What causes HIGH oil pressure? And what should I do about it (if anything)?
On my way home from Cars & Coffee last weekend my oil pressure went to the high end of the meter. It normally moves a bit with RPM ~ varying from about 1/3 to 2/3 of the meter range, but has always stayed well within the normal meter range.
After I got off the highway the pressure dropped back down, but it would still go right back up to the high end if I got the RPM up around 3k for any significant amount of time. At a stop / idle, pressure drops back down to normal ~ right in the middle of the meter.
Oil level checked out ok.
Oil condition is fair ~ dark honey color. I changed oil and filter around the middle of last summer, about 1000 miles ago, and topped it up a quart last month when I started driving it again. Valvoline VR1 20/50.
I checked the PCV and found it was sticking, so I hit it with some brake cleaner, then took the Elim out for a spin today. Oil pressure was still going to the high side, near the max line, but not past it.
Plan for tomorrow is to change oil and filter again and see if it’s possibly a filter problem, but I’ve never had this issue before so I thought I would run it by you knowledgeable folks!
My 70 351C also runs halfway into the very top gauge range on cold start and also above 2000 rpm warm. And it runs right at center of gauge when idling at 600 rpm warm. I put a mechanical gauge on it and measured 70 psi halfway into the very top range of the gauge, and 55 psi with gauge at center. Shop manual oil pressure spec for both our engines is 35-60 psi at 2000 rpm warm. But a little high is better than a little low.
Edit - my gauge tested normal with Bill’s gauge tester, and IVR tests ok as well. So I’m pretty sure my gauge is reading about where it should.
Thanks Craig. It just seemed odd to me that it changed readings / behavior all of a sudden. Usually when something changes from normal, that means something is wrong.
In your case it changed from abnormal to normal - lol! I wonder if your oil pressure sensor is flaky? Think I would put a mechanical gauge on it to see what you really have.
Mike, if you do try a mechanical gauge and the pressure is still high after changing the filter, you might want to use a racing filter that have a higher burst pressure and just go with it.
It could be that the by-pass valve in the block is stuck and holding pressure up.
Gauge is reading too high. If the wire was shorted it would read higher than this. Can’t rule this out. Use gauge tester to verify.
Instrument voltage regulator output too high. All other gauges (temp and fuel) would be uniformly too high. Rule this out.
Bad sender. If gauge tests good then measure resistance. Bad senders tend to fail open or stick at high. If it varies it points to the last option
Obstruction in oil path. The cheap problem is a failure in the oil filter. The expensive option is a spun bearing. Unless you were racing or spinning 6000 and this happened. Very rare.
If you take the sender apart you will find a very interesting electromechanical device. A diaphragm moves against a spring as pressure changes. The. Movement of the diaphragm causes a sweep arm to trace an arc across a wire wound resistor. When the spring fails it reads high. When the contact on the sweep arm fails it tends to fail open circuit. Sometimes the sweep arm wears so badly it can hook a winding on the resistor.
You are getting a ton of bad advice on FB. Makes for interesting reading.
No wonder that sensor is so large. Have you ever seen a failure mode that would suddenly shift the sensor’s operating output resistance range like Mike’s gauge indicates?
Not exactly at least as I understand it. The spring failure does something like this. I have seen gauges fail like this. After 50 plus years all kinds of things happen.
Isn’t that the truth! Its curious because my gauge tests ok using your tester, IVR is good, engine is rebuilt, pressure sensor is new and everything has been cross-checked against a mechanical oil pressure gauge. And my gauge registers the same as Mike’s does AFTER his sudden change. Seems like either something is wrong on my car too, or something on Mikes car was wrong BEFORE when his gauge was only reading 1/3 to 2/3.
There were at least two pressure sending units. if you have an OEM unit it will be stamped 80 or 90. The number represents the reading in PSI to register the high end of the oil pressure scale. Typically Cougars used 90.
Is it correct that the indicated pressure should normally stay out of the upper range (70-90 psi) box on the gauge? If so, mine is reading high too, and I gave Mike a bum steer. My auto parts store sensor could easily be an 80 psi unit. It’s amazing having so many people with such depth of Cougar knowledge on here!
LOL! Yeah, a wide range of advice, for sure! I expected that, and figured I’d have to separate the wheat from the chaff.
I changed the oil & filter today. Meter is still tending to move to the high end at higher RPM, but stays mid-range while just toolin’ around town. It hasn’t gone back into the “HIGH” box since I was on the highway last weekend, but today on the highway for a short trip it did hit the bottom line of the HIGH box.
I picked up a new sending unit, so I’ll swap that out sometime this week when time permits.
Thanks for all the advice fellas, I appreciate it!
FYI, I had to replace the “New” one on the 71. It was reading Zero, tapping it would make it read in the normal range. I had a original one I put in and it works fine.
While you have the sensor off, it’s super easy to screw in a mechanical oil pressure gauge to see what pressure you’ve really got. The gauge I have even had the correct size threaded adapter at the end of the hose.
I drove mine yesterday and it’s not indicating as high as what I had recorded when the engine was first rebuilt. Now the dash gauge just touches the bottom line of the high box (70 psi) on cold start and also above 2000 rpm when warm. Still runs right about middle (45 psi) at warm idle. Seems like healthy oil pressures that agree with the mechanical gauge.
Good idea. Everyone keeps saying check with a mechanical gauge, and it would be a good idea to have a solid reference value.
Question: I’ve had a few people mention a pressure relief valve in the pump that might be stuck. Does this sound like a reasonable possibility? And if so, what would that involve to try and unstick it? Maybe a light engine flush, and if that doesn’t work, drop the pan and repair / replace the pump?
Ford specs to the AMA in 70 show these normal oil pressures:
351W 35-55 psi
428CJ 45-65 psi
351C 50-70 psi
Looks like the Cleveland and 428CJ probably did use a little higher pressure spring.
Well, I replaced my oil pressure sending unit. Started up the car and pressure immediately went up to the high limit.
After warming it up pressure still stayed high even at idle, only dropping down to just below the high limit line.
Still need to get a mechanical gauge and do that check. I figure at least I’ll have a reference value for comparison.
Also need to look into replacing the oil pump too, from the sound of things.
