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diagnosis needed

erich weaver

Well Known Member
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
 
Seems like another fellow posted here with a similar problem about two weeks ago. He also had P-Mags.
 
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.
 
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
 
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
 
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 :eek:
 
Bummer

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
 
True but

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
 
Ok one other thought

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
 
I agree with a post that was apparently removed - the simultaneous indications of high fuel pressure and high flows are probably real. Fuel flow is measured independently from fuel pressure, albeit probably interpreted by the same instrument. Is your engine 180 hp or 200? 180 should see sea level, full throttle and full rich fuel flows of around 16 to 17 gph, and around 18 to 19 for 200hp. There is a graph from an old book I'll dig up that shows where combustion stops on the rich side, and we can probably infer if 22 gph is close.

It might indicate an intermittent stuck relief valve on the AFP boost pump. The boost pump is a constant volume pump, and if the relief valve stuck, I suspect it could generate quite high pressures. Don Rivera could tell you what those pressures might be.

What fuel servo do you have? It would probably be worthwhile to discuss with the servo manufacturer what would happen if pressure really did spike to what you saw as indications on the first flight. If they believe fuel flow could go high due to excess inlet pressure, you might have a diagnosis that would match the data exactly (stuck relief valve). Another problem could be in the engine driven fuel pump - something amiss there. I don't know if there is a failure mode in the engine driven pump that could cause over pressure. Seems not, since there is a spring involved, but I don't know.

Too rich of a mixture will absolutely kill the engine, and when it recovers, it could certainly after fire (bang in the exhausts).

What is the exact description of your plumbing around the AFP pump?

My thoughts are to have AFP look at your boost pump assembly, to see if there is anything that could cause the overpressure.

Good luck, I know you will keep us posted.
 
snip

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

snip

erich

Yes, fuel alone can do it. Pull the throttle too idle quickly on most FI engines during flight, and the pipes will bang.
 
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The problem I have with this idea

is that typically an engine running excessively rich will sort of bog down, splutter and die....as the plugs are then wet it is unlikely to spark back into life without coughing and spluttering some more.

The symptoms imply a sudden stop then restart which means either fuel flow was interupted or ignitions turned off.

Its worth checking though but i would start at the Pmag wiring...at least from the description of the problem.

Your right in that the AFP pump is a positive displacement pump and will make high pressures if the relief valve is clogged.

You can dismantle those relieve valves to see if there is any goop in there...its simply a ball and a spring.

Frank
 
I know

But I'm not sure which our hero experienced...If it was just a bang then that would presumably be difficult to determine.

As others have said...we do get popping in the exhausts at low MP but often that is referred to as "backfiring" but clearly that is not what is going on.

Frank
 
Thanks for the thoughts guys.

I tied the plane down and ran the engine for about 15 minutes today, closely watching fuel pressure and fuel flow while varying RPM up to a maximum of 2000 RPM. I was NOT able to induce the erratic fuel pressure readings during this run - mostly right on the money at about 28 psi. I also experimented with turning the boost pump on and off. No significant changes in pressure from that - never above 28 PSI, a couple of times dropped to 26 or 27 after turning the boost off, but quickly recovered to 28. What DID change when I ran the boost pump was the indicated fuel flow - several GPH higher with it on, even at idle. Didnt really notice any differences in how the engine ran though, which makes me think it may not really be happening. My flow sensor is located inside the cabin, a short distance in front of the AFP boost pump (i.e., between the boost pump and the engine driven pump), and as a previous poster has pointed out, that may be resulting in inaccurate readings. I'll talk to Don at AFP about that.

Pulled the P-mags after the engine run - no loose connections apparent. Im sending them back to Emagair for a complete checkout. While those are out I'll do some more checking of the fuel system. Hope I find something I can blame this on, but Im not too optimistic with respect to the fuel system - the thing seemed to run fine today. I'll talk to GRT about any cases of fluctuating fuel pressure on their sensors and see where that takes me.

With respect to data recording on my GRT system, I ummmmmm, dont know. At any rate, at best, its probably only something I could set up for the future.

best regards

erich
 
Can't say about FI systems but my O-360/Carb does indicate a higher fuel flow with the electric boost pump on than with it off.
 
