Good morning all! I have ran into that I'm hoping the gurus can help with. I have a '74 with a rebuilt 302 with 500 miles on it. Oil pressure was constantly around 50PSI cold and around 45PSI once the engine warmed up with standard 10w30. I have put in 10W30 full synthetic oil and now I run 50PSI cold and once in gear and idling in traffic, the PSI drops to about 25 once warmed up and the engine seems to start to choke. Once I get back on it or put the truck in Park, the pressure jumps back to 50PSI. Does anyone know the cause for this? Is this abnormal? Should I be worried and do away with the synthetic oil?
Community
Buy / Sell
Bronco Tech
3 Arm Wiper Setup
Fix Motor Mount
Roll Cage Braces
Throttle Body 65mm
Wheel Alignment
Heat Riser Replacement
Vent Window Repair
Center Console Mount
Straighten Bumper
Ford 6R80 6 spd
More Tech...
Fix Motor Mount
Roll Cage Braces
Throttle Body 65mm
Wheel Alignment
Heat Riser Replacement
Vent Window Repair
Center Console Mount
Straighten Bumper
Ford 6R80 6 spd
More Tech...
-
Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!If you have problems registering or can't log into your account, please contact Admin.
Low oil pressure after synthetic oil change
- Thread starter ymelnic1
- Start date
Justafordguy
Bronco Guru
- Joined
- Sep 26, 2009
- Messages
- 6,253
Need to know a little more about the engine first. Is it all stock with the stock carb?
Synthetic is great oil. I know a guy that works for Mobile and he told me all about how much better synthetic lubes than regular oil.
Yours I take it is an automatic. So once in gear there is more load on the engine than when idling in park. So you probably need a little higher idle than you currently have.
Just FYI, oil pressure comes from main bearings (and rod bearings) clearance between the bearing surface and the crankshaft. If the clearance is too large (poor machining) or becomes excessive (from wear) between the crank and the bearing surface then the oil pressure will drop. Starting is the hardest thing for an engine due to the fact the bearings can be a little starved for oil when the crank first starts to turn. That is when you can get potential bearing wear. But also crazy reving of the engine can starve a bearing of oil and potentially cause damage.
Anyway, If you know your oil filter is not clogged and your oil pump is doing its job, and the engine is at the correct idle speed, then bearing clearance is the next suspect for low oil pressure. Once an engine warms, then the metal has expanded in there and so the clearances will be at their max, so that is why once warm the engine oil pressure might drop lower than when cold. The clearances we are talking are like the thickness of a human hair. That is the gap the oil film fills between the crankshaft and bearing surface.
You can always plasti-guage the bearings to tell where they are at in clearance. I've done that to engines I was rebuilding, say, installing a crank kit and you want to know what you got. A little tedious (time consuming), but when done you know how good your crank kit was.
I've also used plastigauge with my rifles to check the headspacing. At least on my old Enfields it works rather well to make sure the headspace is not too large.
http://www.plastigaugeusa.com/how.html
Yours I take it is an automatic. So once in gear there is more load on the engine than when idling in park. So you probably need a little higher idle than you currently have.
Just FYI, oil pressure comes from main bearings (and rod bearings) clearance between the bearing surface and the crankshaft. If the clearance is too large (poor machining) or becomes excessive (from wear) between the crank and the bearing surface then the oil pressure will drop. Starting is the hardest thing for an engine due to the fact the bearings can be a little starved for oil when the crank first starts to turn. That is when you can get potential bearing wear. But also crazy reving of the engine can starve a bearing of oil and potentially cause damage.
Anyway, If you know your oil filter is not clogged and your oil pump is doing its job, and the engine is at the correct idle speed, then bearing clearance is the next suspect for low oil pressure. Once an engine warms, then the metal has expanded in there and so the clearances will be at their max, so that is why once warm the engine oil pressure might drop lower than when cold. The clearances we are talking are like the thickness of a human hair. That is the gap the oil film fills between the crankshaft and bearing surface.
You can always plasti-guage the bearings to tell where they are at in clearance. I've done that to engines I was rebuilding, say, installing a crank kit and you want to know what you got. A little tedious (time consuming), but when done you know how good your crank kit was.
