258 Oil Pressure Drop As Engine Warms

79 CJ7 engine oil pressure starts out at 40-50PSI or more, then drops to 6-10PSI at normal operating temp. New SW mechanical gauge. Always run 10W-30. Should I go to a different weight oil or is this a sign of bigger problem?

 

A '79 has the cast iron oil pump, and a separate in-sump pump driven from the distributor. These engines typically don't have oil pressure problems, and do not have a history of cam bearing failure, so I would conclude you have a bad rod bearing. Thicker oil only treats the symptom, not the disease. A thicker multi-vis isn't going to do much. At the Santa Ana barrio parts store, we kept Valvoline 50W single grade oil on the sales floor for such engines. You could try that, but it's going to be thick like tar below freezing. Personally, I would pull the pan and look at the rod bearings. You need 10 psi per 1000 rpm minimum, so you are way below that.

 

Thanks. Sounds like I'm headed for an rebuild. Am I risking further damage by running the engine at the low oil pressure? Guess I'm asking how much time I have.

 

Rod velocity of a 258 is high, because of the long stroke. The higher the RPM, the higher the rod velocity, and the more pressure you put on the oil film in the bearings. You have no noises now, and some oil pressure... if you baby it, you could keep running for quite a while. But it's going to limit what you can do with the Jeep - no cross country trips. Maybe cross town. Once you hear any banging, the end has arrived.

If this is a rod bearing issue - the 258 has a cast iron crank, and you will likely need the crank turned and new oversized bearings fitted. You can buy a new crank and bearings in a kit - a "crank kit" - and it's a bolt-in job to change out the crank and bearings with the engine out. Realize that this engine has the early style crank, with 12 counterweights. These cranks are in demand for stroker motors, so don't let the shop give you a '81-on 4-counterweight crank in exchange. The lighter crank will work, but it's wrong for that engine.

Or look around for a used engine.

Again, I would pull the pan and inspect the rod bearings. Could be something freaky like a clogged pickup, or who knows.

 

Thanks again for the advice. The Jeep is a weekend fun ride awaiting a tub replacement. Hoping to wait until winter for that. I plan to pull the engine and install a new clutch then. But will pull the pan and take a look for now.

 

Timgr is usually spot on. However friend of mine had a 79 acting similar. Turned out the relief was sticking open due to some po poor maintenance habits.

 

If you are pulling the engine for the tub replacement planning a rebuild may be a good idea. Not terribly expensive and it'll give you peace of mind. Hopefully it's a clogged oil screen or sticking relief valve but if that were the case I would think pressure would be low at startup also and not just after warm-up.