Mechanical oil pressure gauges - idle accuracy

[Saturdee]

Well today I was getting a low oil pressure reading. At around 750-800 rpm (operating temp) I was seeing like 7.5 psi. I know that's low, and it should be around 11.4 or something. But I have a friend that told me that he had two mechanical gauges that did the same at idle, and then he went to an electrical gauge and the problem was resolved.

This whole low oil pressure thing is really stressing me out, and I don't want to start ripping stuff apart if it's just my gauge being inaccurate. So, does anyone on here have bad experiences with their mechanical oil pressure readings at idle?

And more specifically, I have an autometer mechanical gauge, connected to the OFH via -4 AN stainless steel line.

 

[dsmgst1996]

Thats a huge line for a oil gauge why so big that might be your problem right there.

 

[Saturdee]

Thats a huge line for a oil gauge why so big that might be your problem right there.

There's a number of people running this setup. It's supposed to be the preferred method. I wouldn't think the line would change idle readings, regardless the pressure should be sent to the gauge, correct?

I also thought mechanical gauges were the most precise with their readings, but now my friend has me second guessing. Any other thoughts or experiences with this?

 

[dsmgst1996]

A good mechanical perform as good as electric signal wire . With that being said with new technology i would bet there almost on par with each other now days. I have noticed scene changing my oil gauge to a led its much more accruate when sudden changes happen but at idle it says i have low presurre but i have verified with mechancal gauge that at idle i have 30psi . Have you changed your oil to a lighter oil for summer this will also affect your presurre .

 

[95blackGsTurbo]

I have never personally heard or had this problem. That line size may be incorrect because thats a massive line. Most gauges come with a small nylon line for the sending unit. Either can be inaccurate but if you get 2 of them that are showing the same reading I would suspect a problem. First I would eliminate that huge line though. You may be losing pressure through it causing a low reading.

 

[LandSpeed-DSM]

I'm using all electrical sending units. Boost/Drive/Oil pressure, IAT/EGT, Coolant temp, etc.

Here's my turbo oil feed sensor:

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[Saturdee]

I have never personally heard or had this problem. That line size may be incorrect because thats a massive line. Most gauges come with a small nylon line for the sending unit. Either can be inaccurate but if you get 2 of them that are showing the same reading I would suspect a problem. First I would eliminate that huge line though. You may be losing pressure through it causing a low reading.

What would you suggest I change to? I like the SS line because it's durable. Do they make a SS line that would be small enough to use for the gauge, in your opinion? Or would I be better off downgrading to a copper line from autometer.

 

[dsmgst1996]

copper line has been a tried and true method for generations.

 

[LandSpeed-DSM]

What would you suggest I change to? I like the SS line because it's durable. Do they make a SS line that would be small enough to use for the gauge, in your opinion? Or would I be better off downgrading to a copper line from autometer.

It's not the line size. If the line is dead-headed to the sensor, the pressure is being transmitted properly. You didn't tell us where you are taking this reading from?

Perhaps that is accurate, or perhaps it is a sensor/gauge error.

I use -3 and -4 for my reference and sensor lines. I use stainless hard-line for my turbine drive pressure (as well as clutch/brakes), but braided SS or Pro-classic for everything else.

 

[Saturdee]

It's not the line size. If the line is dead-headed to the sensor, the pressure is being transmitted properly. You didn't tell us where you are taking this reading from?

Perhaps that is accurate, or perhaps it is a sensor/gauge error.

I use -3 and -4 for my reference and sensor lines. I use stainless hard-line for my turbine drive pressure (as well as clutch/brakes), but braided SS or Pro-classic for everything else.

Which reading? My gauge is fed from my OFH. Where the stock pressure sending unit was.

And I guess I'll get another gauge and see if that reads the same before I go on thinking my oil pressure is low. So mechanical gauges should read completely accurate at idle? I shouldn't have to worry about it being a mechanical gauge issue?

 

[LandSpeed-DSM]

I'm referring to the ~7.5psi reading in your OP.

As far as gauge accuracy.. mechanical or electric of a reputable brand should both be accurate. There are factory defects.. what brand is the gauge? Serial #? With some gauges you get what you pay for. Any gauge can be off, no reason a mechanical gauge would be immune to it.

What oil weight do you run? What are your oil clearances? There are many factors that determine your pressure.

OFH pressure should be higher than 7.5 even at hot idle.

 

[T is for TURBO]

Landspeed is correct. Line size to the oil pressure gauge won't throw off the reading. However, what will effect the reading is the turbo feed line size. A large oil feed line (fed from the OFH) will lower the pressure signal at the gauge.

When was the last time you changed oil? What oil are you running? Are you low on oil?

 

[Saturdee]

I'm referring to the ~7.5psi reading in your OP.

