When I started it up,the oil pressure pegged to the high side.After I drove for 10 or 15 minutes,it dropped to nothing.No drastic rattles,weak,but drove ok.Why would there be no oil pressure?Is there a common problem I should look into,or is it just worn out?
Is there a simple way to restore decent oil pressure? Thanks.

 

I was told dropping oil pressure means engine need a rebuild -- forgot why though. Hopefully you will get lucky!

 

Same happened to me.
Have you immediately checked your oil level?
I was at 1-2 Qts when that happened.

 

If the '89 oil pressure sending units are typical they're not reliable or the least bit of an accurate indication of the real oil pressure. With engine running and assuming there is 7-15 pounds of pressure the dash gauge jumps to some point, supposedly indicating the sending unit is sensing that pressure.

The fluctuating pressure seen in the instrument cluster could be related more to the electrical portion including the sending unit or the oil pressure gauge inside the cluster. There have been a few instances here on FTE where that era instrument clusters have developed problems with accurate read outs of voltage, fuel, water temperature and oil pressure.

Keep in mind I'm not too familiar with that era instrument cluster and oil pressure indication so this might not be 100% accurate---if so my apologies. If previous to this new discovery you know for certain the dash gauge would and did rise and fall according to engine RPM then your situation might be related to the real oil pressure.

In either case I'd suggest connecting a suitable external pressure gauge to the oil pressure sending unit connection point to better monitor the oil pump's health.

Hope this is nothing more serious than a cheap sending unit.

 

When was the last oil filter change? Filters have a bypass valve to continue supplying oil if the filter becomes blocked.

The strange readings seem like an electrical issue. The pressure sensor can leak oil internally causing the high/low swing you described. The wiring to the sensor may be intermittent. jim

 

Oil was just changed,new filter too.It had been sitting,all together about 8 months.Not liscenced,so not driven very far.I bought it from the mechanic,who owned it, (because the owner could not pay for the transmission the mechanic had installed for him), so I took it back,and said what's going on.He has diagnosed a stuck oil pump pressure relief valve.He has offered to split the cost, (750.00), for a new high volume oil pump,pan gasket,oil,and new filter.So my half is 366.00.I bought it "ands is",so ,I guess the best option.When I did my short test drive,the gauge showed great oil pressure,and softer I drove it for 5 miles or so,the pressure relief stuck.

 

The first indication of too low of oil pressure would be lifter noise. Before you drop the pan get a "real" oil pressure gauge and check to see just what your have oil pressure wise. I would get a new sending unit even if the engine ends up having low pressure and you have to drop the pan.
I did that on a Chevy Sub. I had. Added a HV pump, HD pump drive shaft, and all new rod bearings. Went from 10-15 lbs at 2000 rpm to 40-50 lbs.

 

I would do a bit more testing first. I think I have a similar problem developing in my van.
Oncold start my oil pressure gauge needle will work its way up to the A on NORMAL. After the engine is up to temp, and idling, the needle drops to between the O and R. SO I am starting to be concerned about it reading too high.
The oil and filter was changed on mine just before I bough the van so I dont know what weight oil was used or if it has some very thick additives in it. So here is what I am going to do before I think about the pump..

1- Change the oil and filter.
2- Check the oil pressure with a actual oil pressure gauge.
3- If pressure checks out ok, it is probably the sending unit.

In my case my temp gauge is also reading too high and so I think my problem might be in the instrument cluster voltage regulator but that comes after I check the actual pressure.

If it is my pump, then first before replacing it, I might first try to flush and back flush the oil pump. I do not like the idea of a complete engine flush on an older engine because it can cause larger deposits to come loose later and cause major problems..

One way to do this requires you to remove the distributor, which is is actually fairly easy to do. Drain the oil, remove the oil filter, and replace the oil pan plug. Now remove the engine oil dipstick and make a hose, funnel, or any thing else you might think of to be able to pour 2 to 3 qts of kerosene directly into the oil pan. By using the dipstick tube you are bypassing the kerosene draining through the top of the engine and other parts that might loosen something you don't want to break loose.
Next you want to put a pan under where the oil filter goes to catch the kerosene you will be pumping out.

They make a tool (some places rent it) for turning the oil pump shaft that is down in the hole where you removed the distributor from. Use a drill to turn the oil pump and pump the kerosene out. You might want to pump a little out and let it sit a few minutes then pump a bit more, repeat. I would repeat this a couple of times and maybe let it sit over night and do it again the next day

Hopefully the kerosene will seep into the relief valve area and free it up if that is your problem.

There is another method where you can modify a oil filter, fill it with kerosene, and use air pressure to force the kerosene back through the oil pump. I will look for the link later on today and try to post it for you.

