Can anyone help me with an oil pressure question? I have an 89 GTA 5.7L Vortec TPi 5spd. I noticed that my oil pressure drops as engine tempeture increases...it drops to the point where the engine is almost stalling...basically stumbling. Do I need an oil pump or do I need to replace the oil pressure switch again? I changed it last week. Car has good oil pressure on cold start way above 30 on the "not so accurate" oil gauge Any help greatly appreciated.

 

I don't know if this will fix your problem , but I had similiar problems and it was time for me to replace my oil pressure sending unit.

 

If its actually affecting the engine, it should be the sending unit. The sending unit just sends info to the dash. I had to replace my sending unit cuz it was messed up, but the engine still ran fine.

 

True.

 

Isnt the sending unit wired to the fuel pump relay? Because when my oil pressure is down my engine is barely running.

 

I dont know for sure but it shows all wiring diagrams in my Haynes manual.

 

Oil pressure won't effect how the engine runs, unless you have no pressure and the engine is trying to lock up. There is an oil pressure switch that sends voltage to the fuel pump, but its only a backup in case of relay failure.

It could be that because the engine is barely running, the oil pressure is down, since the oil pump is driven off of the camshaft, then if the crank is idling at 500 RPM, then the oil pump drive is only spinning at 250 RPM, probably not enough to make the kind of pressure you want.

Its been stated by many sources that a SBC only needs 10 PSI for every 1000 RPM.

Also note that the dash gauge may be a few PSI off anyway. Best way to know for sure is to put a pressure gauge on the engine and see what it does.

 

As GMTech mentioned, the fuel pump/oil pressure switch connection is only used as a backup. If your oil pressure switch dies and so does the fuel pump, that means that your fuel pump relay is bad. The oil pressure switch acts as a fuel pump relay only as a backup for the actual fuel pump relay.

As he also mentioned, you really only need 10 psi of oil pressure for every 1,000 rpms. At normal operating temp, my car idles at approximately 15psi and with about 650rpms and I'm completely comfortable with that.

As a personal opinion, you might want to check into your oil pump as the culprit.

Just for giggles, start your car and unplug your oil pressure switch. If the car dies when you do this, replace your fuel pump relay, hook up your oil pressure switch and see if the "stumbling" problem is fixed. If this is the case and the new relay fixed the stumbling AND you are still holding 15 psi or more at idle, then you're fine. Otherwise, if the above options don't work or help, replace your oil pump.

 

ok cool, I'll try it this weekend. I had the car running with the oil pressure switch unplugged. Also I noticed that the oil filter I got from work is significantly smaller than the correct filter....could this be part of my pressure problem? Second I scanned the car yesterday and found a code 42....so I think the stumblling is related the ignition moduele. I'll keep everyone posted. Thanks for the help so far.

 

i had this problem, turned out my idle was too low

 

Ok. oil pressure is up. I changed the oil to 20w-50 and installed the correct filter size. At that point there was no change. Something told me to swap ECMs. So since I had a new one sitting around and collecting dust I plugged it in and bam, oil pressure was up and the car was running fine. Unfourtunately I still have the problem of a surging idle. Anything I could check out to repair this problem? It seems to surge at idle only in closed loop. It will surge from 500-1000rpm. Thanks again.

 

Okay,

as far as i know 20-50 oil is a NO NO! I could be wrong though. I was at a GM dealer the other time and asked since i planed to do the same and he said that there was a bulletin about oil thickness for our engines... Maybe some one else here knows?

 

havent had a problem with the 20-50 at all. Oil pressure is still up and stays nominal. I gota fix the surging idle problem though, as mentioned above. Thanks again.

 

okay sounds good but i dont think they ment anything visibly noticable but more internal damage.... Hope i am wrong though.

 

I had a similar problem with surging at idle. After chasing it around awhile (including, but not limited to a new ecm), replaced the IAC valve and that did the trick.

 

Remove the Throttle Body (you'll have to buy a new TB gasket), turn it over, unbolt the IAC passage which is connected by a few SMALL torx bits, buy some good old carb cleaner, and clean the crap out of the IAC passage. While you're at it, clean out the rest of the TB and pay special attention to the back side of the butterflies, there will be a TON of carbon build up unless you've already cleaned it before. Bolt it all back together and see if that fixes your problem.

NOTE: Be careful when removing the torx bits, on my old TB I twisted too hard trying to remove them and I snapped the head off of one of them. They're really small and fragile.

 

Throttle Body and Gasket and IAC are new. IAC passages are immaculate. ECM is also new with NEW GM updated PROM(not custom) Surging still occuring, but now from about 650rpm-1100rpm. Any other thoughts, suggestions or Ideas welcome. Thanks again.

 

Check the Throttle Position Sensor adjustment.

https://www.thirdgen.org/newdesign/tech/tpimod2.shtml