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So I connected my oil pressure gauge up in my 36 gmc. it has a 4.3L vortec engine, I put a "T" in where it comes out of the block so I could use both the electrical sender so the fuel pump turns on with the computer and then ran a line up to the original gauge in the dash. It seems to take 15/20 seconds before it starts to read and is slow when the needle moves. Only goes up to about 20 psi at cold idle, I would think it should go a bit further. I think factory spec is, hot idle is around 20psi and 40psi at cruise. Do you think it's a bad gauge or is it what it is?

Do you have another mechanical gauge you could use as a test, pick up an inexpensive one at a chain store and hook it up. Those numbers are low, how many miles on the 4.3?

Your last sentence is a little confusing. What does GM say the specs are for that engine and what are the corresponding numbers you're getting?
Are you using the specified oil?

The 4.3 V6 is the same structure as the 350 block, just minus 2 cylinders. The oil pressure should be somewhere 30 - 35 psi cold idle, 25 - 30 hot idle, 35 - 40 psi hot at 3000 rpm. Normal oil for that engine would be 10W-30 conventional.

If you have a LOT of air in the line your readings will be slow to change. Try bleeding the line at the gauge while the engine is running. Use a rag as a diaper to catch the oil that comes out.

Pressure regulator could be the issue. You could pinch the return hose and see if pressure comes up, or open the return from the regulator and see if fuel is coming out when gauge is below spec rail pressure is achieved. If no fuel coming out the return look for supply side issue.

How does the fuel pressure regulator affect oil pressure?
Jerry

I'll get another gauge and see if there is someplace else I can tap into the oiling system, maybe down by the oil filter. The motor has 100K on it and am using 5/30 engine oil. I'll report back when that happens, hopefully over the weekend.

If the original pipe plug is still threaded in the block near the oil filter, you won't get that plug out. You are reading pressure at a good location on the top rear of the block. 100,000 isn't too bad, try another gauge and report back.

I've tried to use that port by the oil filter during dyno runs- - - -it's not an accurate way to monitor actual oil pressure- - - - -WAY low for some reason!

Ok, I had time to hook up another gauge and it reads. New gauge cold idle 50psi, original 40psi, hot idle new gauge 25 psi, 2000 rpm 45 psi, original gauge hot idle 5psi, 2000 rpm 5 psi . I'm guessing the gauge needs some TLC. I think the only thing I could do is spray some carb cleaner into the inlet port, but I don't know if that would help

My guess is that the low reading gauge has some resistance in the pivot points, spraying carb cleaner in the bourdon tube probably won't have much effect.

Now you've run up against Segal's Law. Maybe you should get another gauge and see if you can get the closest 2 out of 3??

That's just my luck. I'll see if I can't find another gauge or two and see what happens. It's probably $150 or so to send it out for restoration. I'll update this when I get it figured out.

Methinks--get another gauge, find the closest closest of 2 out of 3, then see how much noise the outlier gauge makes when it hits the bottom of the trash can.

Not sure what you said in your first post. Not sure if you are trying an electric gauge and a mechanical gauge at the same time on a tee in the same port. I'm guessing you want to use the original gauge with the new motor. But you want to be sure the original gauge is fairly accurate.
This post is highly convoluted. Can I just ask some questions? Before you spend $600 on 6 gauges.

1. What electrical gauge/sender are you trying? The vortec original vehicle donor gauge or what?
2. Are you also trying the original 1936 GMC oil pressure gauge?
3. What other gauges have you tried? Mechanical or electric? brand name?
4. Are you doing this to see of orig gauge is working/accurate? so you can use it for the vortec 4.3?
5. Please explain why you mention fuel pump and how it relates to your oil pressure question.
6. Do you have a tachometer hooked up or are you guessing about RPMs?

DON'T spray into the gauge port. That is the inside of the bourdon tube. NO NO NO

Can you pause to digest my thoughts and let the members comment on my thoughts? After you answer.

Please answer using the numbers 1 thru 6
I will have comments after, if you wish.

#1 I'm using the original electric sender to tell the computer to turn on the fuel pump to start the engine and shut it off in case of the lose of oil pressure. original sender and reprogramed ECM.
#2 Yes I am wanting to use the 36 pressure gauge.
#3 I have "T"ed in a mechanical gauge, "off" brand $20 gauge. So there is the OE gauge, the sender for the ECM and the $20 mechanical gauge all plumbed in at the same time.
#4 yes, the OE gauge was very slow at responding when I first started the truck with the OE gauge installed. It only showed 20lbs pressure and would not go higher when RPM's increased to 2000.
#5 The only mention of the fuel pump was in reference to the signal to the ECM to turn on/off the fuel pump
#6 yes I have a tach hooked up to monitor rpm's when testing the gauge's.

After thinking about this I am wondering if I should even use the OE gauge as it only top's out at 30 psi, It would be pegged out at max on the gauge constantly. So maybe just leave it installed in the dash but not hooked into the system and hang the A/F gauge off the bottom of the dash I am trying to keep it as OE looking as possible when looking thru the side windows. Maybe the oe gauge can be recalibrated for the higher pressure by a gauge rebuilding company.

Jim Carter shows a 1936-1939 gauge decal kit for "Pick up trucks". The oil gauge reads 0 40 80. I see Chevy is 0 15 30. There may be some GMC at 0 15 30 or you have a Chevy gauge stuck in there by a PO. They say it's for a 80mph Speedo. 1936 was the first year for GMC "pick ups". before that they only built "big trucks". They did not have a right sized motor yet for a pick up, so they used an Oldsmobile 213 full pressure car motor. The vortec is 262. This is so old it's hard to get info. Anyway I was thinking:
1. The 1936 gauge is designed to show a certain range of pressure.
2. The rule of thumb is 10psi per 1000RPM. I.E. 5 psi at 500 RPM idle. 20 PSI at 2000 RPM. The pressure will fluctuate/not match rule of thumb all the time for various reasons. Oil Wt,, temp, gauge calibration, worn motor.
3. If any gauge shows 5 psi and up, you are OK.
4. I don't know about that fuel pump shut off using oil pressure or if it is working properly or causing issues. Of course you do not want to use that during testing. Just one mechanical gauge, no Tee. Then the 1936 gauge, no tee. Then any other mechanical gauge, no Tee. After you see a consensus, you can decide on what to do/use. The 1936 gauge will not be super accurate. The internet says the vortec gauges were not accurate.
The 'new" mechanical gauge may be the one to trust during testing.


Bottom line is a 0 15 30 may not be calibrated to give you a comfortable reading. As you say It should be maxing out at about 3000 RPM. 5 psi at idle and so on.
YES........Maybe keep the 36 gauge for looks and hook up the "safety" electrical gauge somewhere.

Thanks for the info, as long as I know there is plenty of oil pressure I'm not going to worry to much. I will try to find a 0 to 80 gmc gauge and see how that works. In the mean time I will just use the OE gauge.

Agreed. I wouldn't use the port at oil filter area, as Jerry said.

Happy trails.

I don't know what I was thinking, it is o/40/80. but still doesn't work Will probably run it with the aftermarket mechanical gauge.

If the tube unbolts/unscrews from the gauge, gauge may be fixable.
There is also the possibility to make a new gauge look like an old gauge.