Do I have a problem with my engine?

bcool

bcool

Pre-takeoff checklist
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Nov 25, 2010
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St Charles, MO
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Bud
I've been having a great time flying my new-to-me '79 Piper Archer. It has about 1300 hours on the engine, and at the prebuy all the compressions were in the 70's.

A couple of things I've noticed have me wondering if there might be something not quite up to par with the engine - or maybe it's just because it's getting old :)

First, it's been averaging about a quart of oil every 5 hours. That, by itself, isn't out of line for an engine with this number of hours (or so I think I've read somewhere).

Second, the fuel consumption is out of line with the POH. At cruise altitude, leaned to 100 degrees rich of peak (best power), the POH says I should be able to have a 10.5gph fuel flow at 2650 rpm (75% power). However, if I run it that fast my fuel flow is closer to 12.5gph, not 10.5.

What, if anything, is this telling me?
 
Sometimes the fuel flow gauge can get out of whack.

Are you leaning with a monitor?

Also isn't 2650 near firewalled with the prop?

My 520 cruises around 2400RPM.
 
James331 said:
Sometimes the fuel flow gauge can get out of whack.

Are you leaning with a monitor?

Also isn't 2650 near firewalled with the prop?

My 520 cruises around 2400RPM.
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This.

Are you basing your FF on the gauge or what you calculate your burn to be when you refill the tank?
 
I know, I thought 2650 seemed kind of high, but that's what the POH has for 75% at the altitude/temp I was at.

The fuel flow (EDM-700 with FF) is pretty accurate (based on my refueling quantities), and I lean with the EDM.
 
Are you running an O-360 or 320 in that? 2650 does seem excessively high for 75%.
 
Pilawt said:
An Archer has an O-360, and at a density altitude of 8,000', it takes a skosh more than 2650 rpm to produce 75% power.
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Thanks for posting that image...I'm using my iPad & couldn't post one myself :)
 
I spent about 150 hours flying an Archer (also O-360) around the northeast quadrant of the US. I mostly ran it at 2650 RPM because I was in a hurry (as much as an Archer could do) it wasn't that loud even at that high of an RPM, and it was pretty efficient. Rather than 100 ROP, just lean it to being right on the edge of roughness. It'll do fine with that and burn a lot less fuel.

I'd agree that the quart ever 5 hours seems a bit high. The one I flew consumed a quart every 10 hours or so. It's possible that you have an oil control ring that's broken, which would increase the consumption. The fuel flow difference is probably because the POH is likely referring to a particular EGT gauge (single probe) and you are likely leaning differently with the EDM-700. Also, you may not actually be reaching peak before enrichening to 100F ROP.

I wouldn't necessarily worry about anything for now unless you feel like getting it addressed, but make a note of it at next annual.
 
An engine can have decent compression and still burn oil. If there is cylinder wall pitting, oil stays in the pits and ends up in the combustion chamber. Those pits are often below the point where the piston is set for compression testing. The pitting is from corrosion, and corrosion often comes from short flights or from ground-running the engine without flying it. Cold engines get a lot of moisture blown past the rings into the crankcase. It takes heat and time to get it out.

A dirty air filter will cause a lower manifold pressure that tends to suck oil past the rings and down the intake valve guides.

A worn engine blows more out the crankcase vent, too.

Try a different oil at the next change.
 
Dan Thomas said:
An engine can have decent compression and still burn oil. If there is cylinder wall pitting, oil stays in the pits and ends up in the combustion chamber. Those pits are often below the point where the piston is set for compression testing. The pitting is from corrosion, and corrosion often comes from short flights or from ground-running the engine without flying it. Cold engines get a lot of moisture blown past the rings into the crankcase. It takes heat and time to get it out.

A dirty air filter will cause a lower manifold pressure that tends to suck oil past the rings and down the intake valve guides.

A worn engine blows more out the crankcase vent, too.

Try a different oil at the next change.
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You must remember, the oil control ring has nothing to do with compression. plus the pitting on cylinder walls will fill with carbon and stop hiding oil that the oil control ring can't scrape off.
or
the bearing set has loosened to the point they will spray too much oil on the cylinder walls then the oil control ring will hydroplane over resulting in high oil consumption.
or
carbon has built up in the oil control ring's groove and has caused the ring to freeze in the piston and not do its job.
or
The cylinder and ring set are wore out.
 
Throttle Back to 2450 ,see what the engine does then.
 
I agree that 2650 is too high for cruise and the 12.5 gph is rightfully high.
I don't think you'll see 10gph until you throttle back to more reasonable 2450 or less.

Also, definitely have a mechanic check your tachometer because they are notorious for being off.

And burning a lot of oil is not the best thing in the world but I have learned to accept it. I used to own a 300-hr engine which didn't burn more than a quart or two btw oil changes. Now I own an older engine that is close to 1/2 TBO and it burns a quart every 5 hours like yours. I am not happy about it but nothing I can do short of rebuilding the engine.
 
What color is the oil after, say, 10 hours? If you want to determine the condition of the ring sets, measure the internal crankcase pressure of the engine while running with an a/s indicator or manometer plumbed to the oil filler.

Every engine manufacturer publishes a max value.

If readings are acceptable, try a ring wash procedure to free up the oil rings.
 
It could turn out to be nothing or really something. In any event it will be exciting to fly as the engine could conceivably go another4- 800 hours or more .......or it could fail very soon. It's like going to Las Vegas, it's a gamble. So many variables, hard to tell.
 
BigBadLou said:
I agree that 2650 is too high for cruise and the 12.5 gph is rightfully high.
I don't think you'll see 10gph until you throttle back to more reasonable 2450 or less.
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2650 is fine for cruise if the POH says it is. Too many pilots learn to fly in the circuit and think that 2500 is a good cruise RPM for any altitude. With most of these smaller fixed-pitch normally-aspirated airplanes, you'll need full throttle and near-redline RPMs to get 75% at eight thousand, if you can get even that.
 
