How (in)accurate is the stock oil pressure gauge?

[RedTalonTsi]

So I got curious about what additional gauges I might want to have in the cockpit and it seems a popular one is oil pressure gauge.

So I thought, "hmm, don't I already have an oil pressure gauge?" Well, the answer is "I do!" - but apparently lots of people say, "it sucks!"

The problem is, all people manage to say is, "it sucks!" I am having trouble finding any quantitative data on how much it "sucks". In fact I never yet found any real data at all regarding how inaccurate or accurate it actually is. It's doing something, it's measuring something, it has markings that indicate something. I'm wondering if anyone knows what those somethings are?

Some people even go so far as to suggest the oil pressure gauge is similar to the stock boost gauge in that it is simply estimating pressure based on a formula involving RPMs (I personally find that hard to believe, but hey, I don't know).
I've read that the lowest oil pressure you want at idle is about 11psi and that the highest you want at throttle is about 75psi. Is that true and if so I'm wondering if anyone knows what the approx psi is at each of the stock gauge markings?

I've read lots of people say aftermarket gauges are more responsive. What's interesting about that is being responsive isn't the same as being more accurate.

Basically, I understand there is a general mistrust of the stock gauge but I'm not sure exactly where it comes from. I haven't been able to find any comparisons between the two showing the stock gauge to be inaccurate, just people parroting "it sucks", "it's inaccurate". Well maybe it is, but why don't we know how inaccurate then?

Someone please set me straight on this gauge, how does it work, how well does it work, how much better does an after-market gauge work than it?

 

[mindset]

Yes it is doing something. Idk what kinda thinking you're on, but the stock gauge is inaccurate of actual pressure... idk you're thinking.

 

[buddahboost]

Get an aftermarket oil pressure Gauge and not a cheap ass one. I run an electric sending unit type from Auto meter. It's more reliable and and accurate

The stock one works. But By the time you it reads low pressure, you will have dumped most of the oil and it will be too late

Look on the repair manual for the specs on oil pressure

Vibration from the motor(I've read mostly with those that have poly mounts) seem to kill the sensor. I used a remote oil line relocation kit from AMS or even Jay Racing sell a kit

Good luck and don't count Too much on the stock one and check oil level frequently

 

[DSM1G90]

All remember: the stock gauge doesn't display actual pressure in pounds, or Kpa for our friends up north, but the presence of pressure in the system with the results displayed at the dial. Be the same with the idiot light - displays the presence of low pressure.

All you need is 10lbs of pressure at idle to keep things in the motor at bay..and 10lbs sometimes will not show up at the gauge, but keeps the light off.

-DSM

 

[RedTalonTsi]

All remember: the stock gauge doesn't display actual pressure in pounds, or Kpa for our friends up north, but the presence of pressure in the system

-DSM

I'm having trouble understanding this statement. The stock gauge is displaying units of something on a scale. How is it measuring the "the presence of pressure" but not measuring "actual pressure"?

 

[DSM1G90]

Factories went cheap in this area knowing that no two electrical gauges, that has a glow coil inside the gauge itself to heat up to make the needle move, that are mass produced are alike.

Thus, using a redundant figure of an average: if the needle is at rest, we have zero to up to 10 lbs of pressure and the needle rises to where an average of acceptable pressure is reached, it was called 'at being of required pressure'.

But, as you notice, there is no actual graph of pounds displayed, but just a display of "low", "normal" and "high" are displayed for the lay user to realize that his system's oil pressure is present and the lay person can figure out that if the needle is low, check the oil or add oil, and if the needle is above low and reaching the normal status, oil pressure is achieved for normal operation.

His system is displaying pressure in the system, and that's all is important to the lay user.

If you really want accurate readings, then go mechanical with detailed display (like your speedometer measured in miles/kph per hour) of pressure on the gauge which this will definitely show the actual pressure of oil, not the presence of pressure as with a gauge for the lay user.

