BlackbirdZ07

Two questions:

1. My '96 LT4 has low oil pressure, especially at idle when hot. When I first start it, it has about 40 lbs, then goes down to almost nothing at idle when it warms up. A mechanic put a gauge on it, so the sensors are ok. He then replaced the oil pump; not much change. Everything looks fine in the motor as far as he can tell, no metal in the oil, car runs and drives great, doesn't smoke, doesn't knock or make any bad noises. Any suggestions?

2. Anyone interested in a cheap LT4?

96GS#007

Mileage?

Mods?

Usage (ie 1/4 mile, dd, etc)?

Any oil usage?

Broken oil filter adapter bypass perhaps, or worn bearings but need more data.

BlackbirdZ07

Sorry... 123,000 miles, completely stock. Don't have much history, but very likely used as a daily driver. Haven't had it long enough to determine oil usage, but it doesn't smoke and runs great.

JAKE

If all else is normal, and since you've already done some investigating, I'd simply up the oil viscosity you're running.

If you're currently running 5w30 move up to 10w40, but stay with full synthetic.

My son's 96 was experiencing the same thing. His gauge showed a lower pressure than the gauge on my 96. So I did as above. Now the pressure gauge shows a more comfortable reading.

Jake

bjankuski

There must be some wear in the bearings to cause that low of oil pressure. You could try a high volume oil pump, that should increase the oil pressure but it will not solve the problem only mask it.

jmeyer58

What is low pressure? 6 psi is normal for that motor @ 1000 RPM and 18psi @ 2,000 RPM, so how low is it?

MK 82

My LT4 with 45K holds 35 PSI at 800 RPM.

BlackbirdZ07

Just going by the gauge in the car... At start up when cold, as high as 40 lbs. 1800 RPM on the freeway when hot, about 10-15. Idle when hot, 0-2. But even if I let it idle with no pressure indicated (all the way on zero) it doesn't make noise, the lifters don't start clicking.

96GS#007

I'd check the oil filter adapter to see if the bypass is stuck open for some reason. If it's not that, then it's likely bearings. Before jumping into rebuild mode however....

Did this happen suddenly....which would tend to point to the oil filter bypass....or has it been slowly losing oil pressure over a period of months....which points to bearings.

There is only one other thing that comes to mind. As you're aware, the oil pressure sender screws into a log style fitting at the rear of the block. That fitting screws into another fitting, which itself screws into the passage way in the block. At the bottom of that "passage way" is a press fit ball....it looks like a ball bearing if you were to see it...that seals off another oil passage. If that ball pops free (which is virtually unheard of), you can get 0 oil pressure when hot.

the blur

lt4obsesses

I would first replace the oil pressure sending unit. Inexpensive and from what I understand (haven't done it yet) fairly simple. Mine sits around 20 at warm idle but holds at 40 -50 with even the slightest nudge to the throttle. I have been told this isn't unusal and nothing to worry about. But your readings would cause me concern.

jmeyer58

You said on your first post that the mechanic put a mechanical gauge on the car so the sensors are good. If that is the case, I would first run 20W50 in the car and see what the pressure goes up to. I would agree with what 96GS#007 said if it goes up closer to a normal reading the bearings are probably worn and could use replacement. If the gauge stays low, you may have a problem with a sticking bypass valve. Good Luck!

BlackbirdZ07

Thanks to everyone for the replys. Very good discussion.

I spoke with the mechanic again late yesterday. He told me that he is fairly certain the problem is worn bearings... something about blowing compressed air through the oil passages and getting way too much air (clearance) around the bearings. I didn't completely understand, but I trust him and he eliminated the easy possibilities first.

96GS#007: I don't have a history on the problem, as I just purchased the car (see below). Of course the seller said nothing about it. Oil pressure was ok when I started it, and I didn't notice it during the rest of the test drive. I will be sure to ask the mechanic specifically about the ball valve in the sensor passage and the oil filter adapter bypass. Thank you.

I bought this car as an upgrade from my '92. Unfortunately, I just don't have the time, energy, and money to devote to an engine build (the way I'd want to build it) right now. So go back to my original post and remove the big grin smiley from question two; that's now a serious question. If anyone is interested in a nice black/gray LT4, Z51 coupe let me know. It's a very nice car that needs very little except probable engine work. Right now it runs and drives great. It's certainly faster than my LT1. The sooner I sell it the better, so I may list it here or I may bypass the tire kickers and armchair appraisers and go straight to ebay. Either way, I WILL NOT misrepresent the car so I'm willing to sell it cheaply. $6,500??

Thanks again to everyone.

bjankuski

FYI the oil pressure bypass will not cause low oil pressure, it will only bypass oil around the filter and allow unfiltered oil into the engine. The only bypass that could cause low oil pressure is the oil pump pressure relief and since he already replaced the pump I doubt that is the problem.

MK 82

Mojave

I'd start with replacing the sending unit or hooking up a mechanical gauge.