diagnosis needed

[erich weaver]

Had an event yesterday that shook me up and I would like some assistance in sorting things out. Yesterday was the second flight of my RV-7A. Shortly after takeoff, I had just turned cross wind when I had a momentary loss of power - maybe a couple of seconds - followed by a backfire, then return of power. Got it back on the ground without incident after reducing power and staying in a tight pattern. Confidence was severely shaken however.

Here are the circumstances, as best I know them. Not sure what is relevant to the power loss and what is not. Keep in mind it was only the second flight - its still pretty much a sensory overload experience for me.

Equipment: IO-360B1B, AFP boost pump, GRT EIS and EFIS, dual p-mags

Possibly relevant issues from the first flight:
(1) fuel pressure reading was usually normal, about 25 psi I think, but occasionally went erratic, jumping up into the 40s, 50 and 60s and jumping all over before settling back down to normal again. Ii didnt notice any changes in engine performance when it did go nuts, and I later attributed it to faulty readings, maybe due to vibration

(2)Warning light for high fuel flow went off, although I cant say now exactly what the fuel flow reading was at the time. Light went off after power reduction, but I later reset the alarm to go off at a higher flowrate - I believe it was set to 21 or 22 gph for the second flight.

Second flight details:
During the abbreviated second flight, I remember the fuel flow alarm again going off shortly after lift-off and seeig a reading of 22 gph. This seems too high to me, even at full power. Im not sure now whether or not the alarm was still going off when I had the loss of power a short time later. At some point I hit the Ack button, but dont remember now exactly when I did this.

My limited analysis: Momentary loss of power means a momentary loss of air, fuel or spark occurred. Im ruling out an air problem, at least in the grossest sense - my inlet is clear of obstructions.

The fact that there was a backfire leads me to believe there was a spark/timing problem. I know p-mags have had their issues, but I have a hard time seeing how I could have been so unlucky as to have had a simultaneous dual failure. No product bashing here please. Mag check at runup showed both ignitions to be functioning properly. I checked timing after shutdown and both gave me a geen light at TDC like they should. Brad at Emagair has aready offered to go over them with a microscope to check them out and I will definitely do that.

Not sure about a fuel problem. Tanks were sumped prior to take-off - no water observed. Will check filter in boost pump, but seems unlikely that a clogged filter would result in only a momentary loss of power.

With respect to the occasionally erratic fuel pressure readings, is it even possible that my boost pump/engine driven pump combo could generate pressures in the 40-65 psi range? Is my high fuel flow rate at take-off truly abnormal? If it is abnormal, could that result in momentary loss of power and a backfire?

Any insights would be appreciated. Im hoping to make this project fun. Right now it definitiely is not.

thanks

erich

[Yukon]

Seems like another fellow posted here with a similar problem about two weeks ago. He also had P-Mags.

[szicree]

Originally Posted by erich weaver

The fact that there was a backfire leads me to believe there was a spark/timing problem. I know p-mags have had their issues, but I have a hard time seeing how I could have been so unlucky as to have had a simultaneous dual failure.

Wouldn't have to both fail if one of them went too advanced.

[erich weaver]

Originally Posted by Geico266

Got to be fuel pressure IMHO. What system do you have for pressure regulation? Return lines clear? Are your tanks properly ventilated?

Reading your situation you were in a turn, nose up, full power? Which tank was the selector on?

Trust your guages right now, they indicated an over pressure situation that was followed by loss of power with a back fire. There was raw fuel in the exhaust from something.

I'm thowing stuff against the wall here, but this is a serious situation. Make sure you find the cause before you take flight again.

Believe me, Im taking this seriously.

Dont know much about regulation of fuel pressure. I assume there is some sort of regulation by the AFP boost pump system and/or the engine driven pump. There was little if any turn at the time of the power loss. I was climbing and still at full power setting. Right tank on line.

I do NOT know that there was an over pressure situation at the time of power loss - only that my fuel pressure reading occasionally went erratic during the PREVIOUS 1 hour flight that was completed satisfactorily. Cant rule it out though.


Question for the group: Can a backfire occur from a fuel supply problem alone, or does it always indicate an ignition/timing problem?

Going off to the hangar for a ground run to try and see if I can dupicate the erratic fuel pressure indication while monitoring fuel flow.

erich

[Andy Hill]

Possibly relevant issues from the first flight:
(1) fuel pressure reading was usually normal, about 25 psi I think, but occasionally went erratic, jumping up into the 40s, 50 and 60s and jumping all over before settling back down to normal again. I didnt notice any changes in engine performance when it did go nuts, and I later attributed it to faulty readings, maybe due to vibration

We've a Dynon 180 with it's usual Fuel Pressure Sensor... and see this... in fact steady ~90psi at one point in the cruise! We have been assured that there is no way the engine would run with this, it would leak fuel, and the pump could not produce it. I suspect it is reacting to the "pulses" from the EDP and setting up some wave or something... At idle, if it jumps around the electric pump "calms it down"...

