since 1940

Low Power in Continental or Lycoming aircraft engine. Loss of manifold pressure. Low RPM. Loss of power

Note: Low power can be subjective since there's no one instrument that we can use to measure power on the airplane. With fixed wing aircraft a pilot won't notice a 5 horsepower loss in cruise. Five horsepower is noticeable during single engine climb on a twin. If a constant speed propeller airplane won't reach red-line rpm in cruise then the governor is holding back the propeller and your problem is not low power. At cruise speeds the engine will turn the propeller past red-line even with low power because of the low propeller loading.

Altitude related manifold pressure problems are usually caused by leaks in the intake and exhaust system. Loss of manifold pressure in climb on turbocharged engines is caused by leaks or the butterfly valve in the wastegate is not closing completely.

1. Poor combustion. Check compression.

2. Leaks in induction system. Induction leaks cause poor idle and/or poor engine acceleration. Check induction hoses. On Lycoming engines with the short tubes swaged into the oil sump, check the tubes for tightness. Loose tubes need to be re-swaged. On Lycoming engines that use ring seals to seal the induction tube into the oil sump, check the ring seal for integrity. Ring seals can be inspected with a flashlight by looking into the intake tube housing bore. New seals are easiest installed by lubricating the seal and the housing bore before pushing the tube with the new seal into the bore. Do not use rubber sealants. Sealants don't compress into the bore and therefore don't exert tension against the bore. As soon as vibration breaks the sealant from the surfaces then the joint leaks.

3. Improper fuel flow. Check and clean screens. The August 1974 General Aviation Inspection Aids wrote of an incident of low power caused by an ant that had entered the injector line while it was removed for maintenance. Check for ants!

4. Restriction in air inlet or manifold. Check carburetor air box for protruding gasket. Check air filter. Inspect air duct for evidence of hose collapsing. Check security of alternate air door. Check air filter for plugging.

5. Alternate air door leaking allowing heated air to enter carburetor.

6. Throttle lever not properly adjusted. Check for full travel.

7. Broken baffles in muffler. If section of baffle is loose in muffler, engine may operate satisfactory at times and may be low on power at other times. This indicates baffle is moving around in muffler, blocking exhaust gases sometimes and out of the way at other times. Strike muffler with rubber mallet and listen for rattle in muffler. Inspect muffler per the manufacturer's maintenance manual.
Recently dealt with an old Cessna 180, with 470. Owner has flown 60 hours since buying aircraft 5 months ago. Prior to purchase, the sellers engineer stated that entire left bank cylinders went cold on one occasion only, and after failure to diagnose a reason, it cleared itself after running at 2000 RPM.
The condition reoccured for the buyer, this weekend. This time it was the right bank that went cold, and idle RPM was 1000 RPM. The current owner, during a subsequent run-up attained 2000 RPM, at which point the engine smooted out and all returned to normal. I invstigated today, and found the right muffler flame tube missing. There are obvious signs of very recent failure of the cone. I do feel the exhaust obstruction prevent the entire right bank to breath properly and fire. This same thing likely occured sixty hours ago with the left muffler, but the engineer at that timefailed to identify it. Both mufflers are now just open top to bottom. In reading so many reports etc. on the web it's obvious that this is a little known condition with huge consequences when not inspected and maintained properly. I'd like to suggest that it should have a spot as a possibility in unexplained rough engine operation or loss of power. Below are just a couple of the links that I passed on to this pilot (new customer) regarding his mufflersystem.


Following was copied from Archer Bravo http://www.archerbravo.com/bravotips.php


April 13th, 2006

Don't Get Muffled
When doing your pre-flight walk around, don't forget about your muffler! Here's how to check it: use a flashlight and look all the way up inside the exhaust pipe. You should then see the muffler. If its internal baffling is shaped evenly (cylindrical or conic shaped), with evenly sized holes, it's in good shape.
If it appears warped and distorted, this means the flame cone is fatigued, and it will no longer do its job of vaporizing gas.
Eventually, the parts might begin to shed, and come out of the tailpipe. This can block it, decrease performance, and increase the chance of fire and/or carbon monoxide poisoning. Yes, this is the worst case scenario, but certainly the last thing you want. By the way, some mufflers weren't created with a baffle, so don't panic if you don’t see one initially.
One other important note: if you are in-flight and experience loss of RPM, or rough engine - and your systems check reveals no other problems (magnetos, carb ice, etc), do an inspection of that tailpipe and muffler upon landing.
Submitted by: Flight Instructor Mariellen Couppee
CFI, CFII
info@genehudson.com


Following was copied from http://www.patentstorm.us/patents/5496975/description.html


One type of present day small aircraft muffler currently used has a series of cones that are tack welded to small diameter rods. This assembly is placed within a housing that has an inlet and an outlet. Although this muffler design works well to muffle noise, it has a tendency to fail. Vibration and heat often cause one or more of the cones to break free of the welds. In many cases, the outlet side cone will invert and become lodged in the outlet of the muffler causing immediate power loss due to extreme back pressure in the introduction system. If this power loss occurs during takeoff, a crash is highly likely.

