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#806444 - 01/09/07 08:33 AM '95 GM 3100 oil pressure issues
Ron Nelson Offline


Registered: 02/27/06
Posts: 4
Loc: USA
Hi folks:

I picked up a work beater, its a 1995 Grand Am with the 3100 v6. Car has 120k miles on it, and I live in the Seattle Area, so it rarely gets below freezing here.

I put Mobil 1 5W30 in it, and my commute is about 26 miles (all highway) each way.

Oil has about 2 months/1000 miles on it.

Lately, I've noticed the factory oil pressue guage will drop from 40 psi to maybe 25 psi after the engine has been running for 1/2 hour or more. This is more of an "all at once" drop, rather than a gradual degradation. I suspect one of the contributing factors is that the system only holds 4 1/2 quarts including the filter.

I plan to dump the 5W30 and put in Mobil1 0W40, and see if the oil pressue is better on long drives.

I'm assuming you folks who know a whole lot more about oil than I do can guess why the strange drop.

Any suggestions?

Regards,

Ron

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#806445 - 01/09/07 08:55 AM Re: '95 GM 3100 oil pressure issues [Re: Ron Nelson]
Curious Kid Offline


Registered: 02/20/05
Posts: 1183
Loc: Vermont
Very interesting - is the oil pressure guage factory? If so, I wonder if it has a circuit much like my mother's volvo did for the water temp guage, which would have the meter register "normal" until the sender's signal went out of a pre-determined range, at which point I think it would show such measurement value. I ended up bypassing the circuit because it started acting up and now it gives real-time indications.

I hope others can help.
_________________________
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#806446 - 01/09/07 09:02 AM Re: '95 GM 3100 oil pressure issues [Re: Curious Kid]
Ron Nelson Offline


Registered: 02/27/06
Posts: 4
Loc: USA
It is a factory guage.

Its possible that its a fancy "idiot light", as other GM oil pressure guages have been known to be (94-95 Impala comes to mind), but the sender unit is unique to the guage package, and is rather large, so I'm pretty sure its a linear (rather than a step function) guage. I added the factory guage package to the car and switched the sender unit at the same time...

Ron

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#806447 - 01/09/07 09:37 AM Re: '95 GM 3100 oil pressure issues [Re: Ron Nelson]
dwendt44 Offline


Registered: 05/17/06
Posts: 3571
Loc: Central Wisconsin
I have a 95 GM 3100.
5W-30 SuperTech syn oil.
Gauge reads about 40 cold and when the oil is hot it's just about 25-30; I'm talking at idle.
I think GMs are rated at a lower oil pressure that some others.
Japanese cars/cycles seem to like 50-70 lbs.
_________________________
There's no such thing as:
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or too loud of a horn,
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#806448 - 01/09/07 11:20 AM Re: '95 GM 3100 oil pressure issues [Re: Ron Nelson]
427Z06 Offline


Registered: 12/06/03
Posts: 7409
Loc: Austin, TX
Quote:

Lately, I've noticed the factory oil pressue guage will drop from 40 psi to maybe 25 psi after the engine has been running for 1/2 hour or more. This is more of an "all at once" drop, rather than a gradual degradation. I suspect one of the contributing factors is that the system only holds 4 1/2 quarts including the filter.




Please be more specific at what engine RPM and temperature these oil pressures are being measured. For example:

Cold Idle, cold 2500 rpm, hot idle, hot 2500 rpm.

As an aside, isn't the GM 3100 one of the engines with intake manifold gasket issues?
_________________________
"What do you get when you multiply six by nine? - In base 13."

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#806449 - 01/09/07 11:31 AM Re: '95 GM 3100 oil pressure issues [Re: 427Z06]
Triple_Se7en Offline


Registered: 12/04/04
Posts: 6333
Loc: 1/2 hr. north of Detroit
Sounds about normal for your vehicle oil brand and oil weight.

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#806450 - 01/09/07 03:29 PM Re: '95 GM 3100 oil pressure issues [Re: Triple_Se7en]
Fordiesel69 Offline


Registered: 09/10/05
Posts: 709
Loc: Erie, PA
Don;t dump the oil just yet. Hook up an aftermarket "mechanical" oil pressure guage so you can see whats really going on. I think the oil guages on GM's are also idiot guages.
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99' Crown Vic Police Interceptor (P71) w/Steet apperance package, 138K
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#806451 - 01/09/07 04:10 PM Re: '95 GM 3100 oil pressure issues [Re: Fordiesel69]
TallPaul Offline


Registered: 10/22/03
Posts: 12912
Loc: By Detroit
If/when you get to the point of wanting a thicker oil, don't dump it. Drain off 1 to 1.5 quarts and top it up with 15w50 (Mobil 1 still comes in 15w50 right?). That will give you about a 40 weight for much less cost. Save the drained oil for top ups in any vehicle.
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"There is a slight vibration in it that I always attributed to something I can't explain." --Neil Young

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#806452 - 01/09/07 06:05 PM Re: '95 GM 3100 oil pressure issues [Re: TallPaul]
Patman Offline



Registered: 05/27/02
Posts: 19275
Loc: Oakville, Ontario
25psi at a hot idle is not bad at all! That's about right for an engine running a typical 5w30 oil.

