After driving for 10 to 15 minutes in my 92 z28 the oil pressure drops dramatically. But when you first start it up the oil pressure is nice and high. The kicker is when you sit at idle the engine temperature sky rockets unless you rev up the engine a little bit. I noticed also that when sitting at idle the oil pressure gauge reads 0. Im not sure if this is an oil pump issue or a sending unit issue or what? Any ideas would be helpful thanks!

 

Replace the sending unit and find out.

There's basically 3 things that influence the reading: the guage, the SU, and the actual pressure.

Of those 3, the gauge virtually NEVER goes bad; the SU fails ALL THE TIME; and then of course the pressure goes away when the engine wears out and develops clearance in the bearings. (the oil pump basically NEVER fails either so replacing that is INVARIABLY a waste of time) So change the SU; if that doesn't fix it, it's a sign that a rebuild is in your near future.

 

Would the SU have a different affect whether the oil is hot or cold? Does anyone know about how much they run?

 

Possibly; who knows. Does the oil have a different Effect when it's hot or cold?

www.autozone.com might.

 

Ya heat definitely has an effect on it. When the car is first started up it has a maxed out fuel pressure for about a minute then once you drive for about ten minutes or so then is goes to almost no pressure. At a stop sign or light it does read no pressure

 

So how is it now that you've changed out the SU?

 

Ive not changed that yet I will order one and then post my findings

 

I'm surprised... I posted that like 10 minutes ago... AZ must be a long way away, like a mile or something.

What's the matter with your fuel pressure?

 

I happen to agree with changing the sending unit. I did it with mine to determine if it was the gauge. I found that it wasn't but a new OEM part doesn't hurt. I ended up hooking up a mechanical gauge off the back of the engine to check absolute pressure. Mine was a main bearing issue which eventually lead to a rebuild....

 

I'm leaning on a engine rebuild. when a su goes bad it usually doesn't work at all or is always maxed out on the gauge(even when engine is not running but key on). My car is doing the same thing as yours. good psi (50psi)when cold but when warm it goes to like 5 at idle. I have a new mechanical gauge so I know its true. I did a compression check and got really low readings so all around the motor is worn the f out. to bad cause it runs awesome. going to put 15w-40 oil in mine and see what happens.

 

SU is like $15 and takes seconds to change out if you're super slow like me, maybe even as much as a minute if you stop 2 or 3 times to crack cold ones in the middle of it.

Engine rebuild is like $$$$$$????? and takes ???? to change out.

I think I know which one I'd try first.

 

No, usually they start to read low and nonlinearly.

 

Replaced 3or4 sending units and that's what I've seen.

 

Well thanks for all of the information I will post my my results of the SU change after I change it.

 

Always start with the cheap stuff first to properly diagnosis the problem. Rebuilds aren't that bad. O had one done for around 1300. Gave me the opportunity to clean the engine bay and repair other things like worn motor mounts...

 

Another thing that happens is, the oil pump pickup falls off/out of the pump and causes funny readings..

 

First sign of that is oil pressure diving during braking.

 

So far I have got the sending unit changed and it seems to be holding a good oil pressure at around thirty. It also seems that there was an oil leak around the SU. Also the oil pressure is alot steadier than it was, it doesn't move around as much.

 

Yup, its a common failure.

 

The reading is steadier now, the pressure is the same as it always was.

 

Yep looks like it thanks for everyones help!