Precipitous drop in oil pressure at idle

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Referencing the factory gauge, after the car is at operating temperature, at a prolonged idle (~800 rpm) the oil pressure slowly, slowly drops until it is below the "L", well out of normal range. Ramping up the idle to about 1100 rpm brings it back to the normal, as does regular driving. But it always goes way too far down when idling. I'm currently running 10w30 that *might* have gone 1,000 miles and about one year since it was changed.

I'm bet the first reply is going to be "get a real pressure gauge", which is fine, and yes, the oil sender is relatively new (< 2 yrs old). But I read in a thread about dropping oil pressure from a while back that if the pressure is truly dropping, it's probably either one of two things: 1) the oil pump or 2) the <fill in the blank>. What's the 2nd thing? In the first case the oil pump is likely failing so replace it was the advice and although I don't remember what it was exactly, the second thing was a pretty drastic cause and solution. Anyone have an answer?

I saw the thread when I was just reading up for no reason in particular and it stuck in my mind and I can't find it again most likely because it was in the comments on the thread, not the subject (I think).

TIA,
 

Q1SVT

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I'm bet the first reply is going to be "get a real pressure gauge", which is fine, and yes, the oil sender is relatively new (< 2 yrs old).
LOL, close but no cigar ... But an oil Temp gauge might help.

Referencing the factory gauge, after the car is at operating temperature, at a prolonged idle (~800 rpm) the oil pressure slowly, slowly drops until it is below the "L",
It's a guess but the only thing is the motor is getting hotter & hotter [what is the water temp gauge showing as oil pressure is dropping?] That's why a oil temp might show oil getting hot... less viscosity as it heats up...

Other is an oil change with 20w-40 and see what happens ... if goes away or is slower to drop, then bearing/ other wear tolerances maybe.

jm2c -shot in the dark

edit... been a long time since I've used or thought about 351C connecting rods. Do/did you use the original Ford connecting rods? and you live in a desert too,
long idling in any weather but more so in heat the 351C rods have an oiler built into the big end to shoot oil up into the cylinder and the bottom of the piston directly across each other... 1-5 5-1, 2-6 6-2 etc ... oils cylinder wall and cools the piston. My moneys are on lowering viscosity and the pump cannot keep up enough... Living in the desert is one of the few times I would run 20w-50

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New does not mean functional, especially with electrical items. Less than two years old really means nothing. It is very easy to remove the sending unit and connect a mechanical gauge. Then, and only then, will you know for sure what your oil pressure is. The sending unit might be messing up as the oil warms up. With a mechanical gauge you can watch for fluctuations in the pressure when accelerating and decelerating the engine, indicative of worn main bearings.
 

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Does the resistance in the sender match the gage. An aftermarket oil pressure sender may not match the OEM gauge. Like Don says a known good gauge is the only way to rule out the sender/gauge vs mechanical issues. BTW, my hot oil pressure drops considerably from cold as well.
 

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While we’re on the subject; what is the normal oil pressure for. 351 once it’s warmed up at idle or under driving conditions?
 
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In my old days of street racing back in the 80's and 90's in mostly 5.0 Mustangs when your oil pressure started to drop to zero at idle one of 2 things had happened. You had blown a head gasket and your oil got full of water, or your bearings were gone. Bad oil pump would be a rarity, but what can happen on these old engines, especially if they have never been rebuilt, is that a ton of rubber and plastic can get sucked up into the oil pickup screen and partially block the oil flow. The rubber valve stems after a while harden and break off and end up in the oil pan, plus the plastic teeth from the timing chain will do the same. If the engine did not have regular oil changes they end up with a lot of oil that with time basically becomes like a rock, and it can also end up clogging the oil pickup. The first step would be to put a mechanical gauge and see where you are really at. I would say that you should have a minimum of 14-15 psi at hot idle. If with a mechanical gauge at idle you have some pressure but it suddenly drops to zero and then it goes back up as soon as you hit the throttle, it is time to pull the engine.
 
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Not sure if this typical, but on my car with mechanical oil gauge, it read around 70-75 psi running but when hot, drops to 35-40 at idle and back to 70-75 on acceleration. I run 10W30 Castrol conventional oil in a 351C4V M code.
 
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LOL, close but no cigar ... But an oil Temp gauge might help.


It's a guess but the only thing is the motor is getting hotter & hotter [what is the water temp gauge showing as oil pressure is dropping?] That's why a oil temp might show oil getting hot... less viscosity as it heats up...

