Only 2,500 miles since my last engine oil change when I switched to a Synthetic Blend. Oil pressure has always been good since the switch until lately, regardless of conventional oil or the Synthetic Blend.

But lately at start up & driving, at first, the oil pressure is good. Shows about 60/70. But after a couple hours of driving & the engine temp is fully up to operating temp? At idle, like at a red light, the oil pressure has dropped to just 2-3 marks over the red line on the Oil Pressure gauge. But the gauge goes back up to good pressure as soon as I pull away & I'm back up to 1500rpms & above. And as more driving is done, with then engine temp still "high" but not getting any hotter, the oil pressure gauge continues to drop even more at idle. Drops to the point that the needle is in the red area.

But I have noticed that (automatic tranny) that if I put it in Neutral, like at a red light, the oil pressure comes up out of the red back to 2-3 marks above the red. Still not exactly what I call good, but how can the tranny affect engine oil pressure?

Car feels perfectly fine the whole time so I kinda suspect a bad gauge but obviously am not going to just ignore it & assume that it is just a bad gauge.

Any thoughts on just a bad gauge or something more? Has a TPI 350 that does have over 225k miles on the odometer, but no idea if the engine has ever been rebuilt or replaced at some point. Gets an average of 17mpg, have gotten 21mpg on a pure Interstate highway trip, so I don't think the odometer is correct for the engine, just the chassis.

 

Could be a barrage of things, but I would learn towards weak oil pump. If you can get to it easy, try installing a temporary mechanical oil pressure guage and see what it reads

 

Where would be an easy place to temporarily tap into the engine to add a mechanical oil pressure gauge? I have a cowl induction hood so the gauge face could easily be mounted there to read.

 

I'd just try a new sender first. It's the same work to mount a test gauge as to change the sender, and it doesn't get you any closer to a solution.

 

Isn't the sender just above the Drivers Side engine mount in the side of the block? In other words.....A real PITA to reach!

Although the fact that is seems to be real consistent in its behavior, that seems to lead more towards an over breakdown of the oils constancy. Cold = good, hot = thin.

 

No, above the oil filter. Or beside the distributor in some years.

A sender is cheap and has a chance of fixing the problem. If you put on a test gauge and find that the pressure is actually fine, then you still have to replace the sender anyway.

Either way if you find that the pressure is actually low, you're just as screwed.

 

back of engine next to dist... just unplug the connector and take off sending unit, then you can (plumb) any 10-dollar mechanical gauge to it.

 

Another factor to keep in mind in deciding if it's worth messing around with a test gauge is that if you don't have a clattering hydraulic valvetrain, you don't have low oil pressure.

 

No clattering inside but I haven't gotten out & opened the hood to listen for any either.

Isn't is odd though that the tranny in Neutral or in Drive can effect the engine oil pressure? Why when it reaches red at idle I can shift to Neutral & the engine oil pressure goes up just out of red? I don't understand how the "strain" of the tranny , at idle, can affect engine oil pressure.

 

neutral= slighty higher rpm's

 

It's the slight difference in engine speed from drive to neutral that makes the difference. How many miles on the engine? What weight oil are you running? Maybe you need to step up one grade. Have you noticed any foreign matter(metal) in your oil when draining it?

 

Yes, slightly higher rpms in Neutrals vs Drive stopped, but maybe only a 100 rpms on the gauge changes the oil pressure that much?


As said in my first post about the miles....."Has a TPI 350 that does have over 225k miles on the odometer, but no idea if the engine has ever been rebuilt or replaced at some point. Gets an average of 17mpg, have gotten 21mpg on a pure Interstate highway trip, so I don't think the odometer is correct for the engine, just the chassis"

As for the oil weight....I had a garage do an intake manifold swap & the receipt showed 5w30 Synthetic Blend. No brand listed. Only 2,500 miles since the manifold swap so no oil changes since then to see any metal particles. Yes, 5w30 seems pretty thin, but it wasn't doing this months ago. Just started dropping like this recently. If it was reading like this on start up then I would "know" it was a bad sender. But the gauge reads fine for the first couple of hours of driving. I suppose it could just be the heat affecting the sender.....

I bought a new filter & 10W30 Full Synthetic the other day, but after checking the miles I'm questioning if I really need to change the oil or if it is just a bad sender.

