1986 Z28 5.0L TPI

My oil pressure starts out at 15psi and after driving for 15 minutes it drops to 3psi (almost 0). No warning lights ever come on and I dont "notice" a knocking noise from the motor (my exhaust drawns out external noise). I havent replaced my oil sending unit yet, but am going to within the next few days. If it still reads low psi, what else should I look for?

viscosity question....10W-30....if the oil starts out at 10W and as it gets warm thickens to 30W, why does the oil pressure drop instead of increasing? Or am I totally misunderstanding the "viscosity" of oil?

 

youre headed in the right direction.id bet money on the oil sending unit. theyre known to go bad quite often. not the most accurate gauges either.check the sending unit ,probably has oil coming out of it,later jimmy

 

if you don't any eng. knock or rattles , i am 99.9% sure you just have a bad press. sensor . not too bad to change if you have access to a lift . ( lucky me , i do ) this is a common problem with 3rd. gens. , it's above the oil filter . clean the gunk off the elect. connector and be careful with the 18yr.+ plastic connector , they get brittle . the vis. of the oil is not the problem . 10w30 is the most popular visc. in the world . almost everyone here uses that in their daily drivers . GITERDONE .

 

thanx, that makes me feel alot better....i'll be replacing it soon.

About the viscosity question.....Am I right or wrong in thinking that oil pressure should rise with engine temperature? Every vehicle I have ever owned, oil pressure seems to drop slightly or remain the same after the engine gets warmer....shouldnt it rise? Isnt that how oil viscosity works?

 

oil gets thinner as it heats up i know that...but i don't know anything about the viscosity either...

 

So about the viscosity thing, viscosity isnt actually how "thick" the oil is, its actually based on its resistance to be shed off a surface. Since oil will become thinner, additives are used to make the oil not shed off a surface as easily when it becomes warmer. if that makes sense. think of gear oil, which is classified as like 80w-90 or something like that. if you notice its really not that much thicker than motor oil. the way its designed it will not shed off gears easily as the way gears contact eachother they will shed off oil very easily.

your bearings are probably kind of worn if you have high mileage, but not necessarily bad. 10w30 is kindof thin for a higher mileage engine. my truck has 140000 and i use 10w40 to help fill the excessive clearances. I had a friend with a lt1 z28 that had over 200000 miles and he used 20w50 to maintain the right oil pressure. you just have to remember that when its cold the motor will have to move that thick oil which could consequently lead to hard starting problems. so in my opinion, as long as your oil pressure doesnt go to zero or start making drastic changes youll be ok, but you might want to start thinking about using slightly thicker stuff. but who knows you might have luck with the oil pressure sensor

 

oil visc. is relative to the type of eng., most jet engines use oil that pours as fast from the can as water . ( well maybe not that fast ) but almost , most acft. recip. engs. use 50w , ( not all , i'm talking radial ) anyway 10w30 is fine for a daily driver . it all depends what the eng. what was designed for . much more can be said about this , but flow is more important than press. . too much press. can be bad on an eng. . think about an auto. trans. , it works on fluid press. to the plates . low r.p.m. , low press. . higher r.p.m. higher press. . my radneck neighbor put a high press. oil pump on his phord truck . lost r.p.m. and speed , could not understand why , till i explained how an auto. tranny works . he went back to stock oil pump , got back r.p.m. and speed . ( sorry i can't say red instead of rad , i'm a proud radneck myself ) hope this info helps just one person .

 

the viscosity question was answered fairly well....i appriciate that from all of you.

As for my oil pressure sending unit. I put a new one in today, started the engine and it has the exact same pressure as the old sending unit did ( 15psi cold ) A couple months ago, when i changed the oil, I put 4 quarts of 10w-30 and 1 quart of a thicker "sealer" for worn gaskets and front and rear main seals. It has a small leak (very small, no real oil lossage in 800 miles) coming from the front of the oil pan, near the harmonic balancer. But as for my oil pressure and the oil/addative I am using.....should I change the oil again and go with 10w40? Any addative recomendations?

