IROCtheRoad

I recently purchased an 84 Z28. The previous owner (within the past 1500 miles) rebuilt the original engine .030 over, rebuilt the heads with 'factory' parts, but they did put in a higher profile cam and put in a high volume/pressure oil pump.

To my post:

The gauge is pegged when the engine is turned on (it does go back to zero on shutdown). The Tach/Oil gauge are working correctly. The reason for the pegged oil gauge has to be the oil pump, correct?

Is having high oil pressure ok for the engine? It's throwing the chech engine light because of the pressure.

Aside from installing an adjustable step-down transformer inline of the oil gauge wires, I assume there is nothing that can be done.....

camaronewbie

No, it's not the pump. The wire has become disconnected or broken to the gauge sender.

Air_Adam

I agree. I would verify the connection to the oil pressure sender, and verify the oil pressure as well. The guage *should* read max when the connector is grounded to the engine, and read '0psi' if the connector is disconnected completely (I could have that reversed, off day for me today, but you get the idea). The sender and its connector should be mounted on the back of the block, just about right under the distributor, drivers side. It will look like a big canister screwed into a long brass fitting that sticks straight up about 2-3".

To verify the actual oil pressure, go to your local parts store and buy a mechanical oil pressure guage. The one that comes with a copper or plastic tube, and a brass fitting to hook it up to the block. Hook it up to the engine (either in the same place the electric sending unit hooks up under the distributor or down the drivers side just above the oil filter boss) and start it up. Check it for leaks, make sure there isn't anything significant, and use the reading on that guage to decide what your oil pressure really is. The factory guages are notorious for being inaccurate even when they are working properly, especially now that they are 20+ years old.

General rule of thumb is minimum 10psi per 1000rpm for a small block Chevy, up to maybe 50-60psi at higher rpms. Any more than that is unnecessary. A rebuilt engine will have probably in the area of 20psi at idle with the engine hot, if it was rebuilt properly and a normal weight oil is used, like a 5w30 or 10w30. Thicker oil will obviously be higher. Anything above 10psi at hot idle is generally considered safe though. Just as an example, my 454 which has an all stock, GM assembled bottom end, at idle with everything fully warmed up, has 15-20psi at around 750rpm, and at around 3000rpm is around 50psi. This is with a 15w40 oil in it. A 5w30 or 10w30 would likely be slightly less at the same rpms. My former 350 (a '78 shortblock that had a basic rebuild before I got it) was more or less the same as the 454.

camaronewbie

Yep - oil pressure gauge is pegged at m,ax when there's no connection - my sender broke a few weeks ago, and the wire hangs, and my gauge is pegged out to the max.

IROCtheRoad

Thanks for the help! I'll get under there this weekend and take a look.

Air_Adam

If your '84 is like my '83 Z28, then the factory oil pressure sender is right below the distributor housing, behind the intake manifold. Look there first.

IROCtheRoad

I know for a fact that the oil pressure sender is connected and working. I let the engine get warm and the needle would go to about 30 psi at a stop/idle.

I tried looking through my Hanes/Chilton for an oil sender unit, but no luck.

How hard is it to find the sender? There isn't much room behind the block and the firewall to really see if everything is connected properly

five7kid

iTrader: (14)

If pressure goes down to 30 when warm and at idle, then the oil pump may be putting out more pressure than the gauge can read. Is yours 0-60 psi? Or, the sending unit is weak and is reading higher pressure than the pump is actually putting out.

For the record, the oil pressure gauge will read zero when the sender wire is disconnected, and max when the sender wire is grounded.

Keep looking for the sending unit. It is behind the intake manifold, below the distributor base, unless the PO moved it to a different spot for some reason.

camaronewbie

Mine is currently disconnected, and it reads max, not zero. I'm using an older oil pressure sender (single wire style) from a 1985 car on the china wall as well.

five7kid

iTrader: (14)

Your wire or gauge must be shorted, then.

ASE doc

The high volume oil pump installed in your motor may have an 80 psi relief spring. The Melling HV comes standard with an 80 psi spring. My oil pressure pegs the 60psi factory gauge on startup. Once the engine is warm, pressure will idle at 30, then go back to 60 by 1,500 rpm. Also, the ECM does not monitor oil pressure. The SES light is coming on for some other reason. Read codes and find out why.

A note on the accuracy of GM oil pressure guages: Everytime I've tested oil pressure mechanically to verify the accuracy of the factory gauge, Ive found the GM factory gauge to be dead on. Thats about 30 tests over 35 years. Some of the best aftermarket gauges (AutoMeter) use the same style of electric sender that GM uses. Ive had a few vehicles come to me with oil leaks where the nylon tube for their cheap aftermarket oil pressure gauge burned through on an exhaust manifold.

If you're going to go with a metal tube to a mechanical gauge, use steel brake line for the tube rather than copper. We learned better than to use copper back in the 70s. Copper will only flex so many times before it cracks. When you make a capilary tube from steel, make sure to bend a coil into it before it goes through the fire wall this gives it a place to flex so prevent fatigue in the material. GM did it this way back in the 50s and 60s.

TreeFiddy

Mine does something similar to this. The gauge shows full over when the motor is cold, and doesn't change with revs. Once warmed up, it behaves normally showing 10-60 from idle to hi revs.

Have replaced both the sender, and run a separate wire all the way to the gauge, and that didn't fix it - have always figured it's probably a gauge fault.

five7kid

iTrader: (14)

Maybe your gauge is confused because it's on the other side of the car...

TreeFiddy

And upside down of course! If I connect an external gauge to the sender, it works fine.

ASE doc

Did I forget to say that Im wrong sometimes.

IROCtheRoad

It is the stock gauge, 0-60. I'll take another look whenever I'm able to get back home.

IROCtheRoad

Just an update for those that were kind enough to help me:

The gauge and sender all work fine. The oil pump is sending out a higher PSI than the gauge can read so it's maxed out. When the engine warms up and I go to a stop the pressure will drop to about 30 or so.

Again, thanks for all the help!