'95 GM 3100 oil pressure issues

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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
 
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.
 
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
 
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.
cool.gif
 
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?
 
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.
 
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.
 
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)
 
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.
 
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.
 
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.
 
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.
 
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
 
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.
 
Thanks for the "thick heads" reply...
smile.gif


But no, I just thoght it interesting that the oil pressure seems to drop, not gradually, but all at once, and only after running for 1/2 hour or more at cruise. LIke the viscosity suddenly changes quite a bit, or something mechanical in the engine changes due to heat, or the oil gets aerated (not likely I know). As long as its running I'll just ignore it.

20 psi when hot running at 70 mph is fine, I agree.

I'll drop the subject, it appears to be
deadhorse.gif


Thanks,

Ron
 
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