Topic: Oil pressure - Grand Cherokee

[70CB750]

2004 Jeep GC with 303,000 miles on it.

Fully warmed up, the check gauges light comes on with a beep.  Turns out the oil pressure is 0.  The oil level was little low, so I topped it up but it did not help.   Driving it around I observed:

- the pressure goes to 0 when engine is fully warmed up and rpms < 1000.
- pressure is normal (how I remember it when driving it for 20 years) when the engine is cold - i.e. points to 12 o'clock on the gauge.
- when the pressure drops, it does not go to - say - 11 o'clock or 10 o'clock, it goes all the way down.


The oil pump is original, 20 years old and 300,000 miles on it and I ordered new one from Autozone, but now I am not so sure the oil pump is the problem.

Suggestions, ideas? 

I am inclined to do the SOHC4 thingy and install a mechanical oil pressure gauge somewhere to see what it really reads.

[PeWe]

Oil sludge blocking the oil pump?
(Or other debris that can happen a bike  too)

Long story about it.
Like VAG 1.8T engines got when not changing oil in time and using low quality oil for that hot running turbo engine. 
Sudden oil warning that became fatal.
VAG is the Volkswagen group covering Audi, Volkswagen, Seat and Skoda using same engine.

Synthetic is a must, PAO group IV like a good priced ARAL Supertronic 0W-40 and similar.

It was a class action in USA 2011 when I understood that 15.000 km was not enough.

Here in Sweden nothing.
Lots of people changing oil way to seldom, ca 20.000-25.000km intervals in the fancy name of "Longlife".

That little turbo charged engine contains only 3.7L oil with biggest filter.  Like a CB750...
8000-9000 km is max.  Cheapest "synth" labeled oil max 8000km.

So many cars had to be scrapped too early.
It is not cheap to replace an engine and probably turbo too. More than the value of the car. There are restored engines for people that can do the job. A shop charge min $6000US for the job only, cost for parts adds on.

Toyota had also a class action where they finally claimed max 8000km intervals when driving mostly short travels .
Important to run engine flush in oil just before the change.

[simon#42]

worn main or big end bearings causes pressure loss . if you can get the sump off you can check them . you will probably need to do this to clean the pick up pipe or change the pump anyway . they are not expensive so would be worth changing at this milage .

[70CB750]

Which motor does it have? 2wd or 4wd?

Inline 6, 4.0.

[70CB750]

I always used Mobil 1 synthetic oil and Mobil 1, Bosch or K&N filters.

I am aware of bearings causing low oil pressure, but I would expect it to go down gradually as the engine warms up; the full pressure or zero I find confusing. 

[HondaMan]

This happened on a 1979 Ford straight 6 (which I dearly loved before Colorado emissions rules outlawed running the car here and I had to get rid of it at 470k miles) all of a sudden one day. It turned out that the thin paper gasket between the pickup screen and the body of the pump had finally dissolved in one spot, letting the pump lose suction. The way I tested it first was: I added 2 more quarts of (used old oil from my other car) and started it each time to see if it would pick up the pressure. At 2 extra quarts in the engine it acted normally, so I figured it was that seal, and it was: 2 extra quarts made the oil touch the bottom of the pump, putting the gasket under the oil level.

I was familiar with the setup on the pump because I had rebuilt the engine some 25 years earlier when I got the car, refurbishing the pump's gaskets then, too. I have also seen this gasket split or dissolved in the CB750 pumps, too, and they won't put out more than 10 PSI when it happens. The oil light will flash intermittently at idle then, too.

[BenelliSEI]

When hot and idling do you hear any new noises? I’d screw in a new pressure sensor before ripping to far into it (or even a mechanical gauge as you suggested).

[Tracksnblades1]

After trying a new sensor, If the pressure is still 0, check it with a known mechanical gauge to eliminate wiring.

Still at zero, then  I would first check the oil pump’s regulator valve and see if it’s stuck open..

sometimes on high mileage motors the pump’s regulator valve (piston) can get shoved back into the non worn varnished up bore. Especially on a very cold morning with a teenage driver that’s in a normal redline hurry…

Still no joy, Then do your do diligence and check it with a pressure tank next with the pan off…

Those 4.0 are known for big miles, hoping it’s just your oil sending unit..

[70CB750]

Thanks for all the suggestions, guys.

[BenelliSEI]

My old F350 has been showing “0” for the last five years…… since it never leaves the property I’ve just been ignoring it!

[Really?]

Had an '01 GC 4.7 do that, it was the oil pressure switch. 

[Don R]

 I had a shop truck with a 350 GM crate motor in it, one day going to a job the oil light came on and stayed. The plumbing shop sent out the neighborhood mechanic and he had me crank it while he had a mechanical gauge on it. I saw the gauge go to 40 cranking but he declared it to have no pressure. I told him we would tow it in. I relayed the info to our office, they realized the crate motor had warranty so we took it to a dealer where they put on a new sending unit. The neighborhood mechanic was going to sell a "new oil pump" without ever pulling the pan.
 

[HondaMan]

I had a shop truck with a 350 GM crate motor in it, one day going to a job the oil light came on and stayed. The plumbing shop sent out the neighborhood mechanic and he had me crank it while he had a mechanical gauge on it. I saw the gauge go to 40 cranking but he declared it to have no pressure. I told him we would tow it in. I relayed the info to our office, they realized the crate motor had warranty so we took it to a dealer where they put on a new sending unit. The neighborhood mechanic was going to sell a "new oil pump" without ever pulling the pan.
 

I think that guy moved here some time ago...?

[jlh3rd]

2004 Jeep GC with 303,000 miles on it.

Fully warmed up, the check gauges light comes on with a beep.  Turns out the oil pressure is 0.  The oil level was little low, so I topped it up but it did not help.   Driving it around I observed:

- the pressure goes to 0 when engine is fully warmed up and rpms < 1000.
- pressure is normal (how I remember it when driving it for 20 years) when the engine is cold - i.e. points to 12 o'clock on the gauge.
- when the pressure drops, it does not go to - say - 11 o'clock or 10 o'clock, it goes all the way down.


The oil pump is original, 20 years old and 300,000 miles on it and I ordered new one from Autozone, but now I am not so sure the oil pump is the problem.

Suggestions, ideas? 

I am inclined to do the SOHC4 thingy and install a mechanical oil pressure gauge somewhere to see what it really reads.

do you have a advanced scanner or access to one. Not just a code reader but one that reads the modules and can give live data.
An '04 might allow the capability to read oil pressure. Probably not, and I'm not sure where the ecu gets it's signal if it does. My scanner does that.