suz72vette

Hi --

Is it normal for the oil pressure gauge (which reads a bit over 35psi while
driving) to drop to a reading below 5psi while the car is stopped and the
engine is idling ? It jumps right back up when foot hits the gas. We hear
absolutely no valve clatter, and no oil smoke out the tailpipe ... indeed the
engine seems to run fine ... we'd just like to keep it that way !

Chiltons recommends checking the "oil pressure relief valve" ...there's a
little exploded view of the thing, but nothing big enough to give any clue
where under the hood to look for it !

Would be interested to know others' experience with this (non?) problem
and what the remedy was. Would a higher viscosity oil be a good idea
to keep the oil pressure up(?) or is this a sign of something more serious?

Thanks in advance !

Suzanne & Jim



[Modified by suz72vette, 9:54 AM 7/20/2003]

7t2vette

I can only help you with the location of the oil pressure relirf valve. It is in the oil filter adapter. That is the part that you screw your oil filter to. Remove your oil filter, and you will see that the part the filter was screwed to is held on by two bolts, remove them and the adapter will come off. Hope this helps!

Bearcat

That COULD be a problem brewing. If the clearances in the bearing areas start to wear away, oil pressure drops. You should have about 10 psi per 1000 rpm, rule of thumb. Dropping below 10 is kind of dangerous.

I would recommend dropping the steering gear (pull two bolts off idler arm and drop it), then drop the oil pan and change your pump. You might try the Melling high volume pump as a replacement. If you go with a stock one, make sure to spot weld your pick up in place. The problem COULD be the oil pump wearing out...if you're lucky.

In the meantime, run thicker oil. 20W50 or straight 30 weight maybe.

Geoff H

Just a quick tip, before you start pulling thigs to bits. get an aftermarket guage and fit it up and run it for a while and see what it reads. For the want of $20 or so and 10 minutes it may tell you the engine is ok and the guage is faulty.

rrent

If you idle any motor at a slow enough RPM the pressure will be much lower than at crusing RPM;s -- while sitting at a stop light, with your foot still on the brake, bring the RPM's up a hundred or two and see what the pressure does.

Trying a second gauge is a great idea. If your motor is getting on up in miles you might lessen the time between oil changes --- this probably won't help the pressure readings -- but if they are caused by the normal wear - it will help kee pit on the road.

Chuck Gongloff

Also, depends of the weight of the oil you're using. The "thinner" the oil, the less the oil pressure. Also, the hotter your engine runs....at idle, the lower the oil pressure. Is this a "new" problem, or something that's been going on for a long time? Chuck

Matt Gruber

:iagree: my 67 galaxie had 29 psi @65mph with 10w30.
with 20w50 it had 40 psi.

suz72vette

Thanks to all who've responded. We've only had the car about a month, two
weeks of which it spent in the shop for a new radiator, new brake master
cylinder, and new steering ram & gearbox). We've noticed the drop in oil
pressure at idle :confused: on both of our two short weekend drives.

Oil in there currently is "probably" 10W-30 (maybe 10W-40 ?), so we'll
certainly try a heavier weight oil to see if that helps. Likewise with an
aftermarket gauge and/or new oil pressure sending unit (Chilton's
troubleshooting chapter suggested these also - good ideas !-).

Is this likely the sign of an oil pump on its last legs, or crankshaft bearings
not holding pressure (so a better oil pump [e.g., Melling] is only a stop-gap
measure) ??

Thanks again,

Suzanne & Jim



[Modified by suz72vette, 9:58 AM 7/20/2003]

Matt Gruber

all pumps wear out.
oil, water, fuel
new ones would be nice!
the oil pump tends to lose pressure in hi-mile engines.
nothing serious with 35 at cruise.
i would replace all pumps at the next engine rebuild, run 20w50 until then.

turtlevette

I agree,

I had about the same condition about a year ago with a newer engine. Replaced the oil pressure sending unit and it read fine.

Get a quart of engine flush run it about 5 - 10 minutes then get an oil change with a heavier oil. You could also replace the oil filter adapter for around 20 bucks.

If none of that works, just enjoy the car and don't worry about it. Its not a bad enough condition to have a catastrophic failure, it will just gradually wear out and you can make decisions on a rebuild then.




[Modified by turtlevette, 12:47 PM 7/20/2003]

jerryp58

Have YOU changed the oil?

I had a car that would drop to a very low pressure (at idle) when the oil was old/dirty.

