Weird oil pressure issue-two things. Car starts, cold, indicates 60-ish PSI at 1500RPM. (Even if I use the clutch to pull the RPMs down to 1K or so it remains well above 40PSI) As the car warms up, oil pressure slowly goes down until it's sitting at about 20PSI or so at idle. Also, oil pressure will drop between 3-10PSI or so as the throttle is opened.

EDIT: 1987 GT, V6, stock.

[This message has been edited by Tyrfin (edited 04-25-2012).]

Just making sure, you've checked your oil level, correct? The oil pressure will normally go down as it gets warmer because the oil gets thinner (at least that's been my experience) so the first part of your post sounds normal to me. As long as your oil level is correct, I'm wondering if you could have a pressure sending unit going bad like a bad or weak spot in it's pressure range, similar to a bad fuel tank sending unit. That's just a guess on my part though.

[This message has been edited by Bahrbarian (edited 04-25-2012).]

That sounds pretty normal for an engine with maybe 130,000 miles on it. Where is the pressure at highway speeds? If you maintain 20 most of the time you are OK. The bearings are getting a little sloppy, but no panic.

I have nearly 300,000 miles on my '86 GT and thats typical operation for me even with that mileage. Like Gall said above some wear on the bearings but you're good. Just keep eyes on your oil level periodically and make sure when you change oil you use the longer ACDelco PF52 filter and an even five quarts.

I wouldn’t expect the oil pressure to drop with an increase in RPM’s. The only scenario that I can see happening is a bad bypass in the pump. When the oil is cold it’s to think to go through the filter so there is a bypass that opens when the pressure is high. As the oil thins out the bypass closes and the oil goes through the filter (which helps keep the pressure up). If the bypass sticks open then as the oil thins the pressure drops. (here’s where I’m not totally sure here) I think what might be happening is the pressure sender gets it’s feed directly after the oil filter but with the bypass open it doesn’t so as when the oil is think there is still enough pressure to show high on the gauge. As the pressure thins and you increase the RPM’s the bypass opens more and less oil gets to the sender dropping the pressure at the sender. Again, this is a guess but under all normal conditions the pressure should go up, not down, when the RPM’s increase.

I'm a little confused. With a typical multi-weight oil (10-30) I thought it would be 10W (thin) at cold temperatures and as the engine warmed up it would eventually act as 30W (thick), not the other way around.

Perhaps as the engine warms up the bearings expand causing a wider clearance, hence lost of pressure despite the 30W change in viscosity.

Just thinking out loud..

Spoon


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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

[This message has been edited by Spoon (edited 04-25-2012).]

Watch that you haven't over-filled the oil. With too much oil the crank will whip air into the oil causing a pressure drop with rpm increase.

Definitely checked that-I have a high capacity filter and added an even 5qts less than a week ago. And replaced the oil pressure sending unit two months ago. I'll look into the oil pump when a I get a minute to work on it. Motor does have 120,000-ish on it from the odometer. Maybe I'm just being paranoid because I just had a lifter go bad.

It’s not that the oil is thinner when cold. I found this:

At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

So what its saying is that the oil is as thin as 20 weight when cold and as thick as 50 weight would be at 100°C (which is still thinner than 20 weight when cold). Another way to say it is: when 20W-50 is at 10°C, it’s at the weight that SAE 20 would be at 10°C but when at 100°C (the standard measurement temp) it is what SAE 50 would be at 100°C. SAE 20 is thicker at 10°C than SAE 50 is at 100°C.

I still say that you might have a bypass problem (as some of you others might when you step on the gas and the pressure drops). Keep in mind that it’s not that the engine isn’t getting lubricated but rather the oil isn’t going through the filter.

If you're sure it's dropping oil pressure as RPM increases, then that's not normal, it should noticeably do the opposite.
FWIW, 60degreev6 lists the oil pressure spec as 10psi @ 500rpm, 30-55psi @ 2000rpm, but I don't know what their source is for that. I've never seen that in a service manual. It's consistent with what I get on my car though.

