LOW....L0w.....low Oil Pressure - Oil Pressure Regulating Valve

When I went to buy this truck,  I noticed the oil pressure gauge would go to zero and the check engine light would come on at any rpm under 1000.   Figuring it was just the sending unit,  I went ahead with the purchase and brought it home.

As I started reading on the forums about this problem,  the first thing I found was how that these trucks don't have a real gauge.  It really is nothing more than an idiot gauge,  meaning that at a preset oil pressure the sending unit (actually just  a pressure switch) trips and energizes the gauge which is designed to send the needle to a certain "spot" on the gauge via a resistor which is in the gauge circuit board.  So you really don't have an oil pressure gauge,  what you have a is re-designed idiot light.....  There is a procedure where you can solder a wire around the resistor and replace the switch with a real sending unit to make the gauge work correctly,  but that is a different discussion.

However,  I did the above mod,  replaced the sending unit,  and the results were unchanged.    So I was beginning to think I really gotten taken on this truck......  But it didn't knock or rattle, so I decided I needed to more accurately check the oil pressure.  I attached my Autometer check gauge in the sending unit oil galley on the back of the block.   At 1000 rpm it read 2 lbs, at 1500 rpm it was 8 lbs, 2000 rpm - 15 lbs and 2500 rpm - 20 lbs.  A disaster looking for a place to happen!

Someone suggested that maybe I had a bad mechanical gauge.  So I hooked up a new mech. gauge.  Results were pretty much the same.

Suggested causes were oil viscosity, oil level, plugged pump pickup,  filter bypass valve, and the oil pressure regulating valve.  I knew the oil level was ok,  I felt that the oil (even though it was dirty and needed changing) was thick enough to have more pressure than 2 lbs at 1000 rpm, the pump pickup I felt was a possibility,  filter bypass.....?  As I understand the filter bypass, all it does is open if the filter is plugged to a certain point or if the oil is thick, like on a cold start.  So I couldn't see how that would affect pressure.    I felt like the oil pressure regulating valve was the most likely culprit.

At this point I want to pause and give credit where credit is due.  Member Dave 001 from the Oil Burners Network forum REALLY helped me out on this, and most of the oil regulating valve pictures and info regarding this came from Dave.

The 7.3 IDI oil system works like this:
Several other members also mentioned the oil pressure regulating valve.  OBN member racer30 had it pegged when he said if oil pressure is low and dynamic to engine rpm then it is probably a stuck open pressure regulator.  Once the regulator "sticks open"  there won't be enough pressure to wash the debris out.  (thanks racer30)   Dave 001 mentioned that the pressure regulator assembly is staked in place and "not serviceable" according to IHC. 

So just where is Oil Pressure Regulating Valve located???  In the above oil system schematic you can see how that it is noted as being in the oil filter/oil cooler head.   ALSO notice that there is something called a Oil-Filter By-Pass Valve located in the same filter head.  Don't mistake the filter bypass valve for this oil regulating valve.  Pictures futher down will show details of both valves.

Here is another page from the Navistar manual showing the regulating valve schematic,  and describing the ingenious way that this pressure regulating system works.

What the manual is saying regarding how the flow divides,  and what is really neat about the way IH engineered this is that when the filtered oil pressure reaches a certain psi,  it acts on the part of the valve which relieves the unfiltered  side so that the engine is only filtering the oil it needs.

Dave 001 told me that I probably needed to remove and disassemble the oil cooler.  Even though IH says the valve isn't serviceable,  he said that it can be taken out of the header and taken apart to clean, etc.  
In this next picture, you are looking up into the square shaped hole seen above in picture # 5,  and part of the regulating valve is visable.  Sorry for the poor quailty cell phone picture.  If I had known I was going to be doing this with it,   I would have gone into the house and retrieved the camera!

NOTE:  What we are seeing here is the part of the valve that opens to dump excess oil back into the oil pan.   This is the part of the valve shown in the blue box in picture # 3 above.   Its hard to tell from the poor quality pic, but the valve is closed.  No oil pressure at present, thus the spring is holding the plunger all the way into the cylinder of the valve,  the relief ports aren't open.
Looking at the picture above, notice that  only about 2/3 of the hole is visable,   the other part is hidden inside the housing, and of course the hidden part of the hole is where it begins to open.   With a pick I could feel in the hidden part and tell that there was an edge in there, or the hole was slightly open.

Thanks to Dave 001's  great documenting of the working of this valve and system I was able to understand how it functions and I could see (or rather feel with the pick) that the regulator valve was slightly stuck open.  So I worked the plunger back and forth a little and then I couldn't "hook" the pick on the edge anymore.  So now the relief ports were completely closed.    I put the filter back on, started the truck and now the it's holding 35 psi at 1000 rpm, 45 psi at 2500.   : )    Thanks again Dave 001!!!

happy wrenchin ~ diezelcrazee
After Dave's excellent tutorial I had a full understanding of how this thing works.  I had already removed my filter,  looking to see what I could see from there.  
Following are Dave's great pictures and detailed explanations.  

1. This first one is of the cooler/filter head as seen from the side where it mounts to the block.
Red arrow is the inside passage the unfiltered oil takes from cooler to oil filter.    
Blue arrow is unfiltered oil to pressure regulating valve.  
Any excess oil from the pressure regulating valve gets dumped
through a hole in the block and right back to the oil pan.    
Green arrow is filtered oil from oil filter which travels to the main oil gallery.  
Yellow circle is coolant to the oil cooler.    
Orange circle….this small hole goes through to the back side of the pressure regulating valve.
2. Oil pressure in the small hole (orange) causes the plunger to compress the pressure regulator spring. The excess oil dumps out of the valve (blue) and back to the oil pan. I gotta’ hold it in with my thumb as it is no longer staked and the spring will pop it out.

3. The oil pressure regulating valve. There are two sections to it. The half in the orange box is what “sees” the oil pressure. The small hole pictured in photo # 2 above connects to the small hole (circled in orange) pictured below in photo # 3. The half in the blue square is the actual part of the valve that dumps the excess oil.
4. The pressure regulator valve disassembled. The cylinder (black box), the plunger (blue box), and spring. Notice with the plunger removed from the cylinder, the hole or port (circled in blue) is uncovered. There is 4 of these ports spaced around the cylinder, you just can’t see them in this picture. Actually you can see half of the port on the back side looking through the port on this side. Oil pressure from down-stream of the oil filter travels to the small hole (orange) and pushes on the shoulder of the plunger. When the oil pressure is high enough, the plunger compresses the spring and moves to the right. The plunger moving to the right uncovers the 4 ports on the left. The 4 ports dump the excess oil back to the oil pan.
5. The oil filter by-pass valve installed in the header. The oil filter by-pass valve simply connects the un-filtered side of the oil filter to the filtered side. If the differential pressure across the oil filter becomes too high (usually around 15 psi in a heavy duty Diesel) this valve will open up and by-pass the filter. The differential pressure may get too high because of either a clogged oil filter (not changed often enough) and because of cold oil (at start-up). You are better off sending dirty oil to the engine through the by-pass valve than no oil at all (because of a clogged oil filter).
Click Here for a photo of a disassembled oil filter bypass valve.

(Remember this valve is only concerned with the differential pressure across the filter. It has nothing to do with engine oil pressure. Engine oil pressure is controlled by the pressure regulating valve.)

The square looking hole on the upper right is where unfiltered oil comes into the oil filter.  The pressure regulating valve isn't there in this picture (see next one),  but in this square hole is where part of the oil pressure regulating valve is visable from the filter side.