oil pressure fluctuations

I dug this out of the archives because it is consistent with a problem I'm having. Original thread here:
http://www.vansairforce.com/community/showthread.php?t=25451

Flybipe said:

During cruise at various power settings, the oil pressure needle fluxuates rapidly between about 75 and 85psi - almost (but not quite) so fast that I can barely see the needle.

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mahlon_r said:

Try removing all oil from the oil pressure line between the gauge and the engine. Blow the line completely clear and then hook it back up on both ends. After you start the engine oil will flow into the line, but will also compress all the air that was in the line from blowing it clear. This section of compressed air can act as a shock absorber or snubber to dampen gauge needle vibrations/fluctuations.
Good Luck,
Mahlon

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After having just bled the oil pressure line as part of some routine maintenance, I'm now having the same issue of rapid oil pressure fluctuations of about +/-7 psi at cruise power like that articulated in the original post.

Normally I've understood erratic readings like this to require bleeding of the line, and it has been true of my fuel pressure in the past. However, bleeding the oil pressure line is what seems to have caused this problem and Mahlon's suggestion above seems to be to intentionally introduce air into the oil pressure line in order to dampen such fluctuations. Maybe the air I just bled out was serving this purpose?

I definitely respect his knowledge and am likely to try this. Anyone else see merit in this from their own past experience?

(FWIW, I do have the newer Kavlico sender--just installed last year--so I have no reason to think it is an issue with the sender specifically or the wiring of it given that the only thing I changed/did between flights was to bleed it.)

Well, since purging the line of *oil* introduced the problem, you could go the other way and purge it of *air* and see what happens...?

Update

Update.

Blew the oil out of the oil pressure line, hooked it back up. Lo and behold, just like a Mahlon said, the fluctuations went away! Solid as a rock! Who’da thunk?

Hope this helps anyone else experiencing a similar issue.

alpinelakespilot2000 said:

Update.

Blew the oil out of the oil pressure line, hooked it back up. Lo and behold, just like a Mahlon said, the fluctuations went away! Solid as a rock! Who?da thunk?

Hope this helps anyone else experiencing a similar issue.

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Dang! I pre-oiled the engine last weekend with the oil pressure sender unscrewed from the firewall manifold. Screwed it back in once oil started coming out. The line is supposed to be (mostly) air to damp fluctuations? To think I pay DR to learn how much I done wrong.

Worth every penny.

DrillBit said:

Dang! I pre-oiled the engine last weekend with the oil pressure sender unscrewed from the firewall manifold. Screwed it back in once oil started coming out. The line is supposed to be (mostly) air to damp fluctuations? To think I pay DR to learn how much I done wrong.

Worth every penny.

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You might be fine. I?m guessing some types of pressure senders might benefit from bleeding the air out. Good luck.

I suspect the solid state sender has a high response rate, while a mechanical VDO type may not register the same pulsations. It just wears out.

I am developing a belief that VDO can-type senders do weird things (notably read high) when the can fills with liquid. Next airplane I'll mount 'em neck down so they self-purge, assuming of course the line is filled with air.