Mattkovitch

Okay, so for some time now (9 months and 14k miles) I've been worried that my engine has low oil pressure, bc the gauge was reading very low. I read up on some forums about how the sending units for the gauges go bad all the time, and that they aren't all that accurate really. I noticed my sending unit was leaking oil and assumed that the gauge went bad and that maybe even the leak was causing it to read low. I got a new sending unit for it but before I installed the new sending unit I wanted to be sure about my oil pressure so I checked the oil pressure with a mechanical gauge.
I screwed the gauge into the same spot the old sending unit was in and fired it up. I checked the gauge and it wouldn't even give me a reading, just sat at 0. (The gauge works, we used it to accurately determine oil pressure on many cars before and after I used it on mine) I installed the new sending unit and checked the gauge on the dash, which still read the same as it did before. 0 PSI at idle, about 14 at 1000 RPMs and no more than like 18 PSI at 4k RPMs.
I bought this car in January from a used car lot, it had 110,500 miles on it and since then I've put another 14k on it bringing it to about 124k miles today. I brought a friend of mine who is a technician (I'm a student at an auto tech school) to inspect the car before I bought it. The engine sounded great running, even my friend joe (the tech. I brought with to inspect it) commented on how good it sounded. There wasn't a tick or knock or anything. Idled perfectly, seemed more than happy to be running. We couldn't find a thing wrong with it (aside from the headlight motors not working, and some minor cosmetic issues). Today the car sounds great running still, no knocking, no ticking, revs freely, I pushed it to 95 mph up a hill and it still felt like it had plenty more, all the while the gauge was at about 16 PSI. So how on earth has this thing not siezed up? Is it possible that there's sludge built up (or something) blocking the hole the sending unit is screwed into? Or am I just running on borrowed time, and luck? Does this sound right to any of you? Bc it's blowing my mind.
So basically 14k miles of being beat on, while only holding 0 psi of oil pressure at hot idle, and never more than 16-17 maybe 18 psi at 4 grand.

AlkyIROC

If everything else checks out ok but oil pressure is very low at hot idle, the most common thing I've found to cause that is worn or spun/loose cam bearings. Worn main bearing may also cause the same problem.

Pressure is a restriction to flow. Bearing clearances cause a restriction and help maintain pressure. Excessive clearance and the flow increases which lowers pressure no matter how much oil the pump pushes through the system.

You could try going to a heavier grade of oil but that's only a temporary fix.

Mattkovitch

I know all about oil pressure and where it comes from, but does that sound right? How long it's been running that long on oil pressure? I'm gonna put 10w40 in it rather than 5w30. I'm just trying to think of anything possible that could cause this besides worn bearings, bc it only has 124k on it, and 305s tend to have a much longer life

greenyone

iTrader: (1)

Where does the sender go on the block for your car? Behind the intake manifold or near the oil filter? The one on behind the intake is 1/8 npt. I believe the one on the block near the filter is 1/4 npt. The one not used is most likely capped with a brass or steel plug. You could try and hook up to which ever one is not being used to see if you get a different reading. That could help rule out the port being plugged up. It's a long shot but maybe worth a try since you have a mechanical gauge to test with. If it still reads the same after that then why not throw a new oil pump at the car. They are cheap enough and if it doesn't fix the problem you'll have a new pump for the rebuild.

Supervisor42

First of all, don't panic. Trust the mechanical gauge.
Typical stock idle pressure hot is only 15-20 psi.
If the low pressure were from a bad bearing, you would have heard from it by now.
Many vintage engines had oil pressure of 4 psi at governed RPM when new. They had only supply and flow to the bearings at idle (0 psi).
Since you have 14 psi at 1000 rpm, the bearings haven't starved.
A few quick things come to mind that haven't been covered that will do this:
1. Regulator valve in oil pump stuck partway open. (common)
2. Oil pump is worn out and losing efficiency at low rpm.
3. Lifter failure that is leaking oil.
4. Lower bushing in distributor has failed and is bleeding oil past it.

