Low/fluctuating oil pressure, bad pump?

spoulson · November 2nd, 2009, 10:18 PM

Yesterday on the long drive home from my parent's place, I noticed my oil pressure started fluctuating between the half and quarter marks. A few times it dipped to a hair above red, then came back up. All this happened while cruising at ~70mph.

I verified with a mechanical oil pressure gauge. It does it when the engine is warmed up and has been driving for a bit. I saw steady 50psi at idle on cold startup, then as low as 20psi after warmup with the needle wandering a lot.

So, am I right in assuming the oil pump is failing? It's not very old, only a few thousand miles on a fresh rebuild but over the course of 8 years since it sat a lot. What is the likelyhood I'm facing engine damage?

Did a bit of searching and a high volume pump sounds like the way to go. What brand do you guys use?

C MY 4D · November 2nd, 2009, 11:00 PM

20psi at idle when warm isn't to bad. You should have 40-60psi @ 2000rpm when warm. If it is actually low I wouldn't place blame on the pump yet. It could be a spun bearing causing the pressure to bleed off. Double check your oil pressure.

baine21225 · November 2nd, 2009, 11:08 PM

im close to 40 at warm idle

spoulson · November 2nd, 2009, 11:25 PM

Sounds like I'll be dropping the oil pan tomorrow night to check the bearings. Anybody have the torque spec on the main caps?

What are some other symptoms of a spun bearing on an otherwise running engine?

C MY 4D · November 2nd, 2009, 11:27 PM

what is your oil pressure at 2000 rpm? I'm not sure you actually have a problem yet.

spoulson · November 2nd, 2009, 11:33 PM

I'm not sure that's relevant. What I saw was the stock gauge going from center of normal down to just above red and everywhere in between while I'm cruising steady at ~2300. I was on the edge of my seat the whole way home, ready to kill the engine if it dropped completely.

I drove around today until the fluctuation started, then hurried home, swapped in a mechanical gauge and idled it in my garage, since it's not mounted in the car. The gauge verified fluctuation down to 20psi immediately. I revved it a little (~2k) and pressure didn't increase at all. I didn't let it run for much longer.

baine21225 · November 2nd, 2009, 11:35 PM

fyi, the stick oil pressure gauge is pretty much an on/off switch

C MY 4D · November 2nd, 2009, 11:36 PM

Well sending units do go bad, that is why I ask. If it is a bearing the damage is already done. You might as well verify there is an issue before you go ripping into it.

spoulson · November 3rd, 2009, 07:02 AM

I have a new sending unit on the way just incase.

But, I must not be understanding what you're suggesting. Inspecting the bearings is my idea of verification. Observing oil pressure changes doesn't tell me if it's the oil pump or chewed up bearings (or both).

spoulson · November 3rd, 2009, 09:09 AM

Did some more searching around and this thread on Stangnet has some insight:

The following are more common signs of oil pump failure:

1. Steady decrease in oil pressure over time. Usually the result of gradual oil pump wear where oil pressure will be low at idle and at higher rpm. May also be accompanied by rod/main bearing wear.
2. Fluctuating oil pressure. Most common reason for a truly fluctuating oil pressure is a sticking oil pressure relief valve, but a fluctuating dashboard gauge is almost always caused by a bad sending unit that's sending false information to the gauge.
3. Sudden loss of oil pressure. Almost always the result of a broken or twisted oil pump shaft.

If you have a low oil pressure at idle but at least 40psi from 2000rpm upwards, the most likely cause is worn rod/main bearings. If an oil pump can generate at least 40psi of oil pressure from 2000rpm upwards it's definitely still good.

