Help me diagnose my engine

[M016576]

As of yesterday at about 2pm, my plane is sitting on the ramp away from home, waiting to me looked at by maintenance. I'm wondering if we can isolate the problem here before they even crack the cowling.

As a recap, it's a M20J with an IO-550-A motor

Here's what happened- as I was descending out of 8500, I pulled the throttle back a little (~18"), left the Condition set to 2500 rpm, and left the mixture- the fuel pressure jumped up to 41PSI (two gauges- ships gauge was pegged, CGR-30P showing digital psi), and the motor started to stumble. I richened the mixture up immediately, and the fuel pressure increased to 46psi with no help to the motor. I added throttle back in (I'm now through about 6000') and the motor stopped stumbling and the fuel press dropped back within limits (~35 psi... Redline is 36 on the 550). I kept the descent coming as I was close to the field and continued to try to work the issue. Through 4000", I pulled throttle to 16" and the press jumped up again to 45psi with much stumbling. I then pulled the condition lever waaaay back (2000 rpm), and the fuel pressure dropped down to within limits (30 psi). Of note, if I richened up to 1250 EGT or so the fuel pressure would peg out.

So, in short: low MP (~18" or so), high power (2500 rpm) is equaling 45PSI fuel press and stumbling motor.

I'm thinking it's the fuel pump (safe bet), but I'm wondering what more specifically it could be... Thanks!

[Piloto]

I think you are right. The lower RPM will make the pump to put out lower pressure. Check the pump vent line to insure is not kinked or blocked.

 

Jose

[BigTex]

Did you see anything unusual with the EGT's or CHT's?

[M016576]

Did you see anything unusual with the EGT's or CHT's?

CHT's are all fine (300-350). The missile has a pretty terrific cowling.

Fuel flow is a bit erratic (+\- .2 gal/hr)

Reatime I saw that the EGT's were all tracking how they should with the red knob. But I didn't look at them specifically as I was pulling the throttle back. I have the engine data on a USB, but I left it on the keychain at the fbo.

[pinerunner]

My first thought seeing your fuel pressure take off is some kind of clog after the fuel pump. If the pump itself were screwed I would expect it to be unable to develop that high pressure. I try to thing this way before going into the part-swapping mode.

[jetdriven]

The Continental fuel injection system is a complicated unit, and one that requires a very detailed and methodical troubleshooting session using the factory maintenance manual. Even after you throw a few grand in parts at it until you find the offending item, it has to be set up exactly per a service bulletin.

What you are asking is akin to how you set the E-gap on a magneto. That said, get ahold of the factory service manuals that cover this and I bet you can solve it faster and cheaper than dropping it off. Plus you will know more about ow it works and how to troubleshoot.

[Guest]

Look into TCM SID 97-3, E is the current version. Can you tell us the engine model to be able to offer assistance.

Clarence

[M016576]

Look into TCM SID 97-3, E is the current version. Can you tell us the engine model to be able to offer assistance.

Clarence

TCM IO-550-A

[Alan Fox]

Very well could be a spider , better hope it is as this is the least expensive link in the chain....

[M016576]

Very well could be a spider , better hope it is as this is the least expensive link in the chain....

Indeed...

[PLN_FXR]

You are stumbling at low power (low MP) and high RPM, as though it is too rich; did you try leaning the mixture further?  Were this a Bendix (Lycoming) system, I'd think you had a clogged injector.  On your TCM system, you could have a plugged return line or a fuel pump that isn't regulating properly ... among many other items.  This assumes you are running too rich ... which is a guess, as you didn't mention leaning the mixture - only pushing her back in.  I would also wonder if you had an electric boost pump on?

[M016576]

You are stumbling at low power (low MP) and high RPM, as though it is too rich; did you try leaning the mixture further? Were this a Bendix (Lycoming) system, I'd think you had a clogged injector. On your TCM system, you could have a plugged return line or a fuel pump that isn't regulating properly ... among many other items. This assumes you are running too rich ... which is a guess, as you didn't mention leaning the mixture - only pushing her back in. I would also wonder if you had an electric boost pump on?

That's what it looks and feels like- it's too rich- but the red knob was still leaned out for cruise (-20 lop), to only dropped the MP a couple inches (21-18) and the fuel pressure took off (31psi up to 45+). When I either leaned further, or put the power back in, the stumbling stopped. When I reduced rpm, the fuel pressure came back into limits.

