1991redz28ga

I have really been loving my car lately. Finally got it running right and strong. Yesterday afternoon I was driving home from work and I was beating on it pretty hard. It was 94 degrees, so i was watching my gauges/ temp just to make sure everything was cool. All of the sudden at a stop sign the oil pressure went to zero. Revved the motor up to about 2500-3000 rpm and the needle went to 30psi. If stayed between 15-30 when driving and to absolute zero when stopped and in gear. The oil level is good, new filter and oil change about 900 miles ago. I went ahead and ordered an oil pressure sending unit this morning for about $30, but i am scared it wont fix it. I realize that there is no way for anyone to tell me what is wrong with my car without seeing and inspecting it first hand, but I am just wondering what else it could be (worse case scenario). Bad bearings? Bad oil pump? any chance that if the sending unit is replaced it could then be the gauge itself? I also know that the sending units are notorious for going out on these cars, but I am just want to prepare myself in case it isn't quite that simple. I have a good about of money in this car at this point. I would hate to have a blown motor in a car that I have just spent $8k on. Can anybody give me some peace of mind?

GeneralDisorder

iTrader: (1)

Can't give you peace of mind. I can only give you a piece of mind...... that being the "gauge" (term used loosely - this is by no means an "instrument" despite what GM calls it) in the dash is little more than a fisher price toy. To determine actual oil pressure you need a calibrated mechanical shop gauge. Swaptronics on the sending unit is just a wild a$$ guess and of no practical use unless the sending unit is visually damaged or leaking. Test the actual pressure (there's a slim chance the "gauge" is right) and then proceed with further diagnostics from that point.

GD

sofakingdom

iTrader: (1)

I'm not a big fan of blindly swapping parts either. BUT... I'd suggest swapping the OPSU anyway... costs less than a mech gauge, and is MUCH eeeeeezier. THEN, if the stock gauge still reads low (and there aren't other signs of truth like knocking, metal chips in the oil, etc.), try the mech gauge.

Not a bad oil pump in any case. Put that idea out of your mind.

About 99.99% of the time, low oil pressure immediately following a session of enthusiastic test-driving, is a spun rod bearing. The other .01% of the time, it's just a rod bearing and crank that need to be replaced. Not saying you should bypass troubleshooting and jump instantly to the most pessimistic possible scenario; only, better quit buying lattés and streaming services you don't use, and put those coins into a specially earmarked account dedicated toward replacing your short block in some form or fashion. Then who knows, if it DOES turn out that your enthusiasm somehow ruined the oil pressure gauge instead of the parts that it actually put at risk, or even that the 2 events both occurred in that order at that time but are not related (look up post hoc ergo propter hoc, if you don't already know what it means and you have to because you've already forgotten your grade-school Latin), you can take a vacation to Hawaii or something.

1991redz28ga

Hmmmm. What would the other symptoms of a spun bearing be? The engine sounds and runs fine as far as I can tell. I did notice the temp went from ~180 (I have a 180 thermostat) to about ~200, but like it said, it was hot. There is no knocking or noises.

sofakingdom

iTrader: (1)

Low oil pressure

Metal chips in the oil

GeneralDisorder

iTrader: (1)

Get or borrow an oil filter cutter and (without making more chips) open up the filter and check for contamination.

And yeah a mechanical shop gauge is more expensive than a sending unit but you can always use another tool, and there's no guarantee that the Chinese fisher price toy sending unit you get as a replacement for the one you take out is any good out of the box. That's just adding more variables to an already uncertain equation.

Could be gauge, sending unit, wiring, or actual low oil pressure. You have to start testing somewhere and I would choose to test the oil pressure as this is the most costly possibility and will tell you if you need to STOP running it to limit collateral damages.

GD

AlkyIROC

Rod bearings make noise. Cam bearings do not. Crank bearings don't make noise until the engine seizes up.

A new filter is the cheapest test. Put on a new filter and cut open the old one. I've used a hacksaw to cut the tops off. You won't get filings down at the bottom. Take out the filter material and put a chunk of it in a vise to squeeze the oil out. open up the material and look for bearing material.

No bearing material and you can be pretty sure you don't have a bad bearing causing the low pressure. Now you can go spend the money on a sending unit etc. Any kind of mechanical gauge is more accurate than the electric sending units. Confirm oil pressure is low with a mechanical gauge first.

vinny R

Please let us know what you find out, I have a gauge that acts the same way, it has been this way since I owned the car so I just chalked it up to a cheap gauge on a 253K cluster. Mine will also go all the way pegged and bounce around sometimes when I let off the gas before I stop at the light..

sofakingdom

iTrader: (1)

Yours also sounds like it needs a sending unit. Just swap it out. They go bad all the time.