 

5w30 and 10w30 behave the same at temperature.

 

I know at running temp they do (30) & was thinking the 5 that the garage put in seems kinda low. I was just saying what the shop put in (5W30) & what I bought (10W30).

 

 

I know that. And is why I'm questioning a bad pump vs. bad sending unit vs bad oil.

 

Well the 5w30 oil should be behaving just the same as the 10w30 did, and oil pumps practically never wear out, so try the sender first.

 

My sender busted and showed no oil pressure at all.. I replaced with a brand new GM one .. costed 45 bucks but it works perfectly for 5 years now.

 

I would blame the sender. Ive gone through like 3 of them. Not only did they get more and more inaccurate as they fail, which is occasionally suddenly and occasionally gradual, but none of them give the same oil pressure readings on the gauge.

I wouldnt pay that much attention to it. And mine has been acting weird ever since I switched to synthetic. So I echo the otehr guys, replace the sender, and even if that doesnt change anything I wouldnt worry about it until the engine is making bad noises.

 

First of all, install a mechanical gauge to confirm that the oil pressure is low once the engine gets hot. If the mechanical gauge says pressure is good then you can try changing the sending unit.

If pressure is still low, the most common cause that I've seen is spun cam bearings. Buy a new oil filter and replace the old one. Cut open the old filter. Take some of the filter material out and squish it in a vise to get all the oil out. Then open it up and look for metal in the pleats. Any bad bearings will show up in the filter. Failed or spun cam bearings will not make any noise but will drop oil pressure when the oil gets hot at low rpm.

 

Similar problem to yours, although it's never had any effect. Dropping speed into a stop sign/light, my oil pressure would drop down way low, until I started to leave again. Mysteriously, it went away after about two weeks, and I haven't seen it since. Don't know what I did, but it seemed to be intermittent. Hope your problem (and mine for that matter) are as simple as mine seemed to be.






(Knock on wood)

 

It has been going on for maybe 2 months now, but it also doesn't get driven but a couple of times a month.

I swapped in a spare sending unit I had but I think it might be for the wrong gauge. My gauge goes up to only 60 & it pegs the gauge over 60 at idle. Online it shows 2 different sending units so I think my spare might be the "other one".

Gonna pick up a new one today & see what the gauge reads after swapping it in.

 

I believe the "other one" is the V6 sender for the 90 psi gauge.

 

The viscosity improver in multiweight oil breaks down over time. So as it ages, the hot viscosity of 5w30 can eventually end up not being a 30 grade anymore. 10w30 tends to lose viscosity more slowly because the range isn't as aggressive and it uses a lower concentration of viscosity improver. But quality of the oil is important too, some will age better than others.
Gradual aging might be why you're seeing lower pressure than you did when the oil was fresh.
The oil will heat more slowly than the coolant, so even though your coolant temp has leveled off, the oil pressure will keep dropping as it heats further. It takes a long time for the oil temperature to reach it's peak.

Electric oil pressure senders aren't very accurate, and a new one still won't be trustworthy. If you're worried about the real oil pressure a mechanical gauge is really the best way to get it. The main value of the electric sender is to get your attention when it starts acting different than usual. When that happens it's better to use a mechanical gauge to get the final word.

On a 2.8 car I had a hot idle reading so low it turned the oil light on. But when I checked with a mechanical gauge, when the light came on I was still getting 22psi. I replaced the sender but even the replacement is reading low (just not quite as bad). I've been through 3 aftermarket senders and they all read lower than reality at hot idle, but start out reasonably accurate at lesser temperatures.

 

Yeah, but only 2,400 miles & 11 months for a synthetic blend to break down? Seems more likely to be a bad pressure switch.

I haven't driven it since the swap to the spare used one I had, but I did pick up a new one yesterday. If the spare seems to make the gauge reade wrong too after a long drive? I'll swap in the new one & see what the gauge reads them.

 

According to their site both were for V8s. One for a 60psi gauge & one for a 80psi gauge. I have no idea what psi gauge is in the car that the engine came out of.
http://www.autozone.com/autozone/par...mString=search

My '87 has a 60psi gauge so if the '85 donor car has a 80 psi gauge? That could be the reason for inaccurate gauge readings due to a different voltage range for the gauge sweep.

