MrJ

This has come out of nowhere. When I start the car, the oil pressure jumps up to 35psi, which is normal, but then plummets to zero. The car acts like its going to stall, but then the idle stabilizes and it seems fine. There is none of the ticking that is characteristic for an engine with 0 psi oil pressure. I don't dare drive for for fear of spinning a bearing. I have a small oil leak at the back of the intake manifold, but I've been adding a little oil hear and there to keep the level constant. The dipstick reads fine. What is going on here? Shouldn't the car shut off with no oil pressure? I hope its just a bad sending unit.... The car only has 89k on it, and is running Mobil 1.

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91 Trans Am WS6
Bright White
5.0 TPI auto
Flowmaster 3" 2 chamber catback
Trans Go shiftkit
2000 stall converter
ported TPI setup

Built on Wednesday

RB83L69

You answered your own question. If the gauge reads low but the oil presure is OK, then the gauge is defective not the pressure. Swap the sending unit and see if it helps.

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"So many Mustangs, so little time..."
ICON Motorsports

ColinOpseth

sending units should zero the gauge at the very top not the very bottom..

change it anyway, just to be sure.. most Chevy gauges zero at the top

MrJ

Is there any way to be certain that the sending unit is actually bad? I hope its just the sending unit.... if its not the sending unit, what else could it be? A dead oil pump? If my car really had no oil pressure, would it be really obvious? I am not gonna drive it with that gauge reading zero, its just not worth the risk....

Vader

J,

If you truly had no oil pressure, the lifters and rod bearings would be letting you know.

If you want to verify the engine has at least 4-7 PSIG of pressure, start the engine and unplug the fuel pump relay. If the engine continues to run, the fuel pump would still be getting power through the auxilliary oil presure switch. This switch will interrupt power at anything less than 4-7 PSIG oil pressure. If the engine dies, you might indeed have very low oil pressure, but I'm guessing the gauge sending unit has failed.



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Later,
Vader
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"Let the bodies hit the floor!"
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Brent

iTrader: (1)

The oil pressure switch power for the fuel pump is routed through the fuel pump relay.

If you unplug the relay the fuel pump stops no matter what.

MrJ

any other ideas? What's the worst case scenario here, a spun bearing? I think I may pull a valve cover and see if any oil is squirting out of the rockers.. any help will be MUCH APPRECIATED since I have no way to get to work right now....

ColinOpseth

MrJ, i had this problem and caught it just before the main bearing went. In fact, I had just bought the car in Dallas and driven it back to Los Angeles like that. ******* sold me a car with a blown engine.

Anyways, i would pull off the oil pan and take each main bearing off one by one (torque it when you put it back on) and see if one of them is spun.

you'll know right away by looking in your oil pump's pickup if something went wrong. I did, strips of metal. *ouch*

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Webmaster: www.IROC-Z.org

RB83L69

I wouldn't worry about the worst case scenario until you cahnge the sending unit. I'd give about 10 to 1 odds that it is what has failed, not the oil pressure itself. Go spend $20 and 10 minutes before getting all worked up about bearings and pulling motors and oil pans and all that stuff.

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"So many Mustangs, so little time..."
ICON Motorsports

Vader

Not to start an argument, but I thought the auxilliary oil pressure switch was wired in parallel to the relay wiring. Otherwise, it could not perform its function as a backup if the relay were to fail:



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Later,
Vader
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"Let the bodies hit the floor!"
Adobe Acrobat Reader

Brent

iTrader: (1)

Argh. I was confused once again.

MrJ

uh oh...
The tach is bouncing between around 1 and 2k rpms.... what does this mean? The oil pressure is still reading 0. I can't even get the car onto ramps to get it in the air now, because of the screwed up idle.