Help Oil Pressure Problems
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Freshman User
Joined: May 2004
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From: Alto, Ga
Help Oil Pressure Problems
'79 F-150 4x4
When I first crank up the truck, let it idle, the oil pressure is fine. Holds about halfway between L and H for as long as I want to let it idle. No Problem.
I start driving it down the road, pressure holds fine until I come to a stop sign or even just get behind somebody slow to where I can't keep the RPM's up. The pressure slowly drops at low RMP's. I give it a little gas, the Oil Pressure will go back up a little.
If I pull into a parking spot or something, the Oil pressure will bottom out by the time I get the ignition turned off.
Who's got some ideas?
When I first crank up the truck, let it idle, the oil pressure is fine. Holds about halfway between L and H for as long as I want to let it idle. No Problem.
I start driving it down the road, pressure holds fine until I come to a stop sign or even just get behind somebody slow to where I can't keep the RPM's up. The pressure slowly drops at low RMP's. I give it a little gas, the Oil Pressure will go back up a little.
If I pull into a parking spot or something, the Oil pressure will bottom out by the time I get the ignition turned off.
Who's got some ideas?
Post Fiend
Joined: Jan 2000
Posts: 5,860
Likes: 4
From: Metro Detroit (Redford)
I assume this is a real electronic oil pressure gauge since it is a '79. So pressure should be highest when cold, drop over the first 10 or 15 miles as the oil warms, and vary some with RPMs. But it sounds like you are getting some very low readings ("will bottom out by the time I get the ignition turned off).
What grade of oil are you running (5w30, 10w40, 20w50)? How many miles on the engine? Any other problems with the engine? Low power, smoke, noises, other? Gauge, sending unit, or wiring could be bad. For about $15 to $20 you can get a fair mechanical gauge. Hooks up pretty easy. Then see what is happening.
Not that I am going to have the answer, but if you post some of this info someone on the site should be able to help.
What grade of oil are you running (5w30, 10w40, 20w50)? How many miles on the engine? Any other problems with the engine? Low power, smoke, noises, other? Gauge, sending unit, or wiring could be bad. For about $15 to $20 you can get a fair mechanical gauge. Hooks up pretty easy. Then see what is happening.
Not that I am going to have the answer, but if you post some of this info someone on the site should be able to help.
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Freshman User
Joined: May 2004
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From: Alto, Ga
It doesn't make any noises.
It DOES burn oil.
The guy I bought it from said that he thought that the oil pressure problem was something wrong with the crank shaft (I don't remember exactly what he said but he was just guessing anyway).
I just had the sending unit go out on my Dodge Dakota and the gauge didn't act anything like the one on my Ford is. The Ford's will fluctuate with RPM's (at low RPM's), but my dodge just bottomed out, (scared the he** out of me), but then shot back up to normal.
I guess what I need to do is change out the sending unit, and if that don't fix it, I'll know I've got major problems. Right? But what could it be?
It DOES burn oil.
The guy I bought it from said that he thought that the oil pressure problem was something wrong with the crank shaft (I don't remember exactly what he said but he was just guessing anyway).
I just had the sending unit go out on my Dodge Dakota and the gauge didn't act anything like the one on my Ford is. The Ford's will fluctuate with RPM's (at low RPM's), but my dodge just bottomed out, (scared the he** out of me), but then shot back up to normal.
I guess what I need to do is change out the sending unit, and if that don't fix it, I'll know I've got major problems. Right? But what could it be?
Senior User
Joined: Jul 2004
Posts: 141
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From: Il.
check oil pressure with a mechanical gauge in place of the sending unit.if you verify low oil pressure and it is burning oil, crank could be bad(bearing journal wear) then excess oil from loose bearings will get on cylinder walls and get past the rings.
