I have been driving my model A with a 54 331 hemi since 2019 on average 6 months out of the year. Last year I notice my oil pressure gauge reads 50-60psi on initial start up then after driving for awhile will drop really low after I Come to a stop. The pressure will drop to 5psi when I stop and sometimes will stay there for awhile even at 45mph. But sometimes itll go to 25psi. The pressure never drops as I accelerate. Only when I come to a stop. It won’t go back up and if it does only to 20-25 psi. So I have checked my wiring. I have changed the oil (Mobil 1 vr1 20w-50)and used different filters. I’ve added more oil, 7qts now. I’m using an electric gauge. My dash is out of a 49 Lincoln zypher and I’m using the original gauges. My system is a 12volts so I’m using a convert to run the gauge. My temp and fuel work fine. I think I’m getting good pressure. I would think the motor would be seized by now if I was at 5psi. I don’t hear a loud knock that would resemble bad rod or main bearing. Maybe a little knock at initial startup but goes away before I live the garage. No knocking after that. Granted I have lake pipes but a knock should still be heard right? tonight driving home my pressure was at 5 psi the last 2-3 miles home. It never went up even when I accelerated hard. Then I got home and after 30mins I went back out and fired the car up and the gauge went to 20-25psi and went higher as I rev her up in the garage. I’ve already changed the pump to one hot heads sold me. The motor has never been rebuilt. Had 86k original miles when I put it in my A. May have 100k now. I’m going to try a mechanical gauge this weekend. Any ideas would be helpful.
A quality gauge, autometer/SW, would be a good idea. Overfilling crankcase will only cause foaming of oil , not good for bearings.
You have Ford thermal/electric gauges. They are sort of "approximate" gauges. Before doing to much, go buy a mechanical oil pressure gauge, install it temporarily and drive the car. That way you know if your problem is in the engine or the gauge. I'm betting on the gauge.
Had a similar problem in my '40 pickup using stock gauges. Added a mechanical gauge and it showed that everything was fine. That should be your first step. Hope that is all you need.
If it does the same thing after installing a mechanical pressure gauge, your tolerances are probably smoked. I had a SBC act similarly coming home from Paso one year, 10-15psi at highway speeds, pulled over and checked the oil level thinking it might be low, no joy. Pulled the engine and the bearings fell out of the main caps. If you don't want to pull it apart, you could always increase the viscosity of the oil and hope it serves you another couple years.
I had a customer with a '50s Lincoln. His oil gauge behaved the same way. I wasn't easily able to get a sending unit for the gauge. I added a mechanical gauge for ease of mind, the oil pressure was fine. The original gauge sending unit was causing the intermittent low reading.
After verifying your oil pressure with a known good gauge, Rule of Thum ,minimum 10 psi for ever 1,000 rpms Wide clearances / wear on bearings will cause low or lost of oil pressure , Engine oil pressure will drop after oil is hot compared to cold Reading
You need to varify the gauge and sender. Bad connections could give inconsistant readings. Other than that it could be a sticking pressure relief valve in the pump.
Quick side story, I used to work at a Exxon in South Phoenix in the early 80’s. A lady pulled into full serve. “Check the oil ma’am”? I check, it’s probably 2.5-3 quarts low. I show her the dip stick, ask if she wants me to add some. She says “no, it’s not making that clicking sound yet”.
