'66 Small block -erratic oil pressure

[Dick Kindl]

I recently had the opportunity to get my 66 small block L-79 350 HP up to 90 MPH +/- for a stretch while on a cruise-in with a group of Corvettes in our club. After I backed off I noticed the oil pressure gage was fluctuating up and down somewhere between 30 and 60. I stopped the engine checked oil level, which was almost to the full mark, restarted the engine and continued. If I hold the speed to around 70-75 I have constant oil pressure of 60. I had changed the oil and filter prior to this cruise. The same thing happened again while driving on the interstate and getting the speed up again fo a sustained period of time while passing.Once I back off on the speed and continue the pressure will remain steady again. I have not had this happen before.

I was wondering if the oil pump is the problem...possibly pumping more oil out of the crankjcase than can return to it....What say you experts out there??? What could be causing this.

[Mike McKown]

Re: '66 Small block -erratic oil pressure

High pressure/high volume pump sounds like your problem. A 5 quart pan and the pickup screen too far off the bottom of the pan will also contribute to this.

I wouldn't do this again. When you get in there to take a look at it, check your bearings. You may have already scorched them.

[Dick Kindl]

Re: '66 Small block -erratic oil pressure

Thanx for the response Mike. I had the engine rebuild done when I went to the Quebec Regional. The engine overheated on the way Thru Montreal and blew the head gasket...sooo...henceforth they recommended new pistons, rings correct cam, etc the whole nine yards!. When I picked the car up I was given the oil pump and the pistons etc. I assume they had put in another oil pump. It may very well have been a high pressure, high volume pump. I was told I can drop the pan and replace the spring in the oil pump which might solve the problem. I guess I will have to drop the pan and look into this.

[John H.]

Re: '66 Small block -erratic oil pressure

Dick -

Check the clearance from the pickup screen to the bottom of the pan (should be about 3/8"); if it's too close or touching the pan, it can starve the pump at high rpm, same result if it's too high. Set the clearance correctly and tack the pickup tube to the pump (remove the spring first, then reinstall).

[Brian Monticello]

Re: '66 Small block -erratic oil pressure

Before you dive in, make sure your dipstick is giving you the correct reading. Drain all the oil, fill with the correct amount and see where the level sits on your dipstick. You may be low on oil causing your pump to run dry. I have an aftermarket tube and dipstick and have to fill past the FULL mark on my dipstick.

Regards,
Brian

[Joe L.]

Re: '66 Small block -erratic oil pressure

Dick-----

All of the high volume pumps that I'm aware of are also set for high pressure. However, at 60 psi cruise, that sounds like a high pressure pump to me. So, that means that it could be either a high pressure/standard OR a high pressure/high volume. If your cruise oil pressure had been around 40 lbs, that would have indicated a standard pressure pump and, as I say, if it were a standard pressure, it would, most likely, have been standard volume, too.

If you have a high volume pump installed, the only thing you can do about that is change it; there's no way to make a high volume pump into a standard volume. You will be able to tell if it's a high volume by the longer gears (and pump chamber) which a high volume pump has. Compare it to your old pump for easy identification.

[Clem Z.]

Re: '66 Small block -erratic oil pressure

the high volume pumps require the corvette oil pan baffles to be modified because of it extra length if i remember correctly

[Dick Kindl]

Re: '66 Small block -erratic oil pressure

Thanx to all who responded...I found the oil pump that had been removed. Here are some numbers on the castings..Upper casting.#GM3732796, under the GM # G71.
Lower casting part with the screen tube appears ..3C9 then some other casting numbers H150 (or H159) also C (or G)17. Until I can get the car on a lift and drop the pan I won't know what pump is in there now.

[Gary Tesini]

Re: '66 Small block -erratic oil pressure

My pressure does the same thing idles around 30 but when driving 40-50 mph it's almost always at 60

[Michael W.]

Re: '66 Small block -erratic oil pressure

Dick,

I remember your unfortunate episode very well. Was it not Carey that rebuilt your engine? If so, why not give him a call to see what he installed. Maybe Robert C. would have his latest contact info.

Mike

[Dick Kindl]

Re: '66 Small block -erratic oil pressure

Mike...I don't know of this Carey. The car was left at Syd & Jeffs Auto Shop in St. Laurent, Quebec. I don't have Robert C's E-mail address. Do you reside in that area? I don't know if they would remember working on the engine and the poarticulars that may help. Thanx.......Dick

[Michael W.]

You have mail *NM*