1968 Corvette 427 390HP Low Oil Pressure Hot

[Joseph M.]

About a month ago I decided to do a leak down test on my 1968 Corvette 427 390HP that was rebuilt last February to determine why it has been going through a quart of oil in 200 miles. When I pulled the valve covers I found pieces of the positive stop umbrella valve seals and tension springs around the base of the valve springs. I pulled the heads and took them to the machine shop where they found that the bottom side of the valve spring cap was striking the positive stop umbrella valve seals. The machine shop machined down the bronze guides to the correct height and installed new positive stop umbrella valve seals.

After reinstalling the heads, I installed Mobile One 10W-30 and found that the hot oil pressure was no longer 35 to 40 psi at idle but rather around 13 psi after driving the car for about a 20 minutes. I then pulled the pan and found pieces of the valve seal in the pickup and looking in the pump, saw heavy scratches on the walls and marks on the gears. With the pan off, I pulled the # 3 cap and found no sign of wear on the bearing insert and journal. I then installed another Melling High Volume pump, screen, and Mobile One 10W-30 and the oil pressure at idle hot again was 13 psi, 2000 rpm 40 psi, and 60 at 2800 rpm using an external liquid filled mechanical oil pressure gauge. Prior to pulling the heads, I did have Castrol 20W-50 oil in the engine so with only a little over a 1000 miles on the engine, do you think switching back to the conventional 20W-50 motor oil will bring my oil pressure back up at operating temp at idle or could I possibly have a main or cam bearing clearance issue?

[Duke W.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

I have a suggestion. Use the edit function to break you post up into paragraphs. A solid screen of single spaced text is nearly impossible to read.

Duke

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Hello,

This evening I drained out the Mobile One 10W-30 synthetic motor oil I put in new last night and installed Castrol 20W-50 and a new Fram filter. Using my Snap-On digital tachometer and an external liquid filled mechanical oil pressure meter I made the following measurements:

Temp -----------RPM--------------- PSI
Cool - below 100 1500 (Fast Idle)---- 69
Cool - below 100 2000----------------79
Hot - 200 degrees 950 (idle)----------49
Hot - 200 degrees 2000---------------68
Hot - 210 degrees 960 (idle)-----------44
Hot - 210 degrees 2000---------------66

(1968 427 390HP, Hi-Volume Melling oil pump, stock pickup with 1200 miles on engine)

To simulate driving up the road, I held the RPM around 1800 RPM off and on between 200 and 210 degrees. Also to note, before I took the heads off I did have Castrol 20W-50 oil in the engine and experienced oil pressure similar to the measurements above. Is this normal to have this much drop between these types of oils? In addition, if I take the car up the road tomorrow and after driving it for say 15 - 20 minutes at around 40 mph the oil pressure drops below the first mark on the factor meter at idle, (which I believe to be between 15-17 pounds), do you have any ideal what mechanically would cause this strange situation? I recall seeing a few posts about oil foaming causing fluctuations in oil pressure, but I have the factory integrated windage tray installed and I'm running the factory size oil pump pickup.

Thanks,
Joe

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

I started the car up this morning and the oil pressure running fresh Castrol 20W-50 was close to 70 psi at 1500 rpm cold. After heating the engine and driving the car 7 miles at operating temperature, the oil pressure at idle (950 RPM) was about 11 pounds. At 2000 RPM it is around 40 psi and at 2500 RPM it is 60. What I think is really strange is the oil pressure at idle at operating temperature stays around 40 psi when heating the car up in my garage and the pressure at idle appears to only drop after taking it for a small test drive. I'll probably get with my machine shop tomorrow to see if I can get one of the guys to do a house call prior to potentially pulling the engine and taking it to them. Any thoughts on what is going on would be very much appreciated. Thanks!

[Doug M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Joseph, try another oil pressure line and gauge temporarily to confirm your info to date.

Doug

[Bill C.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

I am by no means close to an expert on this - but

is there any chance that the pressure relief spring assembly (that controls oil pressure) is malfunctioning?

Could there be a tiny piece of trash stuck in some orifice or could the scratches cause some type of unregulated "blow-by"?