A piece of sludge can easily stick the regulator piston in the oil pump not allowing it to close. A broken or collapsed regulator spring as well.

If the engine has hydraulic lifters and the pintle valve in one sticks open, a steady stream of oil will flow up the pushrod. This leakage will be overcome as the pump speeds up.

Blockage of the gauge port will only make the gauge slow to respond. If it follows the changes of rpm, take it off your list.

I think it is unlikely that the crank bearings are bad given the mileage, but possible. Usually when they are worn bad enough to do this, one will spin and you have a knock pretty soon.

This problem needs to be diagnosed by a competent mechanic.

We had one (not a sbc) in the shop a week ago. Same problem. Dropped the pan to change the pump and the bolts that hold it on had gotten loose.
Gaskets, 2 bolts, and out the window went the $2000 for a reman engine.

OrangeBird

iTrader: (1)

You've answered your own question right there , 5w anything is too thin for your 125K mile engine and 10W40 is what I run in mine ....

Supervisor42

How did I miss this.
Wasn't the '91 spec'd for 10w30?
I didn't think they went to 5w until the LS motors.
Someone smack me.

OrangeBird

iTrader: (1)

Hey , no biggie , I miss stuff all the time . That's one of the great things about forums is that with many eyes looking , a good consensus answer can usually be found .

paulo57509

5W-30 has been specified at least as far back as 1987. The intent being for fuel economy (which is debatable, IMO). From the 1987 Owner's Manual:



I've run a straight SAE 30 for a few OCI's years ago (just because I had some quart cans collecting dust)

henryd3

iTrader: (128)

My 92 305 TBI says 5-30 on the oil cap, but I never use anything that thin in my engine

AlkyIROC

5W30 is good for a newer engine but not a high mileage engine. 5w30 is also for climates that never get very hot.

Not a bad cam bearing. It's very unlikely you would hear one of those bearings failing however the oil filter would have a lot of metal in it.

Change out your oil to a heavier grade and replace the filter. Cut open the old filter and take some filter material out of it. Squish the material in a vise to squeeze out the oil then open it up for examination. Lots of metal will indicate bad bearings.

TTOP350

iTrader: (8)

Another thing, what brand filter are you using?

Galaxie500XL

iTrader: (1)

A couple of things...

I'd suspect your gauge or sending unit, or associated wiring. 0 PSI over a short time should have already led to severe damage, or at the very least, siginificant, expensive sounding noises.

Secondly, I believe it's your gauge, because if your oil pressure was truly zero, the ECM would be shutting down the fuel pump. IIRC, if the ECM thinks the oil pressure is below 5 PSI, the ECM commands the fuel pump to shut down.

Keep in mind, your sending unit for the ECM isn't the same as the one for your gauge.

OrangeBird

iTrader: (1)

No my Friend , you are mistaken .

The ECM does in no way whatsoever monitor the oil pressure of the engine since there is no oil pressure sensor reporting the oil pressure to it . The way it works is this , You have essentially two normally open switches connected to the fuel pump in parallel . When the engine is not running , both are open and the pump is off . When you turn the key to the on position , the ECM commands (by activating the relay) it's two second prime and then the relay goes back to being open (since the engine is still not running) . Now you start the engine and the ECM of course does see that the distributor is turning , it's counting those distributor pulses to know when to fire the injectors and as long as those pulses are present the ECM will keep the relay engaged and supplying power to the pump . Of course also once it starts the oil pressure switch wired in parallel also closes and so in effect you have two separate systems keeping the pump powered on while it's running as a point of redundancy . To prove this to yourself , in a properly functioning system , unplug the oil pressure switch that feeds the pump and notice how the engine stays running ? This is because the pump's other switch , the relay that is , is still in the circuit providing power to the pump . Plug back in the oil pressure switch then unplug the relay , see how it stays running ? That because now the oil pressure switch is keeping it running . Remember the oil pressure switch is usually two units in one on most of these right ? One unit is the fuel pump switch and one is the dashboard gauge , right ? Well that doesn't leave one to be telling the ECM anything about oil pressure and the system works instead as I've described above . The site "austin third gen" has the wiring diagrams for the fuel pump circuit and the ECM if you want to check them out , and there you will see that the ECM is blind to the engine's oil pressure .

