Oil Pressure Drop as engine warms

[westsoundred]

My 1992 Silverado has the very odd habit of losing oil pressure the longer I drive it. It will start at about 30, or 1/2 on the oil pressure meter. As I drive it, it will drop until it gets to about 10 while at driving rpms (anything over 1k), but it will bounce on 0 at the stop light (300-600 rpms) and flicker my check engine light. Doesnt matter if the oil is old or new, and the oil level is full.

Why am I losing oil pressure as it warms up? I use 10w-40 synthetic oil. Has anyone else had this issue?

 

[618 Syndicate]

Physics. Pressure is a function of resistance, warm oil is less viscous, causing less resistance and an associated drop in pressure.
That being said, it doesn't explain why (assuming your gauge is accurate) it's dropping so much. That is indicative of a problem. Oil pressure on the gauge in my 400 starts well over 60 when cold (98 454) and doesn't drop below 20. I think you have a gauge or sender issue.

 

[RichLo]

Yea before you worry too much, get an external oil pressure gauge and monitor that

 

[Frank Enstein]

Put a known good pressure gauge on it teed between the block and the sender to see if the oil pressure is really low or if it's an electrical gremlin.
The gauges in our trucks are known to be inaccurate when new and age doesn't help matters.

The connector at the back of the cluster is a known issue.

Also the needles can move on the shaft. They can be popped off with a plastic fork and re-aligned.

If it really is that low run thicker oil maybe even synthetic (synthetic is more viscosity stable so the pressure doesn't drop as much when hot) to get the pressure up while you save up for a rebuild.

A good rule of thumb is 10 psi for every thousand engine rpm hot. Hot means after an hour of continuous driving. So @ 1000 rpm 10 to 20 psi is fine.

 

[stutaeng]

I had the same readings on a 4.3 Vortec. Check your readings with a mechanical guage as suggested. My reading with the guage was a bit higher than the instrument cluster: about 5 psi at hot idle when cluster was below the "0" mark. At high RPM it was actually still within specs.

I changed to 50 weight oil and ran it for another year and a half. Engine still ran fine with no knocking when I pulled it for a replacement.

Check for the actual numbers first...

 

[kennythewelder]

As the engine heats up, the oil does too. This will thin it out somewhat. A cold engine will always have higher oil pressure. How long have you been running that same oil? I see the same effect in my engine, but not to that extreme. I am running Valvoline conventional oil 10-30. I add a container of STP. I know this is a viscosifier and will thicken the oil. The thicker oil, will build more oil pressure. It all makes me wonder what they are doing to oil these days. Formulated for newer vehicles, with aluminum engines? I have run Shell Rotella conventional oil before, and its 15-50 I think. May switch back to that. Keep in mind this fact. When these engines were built, they were designed to be run on conventional oil. As long as your doing your oil changes like you suppose to you should be able to run whatever oil you like, but with these trucks getting old, and the milles going up, there will be ware inside of the engine. Maybe try a high millage oil and see if it makes any difference.

 

[westsoundred]

the engine never rattles or clanks, so I dont think the oil pressure is truly hovering at 0. I did find this, no idea if this guy is legit:

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anyway, thanks for the ideas. I'll need to do some sleuthing

 

[kennythewelder]

the engine never rattles or clanks, so I dont think the oil pressure is truly hovering at 0. I did find this, no idea if this guy is legit:

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anyway, thanks for the ideas. I'll need to do some sleuthing

I have changed my sensor before. Its been a while, but it seems like I did mine without pulling the distributor. I know there is a special socket for this, and I did buy one when I bought the switch.

 

[PlayingWithTBI]

I think (but not sure) the 92 still has the oil pressure sensor on the block near the oil filter. Look there 1st.

 

[DerekTheGreat]

the engine never rattles or clanks, so I dont think the oil pressure is truly hovering at 0. I did find this, no idea if this guy is legit:

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anyway, thanks for the ideas. I'll need to do some sleuthing

Oh gawd, the OG... Perhaps he was relevant ten years ago but most of his stuff is click bait and filled with him shouting at the camera..

This is one of my favorite spoofs of him:

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