The odometer reads 79k miles.......is it original, or is it 179k.....who knows since it only reads in xx,xxx. No, I am not the original owner.

 

For the oil question, you're sorta on the right track - when its COLD it acts like a cold 10W oil... when its HOT it acts like a hot 30W oil.

I'd suggest buying a cheap mecanical oil pressure guage; the kind that has the little plastic tube on it. You can get them for about $15 at any parts store. Hook that up when you take out your old sending unit and see what it reads before you install the new one. GM's guages are decorations, not instruments.

 

Well, I've already installed the new one but it can be taken out if needed. Another bad part about the OPSU.....its mounted directly next to the distributor and you (I) cant get my fingers/hands down there far enough to even touch the outlet. I used a 3/4" socket with extension to take the old one off and install the new one. Since its fuel injected, there are way too many wires and BS around the motor to do much of anything.

 

i'd forget about the sending unit now. the bearings are most likely worn (not worn out) just from normal driving. I've seen results of using lucas oil stabilizer and it actually does make a difference. I'm not sure about other products but lucas products do work well. I wouldnt worry about it until next oil change but thicker definetly wouldnt hurt. for my truck 5w30 was suggested, and my oil pressure was at like 25 psi crusing (i dont know rpms dont have a tach) and next oil change i changed to 10w40 and im now at about 40 psi crusing which was the oil pressure when new. rule of thumb is usually about 10 psi per 1000 rpms so as long as your oil pressure isnt at 3 psi at 4000 rpms youll be ok. is it increasing with an increase in rpms?

 

Yes, the oil pressure does increase with rpms/speed. When the car is warm, at a normal idle speed (5000rpm) with it in drive and brake pressed (sitting at a light), the oil pressure is just above the 0 mark on the gauge. Which may or may not be close to 5psi. I know 30psi is in the center, the next mark down is likely the 15psi mark and just below that starts the red line before hitting 0. I just got back from purchasing 5 quarts of 10w-40 and plan on changing the oil this evening. That should put me real close to where I need to be, pressure wise. If not, I will try draining a tad and adding some of the Lucas that you recommend.

 

There are threaded ports right above the oil filter - take out one of the plugs and hook a mechanical guage up there and run it. Its possible the guage in the cluster iteself has failed - my temp guage did that. Changing the sending unit did nothing, it still read way out from the actual temperature. Put in a mechanical temp guage to see what it would do, and it read right about normal (90* or so) instead of the 115* the factory guage was reading at the same time.

 

I believe you guys are correct on the gauge. I am 99% sure the gauge is reading incorrectly. I have not hooked up a manual gauge yet (not sure if I will or not, just because of the ugliness look it will give the interior of my car), but I did change the oil with 10w-40 and a quart of Lucas Oil Stabalizer. I started the car and watched the gauge throughout it reaching operating tempature. The gauge never once even attempted to go above the 15psi mark, even when applying higher rpm's. With the oil change and Lucas, I am almost sure the pressure has rissen some....i think its just the gauge that isnt reading it. The motor did and continues to run almost perfectly with no dings, knocks or service lights.

Buying a manual oil pressure gauge at this point would only give me piece of mind, knowing that I/we are correct in assuming the stock gauge is faulty. I may at some point attempt changing the tach/oil gauge with another, but since you cant buy them new and spending money on the "hopes" that the one I buy is good....I may just leave it be.

Thanx guys for all your input and assistance.

 

You don't have to hook it up permanently... just hook it up, run it, then take it out and plug the hole again. The guage in the dash, if found defective, can be replaced by itself - just have to find one. They are all over the junkayrd and Ebay all the time.

 

The oil pressure gauge is part of the tach.....not a seperate unit. I am sure I can find one, but the only issue is....will it work any better than what I have.

 

Thats right, it is too.... been too long since I visited my '83

You could replace the whole tach/oil guage assembly... all you ahve to do is take off the black plastic trim piece, then the clear plastic behind it, and then it just slides right out. Its cake. Takes 10 mins.