help! my 302 loses oil pressure once it warms up!

well back in october i pulled my o/t 78 f150 out of the field it sat in for a few years ... blah blah blah well i went through the truck with a fine tooth comb,and it runs really good! but now when i fire it up, it holds 40 psi oil pressure @ idle when cold, but as it warms up, the oil presure slowly falls to about 15psi @ idle when i put it to 2 grand it slowly comes up to about 30 psi and then as it warms up a bit more ,slowly falls once more. what could this be caused by? could i simply run thicker oil to fix it and if so what would you guys suggest? any help would be greatly appreciated cause i need to start using the truck this week and hope to get this problem sorted out . thanks guys

 

Sounds like the main bearings are tired.

 

What is normal oil pressure for a stock SBF?

 

If it were me and everything else seemed ok such as compression and the way it ran I'd drop the pan and put in a set of new rod an main bearings and a new oil pump. Quite often an old set of bearings gets acid etched from sitting for a long time with old oil in it and then when you start driving it hard the bearings go away.

But on an old engine with some miles on it oil pressure fluctuations like that are pretty normal too. The 350 in my 71 GMC does pretty much the same thing every day on the way to work. It had a fresh set of bearings with the rering job about five years ago so I don't get too worried about it.

 

Try a new sending unit before you start tearing it down.Small block Fords are notorious for that.

 

I've been told that if U have 10 lbs for every 1000rpm then U R OK

 

Sounds like time for new bearings. To do it right-you will have to pull the motor and rebuild it. Not only will the crank need turned-but you should have the bottom end align-bored. What happens with the sbf is the lower end will run hot when the oil pressure is low and warp the main caps. I found this out the hard way when I rebuilt my first sbf many years ago. alex

 

Sounds almost like mine. Freshened up a 351W for my 47 quite a while back. Machine shop I dealt with, who is also a distant relative, when discussing the bearing clearance, suggested that the high volume pump I was using would supply adequate oil to the bearings. The clearance were towards the high side of allowable, using standard sizing. My oil pressure drops to 10-15 at idle when hot; I have about 35000 miles on the fresh-up so far. Further to the machine shop discussion, I normally would have went .001 or .002 undersize bearings to get the clearance tighter, but they weren't in stock, so talked me into standard, and thus the lower oil pressure for the last dozen years.
Still going.

 

IMO this yells obstructed oil pickup....

Either pieces of a nylon timing set or good old fashioned sludge....

 

I've had this situation twice with engines. There is a pressure relief spring in the oil pump, or in the adapter to the block. At high pressure this opens and works like a bypass. When that spring gets tired, and hot, it loses tension and bypasses at lower pressure. Mr Gasket used to make different tension springs to swap into oil pumps. I made the mistake of going TOO high tension, about 70 lbs at idle, and would pump the pan dry at high rpm. Easy to check and change. A buddy had the same problem with a 360 a few years ago.

 

an overhaul fixed by 302 last summer. Good pressure now.

 

Bearing clearance is gone. -or- Bearings are all scored up.

Beat on it until it Blows ! ! !

 

I think 40 psi @ 2000 rpm's

 

Check the pickup like others have said, but don't worry about it too much. Unless you're running boost or something, SBFs generally need "flow" more than "pressure". Lots of mustangs, F-150s, crown vics, etc. running around with totally worn bottom ends and no issues other than an annoying noise and a gauge telling you to stop.

 

ran a rebuilt sbc 307 for years-----held 60psi oil press at warm up, ran down to 15psi at a hot idle in gear, thousands of miles later no real issues

 

I have 4 in various rods and they all run approx. 25-30 lbs at idle and 40-60 on the road. Always higher then a standard SBC from my experience.

 

I have had three 5.0L mustangs in the past few years. All of them ran about 45-50psi and start up, and dropped to around 20-25 when hot at idle. I beat the crap out of those cars and the oil pressure never got worse and they always ran like a raped ape. I also had a lot of mods done to them. Heck i knew a guy when i was in welding school with an 89 Bronco with a 5.0 in it, and he drove that thing everywhere, and back out to pittsburgh a couple times a month and that POS had about 5psi of oil pressure for the whole year and half i knew him haha! A freshen up wouldnt be bad anyway, but u still have life in that thing

 

Im a chevy guy and 15 is normal for smallblock chevy's ,Guages are not that accurate to begin with ,If your worried .Do a compression and that will tell you everything............. AS long as your at 15 your ok any lower Id be worried ......If I recall right your ford distributor shaft is like a screwdriver and fits into a female shaft on pump .So there is really nothing that could go wrong ......

 

Compression check won't tell you a damn thing about bearing condition, the main source of oil pressure going away when hot.
Ford's use a hex shaft from the distributor to the oil pump, not a flat blade like a Chevy.

 

First of all, tell us what oil you did change to....If you put some 5W20 in there it is damn sure going to drop when it warms up, even in this weather.

How many miles are on that 78 model of truck you got, and what engine have you got in it?

The size of the engine really isn't going to be your guide though. All of that other stuff that's been offered up??? There's ways to check for those before you even think of doing a tear down. How about doing a leak down test first, not a compression test, to find out the condition of that engine.




HellRaiser

 

You don't have a Fram oil filter on it do you? if so try another brand of oil filter, reguardless change the filter out.