I remember the fuel flow alarm again going off shortly after lift-off and seeig a reading of 22 gph

Our XP IO-360 book quotes Max power 94lbs/hr which I reckon equates to ~16 USG/hr. However, if you have your FF sensor between the Elec Pump and EDP (as we do) it will almost certainly overread significantly when the Elec Pump is on.

Have you any "logging" facilities on your glass units? We have a 4GB (I think - about 40hours worth) card recording all the Dynon EFIS & EMS data, which was great for the odd testing issue (albeit less worrying than yours...)

Good luck! Sorry not to have more useful advice...

Andy & Ellie Hill
RV-8 G-HILZ

[Andy Hill]

Good luck on the testing... From my points, suggest closely monitor at some varying power settings the FF and FPress, with / without Elec Pump on. You may well see, as we do, these readings are not overly reliable in all circumstances

[frankh]

Glad you made it back down safely Erich..

I too have an Emag and Pmag combo and I was one of the customers that suffered 'slipped" timing for want of a better term, but I must say this is the first time I have heard of such symptoms

certainly bad timing can cause bad running together with misfires and the like but never a complete stoppage...I guess its possible but I would expect the backfire to correspond with the loss of power.

You siad the back fire came after the power loss..i.e power went away then there was a bang, then power returned?

That sounds like all the sparks were turned off momentarily then turned back on..The spark then igniting the unburnt gasses in the exh pipe, hence the "backfire"

I have seen this with goofing off with cars in my youth..I.e turn off ignition then turn back on...big bang thru tail pipe.

The only thhing that should be able to cause this on a pair of Pmags is the momentary loss of electrical ground.

Can't be a temp loss of 12V power cus the Pmags make their own.

So first thing to check is the ground to each Pmag..if they share a common grounding point you might want to think about seperating them...and ground them to the engine block, not the firewall...That way the engine it will keep making sparks even if you physically rip the engine off the mount..

So check out the electrical connections...You did't combine your P lead connections by any chance? Of course they would both have to become grounded to shutoff the sparks.

Check this stuff out before you return your Pmags and then have a look for possible obstructions in your fuel supply...Nothing blocking the inlet to your fuel controller?

Physically flush the lines into a bucket...Of course you did do this prior to first flight anyway right?

All the best

Frank

RV7a 260 hours

[frankh]

Originally Posted by szicree

Wouldn't have to both fail if one of them went too advanced.

Thats correct, the mixture gets ignited off the most advanced spark, the second ignition is almost irreleavent in this case.

But too far advanced timing will not have the symptoms described...It may cough, bang, shudder with an associated loss of power..

But it will not completly die then go bang.

Of course this is just what I picked from erich's explanation.

Erich, I would (just for grins) hard time the Pmags..I.e blow into the tube once so you get the flashing LED..then turn off the power..This resets the timing back to the factory default...Then time the Pmags in the normal way...i.e don't use the blow in the tube method..This will at least remove that variable. Although I don't think this is the problem.

Frank

[frankh]

I suppose it is possible to extinguish the flame by going too rich...If the fuel flow really is 22GPH then that is way too high...Mine is about 17GPH and I know that is too rich.

This is grasping at strawas a little and if this was the real problem I would expect the motor to gradually slow then missfire rather than be turned on/off.

Have you calibrated your fuel flow sensor?.....Pretty easy to fo...Get a five gallon jug and put xactly 5galls in it...Mark position on side of jug...Empty jug into fuel tank.

Drop hose from fuell servo and pump into jug shut off at the 5 gallon mark and read the fuel flow from your glass monitor....Divide one byt the other to get a calibration factor..If youdo this you can see if 22GPH is really 18 for example.

As I said I doubt this is the issue bacause by the time you had drowned the spark plugs I boubth if they would have suddenly come back to life.
The AFP pump has a regulated pressure by the blue fitting which allows the fuel to recirulate to the inlet of the pump which has a sppring and a ball in it.

You did flush all this stuff out right?

Is it possible you airlocked the AFP pump?...restriction, air leak (highly unlikely), tank vents.

My money is still on the sparks being turned off but I bet when you find it will be a real DOH! moment..

Good luck

Frank

[asav8tor]

Doesn't that engine monitor record and save data?