I have just recently replaced the head gaskets in a 193k mile 4.2 auto F150 4X4. The engine has been running great but now after driving a distance getting the engine warmed up. The oil pressure will fluctuate when the truck is stopped and in gear (low RPM). The idle RPM dont seem lower than normal and the O.P. drops maybe to mid range. If you put it in park or neutral with the engine idling the oil pressure is fine, or when the oil is cool its fine. It has never done this before as its always had steady, non fluctuating O.P. It has aprox 1000 miles since the repair.
The engine has fresh oil installed, new oil filter and is not low of oil. Should I change the oil pump or is there something else that could be causing this that should be verified first?
Any thoughts?

Thanks

 

Since no one has answered so far, you might try this.
Anytime there appears to be an oil pressure problem with either lights or gauges in the vehicle it is usually a good move to check the oil pressure with a known good mechanical gauge. You can tap into the same place the present oil pressure sending unit uses.
There are times the vehicle gauge is faulty, or the connection at the sending unit is not what it should be or the sending unit itself is faulty. Verifying it with another mechanical gauge should provide you with at least an accurate pressure reading.

 

Thanks for your reply Hanky.
Understand what your saying is its best to verify that you have an actual problem before you spend money. If this an actual oil pressure issue I'm having, is there a way to verify if its a pump issue of if its something else wrong with the engine?

 

There can be several reasons for loss of oil pressure. Some are worn bearings, worn pump, plugged inlet screen to the oil pump, stuck relief (by-pass valve) valve. Best not to jump to conclusions until you can check with a known good gauge. Patience pays !

 

I finally got some oil pressure numbers with the engine warm and hot. This was with a calibrated gage routed into the cab. I also lowered the gage to aprox the same heigth as the oil press port on the engine to see if it effected the pressure indication, but it was about the same readiings.

WARM Idle @ 800 RPM - 18 PSI, Idle in-gear @ 600 RPM - 13 PSI, 1500 RPM - 36 PSI, 2000 RPM - 47 PSI, 2500 RPM - 48 PSI.
HOT Idle @ 800 RPM - 12 PSI, Idle in-gear @ 600 RPM - 8 PSI, 1500 RPM - 27 PSI, 2000 RPM - 40 PSI, 2500 RPM - 44 PSI.

With the oil pressure (indicator) flickering* around 8 PSI I'm assume that is where the O.P. switch is preset to trigger at. I raised the Idle to 700 RPM (in gear) which put the oil press to 10 PSI hot until I look further for my actual problem with the engine.The oil seems some what dirtier than it should be. I guessing possibly due to the oil is probably flushing the old oil/collant that maybe didnt get fully drained? Could this be the reason for the lower oil press?

Now what to look for or do?
A - Sell the truck as-is?
B - Change/inspect the oil pump?
C - Verify the pressure relief is not dirty/stuck?
D - Clean the oil inlet screen?
E - Change oil and filter?
F - Change oil press switch?

I'm not sure I am ready to tackle pulling the oil pan which requires removing the front axle (4X4) or the expense to pay for this? Is there another way to clean the screen say with the oil pump removed by compressed air? (blowing back through the oil supply tube)

Thanks

 

Sometimes a little history is helpful. Why did you need to change the head gaskets?
Did the engine overheat?

 

Never noticed it had overheated. It was using some water over time and finally after sitting for a few hours it had a liquid lock during start-up. The head gasket at the #3 cylinder was jacked. The flatness of the head was within .001" corner to corner when placed on a surface plate and less than that between cylinders when checked with a parallel bar.

 

There is a good possibility of a bent connecting rod which could be allowing oil to go out the sides of the rod bearing and losing oil pressure there. Did you notice any "soup" in the motor oil before you did the work? Oil mixed with water or anti-freeze doesn't lubricate well at all. There could very well be some serious bearing damage.

 

This is the kind of damage I was afraid of. I did attempt to measure the stroke from the water-locked cylinder to a couple other cylinders and couldnt see a difference in them. But this was a rough measurment.

Actually over the last year I had notice the oil pressure would flicker when I would back-up, then put it into drive with the engine warmed up. So I do wonder if the O.P. has been decreasing some over time?

Thanks for your input. Your opinion is greatly appreciated.

 

As you know , some diagnosis can't be made very over the internet.
Until you know for sure, take it slow and carefully measure all the alternatives.
Wish I could have been more help.