87 M998 6.5 NA oil pressure troubles

[joshin24720003]

Hey guys,
I'm having an issue and I figured this would be the best place to go. I've done plenty of searching beforehand i nothing has revealed a solution.
Here's the problem: When i start my humvee and let the oil pressure get up to normal value (60psi), put it in drive, under loads i get 80-90psi pressure readings. During this initial period the engine is noticeably louder. After about 20 seconds of driving, the oil pressure drops back down to 60-65 and the engine is much quieter. This change is immediate not subtle at all.

So far i have one theory. The oil bypass valve is initially closed and hi pressure occurs because of a clog in the system (near/at the filter?). When pressure gets high enough, the oil filter is bypassed and pressure goes back down and oil is supplied to the lifters. I've changed my oil/filter twice since i've been driving it (4 months) back and forth to work (5 miles).

Thoughts?

 

[Dock Rocker]

I would start with the gage and sender first. You should trust a military gage like you trust Hilary Clinton.

Get a new gage and sender.


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[Scrounger]

The oil pressure values don’t seem to be out of the ordinary. And if the louder sound is only during the first start of the day or with a mostly cool engine that doesn’t sound out of place either. The 6.5 injectors are noisy at first.

 

[ken]

Pull and clean the bypass valve. Also change brand of filter if you are using a Fram. They are made of cardboard inside and have sent media into the engine.

 

[papakb]

My suggestion would be to tee in a mechanical oil pressure gauge there where the sender is and see what your oil pressure is really doing. Military gauges and sensors are made by the lowest bidder and some are OK and some aren't. You could also have something strange going on in your oil filter.

 

[Augi]

The engine being loud for the first 20 seconds is the cold timing advance and is normal.

Augi

 

[joshin24720003]

Thanks for all the input everyone. I put a mechanical gauge in and got 55 psi on cold and 60 psi full throttle cold. And now 50 psi warm idle. Looks like the pressure sending unit. I'll keep testing it of the next few days just to be safe...man that engine is loud without the doghouse

Thanks again

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[Retiredwarhorses]

Your IJ Pump is advanced on cold start, once it hits 120, the Pump retards and quiets down...normal