The PS140 shows the same for '85 & '87 for a 60psi gauge.

The PS133 is for a '85 80psi gauge. And the 80psi gauge/PS133 isn't even an choice for '87, on the site.

But I picked up a PS140 at the store yesterday just in case my gauge still doesn't seem to read right. Then I will at least know that the correct pressure switch is in the car. Can't guarantee that would be a good electrical part though! I'd pick up a mechanical gauge afterwards for peace of mind, if it still reads low after a new switch was in.

 

Put 10w 40 in it and see if it makes a difference.

 

I just verified today that the LB9 that I took the pressure switch off of came fram a car with a 60psi oil pressure gauge, so at least I know it is the correct pressure switch.

Still haven't driven far enough to heat up the oil as long as when my low pressure was occuring. At least I know I have the right switch now so the gauge shouldn't be getting incorrect readings.

Next step? Oil change if it still gets low at "hot" temps.

 

Oil change and like I said back up there, cut open the filter for inspection.

 

But if it was just a bad electrical component, then that would be a waste of money.

 

Its not a waste of money if it answers your questions.

 

Seriously guys? Just start swapping multiple items at once in pursuit of a cure? How will you know which item cured the problem? Answer is....You won't!

If I start swapping more than 1 item at a time, then I do not know which item solved the problem. Doing an oil change at this time still would not tell me if it was the filter, the oil, or the sending unit. So yes....It WOULD be a waste of money.

To solve any problem, one must change ONE item at a time then see it that ONE item solved the problem. Not change multiple items at once.

Next solution suggestion will be to go buy a brand new crate engine. That will cure my gauge readings!

 

I'd run 10w40 just based on miles and climate, it may ding you a tiny bit on mpg and performance but I don't think be able to measure it without a dyno. The extra bit of cushion on the bearings may save 'em if you get ahold of some crappy gas.

 

Right! So have you put a mechanical gauge on it yet to determine if the oil pressure stays up when it's hot or not or are you just going to swap out the sending unit and hope that's what's wrong? If a mechanical gauge says that pressure still drops then changing the sending unit isn't going to help.

 

Really? What is my mileage on the engine?

I know my odometer reads over 200k, but I'm getting the same mileage (1mpg less) as the rating was when it was brand new on the stealership lot.

So tell me....You really think that a 200+k mile engine has only lost 1mpg after 23 years? Or do believe the more likely scenario that a previous owner has replaced the engine at some point?

In any case? No.....I'm just taknig ONE step at a time in my pursuit of fixing the gauge reading. I'm not gonna just gonna go start swapping piece after piece out in the hopes of making a gauge read differently.

I guess that is the difference between me & you guys. I swap ONE piece at a time in pursuit of something. I guess my wallet is fatter than most because I don't "fix" multiple items at once. I fix the ONE item that is broken.

I guess I shoulda known better than to ask people's opinion on this matter. A lot of speculation that involves more money & time than actually finding THE problem.

 

The sending unit was FREE. You wanna buy me the mechanical one?

 

Having proper tools is part of doing correct diagnostics. I'm sure your local auto parts store has tools they loan out. If you want to throw darts at a board and guess what's wrong then that's up to you.

 

Which is exactly why I don't go buying random stuff (like most people i see posting here on TGO), or renting tools, to diagnose anything. NOT WHEN I ALREADY HAVE A PERFECTLY GOOD USED PIECE IN MY GARAGE ALREADY!

What was I thinking when I first posted? I KNEW I would get a bunch of useless replies!

I'm Unsubscribing now so that I don't hafta read anymore of these useless replies. I knew what what was needed to be done in the beginning but apparently I had a brain fart & ended up posting.

 

Well, I'll have to admit this is the first I've read this thread. But, I'd recommend buying a mechanical gauge to verify what your oil pressure really is. Relying on the in-dash oil pressure gauge for diagnosis is unwise.

Oh, and my '82 305 with 304k, matching numbers, stock bore, no evidence of any rebuild, got the same mileage as the car was rated new. Of course, with the LS1 in it now, it actually gets better mileage, but...

Since you're unsubscribing, I'll go ahead and lock this thread that is no longer of use to you. It's really hard to help someone who doesn't listen...