Too much of a good thing (oil pressure)

As usual, no good deed goes unpunished. My 2002 started acting up today. Having completed my distributor rebuild, gotten everything running nicely, and washed the car, I decided to take it out for a proper shakedown. Nothing crazy, just a good long drive (~100 miles round-trip) to get things hot and loose for the first time in a while. Everything seemed fine except for the fact that for most of the drive my oil pressure was reading 2-3 bar too high (Mobil 1 15w50 and a Mahle filter, BTW). Normally it peaks about 4 bar at 4k RPM hot, and shows about 1 bar at hot idle; at the moment I'm getting 2 bar at idle and 6-8 bar at 4k RPM. Every once in a while it would drop down to its usual range, only to come back up again with revs.

It's a VDO electric gauge, so naturally I figured it might be the gauge or the sender. Got home, engine still hot, popped the sender out, and screwed in my mechanical test gauge. Same readings. OK, so it's actually high. Pulled the cam chain tensioner out (this regulates the oil pressure). It came out freely and showed no signs of undue wear or dirt, and I could hear the ball shaking. I polished it anyway, blew out the small holes, cleaned up the hole in the chain cover, and slid it back in, making sure it engaged the chain (not spinning). Oil pressure still elevated.

Pulled the valve cover off just to see how things looked. Seemed normal, oil evenly distributed around, nothing odd.

So I'm running low on ideas here. Something is sticking, I suspect, but what? This engine has run the same oil pressures for decades and many thousands of miles. I mean, don't get me wrong, I'm happy the oil pressure went up instead of down, but I'd rather it just stayed where it's supposed to be. Thoughts?

Could you have a plugged oil gallery that feeds a bearing? Not sure how you'd test to find which one, if any.  Maybe do a short oil interval with something high in detergents?

Does the oil pump have a pressure relief valve?  I have seen those stick on late model cars and it cause intermittent high oil pressure which will sometimes lead to a stall as the cam phasers get out of wack.  I realize it may be a bit of apples and oranges in technology but I can't imagine oil pump design being all that different

The pressure sender is at the very top of the engine, on the distributor housing, which is bolted to the back of the head. This makes me doubt it's a plugged gallery, as I suspect if that were the case I'd be seeing lower pressure at the top of the motor.

I believe there is a relief valve on the pump. It may be that this is sticking, though I have to say I'd be surprised if it turns out to be the cause. This car gets fresh synthetic every spring and rarely sees more than 3k miles between changes.

I'm thinking I may dump a quart of something thin and detergent (like Marvel Mystery Oil) into the sump and see what happens.

"Having completed my distributor rebuild" "The pressure sender is at the very top of the engine, on the distributor housing" Your answer is there.

NermalSnert said:

"Having completed my distributor rebuild" "The pressure sender is at the very top of the engine, on the distributor housing" Your answer is there.

I considered that. Problem is the sender bolts into a gallery that is completely visible with the housing off. When I had the distributor out I tanked the housing and it is spotless. It was not previously blocked in any way either, so I don't think it's the source of the issue, though I agree there's reason to think it might be.

What changed, I wonder? Mechanical or electric gauge?

Electric in the car, confirmed with mechanical.

I somewhat doubt that the oil pressure relief valve could cause this as if it was leaking your pressure would be lower than normal, and if it was closed it wouldn't affect your oil pressure until past the point where it was supposed to open (probably ~75ish psi?). Having said that, i don't have any other good ideas! If the pressure sender is before the oil filter housing in the flowpath then i'd look at the filter or its housing/passages next.

In reply to Vigo :

My metric units may be wrong, but isn't 1 bar roughly equal to 1 atmosphere? If so that is approximately 15psi which means 6-8 bar at 4K Rpms is roughly 90-120 psi.

In reply to 02Pilot : I had a similar issue when I rerouted my oil system which eliminated a pressure relief valve in the system.  

After the reroute I had 125-150 psi pressure until the oil heated up and dropped it somewhat closer to normal.  60-80 psi 

thinking it through, as long as I was gentle about my start up so I didn’t blow my oil cooler it really didn’t matter.  It takes the same power to pump the oil no matter if it’s relieved afterwards or not.  

I didn’t spring any leaks, used the same amount of oil per race, and the extra pressure when cold probably was the reason my engines lasted so much longer.  

In short,  Don’t worry, be happy. 

1 bar = 14.5 psi

The pressure sender is the last thing in the line (#10 below), so the pressure throughout the system is elevated. Spec is 4 bar at 4k RPM, and that's where it's always been; I'd like to be able to just accept the higher pressure (as the notes on the diagram below suggest, I'm currently at the top end of the acceptable range), but in order to do that I need to know why it's suddenly so much higher. Internal engine mysteries keep me awake at night.

I can't tell from the pictures but how is the oil pump driven? Higher pressure may mean hight load on the pump driver.  This may or may not be a problem in the making like for example put in a high volume or high pressure in a SB Ford with stock oil pump drive and that Number 2 pencil driver will quickly turn in to a barber pole and stop driving the pump....you have to install a larger diameter drive with allen ends.

Been to long since i was in a 2002 motor.

In reply to 44Dwarf :

The pump is driven by a chain off the crankshaft. Nothing in there has been touched in 20+ years.

Don't overlook the pressure relief valve, it could be sticking. I had two GM oil pumps that the relief valve stuck in the open position on an LS3 (12 Camaro SS), a year apart. So it can happen.