FWIW, I owned a Piper 180 for about a year. I'd cruise at about 2475 (ish) or 2450 and I'd get almost exactly 10 GPH. I put in a fuel flow meter just before I sold it and some days if I throttled back to about 2450 with the right mixture combination I could get 9.5. Which, for a 180 HP O360 is pretty good I might add.

12.5 is way too high, and 2650 is completely unnecessary for cruise unless you're in a real hurry to get there at the expense of fuel.
 
bcool said:
the fuel consumption is out of line with the POH. At cruise altitude, leaned to 100 degrees rich of peak (best power), the POH says I should be able to have a 10.5gph fuel flow at 2650 rpm (75% power). However, if I run it that fast my fuel flow is closer to 12.5gph, not 10.5.
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Are you sure the tachometer is accurate? Buy or borrow an optical tachometer and test it. And has the prop ever been re-pitched? If you have more of a cruise prop, then 75% will be at a lower rpm than in the book.

Does your fuel consumption observation include takeoff and climb? Try filling both tanks, take off and climb on the left tank, then switch to the right and cruise for an hour, then switch back to the left tank and land.

It's tough to lean accurately with a carbureted engine, even more so with a single-probe EGT. About the best you can do at cruise is lean to the first sign of roughness, then enrichen to smooth.
 
As for fuel burn, I typically have found you have to lean almost to peak EGT sometimes even lean of peak to get the book fuel flows. I don't think the POH says anything about leaning to 100 ROP (I could be wrong).
As to how this affects engine longevity, again the POH is pretty silent.

Brian
 
BigBadLou said:
I agree that 2650 is too high for cruise
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Lycoming disagrees. So do I.

I don't think you'll see 10gph until you throttle back to more reasonable 2450 or less.
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I have plenty of hours of <10 GPH in the Archer at 2650 RPM.

Also, definitely have a mechanic check your tachometer because they are notorious for being off.
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Not a bad idea.
 
Ignore the folks who blindly believe that 2650 is "too high." These are usually the same people who ***** that their airplane is slow or doesn't make "book speeds," while ignoring "book" power settings. They just arbitrarily pick power settings that a CFI once told them to use and stick with it, regardless of what the POH or modern engine theory says.
 
bradg33 said:
Ignore the folks who blindly believe that 2650 is "too high." These are usually the same people who ***** that their airplane is slow or doesn't make "book speeds," while ignoring "book" power settings. They just arbitrarily pick power settings that a CFI once told them to use and stick with it, regardless of what the POH or modern engine theory says.
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Any of these certified over bore slow RPM engines can be run full throttle leaned to best power all day long.


I wouldn't count day 2.
 
bradg33 said:
Ignore the folks who blindly believe that 2650 is "too high." These are usually the same people who ***** that their airplane is slow or doesn't make "book speeds," while ignoring "book" power settings. They just arbitrarily pick power settings that a CFI once told them to use and stick with it, regardless of what the POH or modern engine theory says.
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Other they 'might' respect the wishes of the rental facility who asks them to keep the rpm at a certain #. But ur right, everybody is probably just bitching.
 
Thanks, everyone, for your input. I'll experiment some more on my next trip.
 
eman1200 said:
Other they 'might' respect the wishes of the rental facility who asks them to keep the rpm at a certain #. But ur right, everybody is probably just bitching.
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Are there actually rental facilities out there like that?

I always figure most places understood the concept of full rental power and billed accordingly
 
H
BigBadLou said:
I agree that 2650 is too high for cruise and the 12.5 gph is rightfully high.
I don't think you'll see 10gph until you throttle back to more reasonable 2450 or less.

Also, definitely have a mechanic check your tachometer because they are notorious for being off.

And burning a lot of oil is not the best thing in the world but I have learned to accept it. I used to own a 300-hr engine which didn't burn more than a quart or two btw oil changes. Now I own an older engine that is close to 1/2 TBO and it burns a quart every 5 hours like yours. I am not happy about it but nothing I can do short of rebuilding the engine.
Click to expand...


Have you ever owned/flown an Archer?

2650 is fine. I'd burn 10.5 and TAS at 130kts. Pull back on that one armed bandit abit and get 125 around 9gph all day long.
 
On my way to KMKC (Kansas City Downtown) this weekend from KSET (St. Charles County) I was able to tinker with the engine settings. I used the recommendation of some here and leaned just rich of peak (instead of using the EDM-700 Lean Find function), and here's what I got:

Indicated altitude: 8,000'
Pressure altitude: 7,900'
RPM: 2603
GPH: 10.5
True airspeed: 130

These settings kept my hottest CHT at 400 and EGT at 1440. I think I'm happy with these :)
 
Unit74 said:
Have you ever owned/flown an Archer?

2650 is fine. I'd burn 10.5 and TAS at 130kts. Pull back on that one armed bandit abit and get 125 around 9gph all day long.
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This
I own a DA40F, which has O-360A4M and a Grumman Tiger prop on it (same as Archer, bit more pitch)
Depending on the altitude, 75% is somewhere near 2700RPM
Burn is around 10gph close to peak (just rich of rough), 12-12.5gph at 100F ROP
So that part is OK

Oil - I'm in FL and the oil of choice is W100Plus - when regularly flown, the engine (1400 hrs snew) will burn a quart every ten hours. If I don't get a chance to fly it for a month, the consumption goes up to a quart every 6-8 hours. A few hours later it goes back up to quart every 10 hrs.
Hope that helps

PS Check the baffles - I wouldn't be happy with 400 in cruise. Mine - the hottest is 380 at 75% and 360 at 65%
 
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