As far as gauge accuracy.. mechanical or electric of a reputable brand should both be accurate. There are factory defects.. what brand is the gauge? Serial #? With some gauges you get what you pay for. Any gauge can be off, no reason a mechanical gauge would be immune to it.

What oil weight do you run? What are your oil clearances? There are many factors that determine your pressure.

OFH pressure should be higher than 7.5 even at hot idle.

I'm running 10w-30 conventional oil. I just rebuilt my motor. Oil clearences I'm not positive of, the bottom end was assembled at a machine shop.

Landspeed is correct. Line size to the oil pressure gauge won't throw off the reading. However, what will effect the reading is the turbo feed line size. A large oil feed line (fed from the OFH) will lower the pressure signal at the gauge.

When was the last time you changed oil? What oil are you running? Are you low on oil?

The turbo oil feed line I am using is this https://www.extremepsi.com/store/product.php?productid=17334

Since I just rebuilt the motor, it's brand new oil. 10w-30 as stated above, and no, oil levels are fine.



Would I be better off testing whether my oil pressure is on par by revving it to to like 2k or 3k, and seeing if the pressure matches the 10 psi for every thousand rpm rule?

 

[T is for TURBO]

Would I be better off testing whether my oil pressure is on par by revving it to to like 2k or 3k, and seeing if the pressure matches the 10 psi for every thousand rpm rule?

You really want healthy oil pressure across the RPM range.

Did this just start happening recently? Have you tried another gauge to verify the pressure? Cheap gauges fail a lot. I had a cheapo mechanical oil pressure gauge that every time I would close the door, the needle would move to a different location on the gauge. Sometimes when I would shut the car off, it would only drop to 10psi instead of falling back to zero.

As for the 10psi per 1k rule, it's just a guide line for healthy pressure. Everyone's car is different, especially modified cars. Some may make 30 psi at 3k but make 80 psi at 6k, meaning its not perfectly linear

 

[Saturdee]

You really want healthy oil pressure across the RPM range.

Did this just start happening recently? Have you tried another gauge to verify the pressure? Cheap gauges fail a lot. I had a cheapo mechanical oil pressure gauge that every time I would close the door, the needle would move to a different location on the gauge. Sometimes when I would shut the car off, it would only drop to 10psi instead of falling back to zero.

As for the 10psi per 1k rule, it's just a guide line for healthy pressure. Everyone's car is different, especially modified cars. Some may make 30 psi at 3k but make 80 psi at 6k, meaning its not perfectly linear

Well I did put my old sending unit back in and hooked that up to see what my stock gauge showed. Again, the needle barely moved and I didn't really notice much of anything. Grant it the stock gauge is close to worthless, so it's pretty hard to tell. I am in the process of taking off my OFH to see if the valve is stuck open. In the meantime, I'm going to try to get ahold of another oil pressure gauge, just to verify that my oil actually is low, and it's not just an off reading.

I also noticed that my dumby light isn't turning on when the pill pressure is real low. This also makes me thinking it could be my gauge. So I will likely invest in a new dumby light unit, to be absolutely positive.


Oh, and something else I thought I'd add. The invoice the machine shop gave me states "Job Note: Customer supplied own parts, short block assembled on the high tolerances for main bearings and ring gap" What are these "high tolerances" they are speaking of?

 

[NHerron]

First check correct operation of the dummy light. Does the light light up with key on engine off?

Next, you know where the sensor(technically switch) is on the OFH? Take that wire and ground it to the engine or any bare metal.

Stock oil pressure is 11.4psi hot idle (as per the FSM). Did the machine shop do anything with the balance shafts? Did you request the BS delete or anything?

 

[LandSpeed-DSM]

First check correct operation of the dummy light. Does the light light up with key on engine off?

Next, you know where the sensor(technically switch) is on the OFH? Take that wire and ground it to the engine or any bare metal.

Stock oil pressure is 11.4psi hot idle (as per the FSM). Did the machine shop do anything with the balance shafts? Did you request the BS delete or anything?

Took the words outta my mouth

Well I did put my old sending unit back in and hooked that up to see what my stock gauge showed. Again, the needle barely moved and I didn't really notice much of anything. Grant it the stock gauge is close to worthless, so it's pretty hard to tell. I am in the process of taking off my OFH to see if the valve is stuck open. In the meantime, I'm going to try to get ahold of another oil pressure gauge, just to verify that my oil actually is low, and it's not just an off reading.

I also noticed that my dumby light isn't turning on when the pill pressure is real low. This also makes me thinking it could be my gauge. So I will likely invest in a new dumby light unit, to be absolutely positive.