I've also used plastigauge with my rifles to check the headspacing. At least on my old Enfields it works rather well to make sure the headspace is not too large.
http://www.plastigaugeusa.com/how.html
Rustytruck
Bronco Guru
- Joined
- Feb 24, 2002
- Messages
- 10,875
You want 700 rpm with the automatic in gear and the brake on. When you changed the oil to synthetic did you change the oil filter too? Might try a different oil filter like a Wix, Napa Gold, K&N and Motorcraft. Try to stay with a premium brand filter rather than the house or cheep brand filters. Better filters have better construction inside. The difference is only a few bucks. 25 PSI at Idle is not an issue.
Justafordguy
Bronco Guru
- Joined
- Sep 26, 2009
- Messages
- 6,253
What is the RPM while it's in drive? You need at least 700 - 800 RPM in drive to keep the oil pressure up and keep the air moving through the radiator. If it idles smooth you probably don't need to mess with the mixture screws, you need to adjust the idle screw to raise the throttle.
Broncobowsher
Total hack
- Joined
- Jun 4, 2002
- Messages
- 35,713
Oil pressure varies with engine speed. Your engine that is now truggling to idle well (for whatever unknow reason at the moment), is bringing the speed down, which brings the pressure down.
Fix your idle issue and the oil pressure will return to normal.
Fix your idle issue and the oil pressure will return to normal.
25-50 psi is not low oil pressure.
bronconut73
Bronco Guru
- Joined
- Aug 7, 2012
- Messages
- 9,918
6 psi per 1000 rpm is what GM calls for.
I read that in Shop class in High School, so it must be true, right?%)
I read that in Shop class in High School, so it must be true, right?%)
To me synthetic oil in a flat tappet carbed engine is a waste of $$ it doesnt really provide anymore protection than standard oil and in fact probably less as older engines should have oil with higher levels of zinc todays synthetics dont have it. Of course most standard oil doesnt have it either.
Then comes the carb carbs dump lots of raw fuel into the engine it gets into the oil and dilutes it along with adding carbon and other contaminates. So basically at 3000 miles you have to change the oil anyhow the extra $$ for synthetic is not worth it.
My last bit of advice is your engine has not broken in yet 500 miles is nothing. I would wait for about 5000 miles before considering it to be broken in at that point if you really want to run synthetic then go for it.
Newly rebuilt engines tend to run hotter than normal maybe the viscosity of the synthetic is not quite that of the conventional oil at the same temp even tho its has the same rating that could account for the lower oil pressure.
The most likely cause of your oil pressure differances is that your engine was just rebuilt so you had assembly lube/grease ect in the oil possibly causing the higher oil pressure could have even been clogging the filter up some which would cause a higher oil pressure as well. So now with fresh oil and filter the pressure is what it should be.
Then comes the carb carbs dump lots of raw fuel into the engine it gets into the oil and dilutes it along with adding carbon and other contaminates. So basically at 3000 miles you have to change the oil anyhow the extra $$ for synthetic is not worth it.
My last bit of advice is your engine has not broken in yet 500 miles is nothing. I would wait for about 5000 miles before considering it to be broken in at that point if you really want to run synthetic then go for it.
Newly rebuilt engines tend to run hotter than normal maybe the viscosity of the synthetic is not quite that of the conventional oil at the same temp even tho its has the same rating that could account for the lower oil pressure.
The most likely cause of your oil pressure differances is that your engine was just rebuilt so you had assembly lube/grease ect in the oil possibly causing the higher oil pressure could have even been clogging the filter up some which would cause a higher oil pressure as well. So now with fresh oil and filter the pressure is what it should be.
Another thought is your gauge is off. This the factory dash gauge or aftermarket gauge? At times i've connected in a known good reading oil pressure gauge to verify pressure. Really so many variables to consider. I'm running a high volume oil pump in my newly rebuilt 302. My pressures are more like 40 at idle and 60 to 70 at 2500 rpm.
I dont think there are that many varibles. The basic fact is after oil change the oil pressure went down thats it. Considering its a fresh build and he did the first 500 mile oil change the old oil and filter probably just had lots of assembly lube in it which increased the viscosity and was plugging the filter a little so you get a pressure increase. Thats not to say that his guage is 100% correct but its doubtful its off from what it was.
Kind of lost site of the initial post and just started rambling about oil pressure 
.
This is a good read.
https://www.motorstate.com/oilviscosity.htm
.This is a good read.
https://www.motorstate.com/oilviscosity.htm
