Have oil pressure problem

1SILLYGT · 08-04-2001, 03:42 PM

I just dropped a rebuilt 302 bottom end in my 90 GT and put a pair of GT-40 heads on it. I have a 160 thermostat in it, but whenever she heats up I lose oil pressure. Can someone please tell me whats wrong.

Capri306 · 08-04-2001, 04:16 PM

First be sure you have a whole, fresh 5 quarts of 10W-30 or 10W-40 in there. Are you going by the gauge on the dash or a real one? How much pressure loss are we talking about? 5-8 pounds isn't that bad, but 15-20 is. It might also depend on the viscosity of the oil you're using. 10W-30 is almost getting too thin at op-temp for my tired ol' 5.0

If the clearances on the bottom end you "dropped in" were a little loose, that would explain a lot. I always triple-check bearing clearances before installing anything; it doesn't matter who built it since anyone can make a mistake.

------------------
Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch

302crew · 08-05-2001, 12:21 AM

ive been helping him with the car and its realy odd even thow its only the inacurate gauge in the dash as soon as it stops working the motor gets a knock but i doesnt happen when the gauge reads pressure and it only losses pressure when it gets hot and we also tried putting a brand new oil pump in it and that did nothing

exgmguy · 08-05-2001, 09:18 AM

It gets a knock when oil pressure goes away? Inaccurate dash gauge or not, sounds like your motor is toast.

------------------
1988 Mustang GT
11.8 @ 123 (7/14/01)
1992 BadAzz Wrangler
1993 Explorer

302crew · 08-10-2001, 03:28 PM

anybody else hae any clue the motor runs good and stong

Chevyguy · 08-10-2001, 04:14 PM

How low does it go? all the way to 0?? My stock gauge actually works pretty good, reads about 50 psi when crusing, drops to about 20 psi hot while idling at 750 rpm, reads about 65-70 psi when cold even idling. If the pressure only drops at idle may not be super bad, but the knocking thing is really not good. The problem probably lies with the "rebuilt" bottom end. Did you just drop a crank in?? I dropped a Auto Zone crank into my 200k+ miles 305 and I get more oil pressure than the Stang now. All the main and rod bearings were in the middle of the spec range, and I used a new pump. If you have a bad rod, or main cap, the oil press will be poor, or maybe a leaking galley plug, bad cam bearing etc....

------------------
Frank W
90 5.0 LX coupe Daily driver. Silencer removed, K&N filter. Flexalite fan, 3 core radiator. FMS flywheel and Clutch, FMS blue wires Energy Suspension end links
88 Notch 2.3L 5 speed Parts/beater car My two Notchbacks

74 Chevy Laguna Type S-3 305 Finally fixed getting a 454 to put in garage
<A HREF="http://www.chevellepages.com/folingo" TARGET=_blank>www.chevellepages.

PKRWUD · 08-10-2001, 04:14 PM

The only clue you need to understand is if it's knocking, there's a serious problem! Never rely on the stock gauge, but if your description is accurate, then the bearings are probably wrong. Lots of things can cause a knock, but when it's a "new" bottom end knock, and only happens when the pressure drops, I would suspect the bearings. In truth, the engine doesn't require more than 10 psi per 1000 rpm, but we don't know what yours is at because you've got the factory gauge. See if you can come up with some numbers, like what the bearing clearance was, and what the oil pressure is when cold and when hot, and especially when the knocking starts.

Take care
~Chris

------------------
Retired Moderator
MustangNet

My site: JimPorterRacing

RACECAR spelled backwards is RACECAR

HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

stanger8172 · 08-10-2001, 08:45 PM

I have a prob. similiar to yours.

When cold my pressure goes right to 52.

After the oil warms up, at idle, it'll drop to 0 unless I keep my foot on the gas to about 1200rmps.

My buddy says we should try a new sending unit.

My pan leaks in the sensor which detects the amount of oil in the pan. He says all 5.0's do that. I can smell oil after I have been driving it awhile.