You may just need a new oil pump.

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Bill,

Thank you for your response! Tomorrow, I'm going to get my machine shop involved and see if I can get one of their machinist to stop by and take look at my car up on my lift. As for the oil pump, between the original Melling HV pump that had a little over 1000 miles on it and the new Melling HV pump and new pickup screen, I encountered the same oil pressure readings. This afternoon, I'm going to pop the pressure return spring and valve out of the original pump to see if there is any contamination in that one. Also, in trying to figure out if the type and viscosity of oil was contributing to my problem, I flushed the system in the last 2 weeks by changing the oil 3 times. I now have Castrol 20W-50 with 10 miles on it and still have the same problem...

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Doug, I tried another oil pressure meter today and received the same readings. Thanks!

[Duke W.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

First off, I don't recomend the SM oils you have used and are using. (Engine oil discussions populate the archives.) Also, high volume oil pumps are not necessary, but the fact that it delivers relatively low oil pressure at idle may mean you may have excess bearing clearance or a major internal leak.

Being a '68 model is should have a "single hole" rear cam bearing with the oil transfer annulus machined into the block, but AFAIK either a grooved or non-grooved camshaft is okay. The change in BB oiling system circa 1967 has been extensively discussed, but a lot of guys get in trouble over it, and most machine shops are clueless about this issue.

You should do an audit to determine installed bearing clearances and pay particular attention to the rear cam bearing/camshaft configuration and verify that it is correct for you block.

Being as how your machine shop screwed up the heads, it casts suspicion on everything else.

Duke

[Timothy B.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Joseph, Why did you install a high volume pump?? I think your car should have a standard volume/standard pressure pump which relieves at 45 PSI. Take that pump out and install a standard pump with a new screen and braze it to the pump cover at the proper height. Don't braze without taking the lid off the pump and the relief spring out of the cover.

Thirteen lbs at hot idle seems good to me and you report the pressure goes up normal, what's the problem?? If the engine runs good, do some research about the best spec engine oil to use and make the change.

[Duke W.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

45 psi is the upper end of most base small block hot oil pressure specs and some SHP/FI SBs. The '67 AMA specs call for 50-75 psi at 2000 RPM on all big blocks with the engine thoroughly warmed up with the standard OE pump. The '68 AMA specs are probably the same, but the owner should verify.

Though GM does not specify idle oil pressures, at 600 RPM idle speed a L-36 should probably be about 30 psi. Owner's should post their observations.

Duke

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Duke,

Thank you for your feedback. The strange things is why when I idle the car in my garage for practically a half hour and get the temperture up to around 210 degrees, the oil pressure at is 44 psi with fresh Castrol 20W-50 but if I drive it 7 miles at an average speed of 35 Mph and come to a stop, the pressure drops to 13 psi at idle which is around 950 RPM. Today, I'm going to contact my machine shop to get their assistance with resolving this issue. Once we figure out what the problem is I'll update the post. It seems to me that over the years some other people most have ran into this problem.

Thank you,
Joe

[Duke W.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Even though idling in the garage may raise the coolant temperature to fairly high levels, it may not get the oil very hot. To thoroughly warm up the oil, the car needs to be driven.

The difference in idle oil pressure you report might see a contribution from oil temperature, but there may be other issues.

Typically, oil temperature will be at least as high as coolant temperature in normal driving and maybe a little higher.

If you have an IR gun, do the same test, but record pan temperature readings along with oil pressure.

Duke

[Joseph M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

Duke,

Than you for you feedback. I called and spoke to the shop foreman at the machine shop that built the bottom end (not the heads) for me and he stated the same exact thing about the oil heating up more when test driving. Unfortunately for me, he suggested pulling the engine and bringing the short block assembly back to them so they can tear it down and measure everything to determine why the pressure is dropping. I should have the short block assembly to them before the end of the week and will update the posting as soon as I get their evaluation.

[Michael M.]

Re: 1968 Corvette 427 390HP Low Oil Pressure Hot

My 68 L36 runs at about 30-35 lbs at idle when fully warmed up, even when driven in 90 degree plus temperatures and the water temp approaching 200 degrees. Just a point of reference.