Posting Guru
Joined: Jul 2001
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You still haven't stated the engine or mileage but I will assume a high mileage Ford of some kind. There are far too many variables to be able to correctly diagnose the problem on the internet but the likehood of a clogged oil screen on the oil pump would be high on my list. I would suggest running a batch of either Auto-Rx or change the oil to Moble One to clean the engine out. You may even have to change oil several times in short mileage to get the engine cleaned up. Make sure that the PCV system is in good working condition. Make sure there are no internal coolant leaks. Do not use a quicky oil flush as it will strip the engine and do more damage than help. This is where I would start. And change the sending unit.
New User
Joined: Sep 2004
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Oil pressure issue 390 engine
I have a '66 ford 390 engine that has a similar oil pressure problem This engine does not have 3000 miles on it since rebuild. Pressure is great when started, and when under power, however, at idle, the pressure drops very low (mechanical guage). I am told, that the oil galley to the lifter is very large, and reduncing it will increase the pressure in the FE motors. Anyone evre heartd of this.
dmarx@voyager.net
dmarx@voyager.net
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Freshman User
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From: Alto, Ga
Well, I went ahead and replaced the sending unit. Cranked it up and started going down the road and the pressure shot up there where it's supposed to be. Pulled up to a stop sign and the pressure didn't drop like it had been. I drove around for a few minutes, stopping at stop signs, going slow, and didn't have any problems with the pressure. So I figured I had the problem fixed. A measely $7 part. But when I was pulling into my driveway, the pressure bottomed out.
So I went ahead and put a mechanical gauge in there and it reads fine. About 60 psi at start up, gradually decreasing to about 8-10 psi at a warm idle. I've heard some of you say that 351m's are notorious for extremely low oil pressure, so I figure 8 or 10 psi is ok? It sure seems low to me.
So I went ahead and put a mechanical gauge in there and it reads fine. About 60 psi at start up, gradually decreasing to about 8-10 psi at a warm idle. I've heard some of you say that 351m's are notorious for extremely low oil pressure, so I figure 8 or 10 psi is ok? It sure seems low to me.
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Post Fiend
Joined: Jan 2000
Posts: 5,860
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From: Metro Detroit (Redford)
8 to 10 psi at idle is a bit low, but what grade oil are you using? If you are using a Xw30, you could try a 10w40. That boosted my hot idle pressure about 5 psi from 16 to around 20-22. The other thing I read is that the oil pumps can wear a lot and then won't pump up as much pressure.
Elder User
Joined: Jul 2004
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From: Fredericton NB, Canada
I have to disagree on the oil pressure. Ford manuals and any Ford mechanic ive ever known states that a pressure of 10psi/1000 rpm is normal. Thats what mine is at, and I have more miles on the engine than you do.
-Matt
-Matt
Still kicking!!
Joined: May 2003
Posts: 6,061
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From: Kalispell, Mt.
I'll have to vote for the worn bearings myself. Any engine my brother has ever built for me, with even a standard oil pump, will idle at a minumum of 35-40# when warm. At 10-15 # pressure, I'm surprised you don't hear lifter or rocker arm clatter...but then again...hydraulic lifters will stay pumped up for a while too. I seriously doubt it's the oil pump. Methinks it's rebuild time and pretty soon. It could run quite a while if you take it easy, but I'd say the bearings are getting mighty thin.
Post Fiend
Joined: Jun 2001
Posts: 5,228
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From: Running Springs CA
10 psi at idle is not a reason to tear down the engine now, but start saving up for a rebuild. I'm with Paul, put some thicker oil in there for now. In GA you do not have arctic winters, so I would go for 20W-50 since it does burn oil.
Jim
Jim
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Freshman User
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From: Alto, Ga
I am already saving for a rebuild anyway, 150k+ miles on it, burning oil, probably getting less than 100HP, and now the low oil pressure. I think it's time. I've heard several people say in the 335 forum 10 psi is a usual oil pressure for these 351m's. But I will go to a thicker oil, as I am using 10w 30 right now, and maybe that will make it last a little longer, just in case I have some bearing wear.
Thanks for all y'alls input.
Thanks for all y'alls input.