My car runs great despite the pressure and leak probs which I watch religiously. Good Luck!

stanger8172 · 08-10-2001, 08:49 PM

Yes, my buddy says that the main bearings might be shot and I might have to have the crank re-ground but I am starting with the oil pressure sending unit and the oil level sensor first. Good Luck!

exgmguy · 08-10-2001, 10:21 PM

An inexpensive Autometer mechanical oil pressure gauge will do wonders.

We've all done the "maybe it's just....." bit before.

84LX89GT · 08-11-2001, 12:50 AM

My '84 has the same problem except without the knocking. Cold it has about 50 psi and goes down to about 10 or 15 psi when warm. It never varies with rpm (which is scary because i've revved it pretty high). I switched to 20W50 and the oil pressure is about 5 psi higher than when i had 10W30 in there plus it quiets the valvetrain down. Try putting a thick oil in there for now. Chances are if you can hear a knock, the low oil pressure from the main bearing(s) has caused there to be very little pressure to the rod bearings and you may have some munched up rod bearings as well as main bearings. For high performance applications i heard it's best just to get a new crankshaft with standard size bearings because as you go more undersize with the crank, the more likely you are to spin a bearing with it taking up more clearance where the crank used to be. Also, if you dig into your bottom end, clean off your connecting rods and see if any are discolored and replace any that have become dark or even black near the rod bearing end (if that's where the knock is from). I Hate to hear your misfortune.

------------------
previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT

08-04-2001, 03:42 PM	#1
1SILLYGT Registered Member Join Date: May 1999 Location: sloatsburg NY Posts: 28	Have oil pressure problem I just dropped a rebuilt 302 bottom end in my 90 GT and put a pair of GT-40 heads on it. I have a 160 thermostat in it, but whenever she heats up I lose oil pressure. Can someone please tell me whats wrong.

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08-04-2001, 04:16 PM	#2
Capri306 Moderator Join Date: Sep 1998 Location: Grand Rapids, Michigan, USA Posts: 1,001	First be sure you have a whole, fresh 5 quarts of 10W-30 or 10W-40 in there. Are you going by the gauge on the dash or a real one? How much pressure loss are we talking about? 5-8 pounds isn't that bad, but 15-20 is. It might also depend on the viscosity of the oil you're using. 10W-30 is almost getting too thin at op-temp for my tired ol' 5.0 If the clearances on the bottom end you "dropped in" were a little loose, that would explain a lot. I always triple-check bearing clearances before installing anything; it doesn't matter who built it since anyone can make a mistake. ------------------ Capri306, Moderator, The Mustang Works Online 1979 Mercury Capri, 5.0L -- C4 -- 2.73 1987 Mustang LX Notch

08-05-2001, 12:21 AM	#3
302crew Registered Member Join Date: May 1999 Location: sloatsburg Posts: 261	ive been helping him with the car and its realy odd even thow its only the inacurate gauge in the dash as soon as it stops working the motor gets a knock but i doesnt happen when the gauge reads pressure and it only losses pressure when it gets hot and we also tried putting a brand new oil pump in it and that did nothing

08-05-2001, 09:18 AM	#4
exgmguy Registered Member Join Date: Feb 1999 Location: Utica, Michigan Posts: 2,631	It gets a knock when oil pressure goes away? Inaccurate dash gauge or not, sounds like your motor is toast. ------------------ 1988 Mustang GT 11.8 @ 123 (7/14/01) 1992 BadAzz Wrangler 1993 Explorer

08-10-2001, 03:28 PM	#5
302crew Registered Member Join Date: May 1999 Location: sloatsburg Posts: 261	anybody else hae any clue the motor runs good and stong