Sometime back I was researching what would cause oil pressure to drop with increasing RPM. That was on a totally different car, it turned out to be the oil filter, but this is what I have in my notes:

dropping pressure at high rpm
==========================================
collapsed oil filter; stuck bypass valve
dented oil pan (might interfere with pickup flow)
broken pickup tube
clogged pickup
oil too low (starvation)
oil too high (frothing)
high volume pump in stock pan (starvation)

things people try to diagnose:
overfill oil by 1qt
drain oil, hit pan, listen for loose pickup

Mechanical or electric gage? (I'm assuming elec since you mentioned having a new sending unit). Just throwing something out there in th wind--but--what's your electrical system showing as you accelerate? Slipping alt belt on acceleration could result in voltage drop and that can affect what your gage is indicating. Remote, but something to look at if you find nothing else wrong.

Electrical gauge, yes-stock one. Let me clarify-Pressure first drops as throttle opens, then will rise somewhat as RPMs actually pick up. If it's in gear and rolling, and I let the throttle close, oil pressure actually rises for a moment before falling RPM brings it back down. Is it possible that something is somehow applying vacuum to the engine when the throttle opens?

I think it's more likely that you have a grounding issue in your electrical system and you are getting a false reading on your gauge.

Very well could be any of the mentioned symptoms I'd think.

From the '87 FSM:

FOR LE2, ENGINE OIL PRESSURE
69kPa (10 psi) MINIMUM AT 500 RPM AND 207·310 kPa (30-45 psi) AT 2000 RPM. FOR LH7, ENGINE 01 L PRESSURE 345-379 kPa (50-55 psi) AT 2000 RPM.

From what I understand the Fiero 2.8 is an L44, and simply superseded the LH7 which was also an HO motor (LE2 was typical A-Body production).

Ive seen oil pressure drop under acceleration when someone installed the wrong oil filter. Double check the part # on the filter.

The pressure drop when opening the throttle could be an electrical quirk. I'd hook up a mechanical gauge to verify the issue. The electric sender is helpful to see possible problems, but not accurate or well behaved enough for final diagnosis.

quote Originally posted by Tyrfin: Pressure first drops as throttle opens, then will rise somewhat as RPMs actually pick up. If it's in gear and rolling, and I let the throttle close, oil pressure actually rises for a moment before falling RPM brings it back down.

The problem is almost certainly something electrical affecting the OP gauge circuit. Oil pressure will vary with rpm, but there's no way it should ever vary with throttle opening alone.

I would start by checking the grounds for the engine wiring harness, and also check the ground strap between the engine block and chassis. The next step would be to plumb a mechanical oil pressure gauge to confirm (or not) what the electrical gauge is telling you.

Hopefully I'll get that squared away today or this weekend.

EDIT: Only exhibits throttle-open pressure loss when hot-could a wire be expanding with heat, touching something and shorting and then shrinking away from contact when the motor cools down?

[This message has been edited by Tyrfin (edited 04-27-2012).]

quote Originally posted by Tyrfin: Only exhibits throttle-open pressure loss when hot-could a wire be expanding with heat, touching something and shorting and then shrinking away from contact when the motor cools down?

Possible. More likely it is temperature affecting a marginal (loose and/or corroded) connection somewhere in the engine compartment ... such as one of the harness grounds or the ground strap. It could also be a failing connection at a splice somewhere inside the harness, but that's a lot less likely. The main harness ground point on the V6 (as far as I remember) is located at the bell housing near the starter motor, and the ground wiring passes very close to the exhaust downpipe, so that is a potential area for a wire with abraded insulation or accelerated corrosion.

If it were an electrical problem the temp and fuel gauges would also do the same thing (they are the same type of gauge). And, yes an oil pump problem can cause pressure to drop when the RPM’s increase and a momentary drop is the most likely symptom you’d see. Putting a mechanical gauge on is a great idea as is double checking the filter. The wrong filter can either restrict flow to much causing the bypass to open or have no flow causing the same thing. I still think you’re looking at a pump problem (the bypass is in the pump).

Grounds seem okay-resistance from negative terminal to points on the engine/chassis is about 0.04 so that doesn't seem horrible. I'll probably just toss a pump in it this weekend and see if that helps. The filter is supposedly the right one according to the NAPA book.

Try another oil filter and reseat the connector for your oil pressure sending unit a few times.

Same thing happens to me. I replaced the bearings in my car but it does it anyway. Possibly, since I never measured, I put improperly sized bearings in it, causing extra gap. I really don't care too much, it still sits 35-55PSI at cruise, just @ idle it sits at 19-20.

