Oil pressure drops

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kevm14

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First, the vehicle is a 1993 Caprice 9C1 with the L05 engine (TBI 350). 207k currently. Finishing up my final (second) ARX rinse with GTX 5W-30.

Second, the problem: Oil pressure drops (on the gauge) under hard acceleration, generally not until higher rpm. Free revving in neutral will not do it. Furthermore, if I apply the brakes to keep the car at a steady speed but floor the gas, the oil pressure also will not drop. The car has to be accelerating. But before you say it's the pickup (pump being starved), there's a twist - if the oil level is a quart or more low, the pressure won't drop. Now that's curious. A final condition is, it won't drop when the oil temp is low. Only when the oil heats up and thins down does it begin to show the problem. I am going to try a heavier weight oil, like M1 0W-40 or GC (see below).

I made two movies about a year ago. The first is with the car having around 4 quarts in the crankcase, give or take. The second is me shutting the car off and adding about a quart (I think I was running Mobil 1 10W-30 at the time) and repeating the test within 1 minute.

A quart low
Full crankcase

Finally, a little history of the problem. It all started when I changed oil filter adapters. I needed to change to a later year adapter with external cooling lines to clear a driver's side cat for the dual exhaust conversion I was planning (just got it done last week, too). After installing the LT1 oil cooler adapter, I noticed the pressure drop under the above conditions. Then I went back to stock. Same deal. Then I ordered a non-oil-cooler adapter figuring I'd just ditch ALL of the oil cooling. STILL does it. To this day, I am running no oil cooler at all and the pressure still drops.

Does anybody have a clue here? A couple things have come to my mind:

1. Rear main seal blow-by, occuring only under acceleration and only when the crankcase is full. I am finishing up ARX and the problem is the exact same. Plus, I'm not sure how realistic that is anyway (I'd be losing a lot of oil and I'm not).

2. Oil pump pickup. This WOULD have made sense, except that the problem does not occur when the oil level is a quart low. Kind of blows that theory out of the water.

3. Oil pressure sending unit. I ordered a new GM one to rule that out. Both the original sender and the new GM replacement show the exact same problem.

4. Some other weird electrical problem. Not likely an electrical problem is sensitive to oil level.

5. Crank walk. My crank could be walking back on hard acceleration, blowing a lot of the oil pressure right back into the pan. Again, this would explain adequately except the problem does NOT occur with 4 quarts or less oil in the crankcase (5 is the right amount).

I believe I also have a severe pressure loss under very hard, extended braking (like a full ABS stop from 70mph or more) but I do not know if this is affected by the oil level in the crankcase. Haven't done any testing there, as I've been more concerned about oil pressure at high rpm/high load.

When the ARX rinse is done, I am switching to an xW-40 oil. Most likely M1 0W-40 or possibly GC 0W-30. A guy on a Camaro forum claims he fixed this problem in his car by switching to a 40 weight. That may fix it or at least help.

If there is anything else I can answer or clear up, I'd be glad to. It makes me afraid to beat on the car when I see that gauge dip below 30psi at high rpm, especially when it started at 45. Sometimes I wonder if I'd just be better off not seeing the gauge and playing ignorant...
 
I'd say aeration from crankshaft windage ..but there's no way I can tie that in with the exhaust mod and the "one quart low solution".
 
I know it is a big PITA to do, but I would like to see what happens with a real mechanical oil pressure gauge. Those GM gauges are a POS.
Terry
 
I'd agree with Gary in that it has to be the crank weights whipping the oil and causing it to foam. That would explain the one quart low solution.

I wonder if the windage tray has come loose inside the oil pan......?
 
That's really the only thing that makes sense. I wish I could explain how that tray could have come loose when I was swapping oil filter adapters, though. Did you guys watch the movies? Both the original sender (had 180 or 190k on it) and a brand new GM replacement show the same drop. I'd bet money that a mechanical gauge would.

How does viscosity effect crank windage? Seems the colder, thicker oil doesn't get whipped up (if this is the problem).
 
quote:
Did you guys watch the movies?
Click to expand...
Yes, but unless you had already done a run on the one quart low before shooting the movie ..you don't have the impact of a heat soaked engine to compare on the topped up run.

On my wife's 4.0 ..if I wind out the gears under full throttle and then come to an idle (not very exciting ..but you can feel the power
grin.gif
) ..I lose pressure. This is the impact of the massive influx of btu's into both the water jacket and the bearing surfaces (pisons, etc.). If I do repeated runs ..this may drop more.

Now I know it would be very difficult to reverse the order ...but your results may be different.

I think if you installed an oil temp gauge you may see that your oil temp on the first run is 180-190 ..while the second run is probably 220-230.

That's probably why higher visc oil reduces the impact.
 
