Hello Everyone, I own a 1989 6 SPD. It is basically stock. My oil pressure reads zero. When I start the car it says 60/70 but after it's running at operating temperature it will go to zero. I Teed in a mechanical gauge. The mechanical gauge at operating temperature will say 10 PSI at idle. I have changed the switch several times. Still does the same thing. Is there anyway to adjust the reading?

Thank you

 

that seems a little low for warm idle. what did the mechanical gauge show if you bring the RPM up?

 

First thanks for your interest in this post. As I drive the mechanical gauge increases pressure to what I think is appropriate at the engine speed I am driving at. At idle 700/800 rpm the mechanical gauge will drop to 10/8 psi. I am this weekend taking a look at the oil pressure and fuel pump circuit. Right now my FP fuse does not have power. I piggy backed a fuse into the fuse block. I am looking at the MAF 464 through 466 and the FP 469/470. I was looking at the ORN 340 it looks to me like it powers the OP switch. Right now PPL 993 does not have any power. I bought a MAF because the test indicated the MAF was no good. Bu, I was wrong. That is why I am looking at the OP switch. I have replaced that switch a few times hoping to get a better OP reading. OH and ORN 340 does have power. So I am at this opint asking what order should I be tracing the power.
I am thinking
Start at 340 and trace it to the OP switch. Is there a fuse after the OP switch? Or is that the one in the fuse block. What closes the circuit in OP switch?
Thank you all. I'm in Guam the auto hobby shop is only open on Friday and Saturday.

 

All fuel pump power sources go through the fuel pump fuse.
Oil pressure closes the OP switch and sends power to FP fuse.
When ECM sees reference signal from distributor, it closes fuel pump relay and sends power to fuel pump fuse.
Applying 12v+ to terminal G in aldl connector, will send power to fuel pump fuse.
If you not getting power to fuel pump fuse during cranking and your oil pressure is too low to close switch, have a look at fuel pump relay.

 

Well the thing to realize about the oil pressure. If the car is cold both the dash and mechanical gauges read over 45 PSI. I'm wondering if I should just change the oil pump?

 

my personal experience has shown me the low oil pressure reading order of likely failure is Oil pressure sensor, Oil pressure gauge, worn engine parts and lastly oil pump (though it does happen). The pump is an easy job on most GM engine of that era as long as there is clearance to drop the pan (many horror stories on pan clearance in my memories). I'm just saying make sure you check and verify all other possibilities. Best of luck with the process.

 

Thank you for your input. I'm really thinking of changing the oil pump. First thing I am going to do is a little electrical testing. I want to get power to the PPL 339 and I want to get power to the FP fuse. If I get that and I still have low oil pressure on the dash I will most likely change the oil pump

 

If you've changed the switch and it still doesn't read right, its the connector, splice in a new connector before doing anything else.

 

This is a great suggestion. Really? But, what I don't understand. I start the car. The oil pressure is high over 50 psi, I have not looked in a while. By the time the car i at operating temperature it is 25 PSI at idle, 800/700 RPM. After a short while the Oil pressure warning light is on. The car has never stalled even when it says zero. I have a mechanical gauge that always have a pressure reading. Maybe 10 psi at 700 rpm. I'm was thinking of jumping out the OP sensor. Now I might just put female on the OP sensor blade and jump the connector.
I want to thank you all for your input.
I honestly think this is related to PPL 339 not having any power for the MAF. Thank you all again

 

Yesterday I didn't get to do much on the Corvette. But, I did start it. I then probed the orange wire on the oil pressure switch. It didn't have any voltage. The orange wire on on the relays did have power. One of my friends knows more about testing continuity so next Friday we are getting together to find out what is going on. Once again that you all.

 

Friday we did some continuity testing. Everything was good. I connected the oil pressure switch connector and removed the fuel pump fuse and then probed the fuse box slot where the fuel pump fuse is. No power, car is on and not running. I took a careful look at the FP fuse and decided even though it looked good to change the fuse. I started the car. So probably my by passing the FP fuse was unnecessary.

