, 2016 at 11:44 PM, Raptor05121 said: I went and started mine yesterday and she also does this. I want to say my pump is under 2 years old, still looks new under the cowling. At <1000rpm, its near 0 with the low suction light on, but my AI is erect. It takes 1200rpm to get it in the green. I've got a backup system that I can pull and it will instantly go to 5" Suction should make it erect and stay that way. Quote
Marauder Posted January 4, 2016 Report Posted January 4, 2016 Don't discount the quality of the gauge you are using. I had a 2" Mooney original that I replaced with a button gauge that cnoe is showing above. The original gauge would show 4" at 1,000 RPM and the new one less than 3". The replacement failed within 2 hours and I installed another one that is showing 4" at 1000 RPM. Similar to the Mooney original. I'm not sure how linear these gauges are. To be certain, you should be checking it with a vacuum manometer at different RPMs. Sent from my iPad using Tapatalk Quote
Marauder Posted January 5, 2016 Report Posted January 5, 2016 I knew I had a picture somewhere of the vacuum gauge sitting at 1,000 RPMs. Sent from my iPad using Tapatalk 2 Quote
Andy95W Posted January 5, 2016 Report Posted January 5, 2016 On January 4, 2016 at 6:32 AM, ryoder said: Suction should make it erect and stay that way. I don't think I can find a single fault with this statement. 1 Quote
ArtVandelay Posted January 5, 2016 Report Posted January 5, 2016 Suction should make it erect and stay that way. As planes get older sometimes the don't perform like they use to. 2 Quote
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