The Stovebolt Site Forums The Shop Area Engine Shop oil pressure drop

Rebuilt 1956 era 235". Oil pressure gauge is 0-30-60 PSI. When driving the needle stays at 30 but when I slow down the needle drops to almost ZERO.
This is something new as it always stayed at the 30 mark under all conditions.

Also I get lifter clack on start up when it is cold. This lasts a few minutes the noise diminishing and then goes away.

Using 10W30 Quaker State, always have and change it at 1000 miles.

Had the pan off a few years back and it is spotless inside. No sludge in the head either. It could be the gauge but I doubt it.

Any ideas would be appreciated, thanks.

Bill,

I'm sure you have checked...no oil dillution with fuel.

Stuart

There will be dozens of opinions on this but I'm of the opinion you should use single weight grade oil in these old engines. I'd switch to Quaker State 30W.

Now about your gauge. Chevys should have a 0-30psi gauge. Is your truck a GMC? If so is it a Canadian GMC?

It is normal for oil pressure on 235 to drop way down at idle. Are you sure it used to stay at 30 highway and idle? That doesn't sound typical.

You might have a faulty gauge. I've got a fresh rebuilt 235 that 55 at speed and 35 at idle. The 30 all the time makes me think your gauge aint working right. Try a different gauge and see if you get the same kind of readings.

Oil and fuel are fine, no dilution. The truck is a Canadian-built 49 Chevrolet but the source of the pressure gauge could be from anywhere.

A second gauge is one of the options I am going to do.

As far as it always reading 30 I am almost certain because at the time I thought it strange also. The only other time I can recall the pressure drop was after a very steep climb in low gear. When I got to the top the pressure dropped.

Nice to know that the 235 does drop. If I go by the gauge it drops (in my case) about 15 pounds.

I have always been a fan of multi-grade oil but maybe a straight weight might show a difference.

Bill,

Your multigrade oil AND any single grade oil will have the same viscosity at 210 degrees, so technically, it should make no difference to your engine once warm. It wouldn't hurt to try!

Stuart

Olblu I am running a rebuilt 235 with about 30k miles on it. my oil pressure has always stayed at 20 lbs at idle and 45 lbs going down the road at 60 to 70 mph with the engine at operating temperatures. Are you sure you don't have a babbited engine? A change to 30 wt oil will bring the pressure up some but something is not right with nearly zero oil pressure at idle. The clattering lifters are telling you the pressure is not high enough. Install another pressure gauge to eleminate the problem with the gauge. I would check out bearing clearance before something is ruined. A bad oil pump is a possibility. The mains and insert bearings may be too loose?

No this is a late model 235 with insert bearings. I had a look when I pulled off the main cap to change the rear seal.
All of my readings were at operating temps and when I start up initially the pressure goes right to the middle of the gauge, be it 30 or whatever. Only after driving it for a while does the pressure drop when I come to an idle. I will not say it is zero as it seems to indicate between the middle notch and zero.

I could be wrong Stuart but that's not what I've heard. A 10W30 oil is 10 weight oil with the lubricating properties (not viscosity) of 30 weight when hot. A 10 weight oil would be much thinner than a 30 weight when hot. Do you have a credible source I could go to that has data supporting the single viscosity and multi viscosity having the same viscosity when hot? I'm not trying to be contrary, thats just the opposite of what I've been told over the years.

Something that seems to confuse people about muti-viscosity oils is th numbering system. Example-10W30. This oil is essentially a 30 weight oil that has the flow properies of a 10 weight oil at 32 degrees F/0 degrees C. thw "W" in the gradinf stands for "winter". Most think that it means "weight".
I do believe that the 216/235/261 engines seem to run happier with a single grade 30 weight just from my own experience. Especially if they have had performance modifications.

Tiny,

I guess my credible source would be the Society of Automotive Engineers.

Longbox was 'spot on' as they say.

A multigrade oil is formulated in such a manner that it will exhibit different viscosity's at different temperatures.

As Bill noted, the 'W'(WINTER) after the first number of a multigrade indicates its measured viscosity at ZERO degrees.

The second number is the measured viscosity at 210 degrees Fahrenheit.

A single grade oil will not have a 'W' behind it. This means the label viscosity is at 210 degrees.

This concept doesn't mean the oil gets thicker with a rise in temperature, it means with the addition of VI (viscosity index) improvers that it doesn't 'thin out' at elevated temps.

If you 'Google it' you will find a wealth of info on the subject.