Following was copied from http://www.supremecourt.ohio.gov/rod/docs/pdf/8/2009/2009-ohio-5365.pdf

The engine logbook to the aircraft contains an entry dated August
28, 1987 that states “replaced muffler,” which was entered by Edward Ramsey,
an FAA licensed airframe and powerplant (“A&P”) mechanic. Appellants,
through their experts, maintain that the logbook entry establishes that the original
muffler was replaced with a new muffler. Appellants’ theory of liability is that the
aircraft experienced a sudden loss of engine power when the muffler’s flame tube
separated and blocked the exhaust.

Another excellent reference,

AC 91-59 A, 
Inspection and Care of General Aviation Aircraft Exhaust Systems 
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/882287e4ffbbc3ec8625732400584d69/$FILE/AC%2091-59A.pdf
see page 2, 4 b for the following

As an example, we received a safety recommendation concerning the internal failure of a muffler on a Cessna 207 airplane. The cone, which deflects or helps distribute exhaust gasses inside the muffler, came loose and blocked the exhaust gas outlet resulting in engine power loss during takeoff. This mufflerconfiguration is common or similar to many other general aviation airplane mufflers. 

Elsewhere in this same AC,

(3) Partial or full engine power loss caused by loose baffles, cones, or diffusers on mufflers and heat exchangers that partially or completely block the exhaust gas outlet flow. This condition may occur intermittently if internal components are loose within the muffler and move around during subsequent flights. 

Regards John, as always I continue to push your toolbox as an excellent resource for anyone involved in aircraft.

8. Prop out of adjustment (low pitch).

9. Governor linkage not adjusted properly. Adjust for full travel.

10. Crankshaft to camshaft timing off. Note: It is easy to assemble the Lycoming O-320H2AD with the internal engine timing 1 tooth off.

This condition may be checked by first disconnecting starter. Remove top spark plugs and rocker box cover on #2 cylinder. Turn engine to TDC on compression stroke #1 cylinder. Engine timing is checked by first observing the number two cylinder valve rocker arms. Both valves should be closed or nearly closed. Move the propeller slightly in one direction. Rocker arm motion should be seen as one valve starts to open. STOP. Now rotate the engine back to the original position with the #1 at TDC.. Now move the propeller slightly in the direction opposite from the first movement. Rocker arm motion should again be seen as the other valve starts to open. If the two valves started to open as described with only a small amount of engine movement in each direction, the engine timing is correct. If movement in either direction exceeds twenty degrees of engine rotation before motion of the rocker arm occurs, the crankshaft to camshaft timing is not correct and the engine was assembled incorrectly.

11. Camshaft lobes flattened. Remove camshaft followers and inspect condition or examine oil filter for metal.
12. Incorrect carburetor. 

13. Incorrect propeller.

14. (Turbocharged engines) Controllers out of adjustment. Variable absolute controller is adjusted by turning adjustment screw at cam end of controller counter-clockwise to increase boost, and clockwise to decrease boost. Controllers are set to obtain a specific MP and no compensation for density and temperature is necessary. Required MP settings are found in Operator's Manual. Adjust for full throttle setting only. Density controllers compensate for temperature and pressure. Differential pressure controllers are set to maintain 6" (Lycoming) differential between deck and MP.

15. (Turbocharged engines) Damaged turbocharger impeller, binding or tight wheels. Visually inspect wheels for damage and free rotation.

16. (Turbocharged engines) Kinked or restricted oil lines from engine to actuator, and actuator to controller.

17. (Turbocharged engines) Wastegate out of adjustment or stuck open. Lubricate ends of shaft with Mouse Milk.

18. (Turbocharged engines) Inlet orifice in actuator plugged.

19. (Turbocharged engines) Piston seal in wastegate actuator leaking. Noted by excess oil coming out of drain.

20. (Turbocharged engines) Oil pressure to low to close wastegate. Continental engines 30-60 psi limit.

21. Injector and controller linkage not adjusted properly (Lycoming 541 engines).

22. (Turbocharged engines) Variable pressure controller out of adjustment.

23. Inaccurate tachometer. Mechanical tachometers, such as the AC tachometers, have a oil port in the rear. They never get oiled and over the years will start to read slow.

24. Turbulence in air flow causing improper fuel metering.

25. Improper valve rocker clearance. This is important on solid tappet engines (Lycoming O235).

26. Carburetor ice. Will be accompanied with drop in manifold pressure on aircraft equipped with constant speed propellers and on fixed pitch airplanes often a gradual loss of rpm and engine  roughness Carburetor Ice in Continental and Lycoming aircraft engines

27. Worn out spark plugs. Worn out spark plugs that fire but are weak have been shown to reduce engine output by 5 horsepower in test conditions.

28. Air filter dislodged and stuck in intake system thereby cutting off the air supply.

29. Heavy rain - paper air filter swelling.

30. Carburetor heat flapper valve in the air box partly open.

31. HA-6 carburetors- Float pin worn and out of one boss, preventing the float needle from properly seating.

32. Loss of power and vibration suspect magneto problem. Switch from both magnetos to the healthy magneto will regain power and smooth out engine.

