1968 427 L-71 oil pressure drop

[Josh Noser]

Hello all,

Now that spring weather is finally arriving here in Minnesota I am once again thinking about the problem with my corvette. I have posted and received advice from you guys about this topic once before, however now that I am preparing to tackle the project I am unsure what to do.

The oil pressure on my car drops off after the last 1/3 of the RPM range. I have been told it could be many things causing the problem: improper baffling, pick up tube rattled off, all the oil is pumped into the top end, etc.

What is the possibility that the gauge is faulty (original gauge)?

If I go to the trouble of dropping the oil pan I want to replace the oil pump. What oil pump do you recommend?

I really need to solve this problem because I have a lot of money and time in this engine and I do not want to lose it.

Thanks for all your time and help,
Josh

[Jack H.]

Re: 1968 427 L-71 oil pressure drop

No reason not to get a 'second' opinion! You could replace the existing oil pressure gauge with a known good one (temporary fixture), if you had access to a 'spare' guage. Or, you could use an 3rd party brand (e.g. Stewart Warner) to do the sanity check with.

Another thought is 'splice' the engine end of the existing oil pressure tube to a hand pump and verify the in-dash gauge DOES read full scale properly... Sure would be nice if the problem was in the gauge itself, eh?

[Brian Monticello]

Re: 1968 427 L-71 oil pressure drop

I don't see how a faulty mechanical gauge could show a decrease in pressure if pressure was really increasing. I've seen them stick but can't understand why one would show a drop. A gauge check is a good idea but unfortunately, I think you're going to have to drop the pan.

Do you have a high volume oil pump? This could be causing the oil to end up in the heads rather than the pan.

Are you sure you have the right dipstick/tube combo? I ask this in case you have less than the maximum amount of oil in the system. Example - I have a 6 qt system with the wrong dipstick. So... when I add 6 qts the dipstick shows that I am way over MAX. Had someone else built the motor I might think that 6 qts was too much.

I'd put my money on the high pressure/volume oil pump or the pickup falling off.

BRian

[Josh Noser]

Re: 1968 427 L-71 oil pressure drop

I am thinking it is possible I have a high volume/pressure oil pump. Or the pick may have come off. So I guess I will be dropping the oil pan.

This being the case I am going to replace the oil pump. Could someone give me a good choice for an oil pump and possible a part number to be sure I get a good pump in the engine.

Thanks,
Josh

[Michael H.]

Re: 1968 427 L-71 oil pressure drop

Josh,

I agree with Brian. I see no way a guage could cause this erratic reading. Also, I wouldn't attempt any more test runs at high RPM because that's when this motor needs lubricant the most. Sounds like the oil pump pickup tube is not positioned properly or, as Brian mentioned, a problem with baffling or oil level.

[Duke W.]

Re: 1968 427 L-71 oil pressure drop

I doubt if it is the pressure gage. A more thorough description of observed readings might be useful.

What is the normal hot idle pressure?

At what RPM does it achieve full oil pressure and what is that value?

At what RPM does it drop off? Too what? Does the drop off increase with higher revs once it has started or does it drop to another steady state value and remain there to peak revs. Does the reading fluctuate up and down once the drop off starts?

Exact name of oil and SAE viscosity? Are you using an OE type filter, or does the engine have a non-OE filter system? What brand and number oil filter? How many miles ago was it changed.

Is the oil pressure affected by vehicle dynamic loading - cornering, braking, or acceleration?

Are there any other modifications to the oiling system that you know of - external cooler, etc?

Duke

[Joe L.]

Re: 1968 427 L-71 oil pressure drop

Josh-----

Usually, a high volume oil pump will work quite well with street big blocks, although I don't recommend them, at all, for street small blocks.

The last available SHP big block pump (high pressure, standard volume) was discontinued by GM many years ago. So, for a standard volume pump, you can't really go the "GM route" (the high volume, high pressure "ZL-1" pump, GM #3969870 is still available, though, and may be what you have now).

You can use a Melling M-77 with a GM #3876866 spring. Unfortunately, the GM spring is discontinued, but aftermarket equivalents may be available.

[Mike McKown]

Re: 1968 427 L-71 oil pressure drop

Just a suggestion, if you've lost oil pressure on the guage, you may have smoked your bearings. I would check the rod bearings while you have the pan off. If they're good, I'd forget the mains.

The stock oil line size doesn't react quickly enough to detect pressure loss. You may have been running dry while the guage still showed some pressure.

I've never seen anything in writing about how far the pickup should be off the floor of the pan but I shoot for about 3/16's inch.

[William O.]

Re: 1968 427 L-71 oil pressure drop

I had the same problem in a BB, I would drop the pan and look in the oil pump screen and see if there is any blockage.

[Mark #28455]

M77 pump includes spring

The Melling M 77 pumps I have purchased included a separate high pressure spring that can be installed in the pump to duplicate the factory solid lifter pump.

[Terry F.]

Re: 1968 427 L-71 oil pressure drop

Joe, do you by chance know the purpose of the spring that comes with the pump? Does it control the scavaging system when oil pressure becomes very high? I don't know much about them. Thanks, Terry

[Mark #28455]

spring is for pressure relief valve

The spring presses against the piston for the pressure relief valve. When the oil is cold or at higher RPM, you would get pressures high enough that they could burst the oil filter can. The pressure relief valve limits this to 50 or 70 PSI. If you have a hydraulic lifter engine and are not racing it, the stock (low press) spring will be fine.