Oh, and something else I thought I'd add. The invoice the machine shop gave me states "Job Note: Customer supplied own parts, short block assembled on the high tolerances for main bearings and ring gap" What are these "high tolerances" they are speaking of?

They opened up the ring gaps in anticipation of more heat/pressure thsn a stock motor would see if used as the factory intended. Ss far as the bigger main bearing clearances, try running a 5w40 like shell rotella and see what that does for your hot idle pressure. After you check out the items in the other post I quoted

 

[white92talontsi]

Took the words outta my mouth



They opened up the ring gaps in anticipation of more heat/pressure thsn a stock motor would see if used as the factory intended. Ss far as the bigger main bearing clearances, try running a 5w40 like shell rotella and see what that does for your hot idle pressure. After you check out the items in the other post I quoted

High tolerance would be around .0019-.002" in the FSM. I can tell you this, At .002 you should not have any issues with low idle oil pressure. Last motor I put together was a 4g64 with the Mains at .002. No balance shafts and no oil squirters.

Hot idle oil pressure is 40-42psi and I ported to Oil Filter Housing and it never goes over 90psi. That motor runs on Amsoil 20-50 z-rod.


Are you running balance shafts? If not did the machine shop properly remove and rotate the balance shaft bearings? If not you have a failing motor in your car in short order.

 

[Saturdee]

First check correct operation of the dummy light. Does the light light up with key on engine off?

Next, you know where the sensor(technically switch) is on the OFH? Take that wire and ground it to the engine or any bare metal.

Stock oil pressure is 11.4psi hot idle (as per the FSM). Did the machine shop do anything with the balance shafts? Did you request the BS delete or anything?

What would grounding the wire do on the OFH?

The machine shop didn't do anything with the balance shafts. Me and a friend of my did the BSE. We plugged off the oil passages properly. All the machine shop did was some machine work and putting my rods/pistons in.

They opened up the ring gaps in anticipation of more heat/pressure thsn a stock motor would see if used as the factory intended. Ss far as the bigger main bearing clearances, try running a 5w40 like shell rotella and see what that does for your hot idle pressure. After you check out the items in the other post I quoted

So basically high tolerances would be a contributing factor to my low oil pressure?

 

[LandSpeed-DSM]

What would grounding the wire do on the OFH?

The machine shop didn't do anything with the balance shafts. Me and a friend of my did the BSE. We plugged off the oil passages properly. All the machine shop did was some machine work and putting my rods/pistons in.



So basically high tolerances would be a contributing factor to my low oil pressure?

Contribute yes, but as noted 0.0018-0.0020 main clearance wouldn't account for being down ~30psi at hot idle.

Have you tried taking pressure at the head or pre-turbo? Have you drained the oil to see if there is an excess of metal shavings in it from low pressure not separating moving parts from one another?

 

[Saturdee]

Contribute yes, but as noted 0.0018-0.0020 main clearance wouldn't account for being down ~30psi at hot idle.

Have you tried taking pressure at the head or pre-turbo? Have you drained the oil to see if there is an excess of metal shavings in it from low pressure not separating moving parts from one another?

How would a take pressure from the head or pre-turbo?

Yes, I drained the oil after my first two startups - approximately 6-7 minutes of total run time. There were very very tiny specks of metal I could see, but nothing alarming. Just what I would expect from a new motor.

 

[knochgoon24]

For some more info.

I accidentally filled my car with 5w-20 once upon a time. It make the low oil pressure light flicker at idle. My mechanical gauge read approx 5-7psi.

With 5w-30, it would read about 10psi at hot idle. No oil pressure light.

With 5w-40 (Rotella T6), I was seeing about 17psi at hot idle. Then my gauge died (again) and I haven't replaced it.

 

[Saturdee]

For some more info.

I accidentally filled my car with 5w-20 once upon a time. It make the low oil pressure light flicker at idle. My mechanical gauge read approx 5-7psi.

With 5w-30, it would read about 10psi at hot idle. No oil pressure light.

With 5w-40 (Rotella T6), I was seeing about 17psi at hot idle. Then my gauge died (again) and I haven't replaced it.

That's what I'm thinking, I should run a heavier weight oil. A friend suggested rotella T 15w-40. Also, replacing my OFH with one that isn't ported. Hopefully if I do this it will solve my oil pressure issues. If not, well I guess on to the oil pump.

Oh, and the machine shop suggested I check my OFH valve. They believe it could be stuck open, causing my issues. Is this a common problem?

 

[LandSpeed-DSM]

Its possible the relief valve is stuck. Try the 5w40 rotella instead of the 15w40.. you want the thinner cold weight.

 

[Saturdee]

Its possible the relief valve is stuck. Try the 5w40 rotella instead of the 15w40.. you want the thinner cold weight.

Will do. Hopefully I can just swap in a new OFH and be done with it.