I am not a engineer, so some of my statements might not be accurate, but generally , this is how it works.

Stuart

Oldblu I just changed my oil in my maroon 1954, you know it. My pressure stayed pretty steady at about 33lbs. I read in the manual that once the presure reaches a certain point say 33 then it should stay there which mine dose. Any way thats what I ound with my truck. Peter

Just took her for another ride. On start up the pressure goes up to the middle of the gauge. 30 or whatever it really is. Driving for a while with temps normal it seems okay even when I go into idle, but after about 10 miles I get that drop when I slow down or go into idle but it still reads 30 when I am driving. Operating temps never vary.

Read more of the thread blu49 my pressure never dropped at all as stated by someone it stayed pretty constant at about 33lbs at all rpms. Peter

I think we all have to remember how accurate are these gauges. Peter

Oils of differing viscosities having the same "flow properties" (viscosity?) at a given temperature makes no sense to me at all. Viscosity values are a measure of a liquids flow properties. If they were the same they would have the same value. I don't see where a 30 weight liquid could flow as quickly at any temp as a 10 weight. If it does it isn't a 30 weight oil, it's a 10 weight oil.

Conversly it's reasonable to see where a less viscous (thinner) oil could be made to have the lubricating properties (not viscosity) of a thicker oil at higher temps. I've drained a lot of hot 10W30 over the years that was WAY thinner hot than when it went in at room temperature.

I can't do it today because my car club is finishing up a big three day show but I will test the flow of 10w30 vs the flow of 30 weight at the same temperature. I'll make my own Zahn cup and prove to myself at least whether I'm full of BS or not.

Tiny,

I don't think anyone is full of BS. This is my uneducated explanation on this slippery subject.

We take an oil and measure its viscosity at zero degrees, and assign it a grade of 10 weight.

We take that same oil, heat it to 210 and measure its viscosity...wow, pretty thin. Maybe a 2 weight...way too thin for any automotive use.

We take that same oil and pump it full of viscosity index improver's and do the same test again.

It still flows as a 10 weight at zero, but when heated to 210, because of the goodies in it, it does not 'thin out' as much, or loose its viscosity. Now it measures as a 30 weight at 210 degrees.

This is a multigrade oil.

The parameters for determining viscosity at zero are not the same as those for determining the viscosity at 210 degrees.

As the oil is heated and tested, it doesn't get "thicker", it just doesn't thin out as much as a oil that has not been modified by the chemists.

STP is a VI improver in drag. When you are playing with your Zahn cup, be your own chemist and mix some of into your concoction...you might invent a new oil.


Stuart

Good explanation, Atomarc.

Now for the pressure question, a full flow 235 should not have a really low pressure at idle. There is something not right and it is not the gage.

Go buy a mechanical oil pressure guage and hook it up. Then test your pressure.
It only takes about 5 lb's. of oil pressure to float the crankshaft. Make sure you at least have that. I don't know much about the 235 oil pump, I do know that other oil pumps I have worked with use a built in oil bypass valve. If the 235 has one it is possible it is stuck partialy open. Other than that cam bearings are the usual culpret for low oil pressure when it is not the pump.

Here's an explanation of oil vicosities- http://www.performanceoiltechnology.com/saeviscosityclassificationsandmanf.requirements.htm
and here http://www.advanceautoparts.com/ENGLISH/youcan/html/ccr/ccr20040601ov.html

I found the same link and it states that multi viscosity oils, in this case 10W30 is a 10 weight oil that has VIs added to provide 30 weight properties at higher temps. Not a 30 weight that performs as a 10 weight at lower temps as stated above. That's been my whole point all along is when one puts 10W30 in their vehicle they are putting a 10 weight oil in a vehicle designed for a higher weight oil. Yes, it performs equivelant to 30 weight at higher temps but it's too thin during the warm up period. I did discover I was wrong about the oil when it does warm up. It seems the viscosity is equivelant to 30 weight at high temp although my personal experience suggests otherwise.

Tiny,

What is the question?

If I put 10W30 in truck #1 and straight 30 weight in truck #2, warmed them up and took oil samples from each, according to Hoyle they would be identical.

If I did the same experiment with the two trucks, but didn't warm them up before I took the samples, truck #1 would test as a 10 weight oil, and truck #2 would test as a "who knows what", but certainly heavier than a 30 weight.

Is this how you understand it! I think this is how it works.

I guess I don't understand what you don't understand.

