I have had several posts about my oil pressure problem but felt it necessary to start a new thread do to the possible different nature of this issue. I have an 86 GT V6. I replaced my oil sending unit 2 times (to make sure it wasn't faulty). When I start the car up cold and drive it for a little bit the oil pressure is fine. Once the car is completely warmed up and I come to a stop and the car is idling the oil pressure gauge drops and the oil light comes on. If I give it some gas the gauge starts to go up fine again. What could cause the oil pressure to drop at idle after the car reaches normal operating temperature?
I think its normal for pressure to drop at lower rpm when the oils hot,higher rpms will always produce higher oil pressure,as the engine wears the low rpm pressure will become worse,my engine runs about 10 lbs at idle when hot and about 45 lbs at 2500 rpm,I would expect that 10 lbs at idle to drop as the engine wears.So whats causes your engine to run low pressure at low rpm? normal wear? I heard that 10lbs at idle is normal for the 2.8.
That is classic Worn Bearings syndrome. The oil warms up and gets thinner so it flows more easily thru the bearings. Another possible problem could be excessive play in the oil pump. Either way there is only 1 solution, Teardown and rebuild. A drop in oil pressure when it the motor warms up is normal but it should not drop low enough to light up the Idiot light. Some people will say switch to a thicker oil but that will just Mask the true problem.
What rpm are you idling at when warm? You say the light is coming on, are the hydraulic lifters collapsing? As long as they stay up you should have enough pressure. Cheap fix is to remove oil pan and shim up the by pass on the oil filter.
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03:00 PM
Kento Member
Posts: 4218 From: Beautifull Winston Salem NC Registered: Jun 2003
What rpm are you idling at when warm? You say the light is coming on, are the hydraulic lifters collapsing? As long as they stay up you should have enough pressure. Cheap fix is to remove oil pan and shim up the by pass on the oil filter.
That still will not fix the true problem. the bearings are worn to the point that it is causing a major loss of oil pressure and they need to be replaced. Plain and simple. If he replaces them now he is making sure that he does not have a failure such as a broken rod or spun bearing. If I remember correctly the Light comes on at 7psi with GM V6's of our era. When the light comes on you are risking MAJOR and catastrophic failure. A lot easier and chear to pull the pan and replace the bearings then get a hole engine due to a broken rod or spun bearing.
I have had several posts about my oil pressure problem but felt it necessary to start a new thread do to the possible different nature of this issue. I have an 86 GT V6. I replaced my oil sending unit 2 times (to make sure it wasn't faulty). When I start the car up cold and drive it for a little bit the oil pressure is fine. Once the car is completely warmed up and I come to a stop and the car is idling the oil pressure gauge drops and the oil light comes on. If I give it some gas the gauge starts to go up fine again. What could cause the oil pressure to drop at idle after the car reaches normal operating temperature?
Any and All help would be great! Thank you
Just how low does it go? If there is 10 PSI for every 1000 RPM I'd leave it alone. The bypass might kick in at 55 PSI so at 6000 RPM you might only still see 50-60 PSI. If so, all is well and keep going.
As the oil heats up and thins you get less pressure. Nothing odd about that.
TK
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05:00 PM
Fastback 86 Member
Posts: 7849 From: Los Angeles, CA Registered: Sep 2003
Mine did the exact same thing before I put it down. By the time I pulled it, it was burning oil and blowing smoke and oil out the PCV and the dipstick tube. I never bother to tear into it to find out exactly what it was, but we're pretty sure that the piston rings on cylinder # 5 were gone. I suspect cylinder 5 because it had much lower compression than the rest of the cylinders.
I'm not saying you're doomed. Some GM cars seem to do that right out of the factory. Real low oil pressure. But with a 18 year old car, odds are its not good. I would predict that things are going to get worse and you're looking at either an engine swap or rebuild in the future. How long? Who knows. You may make it a year, it might take a dump tomarrow. But sooner or later, that engine is done.
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06:12 PM
black Fiero SE Member
Posts: 341 From: New Hampshire Registered: May 2004
Thank you for all your help so far. I can't say I'm happy about the possable outcome. Do you think if I brought it to Pontiac that could look it over and be able to tell me whats wrong and I could take it from there????
Hi, Im sorry.. What happeneds is when the engine is just started it runs fine. The oil pressure is a little past the center of the gauge. After its been driving a while the oil pressure still runs fine but when I stop the car the needle hovers just above the red line. When I seem to shift it into park it picks up a little pressure so the light doesn't come on but when I go back to drive the light will come on. If I give it some gas the light goes out. My idle is about 1000 to 1100 RPM. I get worried when the light comes on. I just bought this fiero (I have 2 others) about a month ago and it seemed ok. The engine pulls very strong and it doesn't skip RPMs when accelerating. I havn't done my own tune up yet on it. I have checked the oil level and it seems to be at the right spot. It doesn't seem to burn any oil either.. I am nervous it is the engine as a few people have indicated on here. Any advice would be great!
