I have a carbed 84 trans am with the LG4 and 700r4, pretty much everything is still stock. The car has been running fine but after my last oil change the oil pressure has dropped significantly and maybe I'm just being paranoid but it has me slightly worried. It used to stay around 25-35 psi at idle with the old oil and then I switched to Castrol GTX 10w30 High Mileage. I didn't do the oil myself last time so I'm not sure what oil they used but I thought it was the same stuff (almost positive that it's Castrol GTX, possibly a different weight). I didn't change the filter since the last oil change was only about 3000 miles ago and I couldn't get the old filter off (I'll wrestle with it next time I change the oil, didn't have time today). The engine still runs fine with no noticeable leaks or burning of oil, however, the oil pressure has gone down. After starting up it's around 30 psi but once warmed up and the revs drop to around 800 the pressure drops closer to 20. If the car is in drive it will drop closer to 15. When cruising at around 2000 rpm the pressure will rise to about 25-30 psi when it used to be closer to 35-40. The car has about 69000 miles on it (odometer didn't roll over) and the oil was changed shortly after I got it when it was around 66000 miles. I went with the high mileage stuff because the engine is old and I have doubts that it was properly maintained so I figured the lack of care would equate to extra mileage.

I did some searching about normal oil pressure levels and I kept hearing about the 10 psi for every 1000 rpm rule which would seem to put me within safe limits. I also saw a post from a member here saying that a 305 TPI/350 TPI, according to chevy, should have oil pressures close to these:

6 psi @ 1000 rpm
18 psi @ 2000 rpm
24 psi @ 4000 rpm (hot)

If these numbers are to be believed for my engine then my oil pressure would have been way high before and is still a tad higher than necessary. However, my friend who went to an automotive school and knows his stuff fairly well seems to think that my oil pressure is far too low and there might be a problem. So, the first question would be, am I on the road to disaster right now? I have no reason to believe that the gauge is faulty since it seemed to be working fine before but I suppose I can verify it with another gauge. If the pressure is too low, are there any ways to tell that I might be damaging my engine (noises, smells, behavior)? When driving the car feels like it's running the same as before, if not a tad better but that low pressure worries me. However, you guys know far more than I do about this stuff and I don't want to pay some random guy at a shop to either tell me it's fine or try to trick me into paying for unnecessary stuff. There were only one or two people that posted on here with similar problems but I never really got clear answers from those threads. If anybody can help me out, I'd greatly appreciate it.

 

if you are really worried rebuild it... but there is nothing you can do now to save it/make it better that will have any lasting effects other than a rebuild...

so just keep an eye on it make sure it isnt getting worse if it doesnt you have nothing to worry about... if it does you need a rebuild which you were already thinking of doing...

 

If you have a real mechanical engine problem there's nothing you can do about it now. But I wouldn't freak out just yet. The sending unit that runs the gagues in these 3rd gens are notorious for erratic behavior, often reading lower and lower as they head towards failure. And the gague itself in the dash is no precision instrument, although they usually read fairly consistently, if not accurately.

One... get a fresh filter on it. I agree it's probably not the issue, but you need to rule it out, so get on with it and put that to bed.

Two... I agree, you should put a different gague on it, at least temporarily, just for comparison. I have a remote mechanical gague setup that I keep on hand specifically to do these types of comparisons (I just duck-tape it to the windshield temporarily and drive around for a while). If you notice a significant discrepancy, definitely the first thing I'd change out is the factory oil pressure sending unit that runs the gague.

 

Get a new oil filter on first, and check the pressure with a mechanical gauge. Where does your oil pressure read when you first start the engine and the oil is cold. 90% of the time it is the oil pressure switch. I have gone through 3 of them myself.

 

At a cold start it's around 30-35 psi. I'll definitely swap that filter out when I can and I'll try to get myself another gauge but until then do you guys think my pressure is at a dangerously low level or should it be fine for now? I just want to make sure I'm not making my engine tear itself apart or anything. Also, if the gauge is right and changing the filter doesn't help, do you think using a different oil might bring the pressure back up? Thanks for the help so far.

 

When you change the filter, look around the oil pressure sending unit. See if you see some leaks. If it is leaking, that is probably what your problem is. I probably wouldn't drive the car until you have changed the filter. Do you hear any tapping from the lifters when you are idling and oil pressure is at its lowest? Of course, I have no idea exactly how low oil pressure needs to be for you to hear the lifters.

 

Alright, I'll try to jack up the car tomorrow or sometime this weekend and get that filter off. When I noticed the oil pressure dip down to almost 15 I shut the music off and had my windows down so I could listen for any strange sounds but everything sounded normal, no tapping or anything. When I changed the oil I noticed that there was a thin coat of oily grime on the oil pan and whatnot but no serious leaks as far as I could tell. The car hasn't been leaving any puddles or drip marks on the driveway or anywhere else I've parked it and unless the dipstick is lying I haven't been losing any noticeable amount of oil.

 

Often times when oil pressure switches leak, they don't leak sufficiently to cause for puddles to form on the ground.

 

I'm having this same problem. I have a fairly stock 305 H.O. carbed motor in an '84. Up until recently, my oil pressure was very steady. Seems to me it was always about 35-40. Right after a recent oil change (I did change my filter), the gauge on my dash went nuts and started to spike all the way to the top of the range.

I figured it was just the pressure switch, so I replaced that. As soon as I changed it, it seemed to fix the problem. Running it for a few minutes in the driveway, the pressure held at about 35 psi. Figured it was case closed, so I drove it to work the next day. After driving for about 20-30 minutes, I noticed the pressure start to gradually drop - especially when I was stopped and the engine returned to idle. At its worst, the needle drops almost all the way to the red line.

This scared the crap out of me. Given that the pressure before all the gauge foolishness started had been fine, I immediately suspected the new pressure switch so I went and got a mechanical gauge to verify the reading. I put that on last night and let the car run for about 30 minutes. The pressure dropped at idle again. Not as bad as the dash gauge had been indicating, but still lower than I remembered seeing before. At its worst, the mechanical gauge reads about 15 psi at idle. Giving it gas, the pressure jumps back up to around 35 again.

Based on the first poster's info, that sounds like it might actually be normal. Is that correct? It still seems odd that it's changed but if the old sending unit was on its way out for a while, I could have just gotten used to a false reading.

What I was planning to do today is change the oil to a slightly heavier weight. I've always run 10W-30, but a friend suggested I might want to run something heavier to help keep the pressure more consistent at higher temperatures. Any suggestions?

And should I change the filter just to rule that out? Never really thought of that as part of the equation as long as there were no leaks, but would it make a difference?

Thanks in advance...

== Sean