My Corvette runs at 29psi at 200F oil temps at idle with the auto trans in drive. This is with GC 0w30 (which is almost a 40 weight) and with my idle speed 150rpm higher than stock (we set it up this way when we dyno tuned it)
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2005 Corvette (M1 5w30 & Fram Ultra)
2006 Civic EX Coupe (M1 0w20 & Fram Ultra)
2010 BMW 328i X-Drive (M1 0w40 & Mann filter)

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#806453 - 01/09/07 06:20 PM Re: '95 GM 3100 oil pressure issues [Re: Fordiesel69]
Drew99GT Offline


Registered: 10/11/02
Posts: 20600
Loc: Colorado Springs
Quote:

Don;t dump the oil just yet. Hook up an aftermarket "mechanical" oil pressure guage so you can see whats really going on. I think the oil guages on GM's are also idiot guages.




All the trucks I have seen are linear/real readings, like on the 4300 trucks.

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#806454 - 01/09/07 07:13 PM Re: '95 GM 3100 oil pressure issues [Re: Drew99GT]
Gary Allan Offline


Registered: 09/28/02
Posts: 39806
Loc: Pottstown, PA
I agree with 427 here. What's your cold and normalized pressures in all states? Peak and idle cold, hot idle, and while doing any road speed (if it varies over a certain amount).

Just to give you a feel for it on fairly low pressure engine:

Cold start: 42 (this was the peak pressure of the system and remained there until the oil warmed regardless of engine speed or oil visc)
Hot idle: 12-13 (hot idle being after about 15 minutes of operation)
Hot off idle: 13-24 (24 would be the typical highway hot oil pressure)


In your case, if you're starting cold ..then driving 15-20 minutes and THEN look down at the gauge at hot idle and see a near 40% loss in idle pressure ..that would not be abnormal.
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#806455 - 01/09/07 07:19 PM Re: '95 GM 3100 oil pressure issues [Re: Gary Allan]
Drew99GT Offline


Registered: 10/11/02
Posts: 20600
Loc: Colorado Springs
Agreed Gary. On the Blazer I always talk about, and all of them we have had (which all have real oil pressure gauges), the pressure goes to 60 psi at cold start, settles to 20 psi hot idle, and goes to about 45 steady state cruise.

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#806456 - 01/09/07 08:24 PM Re: '95 GM 3100 oil pressure issues [Re: Drew99GT]
m2200b Offline


Registered: 08/28/05
Posts: 1574
Loc: Alamo, CA
Is the whole slow change related to an old and slow acting thermostat. What's the water temp gauge doing over the same 25 minutes.

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#806457 - 01/11/07 08:15 AM Re: '95 GM 3100 oil pressure issues [Re: m2200b]
Ron Nelson Offline


Registered: 02/27/06
Posts: 4
Loc: USA
Thanks guys. To answer a few of your questions:

1/ Yes I replaced the dreaded LIM (lower intake manifold) gasket. Did it myself, and I can understand why some shops charge $600.00 in labor to do it. Did the oil change at the same time, as its impossible to remove the LIM without getting some coolant in the oil. I should add that situation is another reason to change the oil again; to make sure that all traces of coolant are gone from the oil system.

2/ At the same time as the LIM gasket job, I also went through the cooling system; new thermostat & hoses, and removed the radiator and had it gone through. The system now works great, and holds pressue/doesn't lose coolant.

3/ The oil pressue seems to be weakly related to engine speed, and strongly related to operation time. The drop in pressue happens at cruise, which at 60 mph is something like 2000 rpm or so, after running for 1/2 hour or more. Cold start (and up through warm up) it holds at 40 psi. Only after a long run does the pressure then drop, not during warm-up.

It appears that, when the oil pressure drops, if I stop (for a light for example), and then start off, the pressure goes back up to 35 psi.

As an aside, it was about 32 degrees during my drive home yesterday, almost all of which was spent idling in grid-locked traffic for 5 hours due to a snow storm (Seattle is really not prepared for snow). The oil pressure never dropped below 35psi during the whole trip.

I agree, the ideal guage to really monitor this would be an aftermarket one. I may end up going that way soon.

Thanks again.

Ron

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#806458 - 01/11/07 08:43 AM Re: '95 GM 3100 oil pressure issues [Re: Ron Nelson]
427Z06 Offline


Registered: 12/06/03
Posts: 7409
Loc: Austin, TX
Looks like you have the misunderstanding that oil pressure needs to stay at a high level through all operating modes. A brand new engine may have as little as 5-6 psi at idle when the oil in the engine is hot, even though running high RPMs down the highway will produce 35-45 psi levels.

Constantly running against the oil pump bypass valve just means you reduce oil flow to the engine. It's a concept the "thick heads" around here just can't seem to grasp.
_________________________
"What do you get when you multiply six by nine? - In base 13."

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