Oil pressure falling off at high RPM

[sno_boarder4343]

Ok so my 90 laser Rs has not really had any problems since i finished my rebuild almost a year ago. However Now it is doing something odd, when I start my car up the oil pressure goes right up and sits at the first tick, when I start to drive it starts to increase but when I hit 2500 RPM it starts to fall off, and by the time I hit 3500 RPM its barely showing any pressure at all. However the oil pressure warning light does not come on. At first I thought my gauge might just be reacting poorly to the cooler temperatures, but then I realized that it still fluctuates and goes back up when I am at low RPMs so I dont think that is the issue. I have had a little bit of an oil leak, but it has not really been that big of a problem, and my oil level is right where it should be. What do you think the issue could be?

 

[GRAVEDIGGER]

I'd say it's either a loose wire on the sending unit, or the sending unit is bad. I would just replace it with an aftermarket guage and be done with it. Most guages will come with a new sending unit, so that makes it nice.

Here's the one I run. Simple and pretty cheap.
Auto Meter 4327 - Auto Meter Ultra-Lite Analog Gauges - Overview - SummitRacing.com

 

[Black-Out]

You also need to be entirely sure your not having a problem with the oil pressure relief valve sticking or having a faulty by pass which opens up early under moderate pressure conditions. I have seen many a catastrophic failure (although not typical of a 4G63) be due to something as simple as a pressure relief valve (by-pass valve) sticking open on the oil pump when the car should get about 65-75psi oil pressure. You can't go by the light on the dash because as little as 5-8psi can keep it off, but 20psi oil pressure at high rpm can cost you a engine eventually,by a spun bearing. The general rule of thumb is 10psi of oil pressure per thousand rpm I.E. at 7000rpm you should be seeing 70psi.....some cars run a little lower than that but generally keep pressures within 5psi of where they should be in theroy. Try cleaning out your sending unit with carb clean first, if there still some doubt you could still replace the gauge and sending unit pair with aftermarket as suggested above. It truly depends on how much money you want to spend. The factory unit isn't meant to be accurate just read enough to tell you something may be wrong if the needle isn't in the correct position of the gauge while your car is running or while your driving down the road. In that respect it got your attention didn't it?
In the worst case scenario you may be looking at replacing the oil pump assembly to correct the issue.
Good luck with it sir-
William-

 

[sno_boarder4343]

Ok now things are just getting annoying, I did what you said William and cleaned off the sending unit, but now my oil pressure is really quite high. Its not dangerously high, but when I start my car the pressure usually sits right at the first tick mark but now when I start it up it sits right at the end of the little spout on the picture of the oil can. It seems so variable, do you think it is definitly the gauge?

 

[Black-Out]

Ok now things are just getting annoying, I did what you said William and cleaned off the sending unit, but now my oil pressure is really quite high. Its not dangerously high, but when I start my car the pressure usually sits right at the first tick mark but now when I start it up it sits right at the end of the little spout on the picture of the oil can. It seems so variable, do you think it is definitly the gauge?

If by tick mark your reffering to the lower mark off the bottom of the gauge then yea that's fine. Keep in mind on a cold start the oil pressure will usually be almost a 3rd the way up the gauge till the engine is at full running temperature,depending on what weight oil your running in your car. The key is that the gauge moves with engine speed in a linear fashion towards the 2/3rds mark on the gauge and stays there while you drive at full throttle or a cruise till you come to an idle. at full running temp your gauge should be right at or just above the lower most tick mark at idle. As the temperature changes so does the oil viscosity so it's not un common to find that while cold the oil pressure is higher than it would be at full running temp (coolant temps at 205-215 degrees and oil temps at around 180-195 degrees.This is completely normal. Just make sure your pressure doesn't drop off as revs climb or while your driving at a steady speed. Then you got problems. Go for a drive and check your gauge often and see if it reads correctly. If it does you had a contaminated sending unit....If it doesn't then either the sending unit isn't good anymore (in which case you replace it or get another gauge) or you really have a mechanical issue with your oil pump, which again is far few and in between if the oil changes have been done on a semi-regular basis (at once every 5k or so)


Main thing is to be sure is that the gauge isn't bottomed out at idle when it's hot or cold and that the gauge isn't dropping off below half way when your driving down the road....My car always has the needle 2/3 of the way up the stock gauge while im driving it no matter the engine speed (be it 2500rpm or 7000rpm) and never drop below 1/4 of the gauge at hot idle. This just indicates stable oil pressures even though you're denied the actual pressure values,but as I said before....it's simply the gauge version of the idiot light on the dash,a little cooler way of letting you know that something may have gone wrong with the car if this little needle isn't moving. I like the factory gauges personally....they amuse me, then again I'm easily entertained .....

William-

 

[sno_boarder4343]

Yeah you were exactly right, I let it heat up and ran it a little bit and the oil pressure went right back to where it should have been. Do you think I should still do a new gauge to keep a closer eye on things? Thank you guys so much for the help, I thought my turbo was starting to go since that is right when it kicks in.

 

[Black-Out]

Yeah you were exactly right, I let it heat up and ran it a little bit and the oil pressure went right back to where it should have been. Do you think I should still do a new gauge to keep a closer eye on things? Thank you guys so much for the help, I thought my turbo was starting to go since that is right when it kicks in.

I'm nostalgic I guess. I like my stock gauges fine but if your going to mod the car to make a lot more power (say 450 or more,or modding to a point where your going to do a fully built engine and a better oil pump) then you'll definitely want a gauge that's actually relaying real information to a pin point not an approximation like the factory gauges do. That said if you don't wish to clutter up the cabin much beyond an afr and boost gauge (like me) you could always mount a proper gauge in the glove box neatly and use your factory gauge as a warning gauge. Then if your pressures get a little goofy you can always refer to the actual gauge for a hard number to indicate whats going on. In the end it's really going to boil down to personal taste and how much you feel like you need to know exactly what your engine is doing regardless of the state of tune.....

William-

 

[DSM1G90]

Oil loves to go thin when it gets hot...why the drop on the gauge when things warm up. As long as it's in the range, and you pop off the oil cap and you see oil swirling under the rocker cover at idle.....

..take it easy. Take a 'chill pill' ... you're doing fine.

Mine does the same thing, in fact it really drop when it gets hot...and that's been that way for 27k miles now...ain't blown up yet...

-DSM