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Q-talk 130 - Notes From The Net

From time to time I will reprint subjects from the Q-List as I deem them important or informational.

Simon Wilson from the UK writes:

I have recently installed a Grand Rapids EIS model 2000 in my Revmaster powered Q2. I flew the aircraft yesterday for the first time in a year after completing my new instrument panel. The system works well and I am very pleased with it. However, I am having a problem with the oil pressure reading. It starts off in the normal range, but after approximately 15 minutes into the flight, the oil pressure reads zero!!! I have no oil leaks and my oil temps and CHT's are both in normal operating ranges. I have a VDO pressure sender mounted on the firewall with a copper tube supplying it with oil. The case of the sender has a good earth connection. The only thing I think it might be is an air bubble in the copper tubing or sender. Could it be this? Have any of you guys experienced anything similar with a VDO sender or an EIS?

Overall I am happy with the EIS and it is good to get my Q2 back in the air after a year! Looking forward ironing out the little snags and going places in it!

Mike Dwyer responds:

Why are you so sure that you don't have 0 oil pressure? You flew for a while at 0 and the engine didn't seize? An air bubble won't do it.

The sender changes resistance with pressure. Not all senders are the same, but an EDO type, I found had the following numbers; Zero PSI = 10 ohms 40 PSI = 105 ohm

You could test the sender on the ground with an ohm meter. Also, you could apply 40 PSI on it and heat the thing up to 100C, did the pressure drop to zero after a bit?

If that all checks out ok, maybe it's the EFIS. To check that get a volt meter. Hook it in to the same 2 leads that go to the VDO sender. Read the voltage prior to startup (master on). It should be low voltage. Startup the engine, this should be a higher voltage - record the number. Then go fly, did the voltage stay stable, does the EFIS show zero?

Rick Hole then added this comment:

I can confirm Mike's ohm readings. There are several different EDO senders, whatever the max pressure value is will read over 200 ohms. These senders are not my favorites, but they do work well. I have not seen air bubbles to be a problem.

I have seen EFIS systems with oil pressure display problems. Usually these are poor crimps or solder connections in the wiring or in the display itself. If you hadn't said you had a good sensor ground, I would have you double-check that. If this is the single-wire sensor, the case must be firmly grounded. Otherwise, one of the wires must be connected to ground; the same ground as the EFIS. It is best to bring all grounds to a single point at the engine.

For peace of mind, I suggest you rig up a second oil pressure gauge. Much as I hate the mechanical ones with an oil pressure line brought into the cabin, these are inexpensive and suitable for temporary use test usage. You don't even need one that is well calibrated. Just one which will show some needle deflection you can compare to your EFIS when it is cold and when it is hot. If your EFIS shows zero and mechanical shows no change, you know the oil pump is fine.

Just a thought, is the case of the VDO sender mounted in a hot location? You could wrap some insulation around it or better, relocate it.

Here's one from Mike Dwyer on how to test for ethanol in your gas:

Take a small plastic drinking water bottle. Fill it with water to the mold line at the bottom of the bottle (about up or so). Use a small funnel and fill to 2/3 or so from the top with the gas to be tested.

Put the lid back on. Shake well.

In 2 minutes if the water settles back to the mold line, there is no Ethanol in the gas. If there is Ethanol the line will be higher.

You can order a printed copy of Q-talk #130 by using the Q-talk Back Issue Order Page.