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1999 Jeep Grand Cherokee (V8): oil pressure drop and stall

Our Jeep has 278,000 miles and we want it to keep going. For the past few months, the oil pressure gauge falls and sensor lights up. Often the pressure goes back up and everything is fine. More frequently, the car has begun to stall at lights and even when running. Our mechanic has no options and says it is just an old engine and needs to be re-built. Tell me it isn’t so! Any ideas?

First thing is to find out what the real oil pressure is doing. Have a mechanical gauge hooked up and see what it does during these stalls.

The mechanic said they did and the pressure is indeed dropping during both idle and while running.

Then it could be a worn engine. It might be a plugged oil pickup if you’re lucky. But that’s a lot of miles.

So what makes the oil pressure drop? Why can’t that be fixed? Also, how can we check the oil pick-up? My husband is pretty handy…

Worn out bearings throughout the engine cause low oil pressure. A new or rebuilt engine is needed. Not likely worth it.

To check the oil pickup the oil pan is removed.

First off, 275K is pretty good for any vehicle on the original engine. If you like this car and want to keep it, no worries, provided it tests out by a reliable inde mechanic as “worn out”, you can either have your engine rebuilt or get a replacement engine. You’ll be back on the road with good oil pressure in no time. If you price replacement engines for this vehicle out, I think you’ll find they are remarkably economical – compared to the sticker shock of buying a new replacement car anyway.

To address your above query, the oil pump sends pressurized oil throughout the engine via many small passages to lubricate among other things the crankshaft bearings and piston walls. There is a small oil passage that runs right down the middle of the crankshaft for example, with outlets at each of the bearings. Here’s the thing: The overall oil flow rate is limited by the size of the oil delivery openings at the end of the small passages. As the engine wears over hundreds of thousands of miles driven, the bearing surfaces decay a little, so the size of the openings at the end of the oil tubes enlarge. This seems at first to be good, as more oil flows, but it is actually bad, b/c the oil pump is limited in the volume of oil it can pump per second. When a lot of oil is leaking out in certain spots, the other places the oil is supposed to go become starved of oil.

It’s like if you were throwing a birthday party and trying to be efficient and blow up 5 balloons at once, so you hooked all five balloons to the same pipe, and blew into that pipe. It would work unless one of the balloons popped, in which case most of the air would go to the popped balloon, and all the other balloons would get no air at all. That’s probably the situation you got with your engine.

That said, there are a lot of vehicles on the road with more than 275,000 miles on the original unrebuilt engine. How long an engine “lasts” before this happens depends on a lot of factors, including the engine design criteria back at the factor, how aggressively the engine is driven, and the routine maintenance. If you’d like to increase the chance your next car will last more than 275K before this happens, choose a model that has a history of being long-lived, drive conservatively, and follow the manufacturer’s routine maintenance recommendatons to the “T”.

Thank you so much for your detailed explanation!

The only thing I’d add to that excellent explanation is that it’s unusual for a vehicle to make it past 250,000 miles, between crashes, rust/corrosion, transmission failure, and engine failure. Only a small percent make it as far as your Jeep has.

You could buy yourself a little more time by replacing the oil pump. The engine has worn to the point that the bearings have so much clearance the oil pump can’t pump enough oil at idle to keep the pressure up. The pump, too, has worn so that it is not as effective at low engine RPM (idle). I couldn’t find a “high volume” version of your pump, but if there is one available, use that one. It will pump more of the oil you are spilling out from the worn bearings. Its a band-aid to buy you a little time 30-50,000 miles, at best.

An even cheaper alternative would be to change to higher viscosity oil… If you use 5w30, switch to 15w50. That will buy you a little time to save up for a rebuilt engine.

I have a 1999 that has 124000 on it and my son has a 2000 . They both do the same thing . I noticed that on first start up on chilly mornings is when mine does it . Once it’s warmed up it acts normal. I’m wondering if it might just be a poor gage ?

Be very careful going to a heavier weight oil. Pumps are meant to pump a certain weight of oil and increasing or decreasing that can cause problems… thicker oils mostly pumps can’t handle and you’ll starve your engine or blow your pumps, thinner oils and it will pump but it’s not as thick as clearances allow and it can run through the engine too fast… those are just a FEW of the possibilities of changing the manufactures recommended oil viscosity! FYI

Do you have a tachometer? If so does the oil pressure drop coincide with lower than normal idle rpm? If so it may be a dropping idle rpm condition causing your problems.