So I picked up an FRPP high volume pump and hardened driveshaft. I'll give that a shot. If no help, I'll inspect and plastigauge the bearings.

baine21225 · November 3rd, 2009, 10:23 AM

send it back and get a standard volume pump

spoulson · November 3rd, 2009, 10:43 AM

Uh, why? Sounds like general recommendations are for a high volume (not high pressure) to improve oiling.

spoulson · November 3rd, 2009, 11:05 AM

So I'm glad I started this thread because after digging a bit more, it appears you're right. I really should have a large capacity oil pan to support a high volume pump. So, I called Summit back and they swapped it for a Melling standard volume before it shipped. Thanks for the tip.

spoulson · November 12th, 2009, 01:51 PM

For the archives, I was able to complete the oil pump job with only dropping the oil pan against the crossmember. Jacking up the engine and lowering the steering rack was enough room to drop the stock pan. There's enough room to get the two pump bolts and pickup mounting nut, if you don't mind feeling like you did about 1000 crunches by the time it's done.

Also, I went with a FelPro rubber oil pan gasket. It was definitely worth the $28 paid at NAPA. Never before have I seen these plastic gasket holder pieces that screw in the oil pan bolt holes to hold up the gasket. The other end of the piece is a snap that lets you push the oil pan through it and snap into place so you can be sure the gasket is lined up and the oil pan is held up while you're putting the bolts in.

Despite my previous post, I actually did go with a high volume pump. After startup and test drive, both mechanical and stock oil pressure gauges showed normal pressure. (~60-70psi) After driving around a while, the stock gauge did not fluctuate at all. It stayed between the O and R of Normal the entire time. Before, it would start fluctuating as soon as it got up to operating temperature. Good enough for me.

Also, no issues like drained oil pan or high oil pressure as would be suspected with a high volume pump.

November 2nd, 2009, 10:18 PM	#1
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	Low/fluctuating oil pressure, bad pump? Yesterday on the long drive home from my parent's place, I noticed my oil pressure started fluctuating between the half and quarter marks. A few times it dipped to a hair above red, then came back up. All this happened while cruising at ~70mph. I verified with a mechanical oil pressure gauge. It does it when the engine is warmed up and has been driving for a bit. I saw steady 50psi at idle on cold startup, then as low as 20psi after warmup with the needle wandering a lot. So, am I right in assuming the oil pump is failing? It's not very old, only a few thousand miles on a fresh rebuild but over the course of 8 years since it sat a lot. What is the likelyhood I'm facing engine damage? Did a bit of searching and a high volume pump sounds like the way to go. What brand do you guys use? __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears

November 2nd, 2009, 11:00 PM	#2
C MY 4D Black Mamba w/ a New Box. Yeah Baby. 2003 Cobra 11.14@124.36 Join Date: Aug 2006 Location: Hampton, VA Posts: 3,018	20psi at idle when warm isn't to bad. You should have 40-60psi @ 2000rpm when warm. If it is actually low I wouldn't place blame on the pump yet. It could be a spun bearing causing the pressure to bleed off. Double check your oil pressure. __________________ MM Cobra Club Hater Club Founder..... Need work done to your car for a great price in Hampton Roads? Shoot me a PM.

November 2nd, 2009, 11:08 PM	#3
baine21225 MM Fanatic Join Date: Jun 2007 Location: Baltimore, MD Posts: 3,966	im close to 40 at warm idle __________________ "Whip out your dick you'll either get it sucked or get smacked..." "That's how Dad did it, that's how America does it... and it's worked out pretty well so far..."

November 2nd, 2009, 11:25 PM	#4
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	Sounds like I'll be dropping the oil pan tomorrow night to check the bearings. Anybody have the torque spec on the main caps? What are some other symptoms of a spun bearing on an otherwise running engine? __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears

November 2nd, 2009, 11:27 PM	#5
C MY 4D Black Mamba w/ a New Box. Yeah Baby. 2003 Cobra 11.14@124.36 Join Date: Aug 2006 Location: Hampton, VA Posts: 3,018	what is your oil pressure at 2000 rpm? I'm not sure you actually have a problem yet. __________________ MM Cobra Club Hater Club Founder..... Need work done to your car for a great price in Hampton Roads? Shoot me a PM.

