Home » Public Forums » GMCnet » Erratic oil pressure (76-455 stock build)
| Erratic oil pressure [message #171536] |
Thu, 31 May 2012 15:20  |
pickle4k
Messages: 129
Registered: January 2011
Location: San Leandro
Karma: 0
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On my trip last weekend, I noticed that when the engine got hot -gauge ready normal hot, during an freeway run - the oil pressure dropped to about half of normal on the gauge and fluctuated slightly there. It never went to 0. Even stopped on the off ramp and it would not recover to normal state. The engine is making NO unusual noise, the water temp is normal and the oil level is on full.
Today I took a vacuum reading at cold idle = 20, at warm idle = 18. During my test - in driveway - everything sounded OK, but the oil pressure would fluctuate when stepping on the throttle to rev it up to 3000 rpm. It would rise slightly and then drop to normal.
I am confused and need some help to get this figured out. I do not want to loose an engine which has done so well up to now, if there is an - easy - fix.
Thanks,
Nick R. NorCal
76-23'Transmode-Norris
Rear Bath and
75-26' Avion
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| Re: Erratic oil pressure [message #171541 is a reply to message #171539] |
Thu, 31 May 2012 15:41  |
Carl S.
Messages: 4186
Registered: January 2009
Location: Tucson, AZ.
Karma: 13
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| WD0AFQ wrote on Thu, 31 May 2012 13:36 |
Nick, the first thing I would do is to put an oil pressure gauge on it to check the real pressure. They are cheap and you can get one anywhere. If you don't want to keep it under the dash just open the hatch, install it, and see what the pressure is. Should you see what you want to see, then you can take it back out and hook your factory electric gauge back up. It may just be a bad connection somewhere but I would want to know for sure.
Dan
running an electric and a manual gauge
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Good point Dan. I was going to say the same thing. A 35 year old electric gauge is not likely to be very accurate any more, if it ever was in the first place. Mine behaves strangely sometimes too. I should take Dan's advice as well.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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| Re: Erratic oil pressure [message #171549 is a reply to message #171541] |
Thu, 31 May 2012 16:49 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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| Carl S. wrote on Thu, 31 May 2012 14:41 |
| WD0AFQ wrote on Thu, 31 May 2012 13:36 |
Nick, the first thing I would do is to put an oil pressure gauge on it to check the real pressure. They are cheap and you can get one anywhere. If you don't want to keep it under the dash just open the hatch, install it, and see what the pressure is. Should you see what you want to see, then you can take it back out and hook your factory electric gauge back up. It may just be a bad connection somewhere but I would want to know for sure.
Dan
running an electric and a manual gauge
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Good point Dan. I was going to say the same thing. A 35 year old electric gauge is not likely to be very accurate any more, if it ever was in the first place. Mine behaves strangely sometimes too. I should take Dan's advice as well.
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These oil pressure transducers will fail over time. If the needle actually vibrates or moves abruptly, it can indicate a sender starting to go. Also, a poor ground at the gage end can affect the reading and you may be seeing fluctuation due to alternator output, not oil pressure.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] Erratic oil pressure [message #171551 is a reply to message #171549] |
Thu, 31 May 2012 17:03 |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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FREE WIFI @ Mickey D
On May 31, 2012, at 2:49 PM, Bob de Kruyff <NEXT2POOL@AOL.COM> wrote:
>
>
> Carl S. wrote on Thu, 31 May 2012 14:41
>> WD0AFQ wrote on Thu, 31 May 2012 13:36
>>> Nick, the first thing I would do is to put an oil pressure gauge on it to check the real pressure. They are cheap and you can get one anywhere. If you don't want to keep it under the dash just open the hatch, install it, and see what the pressure is. Should you see what you want to see, then you can take it back out and hook your factory electric gauge back up. It may just be a bad connection somewhere but I would want to know for sure.
>>> Dan
>>> running an electric and a manual gauge
>>
>>
>> Good point Dan. I was going to say the same thing. A 35 year old electric gauge is not likely to be very accurate any more, if it ever was in the first place. Mine behaves strangely sometimes too. I should take Dan's advice as well.
>
> These oil pressure transducers will fail over time. If the needle actually vibrates or moves abruptly, it can indicate a sender starting to go. Also, a poor ground at the gage end can affect the reading and you may be seeing fluctuation due to alternator output, not oil pressure.
