MY '87 IS ALL STOCK, 74K MILES, AUTO TRANSMISSION. WHEN FULLY HEAT SOAKED THE OIL PRESSURE WILL BOTTOM OUT WHEN STOPPED & IN GEAR WITH BREAK ON. SHIFT INTO NEUTRAL AND THE RPM'S PICK UP A LITTLE AND RED LIGHT GOES OUT. WHEN DRIVING NO PROBLEM.

SOMEWHERE I SAW A POST ABOUT A SIMILAR PROBLEM THAT TURNED OUT TO BE THE OP SENSOR. MY MECHANIC SAYS SWITCH TO 10W40 MOBIL ONE AND THAT SHOULD TAKE CARE OF IT. IF THAT DOESN'T WORK THEN LOOK AT SENSOR. I'M RUNNING 5W30 MOB1 NOW.

IS THIS A PROBLEM OR JUST AN ARTIFACT?

Normally, when oil pressure drops that low, something is amiss.

When the sensor fails, it will peg the pressure at 80PSI. That's the dead giveaway.

However, dropping like that could be troubling. I would get a good mechanical gauge and test the pressure before making repairs. If a good mechanical gauge reads the same, you have a bigger problem than 10-40 will ever fix.

When you say bottom out does it drop to zero?

Sounds like excessive oil clearances....Hopefully not!

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Yep, put a gauge on it.
Nothing anyone tells you will mean jack until you do this.

Your mechanic...............should be selling shoes.

"excessive oil clearances" COULD YOU EXPAND ON THAT A BIT?

MY MECHANIC IS TRYING TO KEEP ME FROM GOING BROKE ON REPAIRS. IF 10W40 MAKES IT GO AWAY THEN IT'S NOT LIKELY TO BE THE SENSOR, IS IT? IF THAT'S THE CASE TIME TO RECOMPUTE.

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Excessive clearances would be the main/rod/cam bearings having too much clearance.

Checking the oil pressure with a mechanical gauge would take 10 minutes and cost less than an oil change.

If your mechanic didn't check that, he's not much of a mechanic, sorry, you have to do the diagnosis before you start throwing parts (in this case oil & filter) at a problem.

EVEN if he told you to replace the sensor, it would be the wrong way to take care of your situation.

Diagnose first.....then repair.

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Translated into plain english is "WORN OUT". I agree with
the others, put a mechanical gauge on it before you do
anything else to it.

Benny,

Yes. Worn out.

I would also check for presence of coolant in the oil. Generally speaking, an L98 is good for 200k miles without any major bottom end failures. But... at about 70k miles, the head gaskets start to fail.

And if your losing coolant in the oil, you have a major problem on your hands. Sorry, to say.

Oil and ethylene glycol do not get alone one damned bit. EG will eat oil up and break it down to the point where it's liquid sandpaper. Bearings don't stand a chance.

Ask me how I know.

Something else to watch out for. I had this happen to me when I picked up my vette many moons ago. The vette shop I purchased my vette from in florida did a complememtary oil change before they shipped it to me in tx. Probally used a lowball shop. When I received the car it had the same problem as yours hot. I was thinking maybe a lower end tear down or oil pump was in my future on a 29000mile engine even though it was ok on initial inspection but did not know what weight oil was in the engine initially :'( . It turns out the no name generic fram type quality pf25 shorty oil filter was causing a large oil volume restriction without it bypassing. Problem was solved when I installed a good quality oil filter. Its oil pressure is Still good 12 years later. Yours may not be this easy of a fix. I was fortunate So just putting it out their as you never know. Ps If your l98 still has the silver body oil pressure sending unit they are very flaky. I believe there was a service bulliten on replacing them with the revised black body sender. I will have to dig in my sb book later as this is from memory.

Good point. Cheap junk oil filters will cause issues...

I use K&N, mostly cause the nut on the end! But the AC Delco Gold, WIX and Mobil 1 Gold filters are very good, too.

SO HERE'S HOW IT PLAYED OUT: PUT IN 10W40, NEW FILTER. OP WENT UP 8-10 LB ON THE ROAD, STILL BOTTOMED OUT IN THE SAME CIRCUMSTANCES. SO EITHER BAD SENSOR OR MAJOR PROBLEMS. I HAD A HARD TIME BELIEVING IT WAS WORN OUT AT 75K WHEN THE CAR, BY ALL INDICATIONS, HAS ALWAYS BEEN PAMPERED.

MECHANICAL PRESSURE TEST GAVE GOOD RESULTS BELOW & ABOVE. NEW SENSOR AND THE PROBLEM HAS GONE AWAY. OF COURSE THE ACID TEST WILL BE WHEN I GO BACK TO 5W30.

STILL AND ALL, IT BEATS HELL OUT OF A REBUILT ENGINE. I FEEL LIKE I DODGED A BULLET THIS TIME.

the mechanical gauge was the key.. your sensor was obviously dying... but hadn't quit. Once they quit, peg it at 80 PSI.

Glad you found it and glad it only cost the price of a sensor!

I would also bitch slap the mechanic, but that's me.

BOGUS:
I THINK YOUR POSTMORTEM IS LIKELY CORRECT. I FIRST NOTED THE PROBLEM LAST SUMMER ON 100+ DEGREE DAYS AND THOUGHT THAT WAS IT. BUT IT HAPPENED MORE AND MORE EVEN IN WINTER. DYING IT WAS.

MY MECHANIC IS THE ONLY ONE I'VE FOUND THAT I TRUST TO DO ANYTHING AT ALL DEMANDING ON MY 'VETTS BUT HE WORKS AT A GMC DEALERSHIP, WHICH MEANS $$$$. THE OIL CHANGE WAS MUCH CHEAPER FIRST STEP THAN THE GAUGING WOULD HAVE BEEN. IF THE 10W40 HAD MADE IT GO AWAY I WOULD HAVE BEEN PRETTY SURE IT WAS A REAL OP PROBLEM AND STARTED TO DECIDE THE LESSER OF EVILS.

I REALLY DO APPRECIATE YOU AND THE OTHERS WHO RESPONDED. ALWAYS GOOD TO LISTEN TO THEM THAT'S BEEN THERE.

PASSED THE ACID TEST, BACK TO 5W30 AND NO PROBLEMS.

yay!

Wouldn't hurt to stay with a 10-30 anyway on a higher millage car......just saying...

THAT SOUNDS LIKE A GOOD OPTION TO KEEP IN MIND.

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Only if you have 250,000 miles on the motor.