08-10-2001, 04:14 PM	#6
Chevyguy Backyard Mechanic/Chemist Join Date: Feb 2001 Location: Acton MA USA Posts: 435	How low does it go? all the way to 0?? My stock gauge actually works pretty good, reads about 50 psi when crusing, drops to about 20 psi hot while idling at 750 rpm, reads about 65-70 psi when cold even idling. If the pressure only drops at idle may not be super bad, but the knocking thing is really not good. The problem probably lies with the "rebuilt" bottom end. Did you just drop a crank in?? I dropped a Auto Zone crank into my 200k+ miles 305 and I get more oil pressure than the Stang now. All the main and rod bearings were in the middle of the spec range, and I used a new pump. If you have a bad rod, or main cap, the oil press will be poor, or maybe a leaking galley plug, bad cam bearing etc.... ------------------ Frank W 90 5.0 LX coupe Daily driver. Silencer removed, K&N filter. Flexalite fan, 3 core radiator. FMS flywheel and Clutch, FMS blue wires Energy Suspension end links 88 Notch 2.3L 5 speed Parts/beater car My two Notchbacks 74 Chevy Laguna Type S-3 305 Finally fixed getting a 454 to put in garage <A HREF="http://www.chevellepages.com/folingo" TARGET=_blank>www.chevellepages.

08-10-2001, 04:14 PM	#7
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	The only clue you need to understand is if it's knocking, there's a serious problem! Never rely on the stock gauge, but if your description is accurate, then the bearings are probably wrong. Lots of things can cause a knock, but when it's a "new" bottom end knock, and only happens when the pressure drops, I would suspect the bearings. In truth, the engine doesn't require more than 10 psi per 1000 rpm, but we don't know what yours is at because you've got the factory gauge. See if you can come up with some numbers, like what the bearing clearance was, and what the oil pressure is when cold and when hot, and especially when the knocking starts. Take care ~Chris ------------------ Retired Moderator MustangNet My site: JimPorterRacing RACECAR spelled backwards is RACECAR HEY !!! Are you ASE Certified ??? If you are, ask me about iATN. The best tool you'll ever have, and it's free !!!

08-10-2001, 08:45 PM	#8
stanger8172 Registered Member Join Date: Nov 2000 Location: Milwaukee,Wisconsin Posts: 380	I have a prob. similiar to yours. When cold my pressure goes right to 52. After the oil warms up, at idle, it'll drop to 0 unless I keep my foot on the gas to about 1200rmps. My buddy says we should try a new sending unit. My pan leaks in the sensor which detects the amount of oil in the pan. He says all 5.0's do that. I can smell oil after I have been driving it awhile. My car runs great despite the pressure and leak probs which I watch religiously. Good Luck!

08-10-2001, 08:49 PM	#9
stanger8172 Registered Member Join Date: Nov 2000 Location: Milwaukee,Wisconsin Posts: 380	Yes, my buddy says that the main bearings might be shot and I might have to have the crank re-ground but I am starting with the oil pressure sending unit and the oil level sensor first. Good Luck!

08-10-2001, 10:21 PM	#10
exgmguy Registered Member Join Date: Feb 1999 Location: Utica, Michigan Posts: 2,631	An inexpensive Autometer mechanical oil pressure gauge will do wonders. We've all done the "maybe it's just....." bit before.

08-11-2001, 12:50 AM	#11
84LX89GT Mustangs Join Date: Jul 2000 Posts: 1,938	My '84 has the same problem except without the knocking. Cold it has about 50 psi and goes down to about 10 or 15 psi when warm. It never varies with rpm (which is scary because i've revved it pretty high). I switched to 20W50 and the oil pressure is about 5 psi higher than when i had 10W30 in there plus it quiets the valvetrain down. Try putting a thick oil in there for now. Chances are if you can hear a knock, the low oil pressure from the main bearing(s) has caused there to be very little pressure to the rod bearings and you may have some munched up rod bearings as well as main bearings. For high performance applications i heard it's best just to get a new crankshaft with standard size bearings because as you go more undersize with the crank, the more likely you are to spin a bearing with it taking up more clearance where the crank used to be. Also, if you dig into your bottom end, clean off your connecting rods and see if any are discolored and replace any that have become dark or even black near the rod bearing end (if that's where the knock is from). I Hate to hear your misfortune. ------------------ previous name: 84stangLX '84 Mustang 5.0 LX my LX '89 Mustang GT my GT