I have a 00’ C-5 with 51,000mi on the clock. I have been the only driver and wrench since it had 24,000mi and from 24k until I rolled over 50k there were no significant issues with the car.

About 10 mo. ago, while crusing at 50 mph, the "check gauges" warning came on and it was followed by "low oil presure" warning. A quick look confirmed the message. I shut off the motor, coasted off the side of the road and I checked the oil level, finding it to be on the full mark, I restarted the motor and drove off. During the next 3-4 months the condition did not return and I finally wrote it off as a defective OPS unit. Then it happened again….same type of conditions; and again, when I restarted the motor the gauge functioned and everything returned to a “normal” state.

Last November (about 10 months from the initial problem occurrence) I replaced the cam, intake manifold, full exhaust system, and oil pump (ported and shimmed LS-6 pump from Texas Speed). I added a new OPS (remotely mounted) and added a mechanical OP gauge ahead of the sending unit. When the motor is cold, both the dash gauge, and the new mechanical gauge, read about 65psi at start…..after warming , the pressure drops to about 40psi under normal driving conditions.

Unfortunately, the same condition returned within about 50 mi after I put the car back on the street. Almost exactly the same as before I swapped parts in the motor. When the warning sounds, the dash gauge drops to almost nothing (10psi or less) while the mechanical gauge reads a little higher (around 15-20psi.). Both gauges recover and bounce back to around 40psi within about 10-15 sec.; but, they both still drop pressure..

There is not any pattern to when it occurs…. straight and level driving, no turning , no hard acceleration; but, it has happened at least 4 times in 400miles (since returning to the street). The motor is completely dry, does not consume oil (5w/30 Mobil 1), and does not make any unusual noise ( no lifter or oil starvation sounds); although I am watching closely and get the load off the motor within seconds of the warning….I am pretty sure the o-ring on the pick-up tube was correctly positioned and correctly installed.

This is frustrating and I am not looking forward to tearing down to replace the oil pump…again unless there is no other good options…..does anyone have any suggestions regarding the probable cause for this problem????

 

Replace the oil pressure sending unit before you do anything else. It's a very common failure with these cars. Out of 8 friends with C5's, half of them have non functioning OPS units.

 

Possibly, but the fact that his mechanical gauge is also reading low at 15-20 psi isn't good.

 

When putting the oil pump back on and the front cover cant you damage the seal on the pump someway. Which in a little time it get worse and cause this to happen...

 

Weak oil pressure release spring?

Foreign body jamming open the oil pressure release check valve?

Main bearing on the way out?

I don't believe you can pull the oil pan with the engine in the car. You almost never can nowadays.

 

Good point about the mechanical dipping. I was thinking about this and it depends on when the dip happens. If you are accelerating and then let off the gas, there is a tendency for the pressure to drop for a bit. It happens at the track all the time when lifting off the throttle after going through the traps. 15-20psi is fine as long as it is not occurring when accelerating or when cruising around.

If it drops when just cruising around, I take back my previous statement, however, the OPS units are prone to failure.

 

I'm no expert and maybe I shouldn't chime in, but from what you wrote perhaps it's an electrical problem between the oil pressure sensor and your guages. It seems the digital and analog gauges work off the same line coming from the OPS, so maybe you have a bad wire/connection or short going to your gauges, because it doesnt sound like an internal engine problem. Also doesnt seem like a bad OPS since you wrote that you replaced it and are having the same issues as the original OPS. Good luck bro!

 

I seem to recall a HorsePower TV episode not too long ago where they were building an LSx motor and they pinched an O-ring on the pickup tube causing low oil pressure rather quickly.

 

I appreciate the responses. I was hoping to hear something that I didn't already consider; but, I suppose there isn't that many options after all.

This is so confusing because a couple of times, the mechanical gauge did not drop at all when the warning appeared and the dash gauge dropped to zero or very near zero; so, I assumed that there was either a continuing problem with the sender ( another defect??) or possibly an electrical problem that was creating a false signal. Unfortunately, another time it occurred, both gauges dropped as stated, the mechanical gauge just does not drop as much. It does not appear related to acceleration (oil pick-up issues) because it has occurred at a steady speed and moderate (1800-2000 rpm). I guess there is another rack removal making it's way on to my schedule in the near future....thanks for the input.

 

Since the oil pressure problem goes away after you shut the car off and then restart, I am inclinded to think that you have a foreign object lodged or passing through your oil system. I don't think it's the sending unit, as you wisely installed a backup gauge, and I don't think it is an oil pump problem since it has happened with 2 pumps.

It just occured to me that with our 2-piece C5 oil pans you should be able to drop out the bottom half of the pan with the engine still in the car. I have never looked at it from this point of view, I have attached a picture I found on the web. You just may find something hanging around your oil pickup tube...