I have a 1978 460 in my '70 F100 w/ZF 5-speed. It is a fantastic combo. But I have a problem with the oil pressure (mechanical gauge and factory 5-qt. pan.

I installed new mains, inserts and oil pump a short while ago. I noticed that the oil pressure (60+ cold and 35 hot) would immediately fall off to almost zero if I 'stomped on it', then slowly recover to 35psi. Also, once the engine is at operating temperature, the pressure oscillates rapidly at idle. It seems to rise and fall about 5 to 6 lbs at a rapid rate around 10psi.

I searched this forum and found one topic that came close, but didn't mention the 'shaky' pressure at idle.

The engine will run all day at 35psi, but the rapid drop at heavy acceleration and the oscillating pressure bother me. I'm running 10W40 oil. The engine does not run hot, nor does it 'rattle'.

Monsterbaby mentioned, in one of the posts, that a percentage of the oil bypassses the filter. I would really like to know how this is accomplished, and whether it might be a part of my problem?

OldFords

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I wasn't aware of a 5 quart pan for a 460, but then all of mine have been 80's truck motors which are all 6 quart. Are you sure it's a 5 quart pan because you are describing not enough oil in the pan.

 

what led up to changing the bearings ? What did the journals look like ?

 

What difference would the condition of the bearings and the journals have on loss of oil pressure upon acceleration? The only thing that will cause that kind of lass of oil pressure is the oil not staying deep enough around the oil pickup.

 

I agree that the condition of the bearings and journals would have nothing to do with the shaky oil condition. It was only included as information as to the general condition of the engine. Didn't mean to confuse the issue.

OldFords
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Bear, I'm sorry for not responding to your post when I replied about the general engine condition. Somehow I missed your question.

The pan is standard factory. It takes 6 qts. if the filter is replaced.

Ther reason I did the bearings was because it was given to me condition unknown. Since it was on the garage floor, I pulled the pan and inspected the bottom end. The bearings had a lot of miles on them (brass showing). So I took the opportunity to replace them and install a new pump.

Interestingly enought, even with the high mileage, and the apparent lack of regular oil changes (super buildup of carbon around the pushrods, valves, etc.), the engine does not smoke or use oil.

I suspect (hope??) that I may have missed a cracked pick-up tube (probably very near the screen). Is there a possibility that, since the engine is so old, that someone replaced the pick-up tube with one that is a little shorter? Is there a difference between the front sump and rear sump pick-ups?

Since I use the truck every day, I won't be able to pull the engine for several weeks. So I'll just have to put a block under my 'Lead Foot' for the time being.

OldFords
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yep theres a difference in pickups, and theres also a difference in pumps... if youre trying to use a front sump pickup in a rear sump pan it could easily make your oil pressure drop out when you hit the gas because all the oil would flow to the back of the pan.

 

Sounds like oil sloshing away from the pickup during hard acceleration.


Change your oil filter and see if this symptom goes away. By the way, don't get the orange Fram filter but instead spring a dollar or two more for a better filter, such as a Ford Motorcraft FL1A (even available at most Wal-Marts).

Paul

 

Thanks, Paul for the suggestion. Because the engine has a great deal of carbon deposits, I have already replaced the oil filter and changed the oil twice. Still have the same problem.

I had planned to overfill the pan by 2 quarts today to see if the pickup tube was being uncovered under hard acceleration. But my son went to Farmington with my last case of motor oil. I'll have to do it tomorrow. I'm going to make sure the level at the dipstick is correct, then add one quart and check the results. If it is still the same, I will add one more quart (2 qts. over full), and try that. I think any more than that would not prove anything.

Hope I learn something from all this.

OldFords
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The 460 from 70 thru 74 took 7qts. 75 thru 79 took 6qts. my 77 takes 7 qts. so be careful. It depends which pan you have on the engine. You might not be putting enough oil in it. I doubt that is the problem though. It could be a leaky pressure relief valve on the oil pump, the oil pump could be loose, or as you suggest a hole in the pickup tube. You did remember to put the gasket on the oil pump when you bolted it to the block didn't you? You did say that you replaced the oil pump. If it was rebuilt, it could be bad. Some of this stuff you get now days is crap. I put melling pumps in when I replace them. I think they are the best pumps on the market.

 

The condition of bearings and journals has lots to do with loss of oil pressure on acceleration. When the engine is under heavy load the crank is pushed down hard towards the bottom of the journals. The oil gallery holes are at the top of the journals, so as the crank is forced downward a bigger gap is opened at the top of the bearing and more oil can flow past there. This can cause a large drop in oil pressure if there is enough clearance because the crank and cam is the oils first stop as it is pumped through the engine. If you lose lots of pressure here then pressure in the whole engine will drop. Also, the idea that the oil pan is being pumped dry is pretty slim. Unless you have a really bad sloshing problem there is no way hard acceleration would cause the oil pump to pump the pan dry. The oil pump speed goes up with engine speed and not load , so unless you are losing oil pressure at high RPMs then my guess would be that this isn't the problem.

 

Also, if your pan was being pumped dry adding more oil would only delay the effect, because if the pan were indeed being pumped dry that would have to mean that oil wasn't returning to the pan fast enough. Eventually all the oil would have to end up in the top of the engine no matter how much oil you put in.

 

One thought I haven't seen here yet is. when you had the pan off is there any possibility that then or sometime in the past that pickup tube has been bent up very slightly? Doesn't take much of a bend and it moves the pickup screen a long ways which would raise it too far off the bottom of the pan and possibly cause it to be sucking air, which is what your describing for symptoms.

 

Sorry for taking so long to reply, guys. Can you say (L)ong (A)nd (Z)any (Y)ear?

I finally got around to doing the 'ADD OIL" test today. First I checked the oil level (engine off). It was exactly on the full mark (5 qts.). Then I drove the truck to reach operating temp and the oil pressure was oscillating. While my wife watched the oil pressure gauge, I slowly added a quart of oil, making a total of 6 quarts in the engine. Adding the oil had zero effect; the pressure was still "shaky".

If it wasn't for the excellent oil pressure the engine carries, I could almost believe that I have loose bearings clearances. I believe that, with loose bearing clearances, the oil pressure at idle would be low at normal operating temperatur. It is not. Even at high outside temps (95 to 100 degrees), the oil pressure reads around 18 to 20 psi at idle.

Also, if the pickup tube was bent enough to cause a temporary loss of oil flow, the extra quart should have fixed that condition.

I intend to overhaul the engine within the next 3 or 4 months. Maybe then I'll find out what is causing my problem.

I appreciate all the advice you guys have offered. Thanks.

OldFords
bad attitudes

 

We have all been going on the assumption that there is something wrong with the oil pressure. What if there is something wrong with the gauge? Maybe the connector on the block has something plugging it, what if the line is kinked somewhere, what if the gauge itself is loose inside. I would check everything out before I started tearing the engine down. Just a thought!