Fuel Flow/Pressure and Temps

[Richard Lanning]

I have owned my PA24-250 for five weeks now and it has been in the shop every weekend. To say I am shaking out the bugs is an understatement. Very expensive bugs.

I previously posted how the engine would quit on turning off the boost pump. I have been watching this more closely and have a few other details to add.

My most recent flight the fuel pressure was barely reading 1.0 with fuel pump off. The Shadin fuel flow meter was oscillating between 15 and 9 gph. After about 20 minutes I switched to the tip tank and same results for 20 minutes. I leaned till the engine ran rough and enriched slightly. Still same results.

My #2 cylinder seems to be the hottest on this plane which is also disturbing since I would think it would be one of the coolest. Still, after leaning at 7K it reads 325CHT and around 1280EGT which to me are not really that high compared to my 182 flying experience. I did switch probes on #2 and have confirmed it is not a probe problem other than I use to get a shock cooling warning at the slightest descent but since switching the probe I no longer get this.

Anyway after flying another 20 minutes the fuel pressure just jumped to 5 and the oscillations in fuel flow settled down to between 11-13. This is a carbureted aircraft and I was in clear air the entire time. I plan to check this situation on my return flight on Thursday on the opposite side tanks of the aircraft.

Boost pump on startup tends to give me 3psi on the gauge. I must admit, I did not turn on the boost pump when pressure was reading less than 1 to see what would happen. Because of all my previous issues with the engine cutting out in a climb I leave the boost pump on till I level out.

We did find contamination in the left tip tank which we flushed out and we cleaned the belly screen out though my mechanic felt the dirt in the belly screen was not much more than he would see during an annual. This plane was annualed in November. The contamination is an interesting story but for another time. The recent flight was all on the right tanks.

On takeoff I am seeing 20-24gph on the Shadin which based on other posts I have seen seems high.

So.... any words of wisdom? I must admit, this is the first plane I have actually started to be a bit nervous about flying.

Has anyone experienced a jammed control yoke? I can now say I have but that is another story.

[Kristin Winter]

20-24 gph on takeoff at SL seems in the ball park to me.

Are you working with a mechanic that really knows Comanches? I would suggest getting together with Bill Turley at Aircraft Engineering in Bartow, FL. It sounds like you need someone who really knows Comanches, not someone who has just worked on them from time to time.

[Zach Grant L1011jock]

Richard,
Per Lycoming, 250 takeoff fuel flow should be 22.5-28gph std day (29.92" Hg and 15 deg. C) at sea level. It sounds like you are in the ballpark there as you are probably at warmer than std temps. If your fuel pressure just jumped with the engine driven pump only, you probably had a stuck pressure bypass in the pump. It wasn't regulating the pressure, it was dumping it, and therefor was not delivering enough fuel to the engine to keep it running at high power, thus your engine cutout issues in the climb. That issue may now have fixed itself, but could return so you may want to get an overhauled pump and replace it. That's up to you. Did this aircraft sit for an extended period before you acquired it?

As for your boost pumps (yes there are two, are they both working? You can tell by listening. If you only hear one regular "click-click-click..." when it is on, chances are one is not working. If you hear two sets of clicks running at slightly different speeds, then both are working. You can figure out the faulty pump bu touching then when on. The one not vibrating is kaput), should give about 5 psi when you check them before start. Flow oscillations are normal in a carbureted airplane as the float bowl will fill at different rates, but the average will be very accurate.

#2 CHT is just fine. Tweaking the baffling can make the temps more even, but if your hot cyl is only low 300s I wouldn't mess with it!

-Zach

[Richard Lanning]

Thanks. The bypass valve explanation makes sense.

The plane was not flown much the past year. Maybe 20 hours tops. I have put about 20 hours on it the past month. alone.

It has a host of gremlins that I am slowly discovering and attempting to address. Left tip tank fuel gauge appears to only read full to half tank, i.e. at half tank indication it is actually empty. Gear down light does not show when Nav lights are on. Already replaced AI and HSI that both failed in flight in IFR. Oh boy! Discovered mag compass light didn't work when HSI failed at night. Just replaced bulb, will see if it works on flight home since it should be dusk by the time I land. During HSI repair they discovered ELT was not connected to antenna. Beginning to wonder if the ASI is correct. It shows about 120 IAS yet my ground speed is consistently about 160. Stall light tends to stay on all the time. The list goes on.

However, the plane starts real nice and flies beautifully.

This fuel flow issue is definitely concerning me but your explanation makes a lot of sense. Can the bypass valve be adjusted or do I need a whole new fuel pump? Ugh.