New Here. AMC 401 Oil pressure problems.

KarlVP

Hey folks, what a great site. Been pulling my hair out on the rebuild on this 74 Jeep J-20. Should have found this place A LONG Time ago. Anywho, finished a rebuild on the 401 and now I'm having some oil pressure issues. The symptoms are: Truck starts fine. Mechanical gauge reads 40 to 60 psi. Once it warms up, the psi drops to around 20. But if you rev it up, it does hop up to 60 or so. After the trip to work (about 10 miles give or take), the psi hovers around 20, and shoots up when you step on the gas. But when you come to a stop, at idle the pressure drops to about 5. Put it in neutral and rev it up, and it won't climb past 25 or 30. Running straigt 30W oil.

jeepsr4ever

Your oil is too thick for a rebuild. Chances are your oil pump is shot. You are probly keeping your oil filter bypass open when she is cold and then when the oil warms up you are reading the real working pressure without bypassing the oil filter. You could try a bandaid like re-surfacing your oil filter adaptor or a midplate but I suspect you have a pump out of tolerance.

KarlVP

jeepsr4ever wrote
Your oil is too thick for a rebuild. Chances are your oil pump is shot. You are probly keeping your oil filter bypass open when she is cold and then when the oil warms up you are reading the real working pressure without bypassing the oil filter. You could try a bandaid like re-surfacing your oil filter adaptor or a midplate but I suspect you have a pump out of tolerance.

The pump was brand new nickle plated when the motor was put back together. We installed a new cam and lifters then did the break in. Changed the oil to 10W-30. For the first 500 miles or so it ran great. Then we started having problems so we switched to straight 30W. The motor after rebuild has less than 2500 miles on it.

I can check the pump tolerances though.

ironman_gq

take out the spring and plunger on the bypass and make sure it moves freely. 9/10 oil pressure problems with these motors is the plunger sticking or a bad pump cavity.

jeepsr4ever

hmmm. What brand of oil filter are you running? Your plated covers shouldnt be giving you this kind of pressure. That pressure is too low for new bearings.

KarlVP

jeepsr4ever wrote
hmmm. What brand of oil filter are you running? Your plated covers shouldnt be giving you this kind of pressure. That pressure is too low for new bearings.

Running a napa gold filter.

After my research today (nothing as vast as this site) I was under the impression that the oil pressure relief valve spring was/is the culprit. I used the one that came with the motor as the pump I bought did not come with a new spring or valve.

jeepsr4ever

PM me your address! I will get you out a new spring and plunger.

82Waggy

Be sure the plunger moves freely in the adapter. I got a brand new nickle plated adapter that the plunger would hang up in due to dirt being plated in place. Removed the dirt/burrs and now moves well.