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Testing Oil Pressure


Richard S
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I want to determine if my falling oil pressure on my 90 is the pump, the guage, or something in the engine [i already replaced the filter and the sender]. This calls for hooking up a "real" guage directly at the ???. Since the 3.8 pump is in the front of the engine rather than the pan, will this tell me if the pump is bad? I notice the FSM has a procedure for this but it calls for a special tool.

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The oil pressure sender is (I think) a 1/4" tapered pipe thread. Is a standard oil pressure guage fitting & you use a standard guage. I would uses something larger than the miniscule straw cheap guages come with or at least minimise the length and fill with oil, not air.

Thing to watch for is opening relief valve around 40 psi.

What exactly are your symptoms ? (from startup cold through warm idle and 2000 rpm.)

(Padgett on a holel machine).

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Just looking at the guage, Padgett, I get about 3-4 "squares" at dead cold idle. At 2000+ rpm might get 4. As oil [20/50 fresh] heats up, goes down to 2 squares at anything under about 2200 no matter speed/weather. Won't get much more than 3 at higher rpms, though don't really get much above 2800 with the very high 4th gear and CHP laden SoCal freeways. Can hear a little valve clicking at startup but never anything that sounds like rod slap. Try to keep an extra quart in there. Padgett, will a guage at the sender really be reading the actual oil pump pressure, or will it be reading the "system" pressure? I am trying to figure out if there is an obstruction [spun cam bearing or something], or if I should replace the pump.

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Dropped the center cam bearings on wife's 90 Bonneville at 86,000 miles. Pressure would reach 40 when cold, drop to 20 when warm and zilch (light on) at hot idle.

Ran perfecly including 1100 Interstate miles after dealer in Indiana said it was the guage.

If you have changed the sender then the next step is a real guage (got mine home and was same)

Is just an outside chance is a stuck open relief valve but low pressure cold (is four squares straight up ?) says to look inside.

Really outside but GM has several senders that look identical. "L" sender is designed for 60 psi midpoint while early one has 40 psi mid. Correct one is stamped 3532954 on bottom flange.

If all of that fails, I'd pull the pan and check for a cam bearing on the bottom. Have heard of 2 other 90's that did the same thing usually in cold weather. GM claims to have never heard of it.

I wound up replacing the long block.

Note: right now am using Valvoline 10W-30 in all modern cars, personally would not use 20W-50 in post-1980 engines because of very tight clearances. If must use "something else" in a very mild climate I would go for a straight 30 weight racing oil rather than something will really long chain polymer additives (multiweights).

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Sounds like this engine is all worn out! A high volume oil pump is available aftermarket, probably will work but is not a good long term fix. You make no mention of the mileage on this car. Most common on this engone when it gets tired is main engine bearing knock! You can detect this quite easily by power braking the car at abetween 900-1200 rpm with the doors closed and windows up all accesorys off you here a deep down knock in the motor. A good mechanic can easily drop the oil pan and inspect the main and rod bearings for wear. There is no real way to inspect the cam bearings without pulling the engine out. Usually if the cam bearings are shot the whole engin needs a rebuild. This is a tough engine but it can be taken out by years of little maintenance ie few oil changes or quite often the upper intake gasket leaks very common. When this happens it can leak antifreeze slowly into the engine an instant recipe for engine failure. First sign of this is your coolant tank keeps going down and you cant find any leaks!! smile.gif

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Lets see if I can hit all the issues: The car has 145k on it. Was very well-maintained for most of its life. Doubt that it is just "worn out" because this was a sudden problem. Seems to have happened after I added a version of "Slick 50". Before that, oil pressure was fine, very good in fact. The sender that was in there does not have a brand marking, but has 3 different numbers on the flats: 42KPA, 34939, and 552KPA. The new unit is a Sorensen numbered OPS141. At 2000rmp dead cold I am a "square'or two below midpoint. Since the start of this problem I can't get get midpoint on the dash guage under any conditions. No loss of coolant and car still runs cool. If the analogue guage reads low, as I expect it to, then I guess it comes down to wasting money chasing a new pump if that fails to resolve the problem.

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I know this is anecdotal, but I had an ?88 Cimarron and I added Slick 50 to it during an oil change and the oil pressure readouts on the dash went crazy. I Don?t know if it was a coincidence or not, but the immediacy of the failure of the sending unit to the addition of the Slick 50 to the engine made me more than a little suspicious, and I have not used any additive product that claims to coat the interior metals of the engine since.

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I would go a step further. To really clean an engine I change the oil &filter substituting a quart of ATF for one quart oil. In this case I would use 5W-30 because you want fast penetration. ATF is about equivalent to a <span style="font-weight: bold"><span style="font-style: italic">really</span></span> high detergent 20W oil. Would also get the factory oil pressure sender from AC-Delco so there is a known quantity.

If that doesn't work there is one other thing we can try involving kerosine before teardown but lets see the results from this first.

ps if you have lost cam bearings, there is no noise & the engine runs fine (dunno for how long but the roller lifters really reduce the loads) just the oil pressure goes away. If you have some at hot idle, that is a good sign though.

BTW can you readout the actual oil pressure value from the diagnostic scream ?

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Harry, you don't even know how fast I tried that. I was so desperate, I even used some of that dreaded "motor flush" to try to rid myself of whatever migh tbe doing me harm. New filter and oil; no difference. Ego badly deflated. Bruises from hitting head against wall for having added the stuff in the first place are still not healed.

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From the post and reference to lots of cleaning and then go to teardown, do not pass go, I take it you are not much convinced that it could be the oil pump.

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Before teardown I would at least R&R the relief valve. On the 3800 it can be checked/replaced by just pulling the oil filter housing - 4 bolts AFAIR.

Personally would replace with the 60 psi spring & ball from an "L" engine.

As mentioned the first step is to connect a mechanical guage anyway, is the only way to be certain what is really happening and if you know what you are seeing, the relief valve action (or non-action) will be visible.

On a good warm engine, as the pressure rises with rpm it will come up relatively quickly, stop for a moment when the valve opens, then rise much slower.

Have noticed that my 3800s (3 of them at moment) do not change much - maybe 10 psi from hot idle to 2000 rpm, probably high end is relief valve setting.

Spec for a "C" engine is 40 psi min. at 1800 rpm fully warmed up. mine sit more like 50-52 psi.

I have seen too many fixes become multi-year projects that never get done because of someone taking things apart without really knowing what the problem was.

ps 42 kpa = 6 psi , 552 kpa = 80 psi so my guess is that sender is set to acivate the warning light at 6 psi and the guage design range is to 80 psi but the resistance value at that psi is unknown.

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