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Oil pressure dropping with RPM when cold

jeep4x4

NAXJA Forum User
Location
Ohio
OK, I've never seen this before. On a '92 4.0 on 5W-30 GTX synthetic and 189,000 miles, that has always carried 40-60 psi of pressure at start and 30-50 psi in normal driving conditions, it will now start cold (below 40 degrees F)with 40-50 psi at idle and as rpms increase the pressure will drop to 20-30 psi. As rpm drops pressure will build back to 40-45 psi. This behavior continues untill several thermostat cycles are observed and then stabilize around 40-45 psi. I need input.

-is this a:
-clogged filter/pickup screen
-failed (partially open) relief valve
-excessive viscosity at low temperature causing cavitation
-failed sending unit (has been replaced)
-ideas?

Jeep4X4
 
That is wierd. Normally the pressure at idle is lower than at higher rpms. The only thing I can think of would be cavitation. Are you sure you are running enough oil? Actually, too much oil could be a problem as well. If the crank hits the oil too much it can whip it into a froth and cause a drop in pump efficiency.
 
oil level checks out on the line on level ground when cold. I'm scratching my head here and also think that cavitation is probably the answer but has anyone seen this before?
 
Now that I think about it, I have seen this before. The pickup tube for the oil pump was too close to the bottom of the pan due to a dent and it was restricting the intake to a smaller than normal amount and due to the increased volume requirement at higher rpms, the pump was not able to suck enough to maintain the pressure.
 
Your engine is obviously starving for oil at higher rpm and the most likely reason for this is a clogged oil pump pick up screen. I would expect a faulty oil pump pressure relief spring to produce a lowish oil pressure all of the time, not only at higher rpm.
If you've swapped in a high volume oil pump, this may pump the oil out of the pan at too fast a rate at higher rpm eventually leaving too little in the sump and causing the oil pressure to drop.
 
Strange, I had a similar issue with a full size Bronco with a 302. Ran fine at speeds etc but when you decellerated down a long grade still maintaining the same rpms the oil pressure would drop to under 20 psi. As soon as you applied throttle to maintain rpm on flat ground it would come back up to 40. Long story behind the whole thing, but had it torn down and was told when decellerating the cam was walking due to wasted cam bearings and would allow all the tollerances to open and loose pressure. Thats a new one on me in all my days, I trashed the plie shortly afterwards. In your case it seems the oil pump may be starving or has a weak bypass spring. I wonder if throwing in a mechanical guage right at the block location would show any "bouncing" of the needle as in cavitation issues. It seems the stockers are slow to react and are dampened to provide smooth needle movement. Just a thought.
 
I should have been clearer. The pressure stabilizes and acts normally once the engine is warm. A blockage in the pick-up screen would be a constant problem. This problem only seems to happen when the outside temperature is around zero (F). In normal outside temperatures above this, the pressure is good and behaves normally. The only thing that has been done to the Jeep since I know the pressure was normal was an oil change (10W-30 Castrol GTX full synthetic). I will pick up another filter and swap it out to see if maybe the filter has flow problems in cold temperatures. If anyone has another suggestion don't hesitate to chime in.
 
Just an update. It was the oil filter causing a restriction. She thought she was doing me a favor but It's the last time I let my wife get the oil changed!
 
jeep4x4 said:
Just an update. It was the oil filter causing a restriction. She thought she was doing me a favor but It's the last time I let my wife get the oil changed!


why?

did they not change the filter, and it was fairly clogged up?
 
Different filters act different ways, I have run Mopar, K&N, Mobil-1 and Purolator in that order, the Mopar was always consistant thruout the the oil change cycle, the K&N was running about 10lbs low across the rpm range and they also had a tendency of loosen up to not even hand tight. The Mobil-1 filter was the most consistant and thats pretty much all I run now. The purolator I changed at around 2,000mi, it was making me nervous, I had idiot lights at the time and every morning it would take 10 seconds for the oil light to go out on the dash vs all the others that went out almost too fast to see. I tried K&N again a few times when I run out of Mobil-1 filters and they seem to be better, at least they did not loosen up on either my XJ or the wifes Oldsmobile which both had done before, while the loosening was a handy 'feature' on the wifes car come oil change time it's not something I wanted to live with...
With the purolator I figure the anti-drainback was not working the way it is supposed to, no matter, it was just a trial to see what they were like.
 
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