-DSM

 

[91-GS]

Oil pressure gauge in a DSM basically shows "high-low" (relative pressure with no real units of pressure, just comparing). Some manufacturers are even cheaper. They replace a warning light with a gauge where needle moves to "normal" and stays there, tricking the driver into thinking that everything is great and oil pressure stays "normal" no matter what.

As DSM1G90 pointed out, a DSM oil pressure gauge is nothing more than a U-shaped piece of metal with some wire coiled around one 'leg' of it. As oil pressure rises, it causes more current to flow through the coil thus heating it up. That heat affects the U-shape by making it flex just a little. Since one 'leg' of the U is anchored and another is connected to the needle, the needle on the gauge moves with it.

A conventional electric oil pressure gauge is basically an volt/amp meter that is calibrated to show PSI based on what voltage/amperage the pressure sensor is sending out. Some gauges work by measuring resistance of the sensor, not voltage coming out. Some gauges even work off frequency. They can maintain accuracy even if voltage in the system changes some.

A typical mechanical gauge has a pressure chamber that is somewhat flexible, but held back with a properly calibrated spring. In most cases shape of the chamber is curved and acts like it's own spring. As pressure rises, the chamber expands just enough to make the needle move. The face of the gauge is calibrated to show PSI.

 

[Blurred Talon]

Those no actual markings on the factory gauge, it tells you that there is pressure but not how much. A number reading is the most valuable thing when it comes to a pressure reading.

Couple that with the poor connection on the actual sender (it's pretty much a spade terminal); with time, oil and other chemicals, it always breaks off.

 

[Seamus]

I think the simple answer to you questions are as follows

Yes, it is measuring real oil pressure from a real pressure sending unit(when it's working).

No, it is not displaying any quantifiable information to someone wanting to know the actual pressure of their oil.

 

[92AWDHX40]

Fifteen years later I finally replaced my oem one with a aftermarket one but I never had a issue with oil pressue. It did its job.

 

[UofACATS]

The stock gauge does its job by moving.

the sender spade terminal comes off.

Been there

 

[RedTalonTsi]

Thanks for the replies, those were the kinds of answers I was looking for, something explaining how the stock gauge works and its limitations. I don't know if I feel an aftermarket gauge is necessary yet but now at least I have a better idea of what advantages I stand to gain by getting one.

 

[LandSpeed-DSM]

A proper oil pressure gauge is a wise investment for any car, stock or modified. Spend the money on a reputable brand, and with a little forethought you can order some lines, fittings and caps you'll be able to take pressure at several spots by just moving the sending unit around as you please.

You can then check from the OFH, tee'd into your turbo feed, or the head.

When everything went back together last spring and the HX52 went on for the first time, the new oil pressure sensor was on the turbo feed with the stock gauge sender in the factory spot. So when we primed the pump with a drill we could verify pressure was reaching the turbo. Then made sure that turbo oil pressure was both steady and within manufacturer spec on the thicker oil I chose for this setup. (Cold start max, idle min, loaded min, and hot max)

After a few days to get a baseline on the turbo line, pulled the sensor and plugged the tee then moved it over to where the 1G factory turbo feed line hooks up. Used this to check on the Kiggly HLA for a few days, and the onto an open port on the OFH (Aircooled '90 OFH) to see what the actual pressure is vs stock gauge. The stock gauge might as well just a light that just turns on, off and dims or brightens a bit. Barely moves when the pressure swings 10-15psi

Then in late November when the temperature dropped into the teens over in the weeks before I put her away I would get a puff of blue smoke on letting off the throttle from high rpm. Sensor on the OFH was giving me upwards of 90-100psi, decided to check the the turbo feed which was 85psi at the same rpm. Spec hot max is ~75psi per the Holset FSM. Switched to a 5w40 from the 20w50, pressure went down but still met the minimums. Put the sensor back on the OFH and left it there.

Most decent gauge kits will cost > $100, fittings/caps maybe $50 depending on your setup. It's worth it. The only lines I bought were for the feed, which I needed anyways(old pic):

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