I'm not ruling out operator error here, but I've never seen this motor behave this way (in about 50 hours of flying this plane), nor did the io -360 I flew behind for 350 hours ever do this. I certainly have more hours behind a 4 banger Lyc, than a 6cyl conti- so I don't know the motor nearly as well.

I would believe that I made a mistake if the Fuel pressure hadn't have been so far above the 36 psi redline.

The electric boost pump only comes on to start this mooney, it is not used for takeoff or landing- I did not have that on, nor did I try it, as the fuel press was over redline.

[Alan Fox]

This is a very simple system , no return , one line into the mechanical pump , one line from the pump to the servo , which is basically a metering jet , and one line from the servo to the spider , and six lines to each injector.....it draws its gage pressure from the spider , meaning there is high pressure between the  spider and the pump , ...... the spider has a diaphragm in it , and my guess (and its only a guess , is the diaphragm collapsed in the spider......There is also a pressure diaphragm on the mechanical , that may also have failed....

[jetdriven]

Iirc the continental injection system has a fuel return line.

[carusoam]

Another challenge...

Many IO550s in Mooneys don't have fuel pressure gauges...

I have FF only, in an O1.

Looking forward to see what you learn.

Best regards,

-a-

[orionflt]

possibly a slope controler, as stated above Continental's fuel systems are complicated and can be difficult to troubleshoot.

 

Brian  

[Bob - S50]

Did you by chance accidentally leave the electric boost pump on?

 

Bob

[Alan Fox]

@ Tony , the Fuel flow is a pressure gage.....

[jetdriven]

Depending on the system it can be either a fuel pressure gauge (marked in GPH) or a true fuel flow gauge with a turbine wheel transducer. Since he was talking in PSI I'd venture it's a fuel pressure gauge and his FF is a real fuel flow gauge

[Guest]

If the fuel pressure went that high the gauge is reading unmetered fuel pressure not meter or nozzle pressure.

If its really that high the fault is either the mechanical engine pump internal regulator or the electric pump was on.

You should be able to reproduce the boost pump theory by turning it on on ground run up, normally the engie will quit by drowning in fuel, when the boost pump pressure combined with mechanical pressure over powers the regulator Leaning the mixture wil restore normal operation.

Checking the TCM SID for setting and procedure will lead you on the right direction. The TCM system has more parts but is easy to adjust.

Clarence

[M016576]

Update (and close out)

3 separate issues...

1) a partially obstructed injector. This was causing high egt in one cylinder (which looked semi normal once I was LOP). It also was resulting in a single cooler than normal CHT (just so happens that's the cylinder with the ring probe, vice the 5 others with bayonets). I removed the ring probe- now all 6 are on bayonets.

2). The fuel pressure transducer that was installed is optimized for pressures below 30 psi. After talking to EI, they said that it's very accurate below 30, but quickly diverges above 30.

3). Most importantly. The gascolator screen was dirty, so that was replaced... But the piston o-ring in the gascolator was too large. As the demand and fuel pressure increased from the supply side of the pumps, air was being sucked into the fuel line through the gascolator, leaning out the mix. This was also occurring on T/O- causing a steady rise in EGT's during climb. Pulling power for descent caused an overly rich condition which was causing the motor to stumble. Basically, the mixture knob was working exactly the opposite of how it should be working, as air was already introduced into the fuel at an unmetered and unpredictable rate. There was no way to tell this on deck: the gascolator wasn't leaking fuel, even when I charged it chained down during a run up.... Very insidious- it was just a tiny bit too big to seat properly. Held fuel in, but let air in too.

I used the o-ring sizes that M20DOC recommended in a separate post. Not only is the engine now running smoothly and as expected during climb, cruise and descent, but the motor is starting much easier, and idling smoother. For reference, the 1982 M20J gascolator is not the "airight" model mentioned in the gascolator post: those o-rings are too big for the stock gascolator- the ones M20Doc mentioned are the correct size.

And yes, the plane had just come from annual, which was performed by a shop at the field where the new avionics were being installed (and which I had no experience with in the past... That'll teach me).

As an added benefit, the gascolator pull is now silky smooth as well.