C

chris mitchell

Well Known Member
Patron
My RV-8 now has 95 flight, about 80+ hours on it. Problems have been few and I am delighted with it. However, recently I have had a spate of alarms from the Dynon for high fuel pressure. This problem arises in the cruise say 2300RPM, 19-23MAP depending on what I've been doing, just the engine drive pump running - the fuel pressure bounces around. Turning ON the AFP electrical pump stablizes the fuel pressure at 30PSI and the alarm goes away. Pump off and all will be well, at least for a while. Its a very intermittent problem, and seems completely unpredictable.

I replaced the transducer - that was before I had realized that that using the electrical pump would stabilize things - and the problem persists. I wondered about a poor ground on the transducer but the effect of the EP seem to suggest another problem. I wondered if the little O-ring in the relief valve might have swollen and need to be replaced - Don Rivera at FP has kindly offered to supply. But before I start are there any other thoughts?

Engine is a Superior IO 360 , AFP pump in left side of front footwell, EDP in usual spot behind the engine, with T off to the Dynon pressure transducer which is mounted on a Vans transducer manifold. Fuel filter is an ANdair gascolator mounted before the AFP pump.

Thanks
Chris
 
This probably won't help any, but I had a similar problem recently -- high fuel pressure readings (>30psi) on my Dynon EMS. I checked the electrical connections and was ready to replace the transducer, then noticed that I had used restrictor fittings on both ends of the fuel line from the fuel pump to the transducer. I have no clue why I did that.

I removed the restrictor fitting from the transducer end and the FP has been fine since. Just an FYI.
 
High fuel pressure

I have the exact same situation as Chris. I have an XP360 engine but use an AF-3400 Advanced Flight EFIS./engine analyzer. High fuel pressure comes and goes intermittantly without warning. Does not coincide with radios or voice comunication. When i put the electric pump on it drops and goes away for a while.
 
After 350 hours and 5 years, my airplane has started doing this as well.

The problem showed up on the way to Oshkosh. Once the pressure spikes started, they could be immediately controlled by turning on the electric fuel pump. Turning the electric pump off cause the fluctuations to return within a few seconds. Turning it back on dropped the pressure back to a steady 30 psi (fuel injected). The rest of the flight was completed with the electric pump on and no fluctuations.

On the way back from Oshkosh at 9,500' in the early evening (i.e. cooler), I tried turning the electric pump off again when we were about an hour out from Houston. The pressure remained steady for the rest of the flight.

The next day, I took off and flew at 8,500' with the electric pump off. After about 45 minutes, I was descending through 5,500' and the spikes started again. Once again, turning the electric pump on settled things down right away.

I have a new pump on order, but am not convinced the problem is truly in the pump. It seems like it may be temperature related, although in the last 5 years, I have flown in some pretty hot weather.

There are several posts on the forum on this subject, but only a few reporting cures for this specific problem and of those, I wonder if the cure was permanent, given the intermittent nature of the problem. I am not suspecting a problem in the sender itself, as it should be independent of the electric pump running or not.

Has anyone else had this problem and identified the cause and cure? If you previously reported a cure, has the fix been permanent?

Thanks,

Dave
 
One possibility for high fuel pressure is the pressure relive valve getting stock. There can be multiple reason for the valve to get stock but the remedy is relatively easy and cheap. The O-ring in that valve can get swollen and cause it to stock. Different fuel additives that is in different fuel can also be the cause the O-ring to swell.

So, I don't know if this will resolve your issue but since the cost is about a buck or so for the O-ring, I would say start there.
 
Same problem on my Dynon except mine goes to 99 and then starts working again later. I've even had a 99 on the ground with the engine was not running. Before is starting hitting 99 and staying there, it would fluctuate and the fluctuation was getting worse and worse. Seems like the heat of summer was not helping.

Talked to the folks at Dynon at OSH and was told to measure the resistance across the sensor. He couldn't remember if it was supposed to be completely open or closed when it was reading that way. His recommendation was to buy a new sensor since mine is sticking on 99.
 
