As temp rise oil pressure drops
 

ok so I have an '02 2door 4.3L vortec - 50K miles. Haven't had much trouble with it until recently. I took it in for an oil change and the guy says I was missing my radiator cap and the tank was dry ... yes , completely missing. So he changes the oil, adds coolant and new cap. So theoretically, my truck was overheating and I never knew it. The guage never showed it - but "the guy" tells me that with no coolant - there is nothing to measure the temp.

anyway - the problem is this: when I start the truck in the morning all is good. temp slowly climbs to just under 200 and oil pressure starts at about 60 and slowly goes to 40. After about 10 minutes - when the truck reaches its normal operating temp, every time I come a stop and push the clutch in the rpms dip below 500 , bounce back up to about 900 hundred then settle in about 550. the oil pressure guages starts twitching between 20 and 40. It has only stalled once in the past couple of weeks but it seems like it wants to every time I put the clutch in and stop. I have reads a few post on this and the first idea was a bad pressure guage. But the pressure is consistent with the RPMs - as the rpms go up, the pressure goes back to 40. It only drops when idling.

Any ideas - suggestions would be greatly appreciated.

Oil pressure is well above spec for idle and running. I would not worry about it. Maybe clean your throttle body to try and cure the return to idle RPM drop that is responsible for the twitching pressure gauge, but if 20psi is as low as it goes, that is more than enough pressure to protect your motor.

ok thanks _ i will try cleaning the throttle body. So the drop in oil pressure is caused from the drop in RPMs , not vice versa?

The most likely cause in this case, since it happens when returning to idle, is that it is in fact caused by the rapid decrease in RPM. A sticking IAC valve can cause the RPM to dip quite low or even stall when returning to idle which, in the case of the RPM dropping below normal idle speed, can most definitely cause a noticeable drop in oil pressure.

If your oil pressure were causing the drop in engine RPM, your engine would already be toast.

my truck does this at an idle not just when releasing the pedal. just sitting there, it started doing this about 1500 miles ago i had parked it and was getting out and saw the pressure guage twitching(engine still running). would it be the IAC in that case too. i dont have an RPM gauge in my truck

cool oil is thicker than warm/hot oil. The pressure will drop off normally once the engine warms up as the oil flows easier = less resistance = lower pressure = normal.

Rise and fall with rpm is normal too since the pump is driven by the distributor shaft.

My 98 Blazer does this, when I first start it up, it's at about 1/2 way (40) or a little over, but when I drive it a some, it drops to almost nothing. I don't think this is normal, is it? I heard from some people it could be the Oil Gauge Sensor or the Oil Pump it sticking open.

mine does this as well, only instead of decreasing while driving, the pressure will increase back up to 60-80. Like yours, mine starts at 60 at start up and slowly drops to 40 as the oil/motor warm up. I figure this is normal since it happened in every vehicle i owned or rode in.

I am having a similar problem. I have a 2001 S10 Blazer with 4.3 Vortech. My oil pressure stays between 35 to 40 while driving but at idle drops immediately to 0. Then of course my check gages light comes on. The pressure comes back up slowly once I get moving again. Ok so I thought I'd replace the oil pressure switch but I can't find it. The guy at O'Reilly says it's either on the block or by the oil filter but I can't see it. Anybody know exactly where it's located?

It is located on the back of the engine between the distributor and the driver side valve cover. Search for "component+locations" with an author of "swartlkk" and you should find a schematic view of the engine showing a number of sensors including the oil pressure sending unit.

While you have the sensor out, hook up a manual gauge and see if you get the same reading. Low oil pressure isn't something to just throw a part at without verifying what the pressure reading actually is.