Gary, I know exactly what you are talking about and that's not when I'm complaining about. My car loses plenty of oil pressure between cold startup idle and hot idle (it'll idle at like 45 or 50psi cold and 20-25 hot). It's the DROP that concerns me, not the actual numbers. That it drops AT ALL as RPMs go up signifies a problem, regardless of where it started and ended (i.e. if ended < started as rpms go up, then we have a problem).
 
quote:
Originally posted by TallPaul:
Dropping oil pressure as rpms go up is a sign of loose rod big end bearings. As rpm goes up centrifugal force throws more and more oil out the bearing.
Click to expand...
Right, except for the extra detail that the car has to be accelerating. Say I floor the gas and begin to wind out 1st gear. Well, when oil pressure starts to drop (usually by 25mph), if I slam on the brakes but keep flooring the gas, enough to keep the car from accelerating, the oil pressure comes back up. I did a similar test in 2nd gear once. I floored the gas at 35 or 40mph and it kicked down to 2nd. As the car accelerated, oil pressure began to drop off pretty quickly to below 30psi. I got on the brakes and held the car at about 45mph, then slowed it down even, while still flooring the gas, and pressure came right back up. Let off the brakes so the car could accelerate - boom, oil pressure drop again. I could do this all day if I wanted.
 
Now I understand your problem a little better. I thought you were complaining about the post run idle pressures ..not the intermediate oil pressure drop while accellerating. This would bring me back to my original position that aeration is the cause of the drop.

I can't tie this into your swapping out adapters ..but it reasons that since this is only present under hard accelleration that the oil is sloshed to the back of the pan and elevates to a level that whips it up.

I can't integrate long text well (I gotta get checked for that stoke thing
grin.gif
) ..but I believe that you said that you could ...let's say ..maintain high rpms ..let's say in 2nd gear ..and NO LOSS in pressure occurs, right? This would further lead me to believe that the inertia of the oil going to the back of the pan is the cause. One quart of more low ...it doesn't get high enough to be caught by the crank throws.
 
quote:
Originally posted by Gary Allan:
I can't integrate long text well (I gotta get checked for that stoke thing
grin.gif
) ..but I believe that you said that you could ...let's say ..maintain high rpms ..let's say in 2nd gear ..and NO LOSS in pressure occurs, right? This would further lead me to believe that the inertia of the oil going to the back of the pan is the cause. One quart of more low ...it doesn't get high enough to be caught by the crank throws.
Click to expand...
Sorry if I was unclear. I like to type novels sometimes.

* To clarify - high rpms in neutral, car stationary, no problem.

* High rpms and full load, but constant or very slowly increasing rpm (i.e. a full rolling brake torque), no problem. The problem is more evident in the lower gears and won't do it in 4th at all, due to the very slow acceleration in that gear compared to 1st.

* High rpms and acceleration, but only with the correct amount of oil and higher oil temps, pressure drops with acceleration as RPMs come up. I don't think it's really as much of an RPM thing as it is a time-while-accelerating thing. It's just that my Caprice isn't fast enough to accelerate hard without flooring it.

So I guess I buy that I could have crank windage problem. Is it common for the windage tray to become damaged or otherwise not function in a stock 350? The oil pan doesn't have any dents or anything funky like that. Perplexing, no?
 
Does a 350 even have a windage tray from the factory? Most cars I thought did not, like Ford's 5 liter. Windage trays are a huge aftermarket item for hopped up 5.0s
dunno.gif
I like your conclusion that pressure is simply bleeding off somewhere where it shouldn't. Perhaps the releif spring in the pump housing is getting weak or something, and it bleeds off too much pressure all at once, then closes quickly, and you get full pressure untill you romp on it, and the cycle starts all over.
confused.gif
That's weird phenominom! At least it ain't going rediculously low!
 
Hard acceleration, pressure drops. Hard braking, pressure goes back up. Is the oil pickup in the forward part of the pan? And is it possible you are uncovering the pickup under hard acceleration?
 
I think the oil filter is going into bypass a bit causeing the pressure to drop.

I see no real problems here, pressure was good all the time.

I alos noticed that the pressure dropped on the full reading when the engine changed gears.
 
Everything points to "windage"...And rapid airation of the whipped up oil. ANY air getting into the oil pump and the pressure will plunge. Cars that pull high G loads have extra deep sumps (or dry sumps) baffles and windage trays to keep the oil down in the pan..When you are a quart low, it stays off the crank. When it's thick and cold, it stays off the crank. Why it started with the filter adapter change is a mystery.

