mechanical oil pressure gauge help

[mrinnovation1]

I just finished installing a mechanical(0-100 psi) OPG in my 1985 Vanagon using 16' long X 1/8" diameter copper tubing from a refrigeration shop. The entire 16' was used as I left a couple of coils for movement near the OP sensor termination point. I mounted the gauge alongside the auto trans shifter housing(left side) since dash mounting would have required another 3 or 4 feet of tubing. The gauge is easily read from the drivers seat. It took a minute or so for the gauge to react after initial startup. Subsequent startups pretty much give me an instant reading. For comparison, I temporarily left the digital gauge installed to compare readings. Cold start mechanical shows 70-75 psi. Warm idle reads 20 psi and I get around 45-50 psi warm at 2000 rpm. With the digital gauge cold start psi is 80-85 and warm idle reads 20 psi. The reason I went to a mechanical gauge was because the digital gauge used an adapter and a much smaller oil filter. Once I remove the digital gauge and associated hardware installed by a previous owner, I can go back to the stock size oil filter.

[4Gears4Tires]

So what is your issue?
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'87 Syncro Ferric Oxyhydroxide Superleggera Edition
'85 Westy Sciuridae Domus Edition

[mrinnovation1]

[MarkWard]

You didn’t mention bleeding the line. I’d probably start with that. It could get messy, so have a rag or 2 on hand. With the engine running crack the line loose at the gauge that should let any air left escape.

For the difference in readings, not knowing which one is more accurate add the readings together and divide by 2 for an average. My experience is a quality mechanical gauge is more honest, but that’s a long run. Not sure how friction loss would affect readings if at all.

[Ahwahnee]

I always used mechanical OP gauges on my other cars - but those were older vehicles with notoriously unreliable gauges and senders. They also had an engine not far from the dash so a braided stainless steel line was practical. Hopefully the copper will hold up well enough.

Yeah, having two gauges is like wearing two watches... with one watch you know what time it is, with two you're not so sure.

I get it about the oil filter though of course there are many ways to plumb a sender other than a sandwich adapter. I will say that the VDO gauge and sender combo has been consistent for a great many years (I actually think consistency is more important than precision in this application).

BTW - I never had to bleed the line on my mechanical gauges. Somewhat counterintuitive but they always worked well with out messing with that.

[4Gears4Tires]

[mrinnovation1]

I just turned 70 and have always been confused. BTW, I found no need to bleed the line either.

[MarkWard]

You’re not running the copper tube direct from the engine are you? I also didn’t understand you were offering help and thought you were wondering about the difference in readings. There is air in the line with no where to go. The air will operate the gauge just as an air pressure gauge, but the compression rate of a fluid is quite a bit different from air. So yes the gauge would work, but if you are looking for accuracy getting the air out might improve your readings.

Hopefully you aren’t running the copper tube and directly connected to the engine.

[Ahwahnee]

[DanHoug]

[mrinnovation1]

I should clarify that although I did not bleed the line, there was initially some seepage at the gauge fitting. This was not enough to even drip but I did get another 1/4 turn or so out of the fitting. If that qualifies for bleeding, then it was bled. Yes, the copper line runs directly from the gauge to the engine but I did put 3 coils in the line before the engine connection.

[zerotofifty]

I recommend a third gage.
_________________
Sorry About That Chief.

Give Peace a Chance.

Words to live by.

[zerotofifty]

I have a sandwich adsptir but am able to use an oversized filter as i bent the underside pushrod tube protecting metal shield for clearence
_________________
Sorry About That Chief.

Give Peace a Chance.

Words to live by.

[Wildthings]

On a cold morning I would be afraid that if something busted like the o-ring between the oil cool and the block, a mechanical gauge mounted at such a distance might not react fast enough to save the engine. I know it's not very popular on any automotive site, including The Samba, but I am a fan of idiot lights as they are attention getters.

[Sodo]

[raygreenwood]

[?Waldo?]

I ran a mechanical gauge in a daily driver Vanagon for 6 or 7 years. I just used the nylon tubing. The air should definitely be bled from the line. With the line full of air, the reaction time of the gauge is very slow. Even with the air bled, in very cold weather it would take a few seconds for the gauge to start working on cold starts. I have run electric gauges also, but the senders tend to fail very often, maybe from the vibration from diesel engines.

[AndyBees]

I'm thinking the 1/8 inch refrigerant copper tube's ID is quite small. I have a roll of it and decided not to use it because of the small ID. Sixteen feet with the small diameter can affect pressure at the gauge and will definitely be slow to come up to pressure with air inside as others have indicated. But, bleeding out the air is by far of more concern for a quick accurate gauge reading.

Below are some comparisons I did using an old VDO Gauge made in Germany that's about 45 years old (the big gauge with 1 psi increments).

These pressures are using compressed air.

Below, the smaller gauge is an el cheapo made in China.

Below, same as above.

Below, is a comparison with a $50 digital.

Below, same as above.

I used the old VDO Gauge with the 1 PSI increments when working on VWs with the CIS Fuel Injection systems back in the day. It was accurate then and I assume it still is today.
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'84 Vanagon Tin-top, ALH TDI, two trips to Alaska, 2014 & 16. 1989 Tin-top unmolested.
1983 Air-cool, 225k miles, 180k miles mine, seven trips to Alaska from 1986 thru 2003. 1975 Bay hopeful.

[Sodo]

[4Gears4Tires]

I've noticed the Westy's (2wd) fuel gauge goes up and down when I go up and down hills. This never happens with the Syncro. There's still some sloshing. I don't know how you prefer that, it makes me nervous.
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'87 Syncro Ferric Oxyhydroxide Superleggera Edition
'85 Westy Sciuridae Domus Edition