Stuart

Golly folks some of these replies have me worried! I have oil pressure, it never goes down to zero, and the engine runs smoothly.
Suddenly I get the impression I am driving a time bomb engine-wise! Say it ain't so! I do suspect the gauge and am looking into it now.

olblu49, it's been my experience that just about any engine will have higher oil pressure when first started. After it reaches operating temperature it will decrease somewhat, how much would be depending on oil viscosity, engine wear and a number of other factors.
I think Your heading in the right direction with checking the accuracy of Your current gage.

I found among my spare parts another stock OP gauge and I am going to try it now. Of course I realize it could be inaccurate but will see if there is a difference.

@ olblue:

As said, it is normal that your pressure gets down when the engine warmes up, but as the oil get`s thinner when hot the pump can deliver more volume, so you are on the save site as long you got at least 5 lbs. - enough to flow the crank shaft.

At the crank shaft bearings is much more pressure, than you read at your gauge anyways.

Frank

Don't let Pascal know that!


Stuart

Low oil pressure and straight grade vs. multi-viscosity oils subjects revisited....
Maybe it's time to make a tech article on these subjects.

This is really the best and most comprehensive link including test data, I ever found ... Motor Oil Bible

Sufficient oil pressure is mostly important at higher rpm.
The manual of late 235s suggests 35 lbs. @ 2000 rpm.
If you get just enough oil to the rocker arms and the push-rods are spinning at idle and you have the correct rocker arms for your engine block year - you are good!

The pressure can drop a little further in the heat of summer in standing traffic or at prolonged high rpm.
If you really would have too low oil pressure, you would know it very quick....!

I have been driving a late 235 daily for 11 years and am amazed how super-durable these engines are.

If I only knew. Seriously though I had two items I was having trouble getting my mind around. The first was your post stating that multi-vis & straight weight would both have the same viscosity at 210 degrees. As it turns out you were right & I was wrong. (dang I hate it when that happens ) The second was Longbox's post that 10w30 is 30 weight oil made to act like 10 weight when cold. That goes against everything I learned in chemestry and physics. Liquids just don't get thinner as they cool (unless you get to absolute zero where some truly strange things happen). As it turns out I was right on that one & the post was probably a transpositon typo. No big deal, my feeble mind just wasn't computing the incoming data. The system was locking up. :rolleyes:

As the fellow from the state that looks like a mitten has been heard to say..."all is golden".


Stuart

An update. I installed an I could get was 0-100 PSI but it is a wide sweep so the notches are far enough apart to read easily.

When I first started it up it took a second or two to get a reading. The capillary tube is almost clear and I could see the oil coming through. It was empty up till then obviously.

The increments read 0 - I - 25 - I - 50 etc. I figure the notches between the numbers are about 1/2 of the PSI. So the first would be 12.5 more or less.

With operating temps normal and driving 30 mph in 3rd gear I was reading just above the 25, quite near the notch (37.5?). It never got higher than that.

When I slowed down and stopped sometimes it would be below 25, closer to the 12.5 notch. It never went below that.

Similarly so when I geared down to second to go around a corner the pressure dropped.

As an after thought I dug out my 49 Chevy owners manual. It is set up for a 216 with the gauge reading 0-15-30. To quote it says that 15 is normal when driving and if the oil is thin and the operating conditions are warm, the needle will drop below 15 at a slower engine speed. At the same time if the oil is thick and the temperature is cold it can be normal for the needle to as high as 30.

So that is where I stand and further comments, suggestions or words of encouragement would be appreciated. And I also figure that my old gauge was fairly accurate.

Bill

Well, Tiny, I have to admit that I was wrong. Seems that what I was taught wasn't right after reding the various sites on oil vicosity.
I will agree, though, that these engines do seem to prefer straight 30 over a multi-viscocity oil. Actually, I'm running 30wt Valvoline Super HPO in mine, mainly due to both a feul dilutuion issue and a low reading on the pressure guage.
I am looking into a possibility of running full synthetic 30wt, going to do some more research before I make the switch.

I've been looking for a straight 30 weight synthetic. If you find one let me know what & where it is. I know there's debate there too but I'm a true believer in synthetic over petro oil. All I've been able to come across are multi-vis synthetics, 0w* - 5w* & 10w*. As I said I don't like putting 10 weight oil in Sweetie.

Royal Purple is the only one that I've found that makes a straight 30 synthetic. Should be able to get it from Napa, though they may have to order it in for you. For myself, I've got a local parts store that can get a better price on it, I buy almost all my parts from them.

Thanks man, I'll check it out.