X-2 Sure sounds that way to me too. Had a worn out 289 SBF that would drop oil pressure so low the idiot light would 'wink' at me while sitting at a stop sign!! Drove it that way for many/many years........no prob. (It just scared the crap outta me every time I sat at a traffic light) 6sally6
Been there brother, have to go to neutral and bring the revs up a bit to keep the knocking spirits away lol. Never any fun. Once you start trying snake oil to fix the problem, it's time to rebuild it. I'd throw a mechanical gauge on it and tape it to the cowl just so you can see what's actually going on. Could also be the sending unit
Test it with a good mechanic gauge before you get too concerned. It may well be just the gauge or the sending unit, or the resistors and possibly the sending unit ground in the block. Over filling the oil pan can cause more problems then it can fix, but some of the older Hemi's had 6 quart pans. Is the dipstick reading over full with 7 quarts? If you did change to 20 w 50 and the gauge read the same as it did with Mobil 1, I have even bigger thoughts of it being a gauge problem, the 20 w 50 should have at lease increased the oil pressure reading on the cold motor. If the mechanical gauge confirms the inconsistent gauge readings, you have 2 choices: Pull the motor and fix it (an oil pump might solve the problem), or run it until it doesn't run, or makes so much noise it scares you. That last option may take a long time, or based on the pretty sudden change in the oil pressure reading, may not be very far down the road. Without really knowing what the actual oil pressure is, everything is just a guess.
Do not be scared of low oil pressure As long as the oil pressure has 10 psi off idle for ever 1,000 RPMs even a race Application,
If it is down to bearing clearances, running a higher viscosity oil or a viscosity modifier like STP will increase pressure. But adequate lubrication is more down to flow than pressure.
Gauge or sender. Agree get a mech gauge installed for a true read. Past street strip build, set the bearing clearances on the loose side, oil pressure ran about 30 at idle, when it started falling off 20, 10 - time for bearings. Crank was beautiful when I pulled it apart.
Thanks guys. I highly doubt it’s the bearings. Aside from zero knock noise at any rpm, how can the pressure be high then after some miles drop low at a stop and have trouble going back up? Is it how the bearings are spinning? When I changed out the pump I poked at the bearings with a pick and they looked firm in place. This issue has been since last year can an engine continue running with 5psi? Regardless I’m putting in a mechanical gauge today I’ll let you guys know. But just in case is a full rebuild necessary or just change the bearings? And who knows of someone in Arizona who can rebuild he is?
Does the oil smell noticeably of gasoline? May be a failing mechanical fuel pump. I'd try draining the oil and refill with a different brand of conventional 20w 50. And drop the oil filter and replace it with a diferent brand, maybe a Wix or a NAPA Gold. Cut the old filter open and see if it looks like there's anything "funny" going on inside.
I see from the many answers that there are two ways to approach this problem. The first way is to test and verify (mechanical gauge). The second is to guess and load the parts cannon. Both will work, but the second tends to be costly and a lot of work.
The bearings don’t get out of place they just get worn and have excessive clearance between the crank. Oil pressure is generally ok when cold but as the oil thins from heat it can’t maintain pressure due to leakage around the bearings. I have a Chevy motor that I know has more clearance than it should on the crank because I checked it. Cold about 70 psi hot about 20 at idle. Same scenario yours just probably has more clearance.
Before I went panicking about rebuilding the engine I would wait and see what your mechanical Guage says. I bet there’s not a thing wrong with the engine.
I just connected the mechanical gauges. At idle the gauge goes to 55-60psi and goes up with revs. This was also the case with the original electric gauge. I need to drive it around and after several miles I’ll know if my mechanical gauge starts reading low. as far as engine temperature thinning my oil, my car has no hood and our temperature outside is around 50*. Can oil thin out in 15 mins at that temp?
The description of the problem points IMO to a gauge problem, not an actual pressure problem. At an idle? Yes. Under a load, or at higher rpm's? No. Not typically, especially for a 20w-50 viscosity grade.
My race engine on street has 75ish psi Idle Cold with 50wt, 8qts pan , after hot 35ish @ idle , 2,500 50s psi & @ 6,500 rpms 70s psi
Hope it's fine but what was the history of the engine if you started driving it in '19? This sounds like about the right time frame to chuck bearings if it wasn't thoroughly cleaned out before starting. This is why so many harp on the whole "Old engine: Will it start?" or "I think I found a good one" fad. Maybe you pulled the pan and cleaned all the crud and dust powder out but, even if you did, 86000 is a little long in the tooth for a 50s engine.
Could it ever be a oil pump problem? I run one of those high volume pumps and see the same thing from time to time. I always was in the back of my mind that maybe these pumps dont last forever!!!
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