 

Just chasing around replacing parts till one finds the right one isn't really diagnosing the problem. It might be pretty expensive replacing a whole engine, only to find out a person only had a bad spark plug. Not that it's a spark plug in this case, but just replacing parts, is a piss poor way to call ones self a mechanic. IMO




HellRaiser

 

But a leak down will only tell him if it has bad rings or valve guides, nothing about bearing condition which is the most likely cause of his problem.

 

exactly what I think! he needs to run a thicker oil and baby it.

 

Symptoms of Faulty Engine Bearings

Problems with engine bearings will usually result in some sort of knocking noise coming from your car's engine. The type of knocking noise that you hear will use the usually give you a good idea of which engine bearing is faulty or needs to be replaced. So, here is how to differentiate between the types of knocking sounds in order to determine which engine bearing is bad:

• Bad Main Bearing Sounds - If you hear knocking sounds when you start your car engine that continue while the engine is running, and the sound is also coupled with low oil pressure warnings, this is a good indicator that your vehicle's engine has bad main bearings.
• Faulty Rod Bearing Sounds - On the other hand, if the knocking noise has a sound that is more akin to striking a piece of tin or aluminum and increases in intensity as you accelerate your vehicle, the most likely culprit is a faulty rod bearing.

I stand by what I said earlier, there are other ways to determine if it is the bearings going bad.
A leak down test is a "Beginning" on diagnosing a engine problems. A person goes on from there, with other tell tale means of determining what's wrong, before just jumping in and saying that's what it is, without really knowing what it is.

IMO


HellRaiser

 

Years and years ago, I $150 dingle-berry honed/Krylon overhauled the 260 that was in my old '64 Ford Fairlane 500 S/W. Ooooop's, did do a valve job...

Crank had stock bearings in it and the journals had ridges around them that you could feel dragging your thumbnail across them.

Anyway, I did my cheap overhaul using stock bearings and rings and it was funny because the oil pressure never was worth a sh*t even with 10w40. Summertime, the oil light would flicker on and off at idle but once up in rpm's driving, she was fine.

Now all this said, I NEVER beat on that old car at all and I drove her like 10 to 11 years and to this day, I miss it!!

pdq67

 

if the tappets don't bleed down while motor is running,it should be ok. i'd pull the pan and clean it out, and replace the oil pump pickup just to be safe.

 

the cheapest way out is to check the oil pump pickup like everyone is saying. Ford is definately notorious for this. I had a beater "home depot" truck for a while, a 91 F250 with a 351W. ran good, but i gradually lost oil pressure until it went on dead "0". i thought the bottom end was wooped. I pulled the oil pan, took out the pickup tube and it was completely blocked. I soaked it in parts cleaner, made sure there was no obstructions left in it, threw in a new oil pump and it was back to 50 psi at start up. But like a bunch of guys said, doing a compression test isnt going to give you results your looking for. Its the bearings, not the rings or valves. You can have a garbage bottom end and still have good rings and valvetrain.

 

If you are using the factory gauge put a good mechanical gauge on it and see what it reads,the oil light started comiing on in my daily driver 89 crown vic and it turned out to be the sending unit.

 

Well the first thing I would do is change the oil and use a good oil filter . I recommend using K&W or a WIX racing oil filter . Then use a good quality oil pressure gauge and see what it reads at idle at about 800 rpms and then at about 1500 rpms driving down the road and see what the oil pressure is . At idles should be no lower than 15 to 20 lbs -- then at driving at about 35 mph should be no lower than 35 lbs of oil pressure . When around 60 mph then should be no lower than 45 lbs .
I rebuild Ford engines a lot and you really just need a proper rebuild . Anytime I do a rebuild I ALWAYS check the bearings with a gauge and then the strips . If I am off by .001 , then I get a thicker set of bearings . You should be no more than .002 on bearings to get the correct oil pressure and for that engine to live ! Those are my standards and haven't had any problems yet . Just remember the more you hammer that throttle from a stop the more where and tear goes on the main and rod bearings so that difference of .002 to that .003 won't be long so that is why I stick to a clearance of .002 on my bearings . I also don't like the HV oil pump either on most stock and mild up grades . Really would make any difference . On the stronger engines I build I will use the HV oil pump . You also don't want to be so damn cheap on a rebuild and skimp on bearings . Pay for the good ones and your engine will last better for you . Using cheap parts will not save you money anyplace along the line . There are other ways to save some money but bearings is no the place to be cheap ! Also always change the cam bearings when you swap out a cam ! I have seen so many people do a cam swap with 25,000 + miles on their engine and that is a waste of a good cam . When you want your engine to last , use good bearings and rings . A cam you can save lots of money on ! Just because it comes in a Comp or Edelbrock box doesn't mean they are better than a Herbert , Mellings or a house brand name came . When the can is ground they go by specs and one is just as good as the other . Hell you can really save money and just have your cam re-ground . It will still perform as good as a new cam . That is how you can save some money .

Anyway I will say without looking at the engine or hearing it that you need a rebuild , bottom line . If that oil pressure is that bad , you don't have long before it goes .
Getting an engine rebuild doesn't cost that much unless you want that high prise machinist do your work . It's a street engine so just do your homework and get your parts . You shouldn't have $1200 to $1500 in the engine if you do the work yourself !
Just my opinion to think about .

Retro Jim