Other is an oil change with 20w-40 and see what happens ... if goes away or is slower to drop, then bearing/ other wear tolerances maybe.

jm2c -shot in the dark

edit... been a long time since I've used or thought about 351C connecting rods. Do/did you use the original Ford connecting rods? and you live in a desert too,
long idling in any weather but more so in heat the 351C rods have an oiler built into the big end to shoot oil up into the cylinder and the bottom of the piston directly across each other... 1-5 5-1, 2-6 6-2 etc ... oils cylinder wall and cools the piston. My moneys are on lowering viscosity and the pump cannot keep up enough... Living in the desert is one of the few times I would run 20w-50

View attachment 60753 View attachment 60754 View attachment 60755

I've never opened the engine so I can't tell you anything about the connecting rods... as far as I know, at 137k miles, all the internals are original. Your idea about a different oil is easy enough/low cost enough to check out, but while yes, I'm in the desert, we've only flirted once with 90 degrees so far this year. It's mostly been mid-60s to low-80s since last summer. Engine temperature-wise, the gauge barely registers the lowest end of the scale and I have a 180 degree thermostat.
 
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New does not mean functional, especially with electrical items. Less than two years old really means nothing. It is very easy to remove the sending unit and connect a mechanical gauge. Then, and only then, will you know for sure what your oil pressure is. The sending unit might be messing up as the oil warms up. With a mechanical gauge you can watch for fluctuations in the pressure when accelerating and decelerating the engine, indicative of worn main bearings.
Thanks, Don -- that was the 2nd possibility in the thread I couldn't find again: main bearings.
 

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My newly rebuilt bottom end was giving me symptoms like this on 5w-30 (Amsoil Signature, one of the best film strength oils you can buy). I have a mechanical gauge as the electric is just too slow for safety. I had asked my builder about sleeving the lifter bores but he thought it was un-necessary. Turns out I knew more than he did. Anyway, rather than strip it & have it machined I temporarily ran 10w-50 which was ok until it got really hot when it would still drop to 10psi at idle. When I had a spare weekend I pulled the motor & swapped the stock pump for a high volume one & built myself a front to rear (gauge ports) oil line. Now I can run the 5w-30 again & pressure is good. If I pull it again it will get lifter sleeves & fully grooved mains at #2 & #4. By the way, don't run the hv pump with a stock pan as at sustained high speed it can fill the top of the engine quicker than the oil will drain. If you have an electric gauge you will wonder why hydraulic lifters start going ticker-ticker and THEN you will see low oil pressure!
 
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Finally got to putting in a mechanical oil pressure gauge. Getting that sender out would have benefitted from a 1" shorter wrench, for sure, but tediously, got it out. Got the new mechanical gauge installed and just ran the hose out the back of the hood and into the interior via driver's window. And then, went for a drive. Omg.

87 degrees this afternoon and running Pennzoil 10w30. At startup from cold, a steady 42-45 lbs of pressure at idle. In gear, held the 35-40 lbs range until the first stoplight (1 mi from the house), then the stopped at idle pressure dropped to about 18 lbs and stayed there until the (factory) temp gauge registered at low end of middle band (that's as hot as it gets). At each subsequent stop, about 2 lbs less until finally bottoming at 7-8 lbs of pressure at idle. Minimal rebound in pressure at higher revs.

7-8 pounds was the bottom of the range; at 50mph, ~2200rpm, pressure steady at 10-12 lbs. If I get on it, pressure up to 20 lbs but only momentarily.

So -- I think this eliminates the gauge and sender as issues, leaving me with either a failing/clogged pump or worn main bearings. Agreed?

An aside regarding mechanical gauges: there's a couple of great Youtube videos demonstrating temporarily installing a mechanical gauge. So I went to find a gauge and the plastic hose kit with brass fittings. I could not find a decent priced gauge ($75-$80) AND hose kit ($15) on the same website, and I didn't want to pay shipping from two different places. Then I found here in a forum a reference to the gauge and hose kit with multiple brass fittings from Harbor Freight on sale for $29.99. And Harbor Freight is closer than either of the two other auto parts supply vendors.
 