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
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| Re: [GMCnet] Erratic oil pressure [message #171552 is a reply to message #171549] |
Thu, 31 May 2012 17:05 |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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Manifold sucking on the case --again?
Gene
FREE WIFI @ Mickey D
On May 31, 2012, at 2:49 PM, Bob de Kruyff <NEXT2POOL@AOL.COM> wrote:
>
>
> Carl S. wrote on Thu, 31 May 2012 14:41
>> WD0AFQ wrote on Thu, 31 May 2012 13:36
>>> Nick, the first thing I would do is to put an oil pressure gauge on it to check the real pressure. They are cheap and you can get one anywhere. If you don't want to keep it under the dash just open the hatch, install it, and see what the pressure is. Should you see what you want to see, then you can take it back out and hook your factory electric gauge back up. It may just be a bad connection somewhere but I would want to know for sure.
>>> Dan
>>> running an electric and a manual gauge
>>
>>
>> Good point Dan. I was going to say the same thing. A 35 year old electric gauge is not likely to be very accurate any more, if it ever was in the first place. Mine behaves strangely sometimes too. I should take Dan's advice as well.
>
> These oil pressure transducers will fail over time. If the needle actually vibrates or moves abruptly, it can indicate a sender starting to go. Also, a poor ground at the gage end can affect the reading and you may be seeing fluctuation due to alternator output, not oil pressure.
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
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| Re: Erratic oil pressure [message #171554 is a reply to message #171536] |
Thu, 31 May 2012 17:39 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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| pickle4k wrote on Thu, 31 May 2012 13:20 |
On my trip last weekend, I noticed that when the engine got hot -gauge ready normal hot, during an freeway run - the oil pressure dropped to about half of normal on the gauge and fluctuated slightly there. It never went to 0. Even stopped on the off ramp and it would not recover to normal state. The engine is making NO unusual noise, the water temp is normal and the oil level is on full.x.
Thanks,
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I would also check that the oil is really full. After our new engine, the calibration was off quite a bit. Mine was over full, but it could go the other way. When my oil got low because of a massive aluminum manifold leak, mt pressure was twitchy as you describe. The one difference was that when slowing, it dropped to zero as it sloshed forward. On the off ramp were you going up or down?
If it has oil is it draining back properly? With a new engine it certainly should hav no obstruction.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Erratic oil pressure [message #171560 is a reply to message #171536] |
Thu, 31 May 2012 18:21 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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I would follow the advice of whichever guru you most favor :)
--johnny
'76 same as his'n
'76 palm beach
From: Nick Roenick <pickle4k@comcast.net>
To: gmclist@temp.gmcnet.org
Sent: Thursday, May 31, 2012 4:21 PM
Subject: [GMCnet] Erratic oil pressure
On my trip last weekend, I noticed that when the engine got hot -gauge ready normal hot, during an freeway run - the oil pressure dropped to about half of normal on the gauge and fluctuated slightly there. It never went to 0. Even stopped on the off ramp and it would not recover to normal state. The engine is making NO unusual noise, the water temp is normal and the oil level is on full.
Today I took a vacuum reading at cold idle = 20, at warm idle = 18. During my test - in driveway - everything sounded OK, but the oil pressure would fluctuate when stepping on the throttle to rev it up to 3000 rpm. It would rise slightly and then drop to normal.
I am confused and need some help to get this figured out. I do not want to loose an engine which has done so well up to now, if there is an - easy - fix.
Thanks,
--
Nick R. NorCal
76-23'Transmode-Norris
Rear Bath
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: Erratic oil pressure [message #171564 is a reply to message #171536] |
Thu, 31 May 2012 18:42 |
C Boyd
Messages: 2629
Registered: April 2006
Karma: 18
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Sir: my advise is to get an oil analysis done. Go to your local big truck dealer or check yellow pages for oil analysis labs in your area. Locally here it is $12 and I get results in 1 hour. Good or bad you will know.
| pickle4k wrote on Thu, 31 May 2012 16:20 |
On my trip last weekend, I noticed that when the engine got hot -gauge ready normal hot, during an freeway run - the oil pressure dropped to about half of normal on the gauge and fluctuated slightly there. It never went to 0. Even stopped on the off ramp and it would not recover to normal state. The engine is making NO unusual noise, the water temp is normal and the oil level is on full.
