The Car: 1985 GT 4-speed
Bought it without an engine.
Installed a 1987 2.8.

The Problem:
When the car is started, the oil pressure gauge pegs at 80 PSI. The pressure sensor has been replaced and produces the same results.

When the engine is stopped, the gauge remains at 80 PSI, until the ignition is turned back On, without starting the engine. Then the gauge drops to Zero PSI. If the ignition is then returned to Off, the gauge remains at Zero PSI.

Anyone have any knowledge of the problem and a solution? All other instruments seem to function properly.

You may not have an oil pressure sensor that is compatable with your gage. Some sensors were a low oil pressure switch for a light on the dash. You may have to get a stand alone oil pressure gage to see how much oil pressure you really have.

[This message has been edited by Ken_86gt (edited 08-12-2006).]

from OS
The dancing oil pressure gauge syndrome

This is not strictly an electrical blunder. Actually it is the result of a mechanical design flaw that causes an electrical problem. In short, the location of the oil pressure sensor on vehicles with the V6 and air condition is less than ideal. On those cars, the sensor is mounted vertically on an extension tube. The drain hole on the connector lets water enter the sensor, which lets it rot internally.

The effect is the "dancing oil pressure" needle which many of us experience. If you have an 85, you are lucky, because the oil gauge in the tach has a mechanical damper, which hides the problem from you. If you live in a dry climate, you are in luck too. But if yours has failed, it will fail again unless you cure the root of the problem: seal the vent hole in the oil pressure sensor connector. GM has designed a plug to do this, but a bit of silicone will do just fine. Additionally, a shield (P/N 1009 6127) should be installed and sealed with silicone to protect the sensor from water. On the 1988 V6 GM recommends relocating the Oil Pressure Sensor to just above the A/C compressor by using some adapters (see TSB 88-8-22).

quote Originally posted by Ken_86gt: You may not have an oil pressure sensor that is compatable with your gage. Some sensors were a low oil pressure switch for a light on the dash. You may have to get a stand alone oil pressure gage to see how much oil pressure you really have.

The unit I installed is a new AC Delco sensor. I did purchase it for an 87, but the part number listed for the 85 is the same.

UOTE]Originally posted by 3800superfast:

from OS
The dancing oil pressure gauge syndrome

This is not strictly an electrical blunder. Actually it is the result of a mechanical design flaw that causes an electrical problem. In short, the location of the oil pressure sensor on vehicles with the V6 and air condition is less than ideal. On those cars, the sensor is mounted vertically on an extension tube. The drain hole on the connector lets water enter the sensor, which lets it rot internally.

[/QUOTE]

Actually, the gauge doesn't dance. It stays solidly on 80 PSI when the engine is running. The sensor I just installed hasn't seen rain, yet, so I don't believe that's the cause. I am seriously thinking it's an electrical or gauge problem, but if I can find a sensor from an 85, I'll try that,

You may want to approach it the same as you would with the pegging temp gauge--I know the wires & location/connetors will be different and you would need schematics, well worth a look/try..
http://www.fierosails.com/tempgage.html

I just replaced a 6 month old oil pressure sensor to correct the "pegged gauge" problem. The fact that you replaced the sensor and still have the problem *suggests* that it's not the sensor, but this does not *prove* the new sensor is good.

The oil pressure sensor is nothing more than a variable resistor that ranges from zero to 90 ohms corresponding to zero to maximum pressure. The fact that your gauge pegs off scale suggests that it's resistance is greater than 90 ohms. This could be the result of a bad sensor, bad connection at the TAN wire to the sensor, or an open circuit anywhere between the sensor and the gauge..

To check the gauge and the wiring, you can temporarily connect a jumper from the TAN wire at the oil pressure sensor to chassis ground. This should cause the gauge to indicate minimum pressure (you have to run this test with power on), If so, the problem is either at the connector or the sensor itself.

To check the sensor itself, remove the connector and measure the resistance from sensor terminal (that would normally mate with the TAN wire) to chassis ground. The resistance value should be near zero with engine off (no oil pressure). If not, try measuring the resistance from that same terminal to the metal base of the sensor. Again, the resistance should be near zero. If not, you have a bad sensor.

quote Originally posted by Mark: I just replaced a 6 month old oil pressure sensor to correct the "pegged gauge" problem. The fact that you replaced the sensor and still have the problem *suggests* that it's not the sensor, but this does not *prove* the new sensor is good. The oil pressure sensor is nothing more than a variable resistor that ranges from zero to 90 ohms corresponding to zero to maximum pressure. The fact that your gauge pegs off scale suggests that it's resistance is greater than 90 ohms. This could be the result of a bad sensor, bad connection at the TAN wire to the sensor, or an open circuit anywhere between the sensor and the gauge.. To check the gauge and the wiring, you can temporarily connect a jumper from the TAN wire at the oil pressure sensor to chassis ground. This should cause the gauge to indicate minimum pressure (you have to run this test with power on), If so, the problem is either at the connector or the sensor itself. To check the sensor itself, remove the connector and measure the resistance from sensor terminal (that would normally mate with the TAN wire) to chassis ground. The resistance value should be near zero with engine off (no oil pressure). If not, try measuring the resistance from that same terminal to the metal base of the sensor. Again, the resistance should be near zero. If not, you have a bad sensor.

I agree
Don

Thanks, Mark. I'll give it a try.

"+" to all

Well, I've run the tests on the sender unit and it checks out. I've run the tests on the wiring and gauge, and it checks out also. But it still shows high oil pressure.

Someone told me that it may have an oil pump from a 3.4 installed in it. They said they have one in their 2.8 and it runs at about 60 PSI all the time. According to them, the pump for the 3.4 was improved and has a much higher pressure.

