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Discussion Starter #1
Hi there.
By using the search engine in this Forum, I read through many of the threads on this same subject. I want to avoid redundacy.

This car has a poor history (145K miles), not well maintained over the years. With the car in Park, I ran two trials and took the following readings on the oil pressure using an external gauge I set up in place of the sending unit (see attachment):

From what I've learned from other posts/threads, it's apparent that my main bearings have "clearanced" themselves over the years; as the oil heats up, it slips through easier, reducing pressure.
My next step is to replace the 10W/40 with a straight 30 oil. This until, I have the engine rebuilt, maybe 2-3 years away.
Please review the data and comment. Am I proceeding correctly?

Thank you
 

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I'd say you're on the right track. Your car is on the borderline of acceptable Low oil pressure (10 @ running temps). I think the 30 wt will bump it up some and give the bearing surfaces a better cushion of oil. I would think about 10w30 for the winter. Straight 30 wt would be too thick for Conneticut in the winter.

This goes without saying - Take it easy on the old, tired motor or you'll be rebuilding sooner than you'd care to.

Summary of Joz's Spread Sheet Data (in case forum members can't open the xls file -or are afraid to:gringreen)...

"Summary of Observations:Pressure remains at 50 until the fifth minute.The idle slowly climbs as the engine heats upOil pressure reaches a steady-state low of 8-10 after about 30 minutes"
 

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Discussion Starter #3
1967 mustang oil pressure too low.

Thank you "Sick467".

Yes, i am taking it easy on this old engine. In fact, I rarely drive it . I just got it registered and insured about one year ago. I must have put no more than 200 miles on it since then. I'm trying not to further damage the damage that's been done. But I am getting new tires for it next month. I want to drive more. Whatever, I won't dirve this car in any snow next Winter. I'll turn her on once a week or so, for warm ups ....so ...thanks for your 10W/30 weight comment....and thanks for your comment about 10psi being the lowest acceptable pressure this car should run on..I realize that's not good, but only "borderline" acceptable.
I'll post my test results on the straight 30 weight oll next. We'll see just what a differnece this makes.
 

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You could put a new set of main and rod bearings in the engine without pulling the engine. That might help it last until the rebuild. It is a faily easy task and as long as the crank is not badly scored it will improve your situation.
 

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Discussion Starter #5
1967 Mustang - Oil Pressure too low

Thanks PaulS...bear with me here, I'm very much still learning.

So I can put in a new set of main and rod bearings, without having to ...I forget the term....re-bore/re-finish, the cylinders ? This is the kind of stuff I have to read-up on.
I'll add this ....while changing the oil pump last Fall, I had a good look up into the cylinders after I removed the oil pan. And there did seem to be evidence of scoring on the cylinder walls. I should have examined things more thoroughly at that time. I don't recall if I was able to actually "feel" the scratches or scoring marks..
 

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First off, I would like to retract my earlier statement and say that you need to go with a thicker oil NOT from 40 to 30, BUT from 40 to 50 wt. Sorry I'm oil dyslexic. Thicker oil improves oil pressure and the higher the number, the thicker the oil.

Now,

Paul makes a good point by saying you could change the Main & Rod bearings. These bearings are on the crank and have nothing to do with the cylinders. Simply put, the main and rod bearings are the Crank Bearings which is a common place for wear. Replacing them without at least polishing the crank is a temporary fix. The crank will have some roughness to it and the new bearings won't. That roughness will have ill effects on the bearing life, but the new unworn bearings will be thicker and improve your oil pressure and add some time before a full rebuild.

Beware that if the crank bearing surfaces are scratched/galled then replacing the bearings will last about 2 seconds. I'm exaggerating, but it would not be worth doing.

I would bump up my oil to 20w-50 wt and keep a close eye on the oil pressure. If it starts having moments of 0-5 psi - It's time for a rebuild.
 

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You don't say what engine but the Ford oil pressure spec was the same for all of them so its somewhat irrelevant. That spec was also at 2000 RPM and not at idle so its difficult to compare. I agree that 10 psi is possibly a bit low for idle but not necessarily indicating anything all that serious. 35-55 psi at 2000 RPM is the spec to compare against. I have owned more than one new car where it was perfectly acceptable for the oil pressure to approach zero at idle on a really hot day as long as it recovers to normal at 1500-2000 RPM.

Your idle speed going up as the engine warms has little to do with the oil especially with a 10W-40. With a thicker oil the viscosity might be slowing the idle when cold but a 10W is pretty thin for that to be happening. Its the engine warming up and running better that is raising the idle. That is why the choke had steps on the idle cam to close the throttle as it warmed up and keep the idle somewhat the same.

ANY engine will see the oil pressure drop as it warms up even if it has brand new bearings. A higher viscosity oil might raise the pressure but it won't be a lot. AND if you plan to go from 10W-40 to 30W you are going in the wrong direction. 10W-40 acts like a 40W when hot so going to a 30W will REDUCE your warm oil pressure if it changes it at all.

All that said, 145K miles is well past the mileage that these engines were ever expected to achieve before overhaul. And if yours was poorly maintained then its likely time to learn about engine overhauls. A new engine in 1920 was expected to last 20k miles; by 1940 it was up to 50K; by 1965 that was about 100K miles but still far, far below the 200k that owners expect of today's new car. As a kid in the 60s any car with 100k miles on it was expected to be on some fly-by-night used car lot. Much of the improvement in engine life has to do with the crappy motor oils that were used in decades past.
 

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Discussion Starter #8
1967 Mustang - Oil Pressure too low

Thank you for your comments..

Ivy..your comment made me check my spreadsheet attachment again.
I failed to include two additional data points. After the the 34th minute, I raised the idle to 1400 and 3000 RPM for about 15 seconds; I recorded O/P of 30 and 35, respectively. I recorded the same numbers in a second trial.
So your O/P comments make me feel a bit better.

Regarding the oil I plan to put in (I purchased it already), I have Shell Rotella T1 Straight Grade SAE 30 Heavy Duty Engine Oil. I could swear I read in another post here in this forum that "Straight 30" may help to bump up over all O/P.
Isn't "Straight 30" thicker than 10W-40 ? So isn't this what I should try? Sick467...you say 20W-50 wt. Shall I return the "Straight 30" and go with your suggestion? I have to think though, in this matter and these circumstances, there probably wouldn't be much of a difference anyway..
Btw..this is a 289 engine, and, my choke is screwed up as the butterfly stays open and doesn't close after the engine cools down...something else I have to look into. I probably didn't put everything back together correctly after I rebuilt the carb. So that's likely impacting my idle as well. Thanks for your explanation around Crank Bearings..
 

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The W numbers only describe how the oil acts when cold. The other number is how it acts when warm/hot which is defined as 100C - boiling water temp. At any warmish temperature a 30W will always be less viscous than anything of a higher number such as 40 or 50.

This is an interesting site I had not seen previously. I think its in Bolivia so some translation may occasionally be needed. It has all kinds of charts and graphs to explain it all. Motor oil graphs

One thing to note is that on the charts they show, at the upper (warm) end there really isn't all that much difference in the various grades in the grand scheme of things.
 
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