Today I took a vacuum reading at cold idle = 20, at warm idle = 18. During my test - in driveway - everything sounded OK, but the oil pressure would fluctuate when stepping on the throttle to rev it up to 3000 rpm. It would rise slightly and then drop to normal.
I am confused and need some help to get this figured out. I do not want to loose an engine which has done so well up to now, if there is an - easy - fix.
Thanks,
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C. Boyd
76 Crestmont
East Tennessee
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| Re: Erratic oil pressure [message #171589 is a reply to message #171564] |
Thu, 31 May 2012 23:20 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Also with all of the oil pressure suggestions above do not forget to address the over heating issue.
If the engine is truly over heating, then the oil gets thinner, and the pressure will go done.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Erratic oil pressure [message #171655 is a reply to message #171536] |
Fri, 01 June 2012 15:29 |
bwevers
Messages: 597
Registered: October 2010
Location: San Jose
Karma: 5
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Senior Member |
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Nick,
I use 15W-40 diesel oil. The oil pressure is almost 3/4 scale
when cold. After warm-up it drops to around 5/8 or 1/2 scale
when cruising.
And it drops slightly when I go up a long grade, like the Sunol grade.
At idle it drops to 1/4 scale when real hot. That's with 122K
of the original engine. I replaced the oil sender unit a couple
of years ago.
Regards,
Bill
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
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| Re: Erratic oil pressure [message #171701 is a reply to message #171655] |
Fri, 01 June 2012 23:18 |
pickle4k
Messages: 129
Registered: January 2011
Location: San Leandro
Karma: 0
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Senior Member |
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Thank you all for your kind suggestions
I have installed a NEW gauge and sending unit from Sun. It reads cold - 55, warm - 45, at 3000 rpm warm - 50 and at 7mph on the freeway at 3000 rpm it reads - 25 when warm. The engine never got HOT, as in above normal reading going down the road. T add to the problem, there is oil in the air cleaner from the driver side "air intake" for the PCV system. The PCV valve has 20 reading on the vacuum gauge and is not restricted.
It has been concluded that the engine has blow-by, with the rings being the likely issue and it is best to replace the motor, which I will reluctantly do. I want to attend the rallies which are coming up, so time is of the essence.
Thanks again for your help.
Nick R. NorCal
76-23'Transmode-Norris
Rear Bath and
75-26' Avion
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| Re: Erratic oil pressure [message #171715 is a reply to message #171701] |
Sat, 02 June 2012 02:41 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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I guess I am misunderstanding here. Your oil pressure readings are good. I do not understand the PCV reading of 20. What are you measuring there?
Have you taken a compression test? What do the individual cylinder compressions read?
Have you replaced the PCV valve? They are cheap.
If it is indeed blowby what have you done to determine which cylinders are causing the problem?
Have you looked at the spark plugs?
What is your oil consumption like?
I thought you were chasing an over heating problem. What are you doing about diagnosing that?
I suggest that you are a long way from diagnosing the need for a replacement engine.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Erratic oil pressure [message #171743 is a reply to message #171655] |
Sat, 02 June 2012 09:34 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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Senior Member |
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I get much the same results in my 89K miles except it holds a bit better on hot idle than yours. Using Rotella 15W40. They used to make synthetic Rotella 15w40, which as far as I know only was stocked at Wal - Mart. Last couple of times I've looked, there wasn't any. My turbodiesel Jetta Is ran forever on it.
--johnny
'76 23' transmode norris
'76 palm beach
From: Bill Wevers <gmc1975@att.net>
To: gmclist@temp.gmcnet.org
Sent: Friday, June 1, 2012 4:29 PM
Subject: Re: [GMCnet] Erratic oil pressure
Nick,
I use 15W-40 diesel oil. The oil pressure is almost 3/4 scale
when cold. After warm-up it drops to around 5/8 or 1/2 scale
when cruising.
And it drops slightly when I go up a long grade, like the Sunol grade.
At idle it drops to 1/4 scale when real hot. That's with 122K
of the original engine. I replaced the oil sender unit a couple
of years ago.
Regards,
Bill
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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