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07:49 PM
PFF
System Bot
Kento Member
Posts: 4218 From: Beautifull Winston Salem NC Registered: Jun 2003
Hi, Im sorry.. What happeneds is when the engine is just started it runs fine. The oil pressure is a little past the center of the gauge. After its been driving a while the oil pressure still runs fine but when I stop the car the needle hovers just above the red line. When I seem to shift it into park it picks up a little pressure so the light doesn't come on but when I go back to drive the light will come on. If I give it some gas the light goes out. My idle is about 1000 to 1100 RPM. I get worried when the light comes on. I just bought this fiero (I have 2 others) about a month ago and it seemed ok. The engine pulls very strong and it doesn't skip RPMs when accelerating. I havn't done my own tune up yet on it. I have checked the oil level and it seems to be at the right spot. It doesn't seem to burn any oil either.. I am nervous it is the engine as a few people have indicated on here. Any advice would be great!
What you are saying describes worn bearings, I have seen it a lot of times. You can drop the pan without having to drop the cradle or pull the motor. You would need a tourque wrench to get it put back together correctly. You could even do the mains without having to drop the crank. The only other cause would be worn Camshaft bearings and that requires pulling the intakes off and I believe a special tool.
Main bearings oversized .50mm : CLEVITE MS2037P-50MM $33.99 (set) Rod Bearings oversided .50mm :CLEVITE CB1238P-50MM $2.49 (each) Oil Pump, High Volume : MELLING HIGH VOLUME M-95HV $66.39 or Oil Pump, Stock Volume : MELLING Stock VOLUME M-95 $35.99 Oil Pan Gasket : ENGINE_SEAL 82754 $4.99 Plastic Gauge to determine which Oversize to get: $1.79 Assembly Lube, should be red in color, kind used with camshaft installations. $5.00 or so
Replace the above items and you will be surprised how much better the oil pressure will be and how much longer the motor will last.
My first guess would actually be worn cam bearings from his description. Depending on mileage, might want to pull the pan and check crank/rod bearings, if they aren't scored and are just loose (check with plastigauge) just roll new ones in. Otherwise, time for a new engine or a rebuild. Oil pump clearance was also stated, I'd put a fresh on in while pan was off.
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10:23 PM
Aug 3rd, 2004
black Fiero SE Member
Posts: 341 From: New Hampshire Registered: May 2004
When replacing the main bearings do I use standard size , under size or oversize? I checked the Fiero store and they seem to sell undersized and standard? About how long a job does this take. A full day? It doesn't seem to difficult however I've said that before on Fiero and it takes days to do somthing
When replacing the main bearings do I use standard size , under size or oversize? I checked the Fiero store and they seem to sell undersized and standard? About how long a job does this take. A full day? It doesn't seem to difficult however I've said that before on Fiero and it takes days to do somthing
OIL PRESSURE 101- The oil pump picks up the oil and pumps it through the crank first then the rods and cam bearings and hyd lifters. If you have wear in the main bearings you can bet the other bearing are worse off. With the lifters being the last to receive oil that is the reason I said if they are not collapsing you should have enough pressure. You can throw a new pump and main/rod bearings in there and still be covering up the problem of worn cam bearings. My vote is still to remove pan, check bearing on crank furthest from oil pump. If it looks ok I would put a shim in the oil pump by-pass and bump up the oil pressure.
PS: Remember you are worried about the light going on only. If it had 1 more PSI of oil it could go out. Would that make the engine that much better/dependable?
[This message has been edited by fuzz (edited 08-03-2004).]
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09:20 AM
Tugboat Member
Posts: 1669 From: Goodview, VA Registered: Jan 2004
OIL PRESSURE 101- The oil pump picks up the oil and pumps it through the crank first then the rods and cam bearings and hyd lifters. If you have wear in the main bearings you can bet the other bearing are worse off. With the lifters being the last to receive oil that is the reason I said if they are not collapsing you should have enough pressure. You can throw a new pump and main/rod bearings in there and still be covering up the problem of worn cam bearings. My vote is still to remove pan, check bearing on crank furthest from oil pump. If it looks ok I would put a shim in the oil pump by-pass and bump up the oil pressure.
PS: Remember you are worried about the light going on only. If it had 1 more PSI of oil it could go out. Would that make the engine that much better/dependable?
A couple of nit picks here. Oil has to flow through the mains to get to the rods, but it doesn't go to the cam bearings or lifters from there. In fact, in a 2.8 the main oil gallery is a lifter gallery and the cam bearings are oiled on the way to the mains. Cam bearing really don't wear that much but this situation makes it more critical. I doubt you'll replace cam bearings with the motor in the car.
All the shimming in the world will not help your idle oil pressure, the pump simply isn't pumping enough. Shimming only affects top pressure.
