lowflyer

Recently my oil pressure on my '86 L98 seems to be getting lower and lower. It has about 5000 miles since the rebuild last year. It'll start with 60psi of pressure, but as the motor heats up it starts to drop. When at operating temps and at idle (700rpms) the pressure is at 10-12 psi, but at speeds, the pressure pops back up to 40-50psi. At that low a pressure, shouldn't the low oil light come on? No knocking noise or lifter tick is heard when the pressure drops at idle. I've replaced the sending unit and verified the actual pressure with a mechanical gauge and its correct.
I guess I have to yank the motor out again....any thoughts or something that I might have missed? Thanks.

MagRed_04

If I remember, the low oil pressure light will go on at 7 psi... 10 to 12 seems low to me. I don't think I have ever seen mine go below 30 psi.

JK

0ski_dwn_it

That is what mine does too. At idle it drops to ~20psi, but as soon as you touch the gas it goes up to ~55-60psi. This is normal as far as I am concerned. My dads camaro was the same way because I questioned it a while back. I think you are alright. Perhaps some others could chime in.

lowflyer

Man I hope so.... I really don't want to have to pull that motor out again. I don't know if I want to live with 10-12psi at idle. It just worries me. I've had that thing out twice now this year to battle stubborn oil leaks that never seem to end. I've replaced the oil pan gasket, front seal and the intake manifold gasket twice now.
I should just stuff my truck's big block in it (...that dosn't leak!!) and get it over with once and for all...

scorp508

The low oil pressure warning light will come on at different values, there is no one set # for it. It is all relative to the RPM and PSI at a given time. Unfortunately I don't know the forumla for it.

The "rule of thumb" for a SBC is you want 10psi for every 1000 RPM. I have seen my gauge as low as 9psi and no warning light come on, but only when my RPM is at idle after a very hard deceleration when I am a quart low.

LPDesRoche

Check the sending unit first off. They are notorious for going out after a rebuild. I have had 2 vettes do this to me so far. Its worth a shot. Or just hook up a mechanical type gauge to it for test purposes.

CFI-EFI

Scorp508 is correct on this point. Your "new" engine probably has greater than production bearing clearances, creating an easier path for the oil to escape and prevent higher pressures from building. The way I see it, you have two choices. 1) Since you are within specs, leave it alone and learn to live with it. You should be fine. 2) You can pull the pan and shim or replace the pressure relief spring in the oil pump cover. I don't think you have a problem, but if #2 makes you feel better, go for it. Good luck.


[Modified by CFI-EFI, 5:22 PM 3/24/2002]

M2001

I would try a higher viscosity oil just to see if it bumps up the oil pressure. However, as mentioned above, you may have to replace the oil pump or at least shim the spring (provided that you have an accurate oil pressure reading).

87SAM

When I first got this vette and it was 95 outside and with engine hot I would see ~22 psi at idle. Last spring I replaced the sending unit just because I "was there" while replacing the intake gasket and under the same conditions it would be as low as 10-12 psi. This winter I replaced it again and although it has only been as high as 70 outside, the lowest pressure at idle I have seen is 35 psi.

To sum it up, I don't have much faith in their accuracy. :crazy:

merrick

At idle 5 psi would provide enough lubrication, but I would be uncomfortable too. Your oil pressures are ok. As already mentioned, you may want to run a higher viscosity oil since you probably have larger than factory clearances. A good method in your case is to mix oil viscosity. If your using 5w/30 now, try adding 2qts of 10/30 or 10/40 to 3qts of 5w/30.

JAKE

Yep, I do something similar at times.

When I want to run 10w40 and can't find what I'm looking for I buy three quarts of 5w30 and three quarts of 15W50.

In your case I'd up the weight to increase idle pressure.

Oil pressure is REALLY sensitive to high oil temps. When the oil temps begin to climb, man the pressure will fall off a ton at idle.

I've got a crazy condition that I've learned to live with. When sitting at a stop light with my foot on the brake my oil pressure will be around 20/22, but if I put it in neutral (SAME RPM mind you) and take my foot off the brake, pressure climbs to low to mid thirties. At the same RPM you'd expect the pressure to remain the same, but in my heap, 'ain't so'.

Jake

Black87c4

I'd be concerned by oil pressure that low, but maybe that's just me.

your drinking buddy

I have low 20s at idle in D, and mid 30s in P like Jake. Really weird :crazy:

lowflyer

Thanks for all the replies:) I've been using conventional 10w30 to break in the motor, but I used to use synthetic Mobil 1 10w30 (before the rebuild.) I guess I'll try the 10w40 to see if it helps.
I know what you mean by the oil temps really affecting oil pressure. I used to run an aftermarket external oil cooler, but the rubber line started to chafe, so I took it off and now I can really see the temp climb quickly and the pressure drop just as quickly. I may have to put it back on...
As I don't really want to pull that engine out again, I think I'll try the heavier oil and learn to live with it. Thanks again.

JAKE

Actually you won't probably won't see any change since both are 10W oils.

If you want to kick up the idle and low speed pressure you need to opt for 15 or 20W whatever, or a straight weight oil like 30W.

Also shimming the oil pump pressure by-pass spring won't give you any higher idle oil pressure since the spring pressure doesn't come into play until the high end of pressures.

So here's what I'd do. Increase the oil weight by 5W or irenstall an oil cooler and/or do what it takes to lower the coolant temp. Remember, oil pressure drops dramatically as oil temp rises. So the cooler the oil the more pressure it will be created; of course you still want it hot enough to flow freely and to burn off any moisture.

I honestly believe that many guys run their stuff too cold; seems to panic folks whenever temps touch 200F, as if a bad thing, when in fact it's not. See, ring gaps are determined as to what they will be in a fully warmed up engine.

The engine block and other parts have to be hot enough to allow normal, anticipated expansion and contraction of the various parts. In fact, if you desire to run your engine cooler you need to account for that in setting your ring end gaps since expansion won't be the same as in a "hot" engine.

Cool intake charge but with hot parts is the key. Friction, which is a BIG horsepower drag, lessens a lot, but A/F density stay high. I'd love to be able to get mine to run 220F, both coolant and oil, all the time under all conditions.

To get a 5W increase and since you're using 10W30 now, use half 20W40 and half 10W30; that will give you an effective weight of 15W35.

20W40 may not be easy to find, so you may have to go with 20W50 which will give you 15W40.

Mobil 1 comes in 15W50 which would be a choice to consider too.

It's the first number, the 10W or 15W that determines the resistance to flow when the engine is cold and first started, but it also comes into play at very low RPMs like at idle.

Anyway, that's how I see it.

Jake




[Modified by JAKE, 8:07 PM 3/25/2002]

JAKE

Another thing I forgot to mention. It's entirely possible that you've got a mechanical problem.

Pull the oil filter and use a hack saw to cut it in half. Dig into the crevices of the filter material and look for any gold metal sparkling fragments. You'll be able to tell them because it'll look like you've been panning in the Klondike.

Don't worry that you'll need some specialized oil filter cutting tool, just use a hack saw. Any metal from your cutting will be on the surface of the filter material, not deep inside the pleated filter material where evidence of bearing damage will reside. Save your $.

Keep is posted.

Jake


[Modified by JAKE, 8:05 PM 3/25/2002]