I simply do not understand the reluctance on the part of others to just swap that thing out. The point isn't to know the actual oil pressure to the nearest tenth of a psi or something; it's merely to make sure the trim package in the dash is working. The gauge itself virtually never fails, regardless of its "accuracy" (which is, granted, not great, ever); when the stock system breaks, it is almost invariably the SU that goes bad. It's like when a cylinder starts missing, you just swap a spark plug and move on. No need for test equipment to evaluate the spark plug somehow; ya gotta pull it and dink with it ANYWAY, might as well just swap it out and be done with it. Same here. If it suddenly starts reading different, and replacing the SU doesn't restore it to reading about what it did before, then it's a pretty safe bet it's reading different because the pressure REALLY IS different. At that point, then MAYBE it makes sense to start messing with a mechanical gauge. Until then, it makes NO sense to buy a mech gauge, fart around with it, and let it tell you that the stock SU has gone bad, and THEN replace the SU. Just doesn't make sense to go about it in that order.

However, in the OP's case, I do not think dinking around with the gauge, ANY gauge, will resolve his issue... it will only establish that the stock gauge system is telling the truth, more or less. Which since he already has a new SU ordered, he might as well go ahead and just swap it out, and eliminate that possible explanation for what he's seeing, and move on from there.

GeneralDisorder

iTrader: (1)

You're right, sofa. I agree absolutely with your analysis. For a layman this likely makes sense. I will add that you do need to use a QUALITY sending unit. Not the cheapest thing off ebay or rockauto or whatever. Something that's likely to work.

As a professional, I have an obligation to test and verify. Cross I's and dot T's and so forth. I have to practice science or down the rabbit hole I go with my customers wallet. And of course I already own the mechanical shop gauge test equipment. And a filter cutter - which is often the first thing I reach for - I just had a suspected rod knock get towed in on Thursday. I didn't even put the key in the ignition. I drain the (1 quart including the filter) of oil from the pan, cut the oil filter and observed all the constellations of the heavens in the bottom, and then picked up the phone. This way there's no additional collateral damage from my efforts to "locate the problem".

GD

1991redz28ga

I changed the opsu out and now it reads 45-60 all the time. Lol. I guess everybody is right, who knows if the reading is even close, high or low. No metal that I can see in the oil. Haven’t cut the filter, but feel ok about it all now. If the 350 does die, an LS is in my future. So, I kind I have mixed feelings about it being ok.

thanks everybody

GeneralDisorder

iTrader: (1)

45 psi at hot idle? This seems high to me. Like I said swapping the sending unit is a guess, and if you get a faulty part in the box - which is by no means impossible and actually somewhat likely on older cars with poor representation in the auto parts supply chains - you could be just that much more confused or possibly even lulled into a false sense of security. You may have just swapped a low reading sending unit for a high reading sending unit. Which only serves to make you feel better. It actually changes nothing about the actual condition of the engine.

I just don't trust the 30 year old wiring and gauge cluster, nor would I put much trust in a shiny new Chinese sending unit. I've seen way too many bad off-shore parts right out of the box.

GD

OrangeBird

iTrader: (1)

GD makes a very good point here , we are always talking about how the factory "gauges" are in reality nothing more than eye candy , and notoriously inaccurate even when functioning seemingly normally .

If an absolutely accurate measurement of the oil pressure is wanted/needed , wouldn't the best results be obtained with a known accurate direct reading gauge ?

QwkTrip

iTrader: (22)

My year car (1989) the v6 gauge and V8 gauge use a different ohm range. Make sure you have the right sensor for your gauge.

This is why oil pressure reads so low in the v6 cars when somebody does an LS swap. The oil pressure sender is better calibrated to the V8 gauge.

Twin_Turbo

iTrader: (25)

Get a mech gauge to get a real pressure reading.

Reolace sender w correct ine for your gauge (max psi on scale)

mikeceli

iTrader: (2)

If the oil pressure sending unit (OPSU) is leaking, BOTH problems will be solved w/ the new OPSU.

sofakingdom

iTrader: (1)

Sounds like you got a crappy OPSU. Try this one.

https://shop.advanceautoparts.com/p/...tch+%26+sender

Any signs of metal chips in the oil? Pull out your dipstick, wipe it with a clean white rag (or even, a paper towel or toilet paper); examine closely in bright direct sunlight for glitter.