If you haven't changed the oil yourself, I'd go with the "Motor Flush" and oil change idea. Some people don't like using flush type products because of the (probably legitimate) worry that some large chunk of "crap" will break loose and cause worse problems. I used "Motor Flush" on my car for the first three oil changes and was lucky to have no problems. With my current 150k mile engine, I like to run 10W-40.

Good Luck.

suz72vette

Shop that did the repair work also did oil, filter, & lube ... oil change was due
(maybe overdue?) when we bought the vette. I thought they might have
used standard 10W-30, but Jim remembers a discussion with them concluding
a thicker oil (10W-40 ?) would be better given the age of the motor. Still, we
could increase viscosity up to 20W-50 next time (and try the motor flush).

But assuming a lower viscosity oil is "easier" (?) to pump through the engine,
is less pressure required with 10W-30 oil to achieve the same flow rate
of oil ? Naively, isn't flow rate of oil through the engine what we should care
about, whether it takes 5psi or 15psi at idle to deliver adequate volume of oil
to ciritical moving parts of the engine ?

Bearcat or anyone: what's the logic behind "straight 30 weight" ? (We're in
Southern California, so oil getting too thick at low temperature really isn't a
concern for us). Does "straight 30 weight" stay thicker at high temperature?

turtlevette wrote:
Thank you turtlevette(!!) for offering this perspective (makes us feel much
better :-). But zero oil pressure is *BAD* even at idle, right? Assuming the
sender/gauge aren't the problem, what symptoms *DO* we need to watch
for during gradual decline over time, to avoid causing real damage ?

Apologies if we're worrying too much ... this is what happens when you give
someone an oil pressure gauge to look at instead of an idiot light ... they
start asking questions when the gauge reading isn't always the same and
presumed normal !!

Thanks again to all who responded.

Suzanne & Jim


[Modified by suz72vette, 12:09 PM 7/20/2003]


[Modified by suz72vette, 12:10 PM 7/20/2003]

Desertvette

The press relief valve Chiltons is refering to is the valve in the oil pump.
The valve ,under the filter, is a pressure differential valve that is used to bypass the filter if it plugs and only effects oil press a few pounds if at all.
The main cause of low oil press is excessive bearing clearances but you could have oil pump ,pump pickup or contaminated oil problems
Verify the gage and change the oil as advised first and if still low chk out the oil pump.
By the way Wel-come to the Forum and LOL
Bob

Richard Cooper

Try replacing the oil pressure sending unit. I had the same problem as you are having and it turned out to be the sending unit. Its cheap and an easy fix. Got mine from Auto Zone. Good luck. :seeya

bob

Chiltons recommends checking the "oil pressure relief valve" ...there's a
little exploded view of the thing, but nothing big enough to give any clue
where under the hood to look for it !

I think they are maybe talking about the valve in the oil pump. The springs get weak with time. I have had many old chevies that just changing the spring brought the idle oil pressure back up. Last example was a 305 that had 18 psi at cruise and less than 4 psi at idle. I replaced the broken spring and now 32 psi at cruise and 28 at idle. That was the only change and those are the kind of engines most people think need a complete rebuild. Having said that, I have had ones that the problem was much more than just the oil pump spring.

suz72vette

Thanks Bob and Desertvette -- that will be next on our list after first replacing
the oil pressure sending unit (and the gauge too?). Crossing our fingers this
is nothing more serious ....

Suzanne & Jim

loose nut

If you still have low pressure after trying anew gauge ,it is not difficult at all to drop the oil pan in a C3 and pull #1 rod cap. Then if the bearing and crank look good you may want to install a mellings high volume pump in it.

mapman

The owner's manual, for 1980, states that 30-40 psi may be considered normal oil pressure during moderate road speeds of 35-40 mph.

The 2003 service manual for LS1 engines shows, Minimum oil pressure (hot):

6 psi at 1000 rpm
18 psi at 2000 rpm
24 psi at 4000 rpm

Are you sure you have a problem?

jimcork1

The LS1 @ 5 psi is also a low number and it uses synthetic oil. I the oil pressure is not 10 psi / 1000 rpm and you have 10w40 oil I think a rebuild will be necessary soon. You could run 20w50 for a while and stretch it out for a bit. Good luck..

68shark

I agree...sounds like an older engine getting to the stage where a rebuild is required. Using 20W50 will run higher pressures and thus buy you some time, but barring a guage issue (and presuming the engine has not been rebuilt), I would say you are getting close to "that time"......good luck!!