Very low oil pressure at idle when hot

oil

#1


Vehicle: 2000 Jeep Cherokee Sport, 4.0L, 6-cyl, 94K miles, 10W-30 oil (OEM spec)



I’ve always used full synthetic oil in this vehicle, due to the sub-zero winters we have here in Minneapolis that turns non-synthetic oil into molasses. But over this past winter, I noticed a significant drop in oil pressure at idle when the engine/oil is hot. Not too bad, but noticeably lower than the normal mid-point on the gauge.



This only happened when hot. Idle at starting or just a few miles showed no change in pressure. And pressure while driving is fine.



Spring came, and the condition began to get worse, to the point where the oil pressure warning light came on. I read elsewhere that full synthetic oil can be too thin for some oil pumps to keep proper pressure, so I drained it, and replaced it with non-synthetic oil. I definitely got better results, but now in the summer, I get the same significant drop & warning light at idle on hot drives. And again, perfect pressure on idle at starting or short drives.



I gotta think the oil pump is ok, due to the fact that idle pressure is fine when the engine/oil isn’t hot. Furthermore, the pressure is ok while driving when engine/oil is hot.



Do I dare consider using an oil thicker than the OEM spec? Or an additive?



Other ideas?



Thanks in advance!



Joe


#2

The first step is to hook up a mechanical gauge to see what the pressure is actually doing. Often, the problem is the sending unit for the gauge, not the oil pressure itself.


#3

Could the sending unit could be faulting only when the oil is hot and engine at idle speed? I get perfect readings when driving or when the oil is cold.

Thanks!

Joe


#4

Yes, it could still possibly be a faulty sending unit.


#5

You don’t want to use an external pressure gauge to be sure of the oil pressure, and, you don’t want to just change the oil pressure sender; so,what are you willing to do? Using a thicker oil, or additives, are not viable alternatives. What’s left? Jump in and change the oil pump? Then I, for one, can’t help you.


#6

My guess is that the engine is shot.

Here’s the way it works:
Your oil pressure is created by the pump pushes oil through channels and through the tiny spaces between the wear surfaces in your engine, including the main crankshaft bearings. As engines wear out, the spaces become larger and it becomes analogous to trying to maintain pressure in a baloon with a leak. The oil flows through the spaces too easily and the pump can no longer maintain pressure at idle.

The pump itself is simply a couple of impellers that look like two gears through which the oil passes as they’re turned by the engine. The slower the engine turns, the slower the impellars move and the less fluid they pump. That results in less pressure in the pressurized channels and, if the engine is worn, insufficient fluid being pushed in there to keep pressure. Since oil pumps are so simple and so well lubricated they rarely wear out.

To diagnose this, use the remote gage setup that others have recommended. If the pressure drops too low at idle, it’s a safe bet that the engine is suffering from wear. It’s also a perfect application for a higher weight oil. The heavier weight will slipp through the spaces lesss readily and allow the pump to maintain pressure.


#7

I’m not exactly sure I understand your comment. I certainly didn’t say I said I wasn’t willing to test with an external pressure gauge or change the sending unit. I was asking a genuine question.

Joe


#8

Mountainbike is unfortunately probably right that this is a case of a worn out engine. I think you have some misconceptions about synthetic and conventional oil. The thickness of the oil should be more or less the same synth vs. conventional and I’d venture to say most cars in Minneapolis get run on conventional without their oil turning to molasses. The advantage of synthetic is that the molecular arrangement is different, which means it gets into some parts of the engine better than conventional oil and consequently some cars are designed to require it. On a car not designed for it, though, the advantages are somewhat dubious-- the only real one is that the oil has a longer lifespan, so you can theoretically do 10,000 mile oil changes instead of 4-5,000, but it doesn’t protect your engine any better than regularly changed conventional oil. The way to protect your car in subzero temperatures is to switch to a lower-viscosity oil (You’ll have no problems using 5w-30 or even 0w-20 in a Minnesota winter on a car that calls for 10w-30) or better yet by getting a block heater.

Anyways, by all means change to a 10w-40 or even a 15w-40 and this will get the oil pressure back up. You may see a very slight decline in your gas mileage and this will only be a cover-up for the problem. When things get colder out, switch back to 10w-30 and don’t forget to get a block heater for your new car once this one dies.


#9

Does anyone know if this engine is prone to bearing failure at this low mileage.The post reads like it was maintained.GM told us that 7psi at idle was OK on new engines (1998)


#10

i would suspect that you actually have low oil pressure at all times, not just when hot OR, you dont have low oil pressure at all, just a bad sender. this is one of the (many) times a real oil pressure gauge would be nice.

i dont know how much an oil pressure sending unit costs, but the short money would be on replacing it.

if the sending unit is not the problem, then the oil pump is. if the oil pump isn’t, well… new engine time.

so the cheapest thing to check/replace is… the sending unit.

if the sender is relating accurately then the oil pump is shot.

but first things first. check the pressure with a known gauge to confirm it being good.


#11

" Do I dare consider using an oil thicker than the OEM spec? Or an additive?"

Yes! By all means! The 15-40 fleet oil would be a good choice in that old Rambler Motor.


#12

94,000 miles isn’t very much for one of these engines if it has been properly maintained. Oil pressure senders are a known weak spot in the design. As others have suggested, make sure yours isn’t defective before you tear down the engine or go to an unreasonably thick oil. The idea of the mechanical gauge is the usual test.


#13

Thanks! Let me clarify: I can cite from (unfortunate) experience that cars are far easier to start in sub-zero temps with full synthetic oil than non-synthetic. That said, I agree that once the oil has heated, there’s no real difference in running condition.

Joe


#14

Thanks to all for the suggestion of testing via external mechanical gauge and/or replacing the sender. Will indeed investigate.

Joe


#15

Yes, tried to keep up with preventative maintenance on this vehicle as much as possible.

Thanks!

Joe


#16

Oldschool posted an interesting question about bearing failure at low mileage. The low mileage on an 8 year old vehicle indicates to me that the vehicle is used mostly for short trips.

Combined with the Minnesota residence, it could be worn out. When I lived in North Dakota I learned that sub zero temperatures are extremely hard on a car mechanically. All the parts change size and shape beyond what would be normal in a warmer climate, and it can take forever to get them warmed up. We used to put cardboard or a blanket in front of the radiator when temps would get really low or the engine would stay cold, even of it had been plugged in. It would drop sometimes to -30 below and colder at night and get up to zero or less during the day…for days at a time. This weather is extremely hard on engines.

Anyway, the remote gage is the real test.


#17

Theoretically, in terms of how it flows around the engine, a 10w-30 should flow the same dino or synth, which is what affects how much resistance there is to the engine turning over. That’s at least my understanding of it-- maybe some of the other posters with more of a chemistry background can explain it better or know differently. From what I understand, it’s easier to make a synthetic oil that is good down into the dozens of degrees below zero range, hence 0w-30 which is only available in synthetic, but if you’re looking at two oils with the same viscosity rating they should flow equally well.


#18

Hey Joe. Coincidentally, I am seeing the same problem with the same exact car with the same mileage. Can you let me know how it goes? I am praying it is not an engine problem. At 94k, it’s just broken in. Thanks.


#19

Link to oil pressure sender removal & installation. This is where the oil pressure gauge would plug in: http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c15280043182