You can grab the front harmonic balancer and see how far it moves back and forth to check the thrust bearing..It should not move any detectable amount. (.002" is ok)
 
quote:
Originally posted by Fuelrod:
Why it started with the filter adapter change is a mystery.
Click to expand...
Perhaps the adapter if it screws into the block, extends inside enough to contact the windage tray and the new one went a tad farther, dislodging an already loose tray (or if the tray somehow was partly held in by the old filter stud???).
dunno.gif
Or perhaps I ramble too much. Anyway, without the windage tray the oil could slosh a lot more possibly exposing the pickup.
 
quote:
Originally posted by Drew99GT:
At least it ain't going rediculously low!
Click to expand...
Not TOO low but I think GM wants a minimum of 6psi per 1000rpm. When it drops, I am hovering right at that minimum spec. That wouldn't be a bad thing by itself, except when you consider that GM's min specs are ridiculously lax. I think the compression spec is "above 100psi with no cylinder to cylinder variation above 20%." My engine managed ~175psi, with a max 10% variation. Engine had like 190k on it at the time, with police duty at the beginning of its life, and MY duty for the latter half. Now that's a lax spec.

quote:
Originally posted by TallPaul:
Hard acceleration, pressure drops. Hard braking, pressure goes back up. Is the oil pickup in the forward part of the pan? And is it possible you are uncovering the pickup under hard acceleration?
Click to expand...
It's not the braking. I happened to be braking because I was in a 25mph residential.
grin.gif
Simply letting off the throttle brings pressure back up. As I stated above, if I were to slam on the brakes and have the car maintain speed under full throttle, the pressure would ALSO come right back up. I think my pickup is at the rear. I don't THINK I have any lateral G problems, and my car pulls a decent amount on my summer tires (Bridgestone Potenza RE-750 in 255/50ZR17 with a W speed rating - maybe 0.85g or more with my suspension mods).
And I know the oil isn't sloshing away from the pickup because at a 4 quart crankcase level, there's no drop at all. If the pickup was starving for oil, the problem would get worse with decreased oil level, not better. Windage is the word here, I think.

quote:
Originally posted by BlazerLT:
I think the oil filter is going into bypass a bit causeing the pressure to drop.

I see no real problems here, pressure was good all the time.

I alos noticed that the pressure dropped on the full reading when the engine changed gears.
Click to expand...
As I outlined above, since I CAN be at high rpms (theoretically max oil pressure) with light OR high loads and the pressure is fine, it couldn't be the filter. Pressure WASN'T good all the time. It drops when RPMs are still going up. That's never acceptable. And I think everyone can agree that aerated oil isn't very good for the bearings.
On the gear change thing - if you watch closely, it actually starts to drop before the gear change. In the higher gears, it will be dropping long before a gear change.

quote:
Originally posted by Fuelrod:
You can grab the front harmonic balancer and see how far it moves back and forth to check the thrust bearing..It should not move any detectable amount. (.002" is ok)
Click to expand...
This is excellent advise that I have now gotten from two people on the crankwalk theory. Basically, if I can feel play, I have a problem. Feels and acts more like a windage problem, as all of my "it doesn't drop if I"'s still support it.

quote:
Originally posted by TallPaul:
Perhaps the adapter if it screws into the block, extends inside enough to contact the windage tray and the new one went a tad farther
Click to expand...
The windage tray would hang down below the crank, sort of in the middle of the oil pan. The block and filter adapter are much higher. The filter adapter would have to screw into the oil pan to potentially cause any problems.

What sucks is, I pulled all my cooler lines and external cooler out of the car. If I was sure it wasn't the cooler, I would have left it intact. For my driving style, I'd say synthetic is a REQUIREMENT in the summer months, with no oil cooling at all and the factory 3.42 rear.
 
Off the wall...

There is an oil pressure relief spring/valve in the block or in the oil pump itself. If it's in the block, it will be in or near the oil filter mounting area. When you installed the cooler adapter plate, was anything disturbed or changed?? All that stuff has been removed, filter set-up back to stock, right??

More off the wall. The oil pump is not submerged in oil. It must SUCK oil up a tube. If there is any restriction in this inlet path, the pump can cavitate, suck air, big pressure drop. but the one quart low cures it fact rules this out, unless the low oil gets hotter and thinner and flows easier and therefore does not cavitate..I think the sooner you drop your pan the better!
 
quote:
Originally posted by Fuelrod:
Off the wall...

There is an oil pressure relief spring/valve in the block or in the oil pump itself. If it's in the block, it will be in or near the oil filter mounting area. When you installed the cooler adapter plate, was anything disturbed or changed?? All that stuff has been removed, filter set-up back to stock, right??
Click to expand...
I believe the relief spring is integral to the pump. I am not stock at the moment. Currently running a non-cooler filter adapter.

quote:
Originally posted by Fuelrod:
More off the wall. The oil pump is not submerged in oil. It must SUCK oil up a tube. If there is any restriction in this inlet path, the pump can cavitate, suck air, big pressure drop. but the one quart low cures it fact rules this out, unless the low oil gets hotter and thinner and flows easier and therefore does not cavitate..I think the sooner you drop your pan the better!
Click to expand...
One little fact rules this out - the hotter the oil, the WORSE the problem is. As I mentioned above, if the oil is not yet up to temp and still relatively viscous, even with 5 quarts, it stays pressurized to redline in all gears. It would be nice to drop the pan. I also want to replace the timing chain. Then there's this cam I've had for years that I'd like to put in....uh oh.
 
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