Odd oil pressure situation...?

[Krisndan]

Hey everybody,

Ok, so here's my confusion about my restore project. I bought a '75 280z coupe late this past fall. I'm getting no reading from the oil pressure gauge in the dash, and I mean nothing, but the amps are reading just fine (this is a combo gauge, as you probably know already). My first step was to replace the sending unit - check, but still no reading. I put a circuit tester onto the wire going to the sender and saw no power going to the sender (this is after an oil change and fill). -MEANWHILE- I've noticed a bit of chirping on cylinder 5 or 6 toward the cabin, which at first I assumed was a lifter that needed tightening, but it disappears when I open the throttle... So here's the question: could there not be enough oil pressure coming from the pump, which may explain the quietness on the cylinders when I step on the gas and may also explain the lack of reading on the gauge? OR does it appear that I'm looking at bad wiring to the sender? Perhaps a busted gauge on the oil pressure (bottom) half? Or maybe these are separate issues and the gauge is faulty AND I need to tighten a lifter...

Any thoughts before I go on a crusade to replace everything and/or take everything apart?

Thanks everybody, and happy holidays!

[Cannonball89]

L28 doesn't need much pressure at idle. If in doubt remove the sending unit and adapt a mechanical fuel pressure gauge to the hole. If you see 10PSI at idle your fine, and if you see at least 5PSI at idle your likely still fine but it is an indication that bearing clearances are getting large from wear.

 

Also from my personal experience with other engines, a chirping sound coming from a certain cylinder is an indication of a blown headgasket. That chirping may be air getting sucked in/blown out during the intake/compression stroke.

 

I must say though that my old L24 with something like 100,000 miles on it would show very little to no pressure on the factory gauge at idle once warmed up and it never seized up in the time I used it, and as to the chirping sound, I have won several races in a Legends Car with a motor that had a blown headgasket that made it chirp like a canary at idle, so obviously power loss was minimal.

 

So that's my 2 cents, do some more investigating and don't tear the motor apart until your absolutely sure that you have a real problem.

[Tony D]

I had an L26 that sounded like birds chirping at idle. Cam lobes were running dry. When revved up, the chirping stopped. Sludge was in the spraybar restricting flow when at hot idle - faint chirps were heard cold but it wasn't really noticeable until warm. I replaced the spraybar and cam, cleaned everything thoroughly and as far as I know that engine is STILL running today in a red Fairlady Z between Chandler Heights and Luke AFB... that repair was done in 1986!!!

I'd agree with checking with aechanical gauge, around 10 is all you will get at hot idle, maybe 15... More than that on a stock engine and you're wasting HP overdriving the oil pump.

If bearing wear is the reason, a shim in the spring, or turbo oil pump usually restores good pressure (perhaps at the cost of windage) but a loose engine makes more power and there are more than a few 400,000 mile bottom ends circulating untouched out there.

This is usually a sender issue, but that chirp may be a dry can lobe. A head gasket should show on a leakdown or compression check.

You are about an hour away using basic diagnostics to nail exactly which it is precisely and then can take appropriate action.

Good Luck!

[Tony D]

Btw, a head gasket leak would likely get louder under load or when you opened the throttle making mor cylinder pressure, which is why I mention the cam---it does go away when pressure in the oil system rises!

[Krisndan]

Thanks for the advice! This thing has stumped me for the better part of the past month, and now that it's cold outside (I'm in Rockford, IL where the highs are in the 30s) it's probably going to be a while until I can get outside and tinker with it. Now though I at least have a direction to go in when it does warm up. Thanks for all of that! Come to think of it, things did seem a little dry the last time I took off the valve cover, but the car was sitting for a good while before I picked it up - the sludgy cam is beginning to make some more sense... I'll try that pressure reading too, it just seems like I'd get at least a twitch on that OP needle in the meantime. Maybe with a good cleaning I'll get some movement.

You guys rock - have a great christmas!