The car is running, poorly bad isle and a misfire or two. But, the oil pressure at idle goes to ZERO, the mechanical gauge says 8 psi at 800 rpm, a little low but still okay. The Tuner Pro program did not record any codes. I probed the MAF connector C and the decision tree says that if you have no voltage at C the ECM is no good.

So I am wondering should I be buying a new ECM? TunerPro says the MAF voltage at idle 750 rpm is 0.55 volts and the MAF is 7.6.
Injector pulse width is 2.00.
Could the low oil pressure reading be from a bad ECM. If everything is working correctly the car should stall below 4 psi.

I know that this is a bunch of stuff but I'm chasing my tail here.

Thank you for what ever help you can offer.
Next week I'm going to pull the plugs and do a fuel pressure leak down. Maybe an injector is stuck open. But, that does not answer the OIL pressure switch question.

 

Ok there's a lot to unpack here. First off the rule of thumb for oil pressure is 10 psi per 1000 rpm at a bare minimum to not cause severe engine damage. That doesn't mean that it's good. If your pressure is at that point it should be a red flag that something is wrong. Stop trusting the electronic gauges all together in a situation of consistent low oil pressure troubleshooting. Use a mechanical gauge and watch it closely. If I'm understanding what you're saying you are starting the engine and have good pressure when cold but then it slowly tapers off to under 10 psi. IF you can verify that on a mechanical gauge then I would say the issue is probably either the pickup tube on the pump or worn engine bearings. I've had cam bearings do this before. Now don't panic if it's bearings. That doesn't necessarily mean you need an engine overhaul. It just means you're getting excessive oil loss through the bearings. This is the main reason why high pressure oil pumps exist. So that way you can get more life out of an engine that has slightly worn bearings.

So once you verify with a mechanical gauge that you have low pressure during hot idle I would swap the pump with a Melling 10553ST. When you do that make sure to also replace the pickup. Yes I understand this is a band aid to the problem. You are raising the oil pressure to compensate for worn bearings. But to be fair this is exactly why that pump exists and how Melling advertises it on their website. I've addressed low oil pressure lots of times this way. These pumps can also be shimmed to get even higher pressure. So if you install the pump and it still seems a little low you still have options. Also you may have said in this thread what weight of oil you're using. If you did I missed it. But there isn't any fault in putting 10w40 in it to try and get a little more pressure when hot.

 

Phobos84,

Thank you for your input. I have a mechanical gauge on the car. I teed it off the oil pressure switch line. It says 8 psi at 700/800 rpm. But, the dash read zero. My point is why don't the car stall. The oil pressure shut off is supposed to be 4 psi. My 84 shut as soon as the dash said 9 even though the mechanical gauge said 20 psi at idle. I am thinking about the oil pump and a friend of mine that is a mechanic said run 20/50.
Like I said I'm still wondering why the OPS is not shutting the engine. It is running poorly but no codes. I'm going to pull the plugs on Friday and do a FP leak down. I hope that will tell me what is going on with the way the car is running.

My other question is with the MAF testing I did. I don't have any codes but terminal C on the MAF sensor does not have any power. The decision tree says new ECM. I'm wondering if the fact the car will not shut off at low oil pressure readings is a hint the ECM is not working correctly.

Thank you

 

I'm just playing devils advocate here. But are you sure the 89 ECU shuts the engine off for low oil pressure? I'm not saying you're wrong. But I've seen lots of cars (my 84 included) that had a bad oil pressure sensor, show 0 psi on the dash and the engine kept running. If the 89 ECU does shut the engine down with low oil pressure, is it using the pressure sender or the oil pressure switch to tell it to shut down? I'm not sure if all C4's have this but I know the early ones do. They have both an oil pressure sensor and a pressure switch. The switch controls the warning light on the dash. If this is what you have going on then that explains why the engine is still running. The pressure sensor could be reading off a few psi and the switch hasn't tripped yet.

And lastly GM loves timers. Very few safety features from GM were instantaneous. Take the later oil level float switches they started putting in everything starting in the early 2000's. If your oil level got too low it made the warning light come on. But it took several minuets for that to happen. If the 89 ECU can shut the engine off for low oil pressure it wouldn't surprise me if there was either an amount of time required before it shut off. Or maybe even distance. It might be looking at how many miles driven with low oil pressure.