Power Loss in C-172F

Power Loss in C-172F

Hi,

I am hoping someone can provide some insight into a problem I am having with a Continental O-300D engine installed in my recently-purchased 1965 Cessna C-172F.  We have been experiencing a problem where the engine runs fine in flight for about half an hour at 2400 RPM, altitude about 3000 MSL.  Then, the engine slowly and steadily loses RPM over a period of about 3-5 minutes until it quits altogether.  The engine turns over but will not attempt at all to restart after quitting.  Then after
sitting for about half an hour it will start and run fine.  At least four mechanics have tried to find the source of the problem, but
to no avail.

We have:

1) Verified that the fuel flows freely to the carb and that the fuel tanks are properly vented.  Also installed new vented Cessna fuel caps.

2)  Drained and replaced the fuel in the tanks and checked for contamination; none found.

3)  Checked the carb screen; no blockage or debris.

4)  Checked the mags.

5)  Checked the valves.

6)  Before the engine quits we have tried adding carb heat and switching fuel tanks with no effect.

7)  While the engine was in its non-starting mode after quitting we tried three shots of prime, effectively bypassing the carb and injecting fuel directly into the cylinders.  The engine showed no sign of trying to start.


8)  Also while the engine was in its non-starting mode we tried draining fuel from the gascolator- it ran freely.

9) Ran a flow test with the fuel line disconnected from the carb. No problem.

10) Insulated the flex line from the gascolator to the carb to prevent vapor lock.

Any ideas on what to look for?  Has anyone heard of similar symptoms?  I searched the forum and found some discussion of surging, but this doesn't surge-just slowly fades away and quits.  Any help would be greatly appreciated!

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Re: Power Loss in C-172F

It just slowly dies & stops?

My first guess would be vent problem but uit looks like you addressed that with flow rate etc.


A double mag failure would be unlikely but I trust you did VERIFY SPARK

from the leads during "Sleepy Time " ?

Something like this on a Cherokee turned out to be a bad muffler but it

was ERRATIC & also you have 2 mufflers.

How does it "revive"?

Do you do Mag Checks AFTER flight?

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Re: Power Loss in C-172F

I referred you problem to one of our tech reps, here is what he said:

I don't have a lot of experience with piston engines but it sounds entirely like  a fuel delivery problem. At this point I think I would be having the carberator gone through.

Best of luck

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Re: Power Loss in C-172F

Agree with magman, if it does not start with the primer it surely sounds like an ignition problem. Have you performed a mag check when the RPM starts to show signs of degration? It is surely highly unlikely that you are experiencing a double magneto failure, perhaps one is giving up and then the other. A lot of the time magneto's will give up when the coils get hot, you said that you checked the mags. Was it by a shop or a field inspection. The shop that I use heats the coils and checks for break down and suprisingly have found a few perfectly good coils that operate cold but start giving up as the heat up. I have seen a broken impulse coupling spring cause RPM degradation as well. One other item, does the fuel selector have positive detents, if not, fuel starvation will occur if the ports are not perfectly matched. Just some ideas, when you find out please post the findings, good luck...

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Re: Power Loss in C-172F

Thought of this right after I selected the "post" key, Have you checked the wiring and ignition switch for grounding. Try disconnecting the "P" leads, for Slick mags, or insert test pigtail leads Bendix, run the engine and see if you can duplicate the problem, as you probably know with the "P" leads disconnected the mags are "HOT" and the prop will bite if not given respect.

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Re: Power Loss in C-172F

While it does sound like FUEL to me here's thought.

A mag failure in flight will generally be unnoticed; until

the next mag check.

IF the second one fails then the SILENCE is noticed!

Slick did have a problem with capacitors at one time & as Tom (?)

stated a hot coil will do likewise.

Lycoming has Service Instruction to run-up before shut-down.

The intent is to get the LEAD out.

It should be routine for all engines though.

This is where you could find the problem surfacing.


My thought is the "Most-ignored" item in GA is the

Post-Flight Run-up/Insp.

This is where ignition problems, scrubbed tires , oil leaks

SHOULD be found.

Discovery during PREFLIGHT usually means cancelling the flight.


USAF & the airlines followed this to assure the aircraft was ready for the next flight.

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Re: Power Loss in C-172F

Hi Mike;

Just a thought. Because it seems to only happen after the engine has run for a while and gets hot, have you checked the intake manifold for security?  If you are getting any warping of the intake tubes or manifold, it could be leaking air and you would not get enough fuel into the cylinders even with adequate fuel flow to the carburetor.