Thanks for the thoughts guys.


With respect to data recording on my GRT system, I ummmmmm, dont know. At any rate, at best, its probably only something I could set up for the future.



Check out the illustrated guide to recording the data on your GRT system here:

http://www.rvproject.gen.nz/grt/index.html


Then you can use Carl's GRTDecode to decode the data and view in Excel etc... and then I may even let my cat out of the bag and release the software I wrote.. :)

This would be good time to do these recordings..


PS.. My vote's on P-Mags.. and don't worry about your FF now.. you haven't calibrated it yet.. it's not accurate...
 
erich...
My flow sensor is located inside the cabin, a short distance in front of the AFP boost pump (i.e., between the boost pump and the engine driven pump), and as a previous poster has pointed out, that may be resulting in inaccurate readings. I'll talk to Don at AFP about that.
Dynon recommend putting the sensor there, or rather say "lot's of people do"; however, FloScan who make most FF sensors stated in an eMail to me:
The FloScan sensor should not be installed between two fuel pumps. Accuracy of sensor readings cannot be guaranteed if it is. The sensor should always be installed upstream of, or on the inlet or vacuum side of any and all fuel pumps ... I have reviewed the transducer installation sheet and can only say that if you install it between two pumps, your readings will probably be inaccurate....
and our experience, and that of many here backs up what they say. I'd go for ~50% overreading :confused:

Andy
 
When the boost pump is turned on any "over pressure" or "excess fuel" should be returned to the tank via the return line right? I don't know how yours is plumbed, but from what I've seen on others it should be. The boost pump should increase volume so the indications of higher fuel flow are accuare. If you were inflight and saw high pressure readins it seems to me the pressure regulator is suspect.
 
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Assuming a single P-mag issue

Assume that the grounding issue is not a problem and one P-Mag just temporarily did something to cause this. I don't have enough insight into P-Mags to know if this is viable since I assume that if the problem we have seem in the past (loss of timing) occurred then it was a "hard" failure until power was turned off and it reset.

So if a loss of timing on one P-mag occurred in flight, wouldn't a suitable diagnostic procedure be to go from "Both" mags to "Left" and "Right" to isolate the offender. Once that is done stay on the good mag until you can safely land.

NOTE: This does not mean that this case is attributable to a P-mag issue.
 
When the boost pump is turned on any "over pressure" or "excess fuel" should be returned to the tank via the return line right? I don't know how yours is plumbed, but from what I've seen on others it should be. The boost pump should increase volume so the indications of higher fuel flow are accuare. If you were inflight and saw high pressure readins it seems to me the pressure regulator is suspect.

The AFP pump setup uses a "local" loop to handle the constant displacement pump's excess volume. I don't know which FI servo Erich has, but most set-ups on the four cylinder engines do not have the "excess fuel" return system going firewall forward. This topic has been beaten on a lot in the past in this forum.

Something still needs to explain the extremely high pressure readings seen on the first flight.
 
Alex is correct, there is no return line on my AFP injection system.

Planning on a talking with AFP and GRT about fuel flow and pressure readings tomorrow, and p-mags will be shipped back for inspection. Depnding on what they have to say, I may have to move my fuel flow sensor to find out for sure if fuel flow is actually increasing substantially with the boost on. If its true, seems like that is a possible reason for my event.

erich
 
PMags causing backfire (Afterfire)

I had some issues with my PMags. I had an incident where they went to erratic timing and caused a power loss and backfire shortly after takeoff. My #4 cylinder apparently went to a low level of detonation as its CHT and the oil temp spiked. With an immediate power reduction, the problem stopped. My guess is the reduced manifold pressure reduced the timing.
I think my baseline problem was I didn't have cooling on the ignitions (blast tubes). I had the problem on the third quick flight (giving rides). I have since added blast tubes and have not seen a repeat of the problem.
On return to EMag, Brad said the internal recording of the PMag said it got really hot, so I think the electronics went nuts in the heat.

Sebastian Trost
RV-7A
225 hours
Cameron Park, CA (O61)
 
Just to clarify... Our FI Electric pump has it's return plumbed in just before the Fuel Filter, and all aft of the firewall. The FF sensor is FF Fwd... The overreading is some pressure interraction between the pumps... Selecting the Elec Pump on increases the indicated FF significantly, but nothing else alters at all... it is a (well known) pure indication problem.
 
