Looking to find the cause of a severe oil pressure drop off in my 84 Z28 with 305HO. Motor was redone 2 years ago, all new internals, and basic machine work with block prep, nothing crazy. I don't drive the car much, maybe 2 dozen times in the last 2 years, possibly put a 1000-1500 miles on the motor total. Oil changed 3x in this period with good quality oil & filter. Long story short, I noticed the oil pressure gets pegged at 60 psi on initial start up, stays there steady until the choke kicks down the rpm, levels out at 30+/- psi at idle. After the engine temp gets into its second or third cooling cycle(has 180 thermostat, fans kick on at 215 degrees), the oil pressure starts falling until it finally hits zero at idle, and barely comes off the bottom with a slight rev of the engine. I can hear that sick DRY motor sound at this point and get forced to shut her down. Next day same story all over again. Before I do something drastic, I was hoping someone had some experience in this matter and could shed some light on what to do and what to check.

 

Sounds to me if you have lost a bearing.

Long story short. Oil thick when cold on initial start up and when it comes off the choke the oil has started to thin out some. Once it is to temp. The clearance on one of the bearing is too great and the oil pressure goes away.

The bearing might not be all the way spun but probably on it's way. If you pull the motor and change it out not may not have to turn the crank.

 

Mine does the same except mine never gets over 35-40 psi even whjen cold. My car has been running fine this way for at least 1 1/2 years and now someone told me It is probably a bad sending unit. The only oil I use is 1 quart between oil changes and this probale because of bad value guide seals. It smokes for a few seconds when firsr started.

 

ya id say blown bearing, you should change your oil and see if you have particles in the oil, if you do then you can drop the oil pan and replace the bearing. how long was it before you changed the oil and what are your levels at?

 

He said he hears the sick dry motor sound. If the sending unit was the case it would not have that sound.

 

Its possible the problem is internal, I had been watching the oil pressure for the past 18 mos. I noticed in the Summer of 05 the pressure would be great when cold but not so healthy when warm, maybe 15-25 psi at best). I thought I was using to thin oil at the time (5w30)...so when I winterized the car I used some 10w40. The spring of this past year 06 the problem seemed to go away?.....but then again I only drove it a few times and most of that was WOT, and not looking at the guages, just looking for Police!...Anyway the weather has been nice hear in the Northeast so I hadn't winterized the car yet and will certainly change the oil and inspect it for metal before I do anything else. I thought the oil filter adapter could possibly be in 180 degrees out or maybe the bypass is stuck?

 

jsut out of curiosity to understand the situation what is the normal oil pressure for our cars?

 

On my GM Goodwrench crate engine with low miles, running Mobil 1 5w30.

Fully warmed up

Idle = 30 psi
Off-idle = 40ish
3,000+ RPM = 70

 

what is the normal idle? mine is around 800-900
----------
RPM during idle i mean

 

My car idles at 750-800rpm, so do my other 2 cars(in that range)...@Idle psi for my cars warm is around 25-30 psi.. (cold 50-60 psi).......But usually a standard volume/stock sbcV8 oil pump should produce aprox 1 psi per 100 rpm begining at the warm psi/idle rpm as a starting point......Lets say warm my car idles at 800 rpm@28 psi....1000rpm would be 30psi....2000rpm@40psi....3000rpm@50psi....and so on as a baseline. I don't know this to be an actual fact, its just something I read about trying to find answers to my problem.