Street Rat

I've been experiencing low oil pressure at idle. When I say low I mean 0-10 psi. I just changed my oil sending unit to confirm that was not the problem. The pressure gauge has been replaced within the last year. When I first start the engine it gets to 40 psi. As it heats up the pressure drops. This motor is the original '81 350 and has been driven hard since I've had it. I know the rule of thumb is 10 psi per 1000 rpm. But I'm starting to get a knock at idle when the pressure drops . Sounds like the mains have excessive clearance huh? Time for a new motor.

I'm in the process of building a new engine but she's not quite there yet. In the mean time I plan on increasing my oil viscosity to 20w50 from 10w30 to increase the pressure some. Should I pull the pan and put another oil pump in it? I'm sure the pickup could use a good cleaning as well. What do you guys think about the situation? What would you do?

REELAV8R

Thicker oil should work for the time being. If it doesn't thenyou'll have to pull the pan and figure out what's going on.

TheSkunkWorks

Rod knock is never a good sign.

Even tho higher viscocity should increase pressures, since you've already heard it I'd probably drop the pan and check a couple of rod bearings near the front away from the pump. Plastigage is plenty sufficient for this purpose.

Oil pumps rarely fail, but it would be fairly easy to check it out while you're in there. That said, one thing I woudln't do is to throw a high-pressure pump at it. The 10:1000 ratio is all our main journal diameters require. My $.02

Indiancreek

The oil pump doesn't create your oil pressure. The pump creates flow with a pressure valve to limit max pressure. The bearing clearances create the pressure or maintains the pressure to the limit set in the pump.
If you're having pressure issues I suspect you have a bearing problem.
It's like a garden hose. left open the water just rolls out without much pressure. Put your thumb over the end of the hose and you'll have pressure. Your thumb is acting like the bearing clearances. The smaller you make the opening of your thumb the higher the pressure.
Then there is the knock, don't drive it with the low pressure and a knock. You'll only damage stuff in short order.

MakoJoe

Low oil pressure is generally caused by sludge in the engine.

Pull a Valve Cover and see if there is any sludge in the upper end of the engine if there is then clean out the engine with an internal engine cleaner.

Since you just did an oil change I would run Rislone engine cleaner through the engine to clean it out. Run it in the engine for a few hundred miles and change the oil again. You will need to drain a quart out first or just drain the engine without replacing filter fill it up with with 10w30 and a quart of Rislone.



Thicker oils will create more pressure but does not solve the problem or clean out years of sludge. I have used cleaners in the past with good results and they are worth exploring to clean out sludge if you have sludge in the engine after pulling a valve cover.

Les

If he has a rod knock, I'd say sludge is the least of his problems. Time to park it until the new motor is ready...before things go boom.

TWINRAY

Running a "sludge eliminator" I think would do more harm than good. I would see what idle psi is with a 15-40 like Delo 400. I would try that before going to 20-50.

Street Rat

I've had the valve covers off. The engine has no sludge. It is extremely clean.

gerry72

There is no factory specification for idle oil pressure. It's usually oil pressure @2000 rpm or there abouts. Wear in the rod and main bearings are big contributors to low oil pressure issues, but the primary bleeding occurs in the cam bearings. And if the rods and mains are worn, so too will be the cam bearings. While replacing the rod and mains may help -and they are far easier to replace than cam bearings with the engine in the car- they may not solve your problem. And, as others have written, you won't solve this with a new oil pump.

Is what you're hearing rod knock or lifter tick? Even with idle oil pressure in single digits, you won't get a rod knocking unless the bearing is very worn. Lifters, however, will start ticking with low idle pressure. The engine is under very light load at idle and rod and mains don't need very much pressure -five psi or so- to stay quiet.

Street Rat

The noise is definitely rod knock. Headed to the local parts outlet right now to get some heavier weight oil and a filter. I'll see if that puts a band-aid on it for a little while.

TheSkunkWorks

Not the best call IMCO.

scottyp99

Some Sea Foam will fix your engine right up, good as new!

Sorry to make light of your predicament, but I just couldn't resist. I'm with skunkworks on this one, if your willing to drop the pan anyway, definitely take a look at a few bearings. The ones toward the front of the engine are the farthest from the pump, so they will tell the tale.

I don't know enough about, or have enough experience with, engines and how their bearings work to argue about it so I am willing to take gerry72's word for it, but I am intrigued by his statement about cam bearings. I have always been under the impression that since the cam only spins at half the speed of the crank, and the cam bearings don't see the kind of compressive forces that the crank bearings do, that it is not uncommon to see an engine with worn main or rod bearings, and cam bearings that are still within tolerance. Again, I'm not trying to argue the point here, just looking for a litte clarification. I admit, this is knd of off-topic, so feel free to say so, and I'll do a little research of my own, or maybe start a new thread.

Scott

Street Rat

Sea Foam huh? I was thinking more along the lines of some "Motor Honey" lol.

dochorsepower

Don't go adding any "snake oil" remedies. You can add a thicker viscosity for a very temporary evaluation but the best bet is to drop the pan an check for a very worn or spun rod bearing. You will very likely have crank damage also. But since you're building a new one anyway, you may as well try something to limp along.

REELAV8R

A high flow filter right? I've used motor honey on worn out engines, it works temporarily. What you got to loose? You're working on another motor so this one may o may not let loose before you're done. Keep your ears open. Just don't count on reving it real high. If knock gets worse then thicker oil is not the answer. Knock is connecting rod, rumble is mains. If a rumble you can go for quite a while usually.

scottyp99

Check out the link in this thread. It may be a better "temporary" solution than 20w50 oil while you finish up your engine project. 1300 clams delivered, can't beat that with a stick.

http://forums.corvetteforum.com/c3-t...50-engine.html

Scott

Les

Street Rat,
Is the Vette your daily driver, or is there some other reason that you need to drive it before you can finish this other engine? If the answer to either of those queries is "Yes", please ignore the rest of my post. If the answer to both parts of that question is "No", I'm wondering why you'd want to push your luck. If you were ignorant and didn't know that a rod knock is a serious red flag, I could understand why you might happily just drive it until it let go.

That reminds me of a time when I was on a trip about 20-25 years ago when I spotted an old 340 Duster ahead of me on the highway. It was throwing out some pretty good oil smoke through the exhaust. As I neared it I could see that it was occupied by 2 young gals, who were just chatting away. As I got closer I could hear a distinct rod knock which seemed to be getting worse by the second. I decided to accelerate ahead quickly to avoid the possible carnage. Sure enough, within moments it let go in a very ugly way and they had to swerve to the shoulder very quickly.

In any case, I wish you luck and I hope your engine holds up for you if you choose to keep using it.

Street Rat

Put the new high viscosity oil and a new filter on. I gained about 10 psi of pressure at idle when hot. No knock at idle. Definately not a cure all but buys me some time until the inevitable. No motor honey added yet.
Scott I'm going to spend that money towards a new set of AFR heads. I have other vehicles to drive so when the inevitable happens I'm not going to be stranded. I'm sticking to my guns as to my build. The old girl might be sitting up for a while until the new engine is ready. I'm prepared for that.

badrad

I'd stop driving it, I ran one with rod knock while trying to get the money to get it fixed. It came apart instead.

Street Rat

The old girl is not a daily driver. I'd say she is more of a play toy. I've been driving it locally to some shows and such. The weather has been very nice lately. I'm simply trying to get some more enjoyment out of her while I can.