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November 2nd, 2009, 11:33 PM	#6
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	I'm not sure that's relevant. What I saw was the stock gauge going from center of normal down to just above red and everywhere in between while I'm cruising steady at ~2300. I was on the edge of my seat the whole way home, ready to kill the engine if it dropped completely. I drove around today until the fluctuation started, then hurried home, swapped in a mechanical gauge and idled it in my garage, since it's not mounted in the car. The gauge verified fluctuation down to 20psi immediately. I revved it a little (~2k) and pressure didn't increase at all. I didn't let it run for much longer. __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears Last edited by spoulson : November 2nd, 2009 at 11:36 PM.

November 2nd, 2009, 11:35 PM	#7
baine21225 MM Fanatic Join Date: Jun 2007 Location: Baltimore, MD Posts: 3,966	fyi, the stick oil pressure gauge is pretty much an on/off switch __________________ "Whip out your dick you'll either get it sucked or get smacked..." "That's how Dad did it, that's how America does it... and it's worked out pretty well so far..."

November 2nd, 2009, 11:36 PM	#8
C MY 4D Black Mamba w/ a New Box. Yeah Baby. 2003 Cobra 11.14@124.36 Join Date: Aug 2006 Location: Hampton, VA Posts: 3,018	Well sending units do go bad, that is why I ask. If it is a bearing the damage is already done. You might as well verify there is an issue before you go ripping into it. __________________ MM Cobra Club Hater Club Founder..... Need work done to your car for a great price in Hampton Roads? Shoot me a PM.

November 3rd, 2009, 07:02 AM	#9
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	I have a new sending unit on the way just incase. But, I must not be understanding what you're suggesting. Inspecting the bearings is my idea of verification. Observing oil pressure changes doesn't tell me if it's the oil pump or chewed up bearings (or both). __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears Last edited by spoulson : November 3rd, 2009 at 08:00 AM.

November 3rd, 2009, 10:23 AM	#11
baine21225 MM Fanatic Join Date: Jun 2007 Location: Baltimore, MD Posts: 3,966	send it back and get a standard volume pump __________________ "Whip out your dick you'll either get it sucked or get smacked..." "That's how Dad did it, that's how America does it... and it's worked out pretty well so far..."

November 3rd, 2009, 10:43 AM	#12
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	Uh, why? Sounds like general recommendations are for a high volume (not high pressure) to improve oiling. __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears

November 3rd, 2009, 11:05 AM	#13
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	So I'm glad I started this thread because after digging a bit more, it appears you're right. I really should have a large capacity oil pan to support a high volume pump. So, I called Summit back and they swapped it for a Melling standard volume before it shipped. Thanks for the tip. __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears

November 12th, 2009, 01:51 PM	#14
spoulson Newbie 1990 Mustang LX 5.0 Join Date: Aug 2009 Location: Middletown, DE Posts: 46	For the archives, I was able to complete the oil pump job with only dropping the oil pan against the crossmember. Jacking up the engine and lowering the steering rack was enough room to drop the stock pan. There's enough room to get the two pump bolts and pickup mounting nut, if you don't mind feeling like you did about 1000 crunches by the time it's done. Also, I went with a FelPro rubber oil pan gasket. It was definitely worth the $28 paid at NAPA. Never before have I seen these plastic gasket holder pieces that screw in the oil pan bolt holes to hold up the gasket. The other end of the piece is a snap that lets you push the oil pan through it and snap into place so you can be sure the gasket is lined up and the oil pan is held up while you're putting the bolts in. Despite my previous post, I actually did go with a high volume pump. After startup and test drive, both mechanical and stock oil pressure gauges showed normal pressure. (~60-70psi) After driving around a while, the stock gauge did not fluctuate at all. It stayed between the O and R of Normal the entire time. Before, it would start fluctuating as soon as it got up to operating temperature. Good enough for me. Also, no issues like drained oil pan or high oil pressure as would be suspected with a high volume pump. __________________ 1990 Mustang LX 5.0, Edelbrock heads w/ rr, TF intake, AFM power pipe, AFM N-4, 30# inj, MAC headers/catback, AL driveshaft, 3.55 gears

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