So i just bought my first jeep ever, its a 97 xj 4.0 auto. Its got like 250k ish on it, i just bought it to play with so i didnt care abkut miles as long as it ran right and it runs as perfect as you could ask for. But. As soon as i got it home, after two hours of drivong around mind you, my oil pressure dropped to zero immediately and started hanging out around ten ish. When i say immediately i mean it dropped like half the motor fell out. I did a new sending unit and it didnt help. It still goes up to 40 but randomly falls on its face to zero then hovers by 10. Im praying its a collapsed oil filter or a pickup tube or maybe a bad cluster gauge. Havent had time to really check yet. Anyone else have this problem?

 

This is the most common thing talked about on this or any other forum and the cause is basically unknown. HOWEVER, it can be dealt with. Jeep says 13 psi at idle hot is sufficient as long as it comes up at around 1000 rpm. I bought my 2000 with only 125K miles on it and it showed the same symptoms. I switched to 5W-40 Rotella T6 Synthetic Diesel Oil and a new Mopar oil pressure sensor and now have 25 psi at hot idle, 45 psi on cold startup, and 40 psi at anything over 1500 rpm. There have been many explanations and conjectures as to why this happens, but no real points that nail it down. I have a 2000 WJ Grand Cherokee with the same oil pump in the same engine with 252K on it and never in it's life has it showed me any less than 40 psi at hot idle. The only difference in the motor is the oil filter and sensor are mounted on the main oil gallery instead of on an adapter near the oil pan. It uses the identical Melling oil pump that the XJ does, so that's not the reason. The only major differences between the 2 vehicles is the size of the engine compartment, meaning there's more room for cooling air to circulate around the engine and take away heat.

Rotella T6 is a great oil with a boatload of ZDDP in it that is supposed to be good for lifter wear, but is quite pricey at $30 a gallon. I'm thinking of switching back to Valvoline Maxlife High Mileage 10W-40 to see if it still holds that kind of pressure. Maybe in hotter climates 20W-50 would be better, IDK. There are endless posts on this forum that ponder this subject and you should do some reading.

 

Try some 15w40 Diesel oil and a Motorcraft FL1A filter.