D2 5J low oil pressure

what weight oil are you using?

Purchased the D2 back in February, changed the oil after I bought it ,and flushed it with kerosene before putting the new oil in ,iam running mack bulldog 15w40 in it

Everyone will tell you is that you need to run a straight weight in it like 30W and 40W is even better.
I know you probably live in a Cold Winter area and fear not getting it started as the oil is too thick if needed in the winter. Read all the Threads on oil to stay confused even more but some guys run what they have on stock for all their equipment which very well maybe a Multi-Weight. I am not even going to say what I run as the old girl didn't get run this year so the oil was never changed after last winter. It will be interesting to see if next Spring the steering clutches will be stuck or not?
Anyways, you have to figure the hours and age of the engine to realize that the bearing clearances may be pushing the end of their useful life goes as far as "Out of Spec". Then you have to consider that maybe the relief valve and spring on the oil pump is on the weak side which maybe a little tweak will cure or the pump gears maybe out of tolerance due to wear and be in need of a freshening up. Last thing I can think of and would check before the pump is the gauge is wrong as it maybe "worn out" also.
Me?
I'd change the oil to rule that out, then change the gauge to a different one for comparison, then go after the oil pump relief valve, then the oil pump gears.
Bearings would be the last thing on my list!

About the only thing I can add to drujinin's post is that those D3400 engines are one of the "old breed" of Cat engines that used a rather high-volume oil system that would operate satisfactorily at lower pressures as long as the oil pump was in good enough condition to still deliver a good volume of oil. Cat also put out a service bulletin that featured a slightly redesigned oil pressure gauge that had a "normal" range that went lower than the original ones because of customers' concerns of a low pressure reading on a fully warm engine at slow idle. I can't find that particular bulletin right now, but maybe someone else on here has it scanned and can post it.

But definitely try to rule out a faulty gauge, even put a test gauge on it and get a good pressure reading in psi to see exactly what it is.

*Edit*

Found the service bulletin I was looking for, it gives some general specs for satisfactory oil pressure. Since it was put out in 1951 it doesn't list the "J" series D2's, but your 5J should still apply since the systems are almost identical. This bulletin states that anywhere from 4 to 11 psi oil pressure at full operating temp and low idle is satisfactory, as long as the pressure rises to 20 to 35 psi at full load speed (with a 30wt oil). The thing to remember with these old engines; a high volume oiling system coupled with their big wide bottom end bearings means that less actual pressure is required for adequate lubrication at slow, no load operating conditions.

[img]http://i802.photobucket.com/albums/yy310/TRNelson/001_zps1bbf217c.jpg[/img]

I got it all typed out, then got thinking I should edit, I tried adding to it sounded worse. Figured the info is there, just needs decipherin!