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It is time to pull the engine. I would not run it anymore if I could avoid it, you could have a catastrophic failure with that low of an oil pressure. Minimum oil pressure you need is 7-10 psi per 1000 RPM. At 20 psi you should never get anywhere over 3000 RPM. At 2000 RPM minimum is 14 psi, 10-12 at 2200 is well under that. You will eventually end up having a spun bearing and causing damage to the crank, or worse. If you must run it, see if you can get a 20W-50 oil, or maybe even a straight 50 weight oil with some STP additive or any of those oil additives that are as thick as honey.
I would pull the engine and get the oil pan off to see if the pickup is just full or junk, you could get lucky and the pickup could be clogged and just cleaning it may get you back on the road. If I already pulled the pan I would pull the rod caps and main bearings caps to look at the bearings, if they are fine, I would drop in a new oil pump and put it back together. Being that you have an engine with 140K miles, you are probably in need of a full rebuild.
I am going to put this here, but most will probably be horrified by the thought of doing what I am going to tell you that you can do. But if you are short of funds, it will work. If the bearings are gone and the crank is not in too bad of a condition, you can pull the crank and polish it with emery cloth to remove small imperfections, or you can polish it while it is still on the engine (have to be real careful if on the engine as you do not want the metal particles in the engine), get new bearings of the correct size (your engine should be std/std as from what I can tell it has never been apart but check the back of the bearings) and front and rear seals and put it back together and get it back on the road. Just make sure to keep everything real clean. Best thing obviously would be to tear everything apart and send it to be completely rebuilt at a machine shop, but sometimes funds do not allow for that. I have even done this on just one rod bearing that has gone bad.
In the late 80's I had a modified street race 300ZX turbo, it ran high 12's/low 13's back then. One day I got an obvious bearing noise from the bottom end. I went to the dealer and bought some std rod bearings, took the oil pan off the car (on those cars you could take out the oil pan with the engine in the car), found the bad rod bearing by basically feeling the rods, one had quite a bit of play. Removed the rod cap, removed the bearings and polished the crank with some emery cloth as best we could from underneath the car, then put the new bearings in and torqued the rod bolts back to spec, and we were back racing the next day. Sold the car, probably a year later, never had another issue with that bearing. Yes, it is not the correct way to do it, but when you are young and broke, you make do.
 

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You've definitely nailed down an engine low oil pressure issue. I remember back when you had the water pump bolts break in the block, the engine was out of the car. Did the mechanic/machinist not look at the bearings?


71PJ's suggestion is a valid one, lots of "in frame" bearing replacements were done back in the day. The 71-73s are more challenging due to the non-removable engine crossmember.

IMO, I'd drop the oil and go straight for Mobil1 15W50 and see what you get for hot oil pressure. If you get an improvement, then it might be enough for you to get through the summer.
 
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When you accelerate the engine the crankshaft makes contact with the bearings in a different location than it does when the engine is run at a constant speed or when decelerating. You can use this, sometimes, to verify worn bearings when the oil is warmed up. Either accelerate the engine quickly and then let off, or run it at 3000 RPM and let off quickly, and watch your mechanical gauge. A momentary blip in the oil pressure as you change mode from accelerating, decelerating, and steady run generally indicates worn bearings.
 
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@Don C and @71ProjectJunk -- Your optimism that it might not be the near worst case scenario gives me hope, thanks. I am very limited by what I can accomplish just working in my driveway. If only I had a lift ;)

@71ProjectJunk and @Hemikiller -- in other posts a change of oil has been mentioned as an interim option and it was on the trip to get Valvoline VR1 20w50 that I did my pressure testing yesterday. So I will change the oil and filter and hope I buy enough time to get it to the shop. VR1 should be an acceptable alternative to the recommended Mobil1, yes?

Hemi, no, the mechanic that pulled the engine and then sent it off to a heavy-duty metal worker didn't open up anything else; timing chain was already exposed for the water pump replacement (which begat the whole broken bolt fiasco). Have since found a classic-car savvy shop, which is were it goes next.
 
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I like to be as sure as I can before starting on something. I don't like pulling an engine, even though it doesn't take me that long. It's just that when I have the engine out I'm going to pull heads and oil pan, at a minimum, which often leads to something else. Bad old project creep. I pulled the engine out of my '71 M code to install a different cam and wound having to find another block due to hidden damage from previous owners.
 
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Whether it's a "Hallelujah!" or a "Very interesting...", changed out the oil and filter (from Pennzoil 10w30 to Valvoline VR1 20w50) and on first start running at cold idle: 78-80 lbs of pressure! Way up from 42-45. Next will be a test drive to normal operating temperature.

An aside: first time I noticed the oil pan plug/bolt was not in tight; wasn't hand-release loose but with the wrench on it spun pretty easily. It wasn't aggressively leaking, just forming a drop when I crawled under this a.m. Oil filter on the other hand... well, there's no reason for it to be THAT tight.
 
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Test drive results at normal operating temp: Pressure seems to roughly coincide with the earlier suggestion of 10 lbs pressure per 1,000 rpm once above idle (which never dropped below 12 lbs): 20 lbs at 2,000, 25 at 2,500, etc. I didn't get to over 3,000 rpm in my limited drive.

It's a temporary improvement; will have both the pump and main bearings checked late next week (soonest I could get in) but at least I can drive it there.
 
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