Just my opinion but why would anyone run 2 pmag/emags with all the issues they've had, especially on a new airplane :eek:

I hate to be critical of the pmag/emag system but lets get real here, they have had a number of problems and are not a mature product. Also if it was me I'd pull the both fuel pumps and have them bench checked just to be sure all is good there before I flew again. And while you're at it have someone else double check all the fuel plumbing and check valves, chances are if you put it in wrong the first time you won't find anything wrong the second time so get another set of eyes on your bird to double check everything.

PS: If you need a mag to use I'll loan you one if you pay the shipping.
 
Take care to monitor your starter performance. If the timing is going out every now and then with your set up, you could ruin your starter if it happens while you are cranking it. This would help you determine what your problem may be.

I was flying with someone (with the same ignition set up you have) and just after take off the same symptoms you describe happened. We came around and quickly landed. After a brief shutdown, went to start the engine and the starter was sure dragging. Sky tec had to rebuild it. By the way, these same symptoms happened on three different occasions, even after the mags came back from the manufacturer with the "OK" stamped on them. Could have been a grounding issue, etc.,.....I don't know. I have the lightspeed and have not had this problem.
 
Just my opinion but why would anyone run 2 pmag/emags with all the issues they've had, especially on a new airplane :eek:
Because they are some of the best ignition systems going, that's why. Besides, no one seems to have had any problems since they came out with the firmware upgrade last July.

As for the problem at hand...

What version of firm ware where you running on the P-mags? Did you send them in for the latest firmware and hardware updates? Did you wire them per E-mag Air or did you follow Electric Bob's Z-33 drawing? It could be the P-mags but I kind of doubt it. Once they loose the timing, it usually stays lost.

You never did say if you added the blast tubes or if you had to re-time the P-mags after your trouble flight.

What about blast tubes, did you have those installed and aimed at the neck of the P-mags and not the square section?

Based on your description of the fuel problem, I suspect that might be the cause but rule out one thing at a time. The P-mags are easy enough to remove and send in.

Let us know how it comes out.
 
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Erich, I am sorry to hear about your problems, but I do want to offer you a hearty CONGRATULATIONS for getting it into the air! Very cool! Good luck fixing your issue.
 
Just my opinion but why would anyone run 2 pmag/emags with all the issues they've had, especially on a new airplane :eek:

I hate to be critical of the pmag/emag system but lets get real here, they have had a number of problems and are not a mature product. Also if it was me I'd pull the both fuel pumps and have them bench checked just to be sure all is good there before I flew again. And while you're at it have someone else double check all the fuel plumbing and check valves, chances are if you put it in wrong the first time you won't find anything wrong the second time so get another set of eyes on your bird to double check everything.

PS: If you need a mag to use I'll loan you one if you pay the shipping.


Walt,
Your offer of the mag is very kind, and I am taking to heart your advice regarding the fuel plumbing. However, please, please please - lets not rehash the P-mags/E-mags thing here again. Its all been said before, and I made my choice with eyes wide open. Besides, I do not know that they are the source of my problem. I am of course investigiting that possibility

best regards

erich
 
As for the problem at hand...

What version of firm ware where you running on the P-mags? Did you send them in for the latest firmware and hardware updates? Did you wire them per E-mag Air or did you follow Electric Bob's Z-33 drawing? It could be the P-mags but I kind of doubt it. Once they loose the timing, it usually stays lost.

You never did say if you added the blast tubes or if you had to re-time the P-mags after your trouble flight.

What about blast tubes, did you have those installed and aimed at the neck of the P-mags and not the square section?

Based on your description of the fuel problem, I suspect that might be the cause but rule out one thing at a time. The P-mags are easy enough to remove and send in.

Let us know how it comes out.

Version 27 firmware I believe, wired per Z-33, blast tubes installed, pointed at neck. Timing looked ok after flight. P-mags are in the mail to be checked out today. Will post back with all fuel system/ignition findings when I have some information.

My worst fear is that I find nothing. It is no fun being afraid of the airplane that you just spent 3 years building so you could have some fun. Irony at its worst.
 
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