To clarify, it goes down when load increases, which I believe is what he is talking about. Example: Cruising part throttle 2500RPMs @ 35PSI, slam foot to the floor in a standard trans (no RPM increase) and it drops to 25-30PSI instantly, and let go so you are just decelerating, goes up to 40-50PSI. Get it? no real noises, just oil pressure changes depending on load.

With my car, @3000RPMs under light throttle (not decel or accel) it makes a rattle, if I increase the load in either direction, the rattle goes away.

EDIT: Generally it's too much slop in the bearings. I have also noticed my lifters bleed down (get a rattle when I start the car for a fraction of a second), so I don't know if that is related to my problem. I havent noticed any damage to the engine, and it doesn't get worse. Throwing Lucas in the engine makes it keep its pressure a bit better though

[This message has been edited by Racing_Master (edited 04-28-2012).]

So how does too much “slop” in the bearings cause a drop in oil pressure with an increase in RPM’s?

quote Originally posted by firejo24: So how does too much “slop” in the bearings cause a drop in oil pressure with an increase in RPM’s?

its not an increase in RPMs, its drop under load. SAME RPMs, this is with a manual I noticed it more, right after my manual swap. The auto diddnt really do it due to its RPMs rise the more you press the throttle.

the bearings would cause oil pressure to change due to engine load. higher the load, lower the oil pressure.

Also, this is the same thing he is stating:

quote Originally posted by Tyrfin: Electrical gauge, yes-stock one. Let me clarify-Pressure first drops as throttle opens, then will rise somewhat as RPMs actually pick up. If it's in gear and rolling, and I let the throttle close, oil pressure actually rises for a moment before falling RPM brings it back down. Is it possible that something is somehow applying vacuum to the engine when the throttle opens?

[This message has been edited by Racing_Master (edited 04-28-2012).]

Uhm, not. The bearing clearance doesn’t change under load. Worn (on wrong size) bearings cause a drop in pressure because basically the size of the container gets bigger. As to gauges, it could be a gauge problem but again the fuel and temp gauge would do the same thing.

quote Originally posted by firejo24: Uhm, not. The bearing clearance doesn’t change under load. Worn (on wrong size) bearings cause a drop in pressure because basically the size of the container gets bigger. As to gauges, it could be a gauge problem but again the fuel and temp gauge would do the same thing.

I diddnt think the bearings would cause it myself. I had to do a lot of research, and only 2 things can cause it. It appears to be a common issue in worn out engines. Sometimes it happened after people put new lifters in their car, other times just worn out engines. Since mine also has a rattle i suspect bearings. Bearings are one of the only things that sees load like that, that would affect oil pressure as such. Not really increasing clearance, mind you, just shifting slightly. Another thing I heared is a bad thrust bearing could cause this, the crank would move slightly back and forth. However it's all speculation.

If oil pressure is fine, Personally, I would not worry about it. It just moves a bit weird... I can confirm or disprove the theory of bearing clearance causing it when I do my engine swap and rid of my 2.8. I can measure the clearance and provide numbers, provided I remember to do so.

For me, I replaced the sender, and no matter what sender it does the same thing. I switch oil types (Synthetic over conventional, lucas added or no lucas) and it changes its characteristics. I run 5w40 right now in my Fiero. I get 60+ PSI on decel while a gear is engaged, and 25ish during accel. I cruise between 35-40psi

I did just replace the lifters.

quote Originally posted by Tyrfin: I did just replace the lifters.

It very well could be the lifters. I replaced my cam and lifters to get my car going, and it was a cheapo autozone camshaft. Was yours cheap too?

If we have the EXACT same problem, maybe you can do a test to check... Are you a stickshift or an automatic?

Run the car in 1st gear until you hit 3000RPMs (when at operating temp) then let the throttle go so you arent accelerating or decelerating. Have windows up and radio off and listen. if you hear a rattle rattle we have the exact same problem.

Putting new lifters in would have been good info to have at the beginning of the thread. I’d not considered that but thinking back on my days as a mechanic I remember that low end lifters will release their pressure to quickly (compared to specs) and cause a drop in pressure. Fortunately it’s not deal killer if the pressure drop isn’t significant. I’d still pull the filter and cut it apart to see if it’s got debris in it. If the bypass is stuck open the oil won’t go through the filter (that much) and it won’t collect stuff.
Sounds like it’s nothing major, good news!