PS , I'm not looking to be a contradictory dick or a mr know it all here , it's just that electrical devices have always been my #1 strong point and I wanted you to know how this actually operates .

Galaxie500XL

iTrader: (1)

I didn't take it that way at all, so no worries.

305form89

When I purchased my car it's 305 pressure did this, always bothered me drove it from 180K to 240K before the engine went. I wanted to make sure it wouldn't be the oil pump and hope of keeping the pressure in the middle of the gauge on the new engine I installed a new oil pump on the 305 I swapped into it. Still goes to almost red-line at idle. The car actually had mechanical gauges in it when I purchased it because the sender had gone out and they didn't replace it, but I did.
I have a L98 in my 1990 vette 135K on the OD does the same thing.
My brother's 85 monte carlo with a 350 and mechanical oil pressure does this.
93 Suburban 350 same.
96 Tahoe 350 same.
Actually come to think of it every 305/350 car I have ever been in will start with good oil pressure in the center of the gauge then drop off to almost red-line as warmed up. I don't think they ALL could have warn out cam bearings or stuck bypass as they all ran great.

305form89

I run fram filters typically BTW.

AlkyIROC

I wouldn't use a fram filter on anything I own even if they were free.

TTOP350

iTrader: (8)

Ditto

Arctic White 91 RS

My 91 RS convertible with 113k miles would drop to zero oil pressure at idle when fully warmed up say after 45 minutes. The issue was the Fram oil filter and bulk Penzoil 5w-30. The prior owner let the oil change place do their thing - Fram oil filters.

Changed oil to Mobile 1 High mileage 10w-30 & Mobile 1 oil filter. Car holds 12-15 pounds of oil pressure at idle when hot and fully warmed up. SBC engines only need 10 pounds of oil pressure per 1000 rpms so 12-15 pounds is fine t idle.

Mattkovitch

How would I go about checking if that oil pressure relief valve is stuck open?

Mattkovitch

Napa gold or something like that? I don't use shitty Fram filters or anything like that. As far as I know the napa gold filters are decent. What the previous owner was using idk? Pretty sure it was all black with no label, probably some shitty filter a lube shop threw on.

AlkyIROC

You would need to remove the oil pump.

After all this time, have you installed a mechanical gauge onto the engine to confirm the pressure?

Supervisor42

Oil pumps are cheap. Unbolt the motor mounts, raise the engine, drop the oil pan, replace the pump and oil pan gasket. Silicone the corners.
Don't forget to prime the new pump before you install it.
If the problem persists, it is excessive leakage inside the engine (see above).

Mattkovitch

I don't have one hooked up to it, but I have checked it with a mechanical gauge and it reads the same as the one on the dash

Toby T. Tsosie

Bought a 3rd gen firebird with the 2.8 v6 for my son.. it was good oil pressure cold.. After awhile oil pressure went to Zero... I knew w oil pressure was good because I drove I a distance before I noticed.. Of course mechnal Guage is the true test.. But common sense told me the sending unit.. I looked over this forum before commenting.. This year 86 there are two sensors.. On I figured one talks to the Ecu(two prong) ? The other was the pressure Guage(one prong) 80 psi .. Well the dude I bought it from. Had it backwards.. (Two ports on the oil filter housing adapter.) .. The sending u it is a bell shape.. The other was a two prong. Relay to the ecu? Don't know.. But he had the sending unit(bell shaped) on top of the oil filter adapter.. And the other on the bottom... This caused a headache for me..... Switched them.. And perfect.. I suppose.. Not sure the normal readings.. But I known 10 psi is good for every 1000 rpm.. It reads.. 60 psi cold.. 20 psi at idle. (500 rpm) .. And fluctuate 30 45 psi at driving.. This is forced a 1986 2.8 firebird..... Better than the zeroes reading I was getting?