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Dramatic oil pressure fluctuation on cold start

I took a video of it on my phone, but my phone won't let me upload it to youtube. Says the file size is too large. But basically the oil pressure fluctuates dramatically on a cold start up. When I start the car, cold, the oil pressure is about 80 psi. Within a few seconds, the oil pressure slowly drops a few psi, to around 65, then decreases fairly rapidly to 18-20, and then skyrockets back to 80-ish psi. It then repeats the process several times. Once the engine starts to rise in temperature, the fluctuations become less frequent and less dramatic, until the engine reaches operating temp, and the pressure stops fluctuating altogether.

My engine was a problem free, previously rebuilt, 65k stock spec engine. I had the cylinder head rebuilt, performed a BSE, polished the crank, replaced all of the bearings and re-ringed the pistons. I did not replace the oil pump, as I probably should have, because there wasn't anything wrong with the engine prior to disassembly. The engine is equipped with a 1990 OFH, which has a ported relief hole to remedy the excessive oil pressure caused by the BSE. The sender for my oil pressure guage is remotely mounted, on the end of a 4' long, 5000 max psi hydraulic hose.


So here's my thoughts. First, I need to install a second temporary gauge to act as a tattle-tell to comfirm if my current sender is playing nice or not. If the second gauge shows all is well, boom, problem solved, just get a new sender/gauge and be done with it.

But I don't think that is really the problem. Another option is that I shot myself in the foot by reusing the oil pump. Still not likely in my eyes because a pump has so few moving parts that I can't imagine it would almost fail, fix itself, almost fail again, fix itself again, etc, etc.

So we come to the pressure relief valve. I think it is probably sticking in its bore, bleeding off pressure until the spring pressure overcomes the combined forces off the sticky valve and the oil pressure, at which point the piston pops back to the top of the bore and the process repeats itself.

Obviously this is incredibly concerning, and I would greatly appreciate any and all input from anyone.

The engine has roughly 1700 miles on it since my refreshing. I use 10w-30 royal purple oil. The oil pressure never drops below 16 psi @ normal operating temp, and I have not seen less than 10 psi per 1000 rpm (i.e.; at least 30 psi @ 3000 rpm, 50@ 5k), and usually the pressure is 20 psi per 1000 rpm.
 
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turbowop

Well-known member
Joined
Apr 29, 2001
Messages
11,971
Location
Yakima, WA
The oil pump either works or it doesn't. It's just a set of gears that mechanically draw oil up out of the sump. It shouldn't be intermittent.

I bet your gauge sender is acting up. Mine did this a few months ago while driving down the freeway on my way to Seattle. It was at like 60psi, then started bouncing to 100psi and back to 60psi over and over again. It now sits pegged right as I start the car and stays there. I just need to install a new one.
 

I hope you're right Mark. I agree with your assessment of the oil pump, but the more I learn, the more I realize that sometimes it is better to ask the opinion of others first.
 

thecman02

Well-known member
Joined
Nov 3, 2007
Messages
917
Location
Kalamazoo,MI
If it is an auto meter electric unit and is within a year of purchase you can call them up and get a new one sent to you. I also had a similar issue.
 

fuel

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Joined
Feb 23, 2009
Messages
2,165
Location
Toronto, ON, Canada
could be a sticking oil relief valve - as the valve gets pushed open to release excess pressure when the pressure is too high (ie cold and thick oil) it is not returning quick enough and lets the oil pressure dip down then the valve eventually closes up again and repeats the process all over again.
 

So I guess that I'm the recipient of the super-mega-huge-dumbass award. Any guesses before I tell you what really happened?
 

vtecds1

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Joined
Jan 16, 2004
Messages
1,705
Location
Las Begas
While were on topic...what psi are you guys getting at start up?
 

I run 80-82psi on a cold start up. Once everything warms up, I see 18-20 @ 850 rpm.
 

vtecds1

Well-known member
Joined
Jan 16, 2004
Messages
1,705
Location
Las Begas
I think the easiest way, like you said, is get another sender or borrow one from somebody. Shouldn't take no more than 1/2hr to test it out.
 

Quoting thecman02:
low oil?



Bingo. I was about 1.5 qts low, which apparently is low enough to cause the pickup to nearly run out of oil momentarily on cold starts. Seems like cold oil can't drain down fast enough to keep up, especially when it has a big ol' oil cooler with big ol' -8 AN lines to fill.
 
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