# Oil pressure question



## gofight (May 16, 2015)

On my 67 400, with the engine warm and in gear it is idling at 8-9 PSI. If it is in park I am about 16-17 PSI. Driving I get 30-40 PSI and I get over 60 PSI if I get on it. This 400 has about 25,000 miles since rebuild. My question is, is my idle pressure too low?


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## SANDU002 (Oct 13, 2004)

Don't know about idle psi, but driving pressures are ok.


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## allpawl66 (May 9, 2008)

[email protected] idle & 10psi increase per 1000 rpm increase when hot .


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## gofight (May 16, 2015)

Why would my driving PSI be ok and my idle PSI be low. Poor pump. I get 20 PAI at idle in park but not in gear


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## BearGFR (Aug 25, 2008)

Pressure will change with RPM. Lower RPM, lower pressure. That relationship is normal and not something to worry about. 7-8 psi though is a tad bit on the low side, IF that's an accurate number. What kind of gauge do you have and where on the engine is the pressure reading being taken from?

Bear


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## gofight (May 16, 2015)

I was thinking the same thing. It is just a cheapo gauge that connects above the filter.


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## BearGFR (Aug 25, 2008)

Ok then, before getting overly concerned I'd invest in a good gauge (and sending unit, if you go with electrical and not mechanical).

Here's an excerpt from a thread over on PSP (Pontiac Street Performance) that discusses the topic:
"Rocky Rotella responds:Bret, it seems the most common location to plumb a mechanical oil-pressure gauge into a Pontiac V-8 is near the distributor hole on the driver side of the block. There you will find a 31/48-inch-diameter pipe plug with a recessed 51/416-inch square opening. The plug covers a direct path used during initial machining to bore an internal oil passageway that was subsequently tapped and sealed off during production. Because of this and its close proximity to the instrument panel, it makes an ideal location for accurately retrieving an oil-pressure reading.

The pipe plug is commonly removed during engine rebuilds to access the internal oil passageways for cleaning and is typically replaced with a new unit. If your 455ci has been rebuilt in the past, the plug may come out easily. If the engine hasn't, you might find the original plug very tough to break free, possibly rounding out during the attempt. To prevent this, I suggest heating the plug for a short time before hand, it typically makes removal much easier. This assumes, however, that your engine isn't in a vehicle and you can fully access the plug.

If your engine is installed and you don't have plans on removing it, an alternative source for an accurate reading may be the oil-filter adapter. Simply remove the existing electric oil-pressure sending unit, and connect the mechanical gauge's hardware. A drawback to this location, however, is the lengthy distance required to plumb the gauge into the passenger compartment. If going this route, be sure to securely route the gauge's oil line away from the entire exhaust system to prevent burning the line and causing an oil leak.

Source:
Pontiac 400 Engine - High Performance Pontiac Magazine

And a fwiw, there can be quite a bit of differance in the reading between the two differant locations."

And here's a link to the thread:
Pontiac Street Performance - Where to take oil pressure gauge from

Bear


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## gofight (May 16, 2015)

Thanks Bear. To follow up with your last commentI would believe the pressure reading would be higher at the oil filter...correct? I aldo just checked the service manual and thr idiot light in the cluster is set to illuminate at 5psi +/- 1.5 psi. I am going to buy a better gauge and work from there. What is your opinion on pressure differences between locations?


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## BearGFR (Aug 25, 2008)

I don't really have an opinion one way or the other. I don't think that 2-3 PSI one way or the other is going to have much difference on the price of frijoles in Tijuana ... unless it's at the extreme low end of the range. I like to see around 10-15 psi or more at idle on a hot (fully warmed up) engine, but I'm more concerned about pressure under load at rpm because that's really where it matters. The engine is dependent on that thin film of oil between the bearing surface and the crank surface to keep things from ever being 'metal to metal', and that film is much harder to maintain "under load at rpm" than it is in an unloaded engine at idle. If those two metal surfaces ever actually touch when the engine is under load, neither one of them is going to survive very long. ("Very long" meaning milliseconds, not days or even minutes).

Bear


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## gofight (May 16, 2015)

Agreed. I get great pressure under acceleration and cruising


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## gofight (May 16, 2015)

Do you use the 10psi per 1000 rpms theory


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