33. Dirty or restriction in fuel nozzle. Will be accompanied by oily spark plug.

34. Lycoming O235 refer to latest edition of Lycoming Service Instruction 1388 "Troubleshooting of the O235 Series Engine".

35. Exhaust pipe becoming disconnected in turbocharged engine. For Beech Baron model 56TC serials TG-2 thru TG-94 and Duke models 60,A60, and B60 serials P-4 thru P-596 comply with Lycoming Service Bulletin 479 or latest edition and refer to Beech Service Communiqué No.87 dated January, 1988.

36. Dragging valve.

37. Fuel siphoning into the cylinders past damaged "O" rings on the primer plunger causing an overly rich mixture. 38. (Turbocharged engine) Exhaust system leaks.

39. Plugged fuel vent creating vacuum in fuel tank.

40. Carburetor blockage in the channel at the base of the mixture metering sleeve where fuel pick-up occurs for the fuel discharge nozzle. Will cause the float to be unusually high.

41. Float stuck in carburetor

42. Continental "A", "C", and O200 engines - fuel flow restriction caused by a clogged screen in fuel pump. Reference Continental Service Bulletin M81-8R1.

43. Lycoming engine with mechanical (diaphragm style) fuel pump. Textron Lycoming issued their Service Bulletin (SB) No. 548 determining that several lots of diaphragm-type fuel pumps may have a potential for an internal failure. When an internal failure occurs, the check valve seat separates from its shaft due to an inadequate or improper assembly crimp. This allows the liberated parts to move within the housing and may cause a blockage of fuel flow. This failure may not be evident on the exterior of the pump, but it may result in a decrease or loss of fuel flow and a corresponding decrease or loss of power. Textron Lycoming estimates that approximately 1,252 fuel pumps were manufactured between September 18, 2000 and February 18, 2001 with a potential to fail, but they can not predict if or when the failure may occur.

44. Power interruptions in the IO-360-L2A engines used by the 172R were related to a combination of over-rich operation and fuel system vapor formation during prolonged ground operations in summer heat. Service Instruction #1498 states that "vapor may form in the fuel system during ground operations when the ambient temperature exceeds 85 degrees F," An alternative explanation is that water is being trapped in the integral wing tank.

45.  Water ingestion into engine. Water trapped in integral wing tank.

46.On 07 November 1991, Schweizer issued MSB B-248, pertaining to compliance with the Precision Airmotive Service Bulletin No. PRS-94 issued 21 September 1989, and states that, "there are persistent reports of low power and high engine temperatures (cylinder head, oil, and EGT on new aircraft, and on overhauled engine/servo combinations". It also states that, "failure to comply with this Service Bulletin could result in engine failure, which may lead to loss of control of the helicopter and subsequent injury or death".

47. Loose carburetor bowl attachment bolts allowing air to suck into carburetor.

48. Fuel leak from the engine driven fuel pump. Continental SB 01-1

49. Improper float level. Refer to FAA Special Airworthiness Information Bulletin NM-02-47 September 30, 2002. and Precision Airmotive   Service Information Letter MS-4, Revision 1, dated July 18, 2002, Installation Instruction E-954 Revision 2 dated June 18, 2002, and E-955 Revision 2, dated June 14, 2002 to expand and clarify these instructions.

50.Valve float caused by not enough dry tappet clearance, insufficient lifter bleed-down, or heavy rocker arms (roller rocker arms). Occasionally occurs on Continental 520 engines. On these engines if the lifter pumps up and holds the valve open the rocker arm will strike the inside of the rocker cover causing a crack. If you are using the "full flow lifter" (constant registration) then check to see if you have the correct rocker arms installed. Also, replace the lifters as they may have low bleed-down rates due to varnish build-up. Valve float is more typically at higher rpm's and with cold oil as both of these reduce the rate of lifter bleed-down.

51. Weak or broken valve spring. 

52. Hot air from alternate air door or carburetor heat. As a rule-of-thumb for every 10 degrees F. heat above the standard 59 degrees F. there is a 1% power loss. At 100 degrees this adds up to 10% power loss. Tests at Lycoming yielded a 15% power loss with the full application of carburetor heat.

53. Hydraulic lifter pumping up and holding the valve open.

Valves can leak if the hydraulic lifters pump-up. This may result in the situation where the compression check is good, the engine starts to run OK but then a cylinder drops-off or the engine becomes rough. This happens when the hydraulic lifter pumps up and holds the valve open. An example of this happening is on the A-75, A-64 engine installed in an aircraft that originally used an oil cooler. Then the engine is removed and installed in an aircraft without an oil cooler. Now the engine drops two cylinders and runs rough because two hydraulic lifters on one side of the engine are pumping up and holding the valve open. The reason its doing this is that the 22130 sleeve (oil restrictor) was not removed from the crankcase. This oil restrictor diverts some of the oil thru the oil cooler. If you remove the oil cooler and don't remove the restrictor then pressure builds up on one side the the crankcase and the extra oil pressure causes the hydraulic lifters on that side of the engine to pump up and hold the valves open.

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.