2000 Grand Cherokee 4.0L no oil pressure @ idle

Messages
918
Location
Florida, USA
Hello. My friend has a 2000 Jeep Grand Cherokee 4.0L with 242k miles. The oil pressure gauge reads 0 PSI at idle, and the check engine light comes on. The oil level is full, and the pump is working (confirmed by a shop). When he accelerates, the OP rises to around 40 PSI and immediately as the gauge moves away from 0 the CEL goes off. Could this be the oil pressure sending unit, or symptoms of excessive wear elsewhere?
 
Messages
5,091
Location
USA
Replace the sending unit first,if that doesnt solve the problem,you either have some bearing wear (not unusual at 242K) or internal oil pump clearance (or a screwed up pressure relief valve) thats out of spec (worn).
 
Messages
990
Location
MA
Put a mechanical gauge on it and see what the pressure really is first. Then you'll know if the sender is bad.
 
Messages
19,686
Location
Sunny Florida
If there truly was zero oil pressure the PCM would likely pull the spark as most do. NEVER trust a factory gauge. Put a known mechanical one on it and get a true reading. The check engine light may or may not be related to this problem. Did you pull the codes?
 
Messages
25,984
Location
Upstate NY
I agree, everyone should have a mechanical pressure gauge and DVM, as one cannot really trust the dash gauges. And make sure when buying a mechanical gauge it has the right fitting. The gauge I got from HF 4 or 5 years ago had 2 fittings, the ones from HF now have a bunch. Also consider throwing on a new ST oil filter as a test. I have seen a clogged oil filter cause problems they should not have.
 
Messages
5,651
Location
Iowa
Cheapest, quickest and easiest is to replace the sender with an OEM part- they commonly fail. I have seen a 4.0 Jeep come in with indicated low oil pressure but was whisper quiet... It really was dropping to almost no pressure at hot idle. ~5psi at idle and 35-40psi max. Also, FWIW, it's much more likely the "Check Gauges" light is coming on, not the "Check engine" light.
 
Messages
1,283
Location
Canada City, Canada
I had the same thing on my old 2000 Cherokee. It also dropped to 0 at idle and the light came on, however the engine still sounded fine. I ended up putting in a 40 weight oil just to keep the (malfunctioning) gauge and oil light happy.
 
Messages
8,859
Location
Texas
+1 to all the advice except "ignore it." :-) Try the sending unit (that was the fix when my 2001 was doing this) Try a different brand of oil filter and get an FL1A interchange size oil filter (Wix 51515, Purolator L30001, its bigger but same mounting and valving specs) so that you get less pressure drop across the filter. Try 10w40 or 5w40 oil.
 
Messages
8,859
Location
Texas
Originally Posted By: jeepman3071
Is there any coolant loss?
Good point. To clarify the reasoning here, model year 2000 and 2001 Jeep 4.0 engines used a new head casting denoted by a casting number ending in "331" cast between the #3 and #4 intake ports on the top of the head. It was prone to cracking from the water jacket into the area under the valve cover- no compression loss or coolant out the tailpipe, but oil COULD leak into the oiling system where the silicates turned to grit and wiped the bearings. Sometimes the first sign of a problem was sinking oil pressure. Its always important to NEVER let a 2000 or 2001 Jeep 4.0 overheat, keep an eye out for unexplained coolant loss, and maybe even have periodic oil analyses looking for coolant.
 

Towncivilian

Thread starter
Messages
918
Location
Florida, USA
The battery was failing. It was replaced under warranty, and it seemingly solved the issue of showing 0 PSI at idle as well as a rough idle! This confused me. Anybody know why the battery was causing this issue? There is no apparent coolant loss, but I removed the radiator cap and saw some rust. How should I deal with it? It's filled with some green coolant. The radiator cap doesn't look too great, I'll probably have my friend buy a replacement OEM one. The blower motor and A/C haven't worked for the entire time my friend has been driving this vehicle. Would the blower motor resistor be a potential cause of the blower motor not functioning? The controls seem to work and they light up when one selects a mode, but the blower motor and compressor do not turn on.
 
Messages
2,750
Location
Rochester, NY
Jeeps, WJ (99-04) Grand Cherokees in particular, are picky about batteries. They just do weird stuff when they get old, such as headlights randomly turning on when opening a door, ABS light coming on for no reason, etc. I'd give the cooling system a thorough flush, and refill with either old school green, or G-05.
 
Messages
8,859
Location
Texas
Originally Posted By: Towncivilian
The battery was failing. It was replaced under warranty, and it seemingly solved the issue of showing 0 PSI at idle as well as a rough idle! This confused me. Anybody know why the battery was causing this issue? There is no apparent coolant loss, but I removed the radiator cap and saw some rust. How should I deal with it? It's filled with some green coolant. The radiator cap doesn't look too great, I'll probably have my friend buy a replacement OEM one. The blower motor and A/C haven't worked for the entire time my friend has been driving this vehicle. Would the blower motor resistor be a potential cause of the blower motor not functioning? The controls seem to work and they light up when one selects a mode, but the blower motor and compressor do not turn on.
Low voltage gives some older Chrysler PCMs fits. They'll misfire at idle, giving a low idle speed, which in turn drops the oil pressure. To add insult to injury, the reference voltage for the oil pressure sensor might drop too, or it might simply trigger the zero oil pressure warning due to the PCM going through its fits. +1 to the suggestion to do a coolant flush and re-fill with either green or G-05. I've switched my whole fleet over to G-05 for convenience, but green is just as good for these vehicles. Silly question... have you checked the HVAC fuse? The control panel indicators are not necessarily on the same circuit as the fan and compressor clutch.
 

Towncivilian

Thread starter
Messages
918
Location
Florida, USA
I'll ask my friend to check the HVAC fuse. I've never flushed an entire cooling system, just a drain & fill of the radiator and overflow reservoir on my Pathfinder. I'll do a search here on flushing methods. Another issue: the driver's side brake light turns off when the brakes are hit and the parking and head lights are on. The taillight part of the bulb lights by itself, and if the parking/headlights are off, the full intensity brake light works. Also, the turn signals on this side flash quickly as if one of the bulbs is out, but neither are out. All of the bulbs look fine. Is there some wiring fault, or could one of the bulb sockets be bad even though they all look fine? The transmission probably needs a fluid exchange too. I believe there is an in-pan filter that isn't a rock catcher, right? That'll be fun to replace... I'd probably do a pan drop, then a cooler line exchange (there is a cooler, right?) and install a Magnefine. ATF+3 is spec'd, so any ATF+4 will suffice in its replacement? Does the power steering really need the special fluid mentioned in the FSM? It says specifically to not use automatic transmission fluid.
 
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