Another thought is with the valves, which can be a problem on the O-300 engine (mostly due to the higher lead content of 100LL).  If you have enough lead buildup on the valve stems, they may be paritally seizing when the engine gets hot. When the engine cools, they are free to move and the engine will start and run normally again. The valve lifters could also be sticking in their bores, and causing the valves to not close completely when the engine gets hot.

Some people have had success using Marvel Mystery Oil in the fuel to free up sticking valves. Otherwise I would think you would need to pull the cylinders and remove the valves and lifters to check the valve stems, guides, and bores for lead buildup and clearance.

If you have fuel/air mixture, a timed spark, and compression, you should get combustion. A leaking manifold would affect the first, and the valve problem the last. The magneto checks above should cover the timed spark. Hope this helps.

Jon

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Re: Power Loss in C-172F

Might be a chance of finding one of those valves leaking by doing a compression check.  You could do one when the engine is cool and then another right after flight.

Tom
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Re: Power Loss in C-172F

I had a very similar experience with my 172H.  The engine would run fine in flight, and then occassionally would just stop during the landing roll and couldn't be restarted. 

I am on a farm strip and would have to tow the aircraft back to the hangar. I did all the tests you did and got all the same results. Good fuel pressure, spark, timing correct etc. 

Like you, I also wondered about heat soak in the main fuel hose. 

I the end I put on a new Slick ignition kit from Aircraft Spruce and have not had a problem since.  I can only conclude that the coils in the original Slick 664 mags, which had been on the aircraft since 1972 were breaking down when hot and at idle revs.

The new mags have made a world of difference to how easily she starts too.

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Re: Power Loss in C-172F

All,

Thanks for the input.  I'll share these ideas with my mechanic. 

We have not tested the mags when the engine is losing RPM, as we have been too busy focusing on a power-loss then power-out landing (twice now), and the engine has been dead after landing.  An AP/IA heated and checked the mags, and says they are OK except for needing new points, which we are replacing. We have not checked the spark during the failure mode--by the time we could get the cowling off, it'd probably start. 

We are also installing a new carb. 

Today the mechanic commented that, while draining fuel from the tanks into a container air bubbles were observed in the fuel with the drain line below the surface of the fuel, he is suspecting a primer line leak. We also found two loose valve guides but the mechanics say that wouldn't cause the problem (fixing them anyway, of course).

We haven't been able to duplicate the problem on the ground and I don't have a glider rating, so I'd really like to find the root cause before flying the plane again!

Stay tuned, I'll post any updates.  Thanks Again!

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Re: Power Loss in C-172F

Well, gang, here is an update. Since the last power-out landing we:

1. Replaced the carb with a factory-rebuilt unit.

2. Found and fixed loose valve guides in two cylinders.

3. Installed a new gascolator (Steve's Aircraft).

4. Replaced the flex hose from the gascolator to the carb.

5. Installed a carb air temp gauge.

6. Repaired carb air box.

7. Mags tested by IA and new points installed.

I'm going from memory, but I think that's it.

We taxi tested on Monday for about an hour and ran it up several times.  We also did a few high-speed taxi runs. No problems.

Today we did some more run-ups and taxi tests, no problem.  So, we took off and climbed to about 3300 feet MSL and stayed over the airport.  The engine ran fine--for about half an hour, then a few occasional misses (nothing violent), followed by a rapid loss of power. Carb air temp was well above the yellow arc.  Landed uneventfully and rolled to the taxiway, where the engine died and would not restart.

We checked the new gascolator, plenty of fuel at its drain.  Turning over the prop by hand felt OK, suggesting normal compressions.  Tried primer and cranked-it showed no sign of starting.

The only change this time is that it would not restart after sitting for about half an hour, unlike the previous episodes. Hoping whatever is acting up has failed altogether, so we can find the problem!

We need to revisit you folks' previous suggestions in this thread--just wanted to provide an update.  We're going to uncowl the plane tomorrow evening and investigate.  Stay tuned, I'll keep y'all posted! And if anyone has any additional ideas, please chime in!

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Re: Power Loss in C-172F

May want to think about those mags.....not sure how the IA heated them, but they have been known to do a lot of strange things when they get hot.

Tom
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Re: Power Loss in C-172F

First, my apologies for taking so long to give you folks an update.  The problem turned out to be...you guessed it--the mags, as suggested by many. I was under the impression that the mags had been bench-tested while hot, but that turned out not to be the case.  When they were tested hot, each of the coils failed within about 10 degrees of each other.  We installed a new Slick system and that took care of the problem.  So, while it is *unlikely* that both mags can fail in the same way at the same time, it is not *impossible*  !!  Thanks to all for the advice.

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