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  #1  
Old 11-11-2005, 05:50 PM
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Default Fuel pressure puzzle

OK, lets see if any of the wizards here can get their head around this....

My fuel pressures were uneven on the last runup. The left one was 37 and the right one was 43 psi. (14.4V on the buss). Switching pumps made enough difference that the mixture went a little lean on left only. The pumps are served by direct feeds from seperate batteries. Both batteries are fully charged and each shows 12.6v on the buss when the engine isnt running. The pressures dont change appreciably when the alternator is turning. This morning I checked the pressures again without starting the engine. 42/43 on the right. 37 on the left. All pressures are verified by two seperate fuel pressure sensors which read the pressure on the rail.

The left pump was cleaned just before the above readings. I removed both pumps, cleaned them both, blew them out with an air line, and put them back reversing left to right. I was expecting to find out if it was a bad pump, or a blocked filter. Now I get 45 psi on the right and 44 on the left. i.e both pumps are now behaving correctly.

The only answer I can come up with is that there was a small piece of crud in the left pump that I didnt get with the first cleaning.

I hate things that don't make sense.
Anyone have any other theories?
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  #2  
Old 11-11-2005, 05:59 PM
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I would suspect the sensor - BUT the condition was verified with the engine running lean.

Gunk does not make sense to me - where da gunk come from? i suspect a binding on the bad pump that was straightened out with the second cleaning.

you didn't see no gunk? what did you clean with. i would suspect the motor shaft to impeller or whatever seal moves the fuel seal or maybe a bad ground
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  #3  
Old 11-11-2005, 06:13 PM
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I'm kinda with dust on this one, but I'm leaning towards a bad ground.
I've seen many a pump work fine until it gets loaded up, where it will want to draw a little more current, a bad connection, ground or otherwise will affect this.
If the problems continue, try clipping an ammeter in the line of the pump power supply to see what the current draws are, a bad joint will have a markedly higher drain under load than a good one.
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  #4  
Old 11-11-2005, 06:59 PM
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Quote:
you didn't see no gunk? what did you clean with
A tiny bit on the first cleaning, nothing on the second. There's no way to get inside the pumps. I tapped them on the bench to get any crud out of the inlet, then blew them out with the air hose.

Yes, a weak connection makes sense. I remade the connections when changing the pumps over.

Thanks. I feel better now.
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  #5  
Old 11-11-2005, 07:19 PM
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What's the pressure setting of the regulator? The pumps should provide fuel pressure far in excess of the regulator setting you have.

Are you saying that one pump is OK and the regulator is keeping it at 42/43 and the other pump can't even meet the regulator?

But wait, after cleaning and swapping them, they are now at 44/45 psi?

Does this mean that before the cleaning that neither was even meeting the regulator?

Somethings not right. How are they connected? Is it possible that either/both were not getting a good battery/ground connection?

For pressure, there has to be a restriction of some sort. (Analagous to resistance in an electrical circuit) The pump has to be able to provide a certain amount of fuel at a certain pressure. What are the ratings of the pumps you use?
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Old 11-11-2005, 08:27 PM
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Well done mr lafleur - now that does seem to better define the problem
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  #7  
Old 11-11-2005, 11:06 PM
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Good point, Mike. The pumps are rated at 95 PSI max. I think they put out about 55 psi when unregulated.
They're wired with 16G wire and circular crimps. Maybe the humidity down here got to the crimps, which were done a long time ago. I'll replace all the crimps in the am and check the amp draw on each pump. I'll also test the resistance of the feed and ground under load with my load tester gizmo.

The regulator is vacuum controlled by manifold pressure. I'm not suspecting the regulator since the difference was on one side, and the two lines T just before the rail, then go via the same regulator to the return.
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  #8  
Old 11-11-2005, 11:28 PM
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YOU GOT 10 ON ONE,,,,,100 ON ONE
YOU CLEAN BOTH
YOU NOW HAVE 90 ON ONE,,,,110 ON THE OTHER
SOUNDS LIKE FINE DIRT, FUZZ I CALL IT.
TO SMALL TO SEE, I SEE IT ALL THE TIME IN RACING CARBS
MY THOUGHT IS CLEAN EVERY TWO RACE

but in your case, I'd set up a test bed that could be hooked up to the pumps
easy with out removing them from the plane

lets see, hot wire from battery not on plane
gal can with new fuel and a small lead to pump
small lead away from pump going to a carb jet(just a small hole in the fuel line)
and a gage fit in the line to read the pressure
be for fight hook up left pump,spray 1 min(read pressure)
then do the same for the right pump(read pressure)
lets say you do it now that you just cleaned them and you get 72 pds and 68
as long as they stay clean you should get that the next time you test.
but heres my point,the next time they ack up dont clean but test and if you get 20 and 60 you mite look at the fuel,but then clean and retest

get a glass test tube fill and let stand for 12 hour,your fuel and some from the gas station and see if they look the same
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Old 11-11-2005, 11:40 PM
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You wanna now what's amazing...

I completely understood all of that.
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  #10  
Old 11-12-2005, 12:21 PM
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I'd go along with MP on this one. You need to perform a volume @ pressure test as the pressure itself is rather meaningless without knowing volume. Easiest way is to disconnect the return hose from the reg, fire up the pumps and measure the volume of fuel put out in 1 minute. This will force the pumps to put out reg pressure. You should see a minimum of 2L/ 2QT per minute in this test and both pumps should be very close or you have a problem with them on a line or filter restriction. As a further test, apply regulated shop air to the manifold reference port on the reg, say 10 psi. Fuel pressure to go up 10 psi and pump volume will decrease slightly, just make sure you still have the volume required to feed the engine.

If you have proper filters before each pump, you should never have a problem with crud in the pumps, only crud in the filters. A simple mouth blow test through the filters can usually verify condition. If you turn blue in 5 seconds, the filter is bad, if you can blow easily, it's likely ok.

Again, I've seen more than a few EFI pumps pass the pressure test, pass a free stream volume test but flunk the combined test. If fuel pressure falls off at WOT and high rpm on the runup, don't fly.

Just had a similar problem this year on my Nissan 240SX turbo. Intermittent lean out at WOT. Sometimes ok, sometimes not. Taped a fuel pressure gauge to the windshield and drove it under boost. Sure enough, fuel pressure would sometimes start to fall above 4000 rpm and it would lean out. Fuel pressure was always fine at idle where no volume is demanded. New Walbro pump cured it and it storms like it used too again.

Checking connections is also a great idea and current draw if you can. Eliminate everything before you fly again.
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  #11  
Old 11-14-2005, 04:09 AM
Hennie Engelbrecht Hennie Engelbrecht is offline
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John

Maybe you were sucking air and thechanging of the pumps cured the leak

Hennie
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  #12  
Old 11-14-2005, 08:00 AM
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No, I think it was bad / corroded crimps. Both pumps are now performing purrrfectly. I flew her for an hour yesterday with no issues. Thanks, everyone, for all the input.
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