You could use a HV pump as a band-aid but it would be better to replace the main and rod bearings. Use standard sizes, undersize bearings are to match a turned crank.
GL
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10:02 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
yup - I agree with all the above. drop the oil pan and look. replace the oil pump while you are down there. when you replace the bearing, unless your crank was machined, standard size bearings is what you want. its not to bad of a job. I did it for mine last year - worked out great. lost pressure, started getting some knocks @ 2000 rpm. had 1 worn bearing. dropped the pan, sanded the crank with 1000 grit, got 4 bearings, plastigaged untill I got the best fit, took back the other 3 bearings, put it back together, and away it went. checked on the bearing last month, while doing a new cam, and its just fine.
No one asked so I will and offer this suggestion. What weight oil are you trying to use. I have an 86gt which will not hold oil pressure on anything lower than 10w40 and the motor has over 113k on it and runs fine. I would not even consider a rebuild until 20w50 oil would not hold pressure or the engine started smoking from the exaust or out the top. The other thing that a lot of people overlook is when changing the oil pressure sending unit is to take a coat hanger and clean out the tube the sending unit screws into it gets cloged and will not allow the unit to function by blocking off the path to it.
If you are using 5w30 or 10w30 put in a bottle of STP and it will be fine untill your next oil change.
Do the above and enjoy driving. It's a fun car the more you drive it the more you like it.
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11:15 AM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
While the bearing may well be the problem, no one has mentioned the gauge itself. When cold, the oil pressure should be between 60 and 80 psi. When warm, 40 to 60 psi. Have you tried hooking up a seperate oil pressure gauge? You've never mentioned if your motor stalls out when the oil pressure gauge drops. Since the pressure switch is connected to the fuel pump, low oil pressure will cause your motor to stall. Before you pull the pan, you may want to buy a cheap mechanical gauge and use a couple of fittings to see if your gauge is correct. From the wiring diagrams I've seen, I see no seperate wire feeding the oil light. Therefore, I assume that the light is triggered by the gauge (or the low voltage feeding the gauge). You should be able to test the gauge by grounding the tan wire at the gauge. The gauge should deflect from max to min (or visa-versa). Then, only after you're satisfied that the gauge is functioning properly should you proceed to pull the oil pan.
I would put a mechanical gauge on to see whats really going on,I had a leaking sender (make unknown) it always read 50-60lbs cold 40lbs hot and about 20lbs hot idle,I replaced it with an AC Delco and my gauge read max 40lbs cold 30lbs hot and zero hot idle,I then bought a Niehoff sender and it reads 50lbs cold 40lbs hot and about 10lbs hot idle,I would guess that the Delco was wrong as the car runs fine with no odd engine noises and would doupt it would at zero oil pressure.
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04:40 PM
ka4nkf Member
Posts: 3702 From: New Port Richey, FL USA Registered: May 99
I have always said there is something about this oil pressure that we have not found yet. I just can't see so many of us that has 70 pounds pressure at start up and then drops to 10 pounds at hot idle. think about it all of us can't have bad bearings. I noticed one forum member said he had a new engine and it did the same thing. I have the same problem and I could not ask for a engine to run any better than mine also it is very quiet. iI am going to start a new post and see how many V=6 owners have this problem. Don
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10:07 PM
Aug 4th, 2004
black Fiero SE Member
Posts: 341 From: New Hampshire Registered: May 2004
Ok.. Here we go. I brought the car to Pontiac today and had them check it out. They found nothing wrong with any electrical devise but they changed the oil to 10W-40 and said they got the oil pressure up. They did recommend changing the main and cam. bearings. I drove it home with no problem. I went for another spin after and as I pulled it in the garage and sat at idle the oil light came on !!!!! I am going to change the Main bearings and oil pump in a couple of weeks (I wont use the car until I fix it). I'm not going to do the Cam bearings because I really don't think I can do that in my garage without dropping the engine or getting very involved with it. Most of your indications here said to just look at the main bearings and crank shaft and so far usual everyone's right in the forum when everyone agrees.
it only takes about 3 lbs. of oil pressure to put out those oil lights so you need to put a good gauge on the block and check the pressure, if the pressure is below 10 to 12 lbs. you need some new bearings. thanks mike
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10:53 PM
Aug 5th, 2004
Doug Chase Member
Posts: 1487 From: Seattle area, Washington State, USA Registered: Sep 2001
Why not? That's the simplest, most logical explanation.
quote
Originally posted by black Fiero SE:
I am going to change the Main bearings and oil pump in a couple of weeks (I wont use the car until I fix it). I'm not going to do the Cam bearings because I really don't think I can do that in my garage without dropping the engine or getting very involved with it.
First, I'd suspect rod bearings before main bearings, but if you're in there you might as well do them both.
Second, you're right about the cam bearings. In order to do this the motor needs to be out of the car and stripped to the bare block.
------------------ Doug Chase Chase Race Custom roll cage and exhaust fabrication
