Oil Pressure Guage Mystery

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JamesSam

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I know tons of discussions already exist about the oil pressure gauge and sending unit. I have already searched the existing forums that helped me to trouble shoot but I'm hoping to get more info with some details in my trials. Here goes:

1987 TBI 350 4WD

When I bought my truck several months ago the pressure gauge reacted to driving and was at a seemingly healthy range between 35-55 PSI and eventually dropped down to around 24-27 PSI and wouldn't really move much at all. I then started checking my oil often and have really only needed to add a quart once. Then I started researching and thinking my sending unit could be bad, or there was a kink in the hose, or a ground wire...most of you all know by now. I crawled under the truck after locating the sender and sending unit to see if I could find a kink in the hose that leads out or obvious damage to wires or leaks etc.. I didn't notice anything that looked out of place but I did move wires slitly and the hose to ohysically see if there was anything not immediately obvious. I left it thinking about what to do next. About an hour later while driving around I looked at the oil pressure gauge and it was on 0. Having never seen this I was immediately alarmed but the truck was running fine so I figured it had something to do with me messing around earlier. When I got home I let the truck cool down a while, unclipped the wire from the sender then turned the ignition to on, but not cranked, to see if it was the sender unit and somehow the oil pressure returned to where it was at 24-27 with the wire unclipped! I cranked the truck and the gauge actually moved the needle ever so slightly. I turned the truck off clipped the wire back in and cranked it up again to see if there would be an adverse reaction(?) but the pressure auge remained reading the low pressure. I revved it a few times just to see if it would move the needle and it did. Happy as I could be that the gauge actually shows there is oil pressure I turned the truck off and decided I would call it a win.

Anyone please respond with conclusions or best guesses!! Any insight will be greatly appreciated.

-Sam
 

JamesSam

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Please forgive all the typos. I was so over the top to send the info and I didn't proofread.
 

JamesSam

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I've read lots of suggestions to check with and install mechanical gauges. How do they work versus the GM OEM original design?....and...Is there a way to install a mechanical gauge in the instrument cluster where the original oil pressure gauge lives?
 

PrairieDrifter

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My gauge was acting up, compared to what was "normal" after I installed an autometer mechanical gauge I found out my factory gauge was never correct. Thank god it worked somewhat, because I have great oil pressure. Im glad I installed the mechanical one because I would have never known.
 

JamesSam

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I like to "T" in a mechanical gauge to get a second opinion.

My gauge was acting up, compared to what was "normal" after I installed an autometer mechanical gauge I found out my factory gauge was never correct. Thank god it worked somewhat, because I have great oil pressure. Im glad I installed the mechanical one because I would have never known.
How and what do mechanical gauges do differently? What is the time frame to work out for install of a mechanical gauge?
 

squaredeal91

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chevy trucks used mechanical oil gauges up till around 1980. Maybe swap in one one of those?? I don't think the clusters changed much till around 85 or 86 when they went to electric speedometer. Maybe someone else knows better or has swapped one over.
 

JamesSam

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chevy trucks used mechanical oil gauges up till around 1980. Maybe swap in one one of those?? I don't think the clusters changed much till around 85 or 86 when they went to electric speedometer. Maybe someone else knows better or has swapped one over.
Anybody know? This truck is a 1987 model.
 

Randy and Easton

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@JamesSam , hang out, someone will have this answer I can assure you. Bunch of real sharp guys here… I’m just not one of them. Great thread! , we are learning a bunch

Randy and Easton
 

Redfish

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@JamesSam I went through 3 brand new sending units before the oil pressure gauge in my '87 started working correctly. Once you get a decent one they work just fine.


This one was bad and you can see it was still almost new when I changed it.

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75gmck25

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My ‘75 has a mechanical oil pressure gauge, but I think they switched to electric in about ‘78 when they also switched from an ammeter to a voltmeter.

You can buy the mechanical oil pressure gauge and line from LMC, but but it’s probably easier to just buy an aftermarket gauge and put ita under the dash.

I have both in-dash and an aftermarket mechanical temp gauge because in a ‘75 cluster the temp gauge has no degree markings. The first aftermarket temp gauge I got was electric, but the electric sending units couldn’t seem to tolerate the heat from the cylinder head. They both worked fine for about a month and then died.
 

dvdswan

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You have an electric oil pressure gauge unless the PO installed a manual one. GM started using the electric gauges starting in 77.

A manual gauge will have a 1/4" copper tube or plastic tube from the engine to the gauge.

I would look at the oil pressure gauge and see if it is a 60 or 80 psi gauge. IIRC, the non-labeled gauges are 80 psi gauges. Buy a new sender, due to its age it's probably failing, for the gauge; either 60 or 80 psi. Be sure to check the secureness of the clip for the gauge that goes on the sender. If you apply teflon tape on the sender to keep it from leaking be sure not cover the hole in the sender.
 
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