Try "bleeding" the oil line to the sensor. You probably have air in the line which will cause high pressure readings. It also will cause the reading to "bounce".

I see this a lot. Next time you have the cowl off, open the line at the manifold and (with the mags COLD) have a friend spin the starter until oil comes out of the manifold, then simply replace the plug.
 
We had intermittent high fuel pressure readings from the Dynon too. Turned out to be poor ground connection on the transducer that is mounted on a manifold/bracket on the firewall that connects it to the fuel flow transducer also mounted there. Just relying on the mechanical mounting bracket alone for an electric ground was not good enough. I added a stainless steel hose clamp ("breeze clamp") around the body of the fuel pressure transducer and a dedicated ground wire from the hose clamp to my ground bus and no more funky fuel pressure readings. On the fuel pressure transducer, the higher resistance - the higher the reading, so if the ground connection opens up momentarily the electronic pressure reading spikes.
 
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solution with grt

I had similar problems with intermittent high fuel pressures readings using the grt efis equipment. After much fussing around and 2 replacement vdo fuel pressure senders, I got the more expensive, 3 wire fuel pressure sender and haven't had a problem since. I can't remember the part number but it's on Grand Rapids' web site. I don't know if it will work with or fix a similar problem with Dynon's equipment...

Jeremy Constant
rv7a 120hrs
 
dforster said:
By "pressure relief valve", do you mean one of the two check valves inside the mechanical fuel pump?
Click to expand...

No, the pressure relief valve is part of the AFP electrical pump. I had to replace this o-ring and have not seen the issue since.
 
Someone once said to try the simple things, first. With this in mind, the fuel sender was moved so that instead of the fuel line being routed up to the sender, it is now routed down to the sender.

After the move, we flew for 1.4 hours at 28 C OAT. No problem, so we landed, shut down, waited 30 minutes for everything to heat soak and then took off again for 20 minutes. No problem.

No victory dance yet, but it may be that the simple cure is all that was needed - though it seems odd that turning the boost pump on (which is upstream of the mechanical) would cause a reduction in pressure seen by the sensor . . . and that this would show up for the first time after 350 hours of operation.

Thanks for everyone's help. If it turns out the problem is not fixed, I will report back (I fly about 75 hrs per year, so should hopefully see it again soon if it is not cured).
 
I had a problem like this on my RV. The setup it has is the AFS 3400 Engine monitor, which uses a VDO resistance type fuel pressure transducer. The way these work is that the pressure diaphragm inside the sender moves a little wiper along a coil of resistance wire. Because during cruise the wiper sits at a very similar position and "wiggles" a bit due to pressure fluctuations, it wears a spot on this coil until it wears through. This means that the normal resistance gradient it would normally encounter is now discontinuous and, depending on the position of the wiper, can show either very low fuel pressure or very high pressure. It can also show close to normal at times when the wiper happens to bridge the gap or the gap temporarily closes due to temperature, mechanical perturbations, or who knows what else. Turning on the electric pump generally results in moving the wiper elsewhere, especially if the boost pump provides higher pressure than the mechanical pump, which then causes a different reading.

A bad ground to the sensor can also cause erratic readings, but I don't think turning on the boost pump would stabilize these.

I cut my defective sender apart to verify this and indeed there was a noticeably worn spot on the coil.

Replacing the sender cured the problem I was seeing, but I anticipate it will come back when the new sender wears in a similar way to the old one.

Mark Olson RV-7A F1-EVO Rocket
 
New transducer

I've had fluctuating fuel pressure readings almost since Day One on my Dynon EMS. It would bounce around from 25.X to 28.X in flight. Nothing to be concerned about, but enough to get my attention.

The last several flights have resulted in FP alarms on the Dynon in excess of 30 psi. So, having read a thread here showing how the mechanical arm in the transducer wears the gizmo, causing erroneous readings, I bought a new transducer.

Today I popped the cowling and swapped out the transducer. Post-maintenance flight showed a rock-solid 28.6 psi reading on the EMS. Yippee!!

Just an update.
 
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