81cpcamaro said:

Don't overlook the pressure relief valve, it could be sticking. I had two GM oil pumps that the relief valve stuck in the open position on an LS3 (12 Camaro SS), a year apart. So it can happen.

Not overlooking it, just hoping it's something else. I really don't want to have to drop the pan.

How does the oil travel in and around the distributor?  Did you change a pressurized bushing in the distributor when you rebuilt it?  Since that's where you were last I'd be taking a close look in that area.

APEowner said:

How does the oil travel in and around the distributor?  Did you change a pressurized bushing in the distributor when you rebuilt it?  Since that's where you were last I'd be taking a close look in that area.

This is the oil flow through the distributor. There are two circuits, though one (the pressure sender) is really just a gallery getting oil from the intake rocker shaft. As I understand it, the oil enters the distributor housing from the exhaust rocker shaft at the distributor gear and flows through the large central opening where the camshaft enters before exiting through the rear cam bearing and back into the head to drain to the pan. The only passage not visible in the photo is a tiny one (~1mm) that separately drains from below the distributor gear back to the head. I'm not entirely certain what the purpose of that drain is, but it's clearly too small to be the primary drain for the oil entering the housing.

The fact that the pressure sender is completely separate from the other oil entering the housing tells me it's a system-wide issue, not a localized one.

My metric units may be wrong, but isn't 1 bar roughly equal to 1 atmosphere? If so that is approximately 15psi which means 6-8 bar at 4K Rpms is roughly 90-120 psi.

Yes, but i wasn't converting his numbers, just giving a ballpark number for when a relief valve opens. Most cars that have gauges won't display more than about 80 psi no matter what, because their relief valves won't let pressure go higher than that. 

It's possible that the relief valve is stuck closed in this case, but even if it were, the only way that would affect pressures LOWER than the valve's opening pressure would be if it wasn't sealing when closed before, but now it somehow does. I think that's unlikely.  

All I've got is a random piece of 2002 trivia which I can't find a way to make related, but I'll state it just in case it reminds someone of something.

I really wanted a mechanical oil pressure gauge in my 2002 and had a nice braided stainless line made up. That's how I found out that the pressure at the fitting for the sensor pulses A LOT. It hydraulic-hammered two VDO gauges' wildly fluttering mechanisms to smithereens before I accepted that I had to go back to something that didn't drive the needle directly off that signal.

Maybe the closest thing to a pertinent point is to wonder whether you've checked this car with that mechanical gauge before. Based on your username, I'm guessing so :) But if it's fluid-filled or otherwise damping the pulses, I wonder whether we could be seeing a failing electronic sender and a false high from a mechanical based on peaks above average. Occam should be spinning in his grave.

I don't recall what part of the system causes the pulsing (I think it's just a matter of the feeds in the cam alternately lining and not lining up on the adjacent branch, feeding back through the system)

Okay, that's plenty of text on a probably-unrelated tangent.

02Pilot said:

APEowner said:

How does the oil travel in and around the distributor?  Did you change a pressurized bushing in the distributor when you rebuilt it?  Since that's where you were last I'd be taking a close look in that area.

This is the oil flow through the distributor. There are two circuits, though one (the pressure sender) is really just a gallery getting oil from the intake rocker shaft. As I understand it, the oil enters the distributor housing from the exhaust rocker shaft at the distributor gear and flows through the large central opening where the camshaft enters before exiting through the rear cam bearing and back into the head to drain to the pan. The only passage not visible in the photo is a tiny one (~1mm) that separately drains from below the distributor gear back to the head. I'm not entirely certain what the purpose of that drain is, but it's clearly too small to be the primary drain for the oil entering the housing.

The fact that the pressure sender is completely separate from the other oil entering the housing tells me it's a system-wide issue, not a localized one.

I agree that the sender side of the housing is unlikely to be the problem. 

Here's what I'm thinking.  From a hydraulic perspective the oiling system is a pump (with bypass), a filter and passages going to a collection of leaks.  If you reduce any of the leaks then the overall system pressure is going to go up.  That's why wide main and rod bearing clearances lower oil pressure and tight ones increase it.

It looks like the lower bearing in the housing is lubricated by the pressurized oil in the main cavity and that oil that passes between the bearing and the shaft drains back into the head from the hole you have labeled as the drain from the distributor shaft.  The clearance between the shaft and the shaft bearing will effect the amount of oil that leaks through and therefore the oil pressure. So, if it works like I've interpreted it to from the one picture then oil pressure would go up if you replaced the lower bushing.

Agreed, somehow an oil path clearance got smaller or clogged up. If you just did the distributor, it could be that. Worst-case is you have spun a bearing, which is blocking the oil leak to that clearance. 

Too much oil pressure won't really hurt you other than increasing parasitic load on the engine, as long as that pressure is actually getting to every important surface. 

maschinenbau said:

Agreed, somehow an oil path clearance got smaller or clogged up. If you just did the distributor, it could be that. Worst-case is you have spun a bearing, which is blocking the oil leak to that clearance. 

Too much oil pressure won't really hurt you other than increasing parasitic load on the engine, as long as that pressure is actually getting to every important surface. 

I disagree, the oil pump load is constant (yes RPM dependent)  but the relief occurs after the pump not prior to the pump.  

In reply to frenchyd :

Pump volume is constant for a given RPM, but load increases with pressure (as it takes more energy to force the same amount of oil out of the pump against a higher pressure).  The relief valve reduces pressure (and load on the pump) by dumping some of that volume right back to the sump without it going to the rest of the engine.