Should oil in pressure line for gauge go all the way to the gauge?

muzician21 wrote in news:73fba85d-edcd-4d8e-9305- snipped-for-privacy@c3g2000yqd.googlegroups.com:

Huh. Useless and dicey aftermarket gauge.

ALL the way! /Any/ air is unacceptable.

It shouldn't matter.If there is air in the line, it will be compressed so that the "air" pressure is the same as the oil pressure.

I don't really like gauges that require running oil lines to the dash -especially the sort sold in most mass market auto parts stores. I prefer good quality electical gauges.

Ed

7MGE inline 6 in a Toyota Cressida, the oil pressure light has been coming on when hot and at low idle, about 30 - 40k out from a rebuild. Oil sender was replaced new when the engine was built.

Gauge from Advance which I realize you're going to turn up your nose at is showing really low - like about 5 psi at about 600 - 700 rpm. Jumps up well over 30 psi at highway speed.

I've always heard 10psi per 1000 rpm as a ROT, seem to be getting enough pressure at speed but seems low at idle. Am probably going to change the pump out and see if that cures it. Got a Clevite pump, anyone have any feelings about them one way or the other?

Thanks

Are you hooking up the gauge temporarily to check oil pressure or are you trying for a permanent installation in the dashboard?

In either case, the oil needs to go all the way to the gauge unless you are using an electrical sender type gauge.

The downside to using a direct read gauge is that there are additional potential sources of oil leaks, and leaking hot oil in the instrument panel can cause burns and make a huge mess.

The factory repair manual has specs for clearances between the crescent, impeller, and pump body. Check the clearances before you swap the pump. Besides a bad pump, excessive bearing clearances can cause low oil pressure.

Why would having the oil go all the way up to the gauge be a big deal? Would there always be oil in the line?

It wouldnt normally make much difference. If the line, partially filled with air, experienced heating or cooling, I guess the expansion or contraction of the air could be more of a factor than it would be with a tube filled with oil alone.

"hls" wrote in news:n7OdneWHlYElMqHWnZ2dnUVZ snipped-for-privacy@giganews.com:

Air is compressible. Oil is not. Air in the line will result in a falsely low gauge reading. At least that's what happened to me many years ago when I added such a device to my car.

How did you make the leap from air is compressible to it will result in a falsely low gauge reading? You say that as if everyone is supposed to automagically see the connection.

-jim

jim wrote in news:qqWdnS6fv-RgJ6HWnZ2dnUVZ snipped-for-privacy@bright.net:

most gauges are dampened so the air is a non issue, and besides every one I have ever had had air in the line there is no way to keep it out, every time you shut it off it will drain back and have some air on restart. KB

As his previous posts and replies have conclusively proven, he wouldn't pass a first year class in statics. Gawd forbid his next attempt to pointificate on the Navier-Stokes equations.

The MG manual says that it isn't a leak, it's just "normal seepage."

--scott

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The pressure is transmitted through the "fluid", both air and oil being fluids. You dont lose pressure just because the air is compressed.

What Ray said. And IF it's an electrical guage, the sender will be screwed directly into an oil galley somewhere, and the wire will go all of the way to the guage.

That's an understatement ...

Of course there's oil in the line. How else would oil pressure be measured without oil in the line measuring the pressure?

You wouldn't lose static pressure but the air would buffer transients. So air in the line is not necessarily a bad thing.

Oil going all the way to the gauge is only a big deal if the gauge is mounted in the passenger compartment or instrument panel and it leaks and causes burns or worse, catches fire. I suppose another downside to having an oil line in the passenger compartment is that the oil line would radiate some heat from hot oil into the passenger compartment - not a big deal in the winter, but annoying in hot weather.

Whether or not there would always be oil in the line depends on how the line is installed but most likely, oil would drain out when the engine is shut down.

I got the impression that the OP was installing an oil pressure gauge to diagnose an oil pressure warning light, so the installation was temporary and not a permanent instrument panel installation, making most of this thread a moot point.

This is traditional British design, and indeed in MGs it does all of the above. It's part of the charm of the vehicles, much like switching the current to the headlights directly through an undersized switch on the dash rather than with a relay.

--scott

Add the fun of truing spoke wheels and non synchromesh transmission...