My mothers van was having oil pressure problems and we found the problem to be a combination of the sending unit and the adapter for the sending unit that comes off the timing cover to be clogged.
Before I spent $300 i wold do a bit of testing first..
I also believe that the engine can be raised up enough to drop the oil pan enough to replace the oil pump. You wont be able to get the pan off and it wont be easy working in that tight of a space, but it can be done if I remember correctly. You would probably have to remove the upper intake but that would be a good time to clean it too..
good luck
Anna

 

I couldn’t find the article where a guy modified a oil filter, so here is another method you could try which would probably be simpler then turning the pump itself.

This would be back flushing the pump. The pump itself is probably pretty clean and may be in good shape, it is the pressure relief valve you are concerned about.
In this pic you can see where the valve goes in the pump.

Here is a pic of the pump taken apart and you can see the the valve much clearer.
http://diyford.com/wp-content/uploads/2015/11/61.jpg

The valve can have a broken spring but then it would probably stick opened and you would not have any oil pressure all of the time. If the spring was just weak, the pump would not be able to build up the high pressure and as the oil heated up and the pressure dropped, the relief valve at some point, should close and you would have some pressure..
So if it is the pressure relief valve, it would have to be sticking closed. When the pressure rises enough it pops opened and locks open, or at least partially opened and you loose pressure.
In this last case, cleaning it might very well help. It could be that the bore the valve piston slides in is worn, crud built up inside it, or a small piece of metal or plastic stuck in it.
Again before changing the pump, if that is what you determine it is, i wold at least give cleaning it a shot.
So the idea is to get some kind of cleaner into the pump so it can soak into the valve. Trying the back flush method might be simplest.

Here is what you need to do.
Drain the oil and leave the lug out for now. Remove the oil filter and screw on this oil filter adaptor.
Derale Performance Oil Filter Adapters 15746 - Free Shipping on Orders Over $99 at Summit Racing
You will need two more plugs because you want to plug all but one inlet to start with. You will need some kind of threaded adapter to connect a hose to the opened inlet on the adapter. With the end of the hose above the engine, devise a way to pour kerosene into the hose. It will run directly into the pump, on down through the pickup tube, and down into the oil pan again avoiding washing off bearings and loosening crud except some that might be in the bottom of the pan.
It might be a good idea to put the plug in and pour in a qt at a time, draining it each qt to flush out any loose crud. Hopefully this will get some kerosene in the valve itself and soften up any crud that might be in it.

There are only one ways I know to get the pressure relief valve to move. I would probably try this first one myself, on my own car, but not sure I would advise someone else to do it.... If you removed the hose adaptor from the screw on oil filter adapter and used the 4th plug to plug the hole in the adaptor... If you had about 2 quarts of kerosene in the oil pan and cranked the engine for just a second or two, it should build up enough pressure to pop the relief valve opened forcing the kerosene through it. When you turn the engine off, the pressure would drop and the valve would close. I would think that only once or twice for just a couple of seconds wold not harm the engine... The kerosene wold not be pumped throughout the engine because the adaptor is blocking it from doing so.

The second way would be to remove the distributor and turn the pump with a drill as in the first post, and that would be safest and you could run the pump longer. you would probably need more then a battery drill..

After cleaning I would drain all the kerosene and use cheap oil and a filter to check the oil pressure and as a flush to remove any left over kerosene. If it worked, I would change the oil one more time.

If it doesn’t work then you are back to changing the pump if it is not the sending unit..
Do you know this mechanic very well?
You wouldn't want to pay for a pump and that not be the problem. Make sure a actual gauge was used to check the pressure and that the passages were cleaned out where the oil pressure gauge screws in.. At least drain the oil and check to see if it looks like there are thick additives in it. If it looks good you can always put it back in and put on a $4 filter...


You said the oil was just changed... how long ago?
watch this video “Low oil pressure, finding evidence”

 

Worn out main bearing on the crank shaft would cause this.

On some engines, like the Jeep 4.0, the main's can be replaced and normal oil pressure will come back.

 

I'll agree that the problem could be the oil sending unit.

If at first start up, the oil pressure spikes to the very high side on the the dash gauge, and then drops, this would be a very inexpensive item to replace.

I've experienced this with a '92 5.8, and I would have the dash gauge drop, and then back to normal, replaced sending unit, and problem solved.

As far as spending the coin that the guy that sold you the van wants, try this first. Replacing the oil pump with a "high-volume" pump could cause other problems with an older engine.

 

If someone runs up to you screaming "THE SKY'S FALLING!" Would you start running and screaming or look up to see what's actually going on???

Before he goes freaking out thinking he needs to tear into the engine he needs to install a good mechanical oil pressure gauge and see what the pressure actually is. Then go from there.

More then likely it's a sending unit or instrument cluster issue.

If it actually had high or low oil pressure there should be additional signs, like noise or leaks.

But find out what the pressure really is then go from there.

 

$12 at Oreillys auto. Not tough to replace. Try this first.
BWD S334 - Oil Pressure Switch | O'Reilly Auto Parts

jim