falling oil pressure

Okay gents, here is the situation.....88 MJ 4.0, 168,000 miles. I just bought it and thought I would clean out the engine, so I changed the oil and put in a bottle of Auto Rx. Auto Rx is supposed to be a gentle cleaner and it has been in the engine for about 300 miles or so.
Prior to introducing the Auto Rx, the oil pressure stayed up around 40 at highway speeds and dropped to maybe 20 at idle. Today I drove it about 100 miles on the highway and noticed that at 60-65 mph the oil pressure dropped to just above 20 psi. When I stopped at a light it stayed at about 20, but when I moved away from the light, the pressure went up to almost
40 again...after about 4 to 5 minutes at speed, it dropped again to just above 20.
My thought is that maybe the pickup screen is clogged with junk that the cleaner broke loose. As I am stopped at a light, the reduced flow allows the oil pump to "catch up", but as the speed goes up, I'm not getting enough flow to maintain the healthy 40 PSI.. any ideas?

The screen may be plugged as your thinking. But first, I would test the stock electronic oil gauge with a mechanical unit. You can install one just long enough to test the pressure, so don't worry about how it looks. If the pressure is low, then its time to start digging some more. Even if the screen turns out to not be plugged it's not the necessaraly the end. A rig can run a long time w/o a lot of oil pressure, but it is a sign that it may be time for some serious engine work. With 160K miles that may be the case. Good Luck

Woody

I have read on this forum that the 4.0 will run down at 14 PSI. I just replace the conrod bearings and rear seal on my 92 with 201K miles. Oil pressure went pu to 50 at higher RPM and settles to about 35 at idle. I am happy!

tugalo said:

My thought is that maybe the pickup screen is clogged with junk that the cleaner broke loose.

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That would make sense so it would be a good idea to drop the oil pan and have a look. While you're there, clean as much of the gunk from inside as you can (including the oil pan itself). If you still have the original oil pump, it might be a good time to replace it with a new one.

Had this happen to me also, it was in an older ford. I thought I would run engine cleaner through it, mistake! Since then I have worked in auto parts and see more trashed engines because of those cleaners. Clean your screen asap and change your oil. The cleaner will thin your oil causing lower oil pressure too. Hope this helps.

thanks for the replies...I have used Auto Rx in other engines and not had any problems with it....according to the company it is a form of soap that works with the oil and is not prone to breaking off large chunks of crud.
I wonder if I could blow compressed air into the oil feed tube at the filter to blow the junk out of the pickup screen? anybody tried this? I'm gonna change filters today to see if that might be the problem.....

Dr. Dyno said:

That would make sense so it would be a good idea to drop the oil pan and have a look. While you're there, clean as much of the gunk from inside as you can (including the oil pan itself). If you still have the original oil pump, it might be a good time to replace it with a new one.

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While you have the pan off ... you might as well put in new conrod bearings, rear oil seal, and a new oil pump. That should make a good difference.

montanaman said:

While you have the pan off ... you might as well put in new conrod bearings, rear oil seal...

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Good eye deer.

tugalo said:

thanks for the replies...I have used Auto Rx in other engines and not had any problems with it....according to the company it is a form of soap that works with the oil and is not prone to breaking off large chunks of crud.
I wonder if I could blow compressed air into the oil feed tube at the filter to blow the junk out of the pickup screen? anybody tried this? I'm gonna change filters today to see if that might be the problem.....

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Filter is a good idea, if the stuffs working then the filter will be plugged.
I would change the oil also and add RX again.

changed filter yesterday, drove maybe 10 miles in town, oil pressure stayed at 40 psi. The oil is really dirty after only 300 miles so I know the Auto Rx is working, but the directions say to leave it in for 1500 miles and then change it out. Has anyone tried my idea to blow the pickup screen out from the filter fitting?

Don't think you will achieve much by clearing the oil pick up screen with air via the filter. Let's say the pickup is plugged, and you manage to clear a small spot on the screen. I think it will just clog up again. Personally, I'd follow the suggestions of Wa Woody and "T" in a mechanical gauge at the oil pressure sender fitting. If the oil pressure is approximately the same as the dash gauge, I'd pull the pan and replace the oil pump and pickup, rod bearings, and rear main oil seal.