Anyone substantiate this? If so, I'm having one installed in my 87 that has sagging oil pressure.

Thanks
Charlie

Charlie , That doesn`t sound right to me, it would be the first Iv`e heard of it also, mainly due to the fact that your gauge is doing what you said at the top post .
** When the engine is stopped, the gauge remains at 80 PSI, until the ignition is turned back On, without starting the engine. Then the gauge drops to Zero PSI. If the ignition is then returned to Off, the gauge remains at Zero PSI. ***
I`m still thinking this one through , the engine being off and the gauge is still reading 80psi , wonder if you swapped in the other gauge from your 87 --what would happen ?

The 87 gauge is on top of the radio. The 85 gauge is in the instrument panel. As I said, the tests outlined by Mark, and I got results that indicate everything is good. The only concern I have with the high indication is that the pressure may indeed be low, or drop too low and I wouldn't know it. After all, the engine has about 145K on it.

I'm taking it up to the East Tennessee Fiero Club cookout tomorrow and I'll bet Robert to put his mechanical gauge on it and then we'll know the reading, for sure. I'll let you all know later.

BTW, anyone have any thoughts on the 3.4 oil pump?

Mine will do some of these symptoms occasionally as well...

Mine pegs all the way over, off the scale. I can pop the decklid and jiggle the sender (the extension tube wiggles a little) and make it work right. If I keep jiggling, it will go peg/correct/peg/correct really fast.

It seemed like I could consistently do it when I touched the bracket (that's probably supposed to be bolted there but isn't on mine) to the mount arm for the A/C compressor. When I would touch those two toether, the guage would peg every time.

My sending unit is less than 6 months old because the original one I had would "dance". This one was just fine for a long time, then started doing this about a month after I had my A/C fixed.

EDIT: I have a 1986 SE 2m6 v6 with 4spd manual and A/C.

[This message has been edited by squisher86SE (edited 09-07-2006).]

Charlie,
Was the harness that was installed originally from that car or another 85?
I have heard that there are some minor wiring differences between the 85s and later years. (Not nearly as dramatic as the 84s, but there, nevertheless.)
I *know* this is true of some of the taillight harnesses. Perhaps the engine harness is different too.

Just a shot in the dark. Take it for what it's worth.

Raydar: I'm talking about the 87GT. As you know, it's all original. While I was on my way to the ETFC cookout, the alternator went out. I was taking it up there to have him replace the oil pump anyway, so I just let him repair both.

He put the high volume 3.4 oil pump in it, and it did boost the oil pressure. Before doing that, he checked it with a mechanical gauge and it was only pumping 8 PSI at hot idle. The engine is definitely bad. Right now, with the new oil pump, it's putting out about 15 PSI at hot idle. Just on the edge of the warning light flickering on and off. I've got to try to find a tow dolly and go up there next week to tow it back. It probably could be driven back, but I don't want to take the chance.

Anybody have a super-cheap deal on a 3.4 short block? I can get hold of a 3800SC Series I for a couple hundred, but don't want to go that route with the 87.

Okay. I got confused. I thought we were still talking about the 85 gauge pegging.

I don't know of any 3.4s, but Aceman had a post in the mall for rebuilt 2.5s.
To the point... He also has several minivan iron-head 3.1s, among many others.
An acquaintance of his bought out an engine rebuilder who went out of business. IIRC, the price was ~$500 shipped for a long block. Has the hi-po heads, but a bit milder cam.
I was seriously thinking about doing one of these before I had Ed do the 4.9, but I lost contact with him.

[This message has been edited by Raydar (edited 09-08-2006).]

Steve, actually, I did change discussion to the other car. The 85 has Chris Ackerman's 87 engine with the 87 wiring harness. That is the gauge that pegs. My 87 is just the opposite. The oil pressure has gone away. That's the one I have to go up to Tennessee to tow back. The 87 is the one I need a 3.4 for.

The 85GT is running very well. Ran all the tests outlined above on the sensor and wiring and they check out to be good. I'll eventually get a chance to put a mechanical gauge on it to determine it's true reading.

quote Originally posted by 3800superfast: from OS The dancing oil pressure gauge syndrome This is not strictly an electrical blunder. Actually it is the result of a mechanical design flaw that causes an electrical problem. In short, the location of the oil pressure sensor on vehicles with the V6 and air condition is less than ideal. On those cars, the sensor is mounted vertically on an extension tube. The drain hole on the connector lets water enter the sensor, which lets it rot internally. The effect is the "dancing oil pressure" needle which many of us experience. If you have an 85, you are lucky, because the oil gauge in the tach has a mechanical damper, which hides the problem from you. If you live in a dry climate, you are in luck too. But if yours has failed, it will fail again unless you cure the root of the problem: seal the vent hole in the oil pressure sensor connector. GM has designed a plug to do this, but a bit of silicone will do just fine. Additionally, a shield (P/N 1009 6127) should be installed and sealed with silicone to protect the sensor from water. On the 1988 V6 GM recommends relocating the Oil Pressure Sensor to just above the A/C compressor by using some adapters (see TSB 88-8-22).

When you say "dancing", does that refer to it bouncing back and forth? That's what mine does and it really gets on my nerves.

quote Originally posted by krackley: When you say "dancing", does that refer to it bouncing back and forth? That's what mine does and it really gets on my nerves.

Welcome to the forum, The fix above will work, or you can buy a 88 sending unit. I would check the pressure with a mechanical gauge first to verify thats the problem, hope this help some..

Thanks 3800, I'll give it a try.