Hey all, sorry I haven't posted in a while. Not too much has been happening with the old black 'vette. Recently, though, I noticed that the oil pressure would drop off (no pressure) at idle and pop back up if I gave it a little gas. After a few minutes, though, it goes back to zero. I'm not driving it because of this and think I may have a bad oil pump but would like some input on this. Any ideas?

 

Install a manual test guage in conjunction with the dash guage compare them and make sure your reading accurately on the dash (not a sending unit or guage problem). If this test proves you do have a pressure problem then go further.
Better detailed explanantion. Pressures-rpms-temp.
I have run into a problem similar to this before and yes it was the oil pump (bypass dumping pressure for no reason), but I've also had oil pressure sending units display similar symtoms. So verify actual pressure problem first.

 

Assuming the 90 has the same oil pressure fail-safe my 85 does when pressure drops below 6 psi the fuel pump shuts off. Done by a pressure sensor that lives beside the oil pressure sensor. Do the mechanical pressure check but I'll wager at least a buck your pressure sender has gone away.

 

Hey Muffin as much sense as that would make being a fuel pump shut off, it isn't. It's actually a back-up fuel pump circuit. If your fuel pump relay fails the oil pressure sending unit routes power to the pump. So if the oil sending unit for the fuel pump doesn't see 4 psi (thats the amount of pressure required to close contacts) the relay is still powering the fuel pump. It's just a back-up.

 

Its just a backup circuit. Your engine will not shut off.

 

Well I bought an oil pressure sensor, figure I may as well use process of elimination to decide this one. Can I replace that without a jack, or do I need to risk taking it to the base and using a lift?

 

It's not under the car, so no you don't need a lift. It's behind the intake, back of the block, next to the distributor. It's kind of a b!tch.
That's one of the reasons I suggested verifying the sensor was the problem before you started throwing parts at it. To test pressure you could just pull the oil temp sensor above the filter and install manual guage to verify.

Hey Vader that's what I was trying to say in the post before yours I just ran on a little.

 

Gotcha, I didn't know where it was at. Good idea, I'm gonna try that on Monday. Unfortunately plans interfered with doing it today *grunt*. Thanks for all your assistance guys!

 

I stand corrected. Learn something new every day. Thanks guys.

 

I apologize for the delay in the reply. The good news? I am now the owner of 2 good oil pressure sensors. I guess you can guess the bad news... Yeah, I still have 0 psi on the oil pressure. I bought a performance oil pump and hardened input shaft. Now all I need is some time. Do you have any tips for me about replacing this? Thanks for all the help so far !

 

Have you checked the pressure with a mechanical gauge? If you have an AutoZone close by, they should have one to rent (it's no cost, just pay a deposit and you get all of the deposit back when you return it)

With zero pressure indicated, you should have heard internal noises. The lfters would have bled down with the motor running and you would have heard that almost immediately.

If you don't hear any noises from the motor, I would think that the dash guage is bad or there is a problem in the wiring harness to the gauge. The Electrical Diagnosis book (book 2 of the GM SErvice Manual set) has diagnosis procedures for checking the dash guage.

 

OK, I like that idea. I'm going to give that a try. I did notice a slight change in noise from the engine, but that may be my spidey senses gone paranoid. I'll give that a try... I miss driving my toy...

 

Sure you go the right sensor?

I'm not sure about your vette, but I know some systems have 2 sensors,,,, one for the guage, and one for the light, or alarm system.

Wishful thinking maybe,,, but worth a check.

I would also start with the manual guage test.

GOOD LUCK !!!!!!!!

I guessing you would have alot of lifter noise with 0 oil pressure though

 

There are 2 sensors coming off of a block of some sort near the distributor. I replaced the one that most closely resembled the sensor I bought, I figured the other was an oil temp sensor (the one that ran parallel to the hood, the other was mounted more upwards, perpendicular to the ground). Looks like the deciding factor requires me to get off my *** and check with a manual gauge.

 

A few thoughts:

Before you throw anymore parts at this, test the oil pressure with a manual gauge. You have really tested nothing by replacing the pressure sensor. You need to see the real pressure.

If the manual gauge proves to be matching the electronic one, then read the doom and gloom below:

1) When the oil pressure sensor fails, it results in a constant 80 PSI reading. Not zero, and not a reading that comes up. More accurately, they will stick at 80, the drop to 50 for a time, then ultimately stick at 80.

2) Oil pumps don't normally quit. It is a rather durable and reliable part.

3) Which leads to the ugly news... I would drop the oil and check it for metal. The easiest way is to put a rag over the catch pans drain hole, and pass a magnet through the oil. Remove the oil filter, and carefully cut it open. Check for metal in the pleats.

4) The reference above about the failsafe is correct... it is a failsafe to keep the engine running! Not to shut it down. It runs a parallel circuit to the fuel pump incase the fuel pump relay fails.

My guess? A bearing has failed somewhere... the main and rod bearings have more to do with oil pressure than anything else (short of the pump itself). If one decides to fail, that's about all it will take to drop pressure to zero.

Also, the high volume oil pumps are not a good idea... they can suck a crankcase dry in no time flat... and if you are a little low one morning, it could do just that. I would stick with a stock style pump.

 

one is long, one is short. The pressure sensor is the long one. The short one is the backup fuel pump switch.



The oil temp is located (as mentioned above) on the block, just above the oil filter. It is a bit tricky to reach, tho...

 

eh heh heh... thats not cool... Sounds like a new engine is on order if the test results show no pressure... Ya, gotta check that damn pressure... or get a reeealy nice new loan... and no Corvette for a lil while... Ok more than a lil while... By the way, I offer network consulting for engine parts!

 

Get the pressure checked manually, then panic.

Until then, my post was simply an alert to a maybe, not cast in stone.

If it's a problem, we can address it...

 

OK, well I bought an oil pressure gauge at Autozone (like 15 bucks) and hooked it up in place of the oil pressure sensor. Sure enough, the readings were correct. There wasn't even enough pressure to push the oil all the way through the tube, barely any pressure at all. It didn't read anything unless I revved, then it moved only slightly. Whatcha think?

 

Well I took a gamble and towed the car to the base's auto hobby shop and put it up on a lift. Sure enough, dropped the oil pan and no metal. After a sigh of relief I replaced the oil pump (that oil pan is a pain in the @$$ to get off of a '90) and kicked her over. RFI IAW MIM. By the way, don't ever follow that Chilton's \ Haynes manual, you don't need to disconnect the transmission cooler lines (thank God). End result: 'vette workey