Fuel Flow/Pressure/EGT - Part Two

[Richard Lanning]

I replaced both boost pumps this past weekend in my PA24-250. They were the original boost pumps.

We checked the direct pressure output of the boost pumps and we had 4#. We then checked the pressure in line with the engine driven fuel pump and still had 4# on an external gauge. So far so good.

So we buttoned everything up but alas I still only show maybe 1.5# pressure with only the boost pumps on with the cockpit fuel pressure gauge. When I crank up the engine I see the same thing even during run up. On takeoff I see the same thing. But... once I level off to cruise the pressure gauge creeps up to 4# (boost pumps off) and stays there pretty much throughout the flight.

Shadin fuel flow is still somewhat erratic when pressure reads 1.5# but again steadies out in cruise flight.

I am told I need to replace the pressure gauge but I am not so sure. Why would cruise flight have any bearing on the indication?

On a separate note. My #2 cylinder is around 320 degrees CHT and pretty much in line with the other cylinders. However, the EGT is around 1400. About 75 degrees warmer than the others. Should I be concerned? What EGT reading should I be leaning to? I know when I flew a C-182 we typically leaned to 1430 EGT on the peak cylinder. I have temp probes on each cylinder. I am beginning to think I might have too much information. I am still trying to figure out what the #7 probe is reading. Clearly it is showing OAT on the EGT side but the CHT side had a reading of 117. The cockpit oil temp was reading 200 so this can't be oil temp. When I add carb heat this reading does not change so it is not that.

[Ed Asmus]

Hi Richard,

I am no expert but I highly recommend you read everything you can by Mike Busch (savvymx.com). He writes for the EAA and owns and flies a 1979 T310. He has free webinars on the website just mentioned and teaches folks how to lean their engines. From what I understand, leaning to any particular EGT value would not be the goal. Leaning by reference to the peak EGT would be a better way to go. The most important numbers are not EGT at all but CHT are extremely important. Leaning impacts CHT dramatically and keeping these numbers at or below 380 degrees F is what Mike Busch teaches. CHT is by far much much more important than EGT. Your 1400 EGT reference is absolutely nothing to be concerned about. In fact, EGT differences across your cylinders really is of no great concern (assuming all cylinders and spark plugs are firing) because so many things impact the number you see on your display. In a perfect world, as you lean, all your EGT's would rise, peak, and fall at the same time regardless of what the absolute numbers are.

Read what Mike Busch has to say. It will take you many hours of research but the education will be well worth it.

Hope this helps!

Good Luck!

Ed

[Kristin Winter]

Mike Busch is worth reading, but a note of caution. Mike is a Continental/Cessna guy. He is not a Lycoming or Piper guy, and his recommendations are not always entirely valid for us. While lower CHT's are good, Lycomings often run warmer. I have personally run a set of Navajo engines from day one to TBO and the CHT's were usually around 400F and in the summer could be as high as 425F for periods. The engines had zero heat related problems.

[Richard Lanning]

Thanks for the tips. Sounds like I can lean a bit more since my CHTs are all under 350.

I may resort to old school and lean till the engine runs rough then enrich a bit and see what the JPI instrument tells me.

Initially I was constantly getting an alert that I was shock cooling the #2 cylinder with the slightest nose down pitch or power reduction. Swapped the leads with #4 cylinder and that problem has gone away though #2 is still my hottest cylinder usually which is strange being up front. The baffles are all in great shape so that isn't the problem.

Still working out the gremlins of which I have plenty in this aircraft.

[Ed Asmus]

Hi Richard,

In my little opinion (which is probably only worth what you paid for it), I would agree with your "old school" method of leaning in a naturally aspirated engine. If CHT's are good, fly along happily.

I would agree with Kristen's comments as well. Rarely is anything "absolute." Normal engine ware is cumulative. It does not happen on one flight. Good operating practices with attention to temps can reduce those ware rates and extend the life of the engine.

As for shock cooling, Mike Busch discusses this also. He seems to believe it is rather overrated. Again, in my little opinion, if my choices are gradual changes versus abrupt changes, I choose gradual every time. It just seems logical to me that over the life of the engine, gradual would be better than abrupt.

One last comment about CHT's. Kristen mentioned as high as 425 F. Busch discusses the CHT threshold as being somewhat variable depending on outside air temp. The 380 number would be for a day in the vicinity of standard. He discusses using a slightly lower threshold in the winter time and somewhat higher threshold in the summer time. In the ballpark of +/- 20 degrees. If it's me, and again I am no expert (I just read a lot), I would have an uneasy feeling looking at a 425 (note: I have zero experience in a Navajo). The engine certainly is not going to come apart at that temp, I would just like to bring that down somewhat, assuming a normal flight and not an extenuating circumstance. My personality is quite conservative by nature however. I also believe, there is a little (not a lot, but a little) luck involved in engine life also.

Fly safe!

Ed

P.S. Keep those baffles in good shape!