So, here's what happens:
This only happens if my truck has been driving in traffic jam conditions, and overheats a little.
When the engine RPM's go down to around 500 (Which only happens when the truck is stopped and sitting still held by the brakes. (When in park, the RPM's are around 750)), the oil pressure gauge will read 0. At any other time (including: stopped with brakes without overheating, normal driving, stopped with transmission in park) the gauge will read normally.
Now, overheating a little is normal in traffic jam conditions (although I think mine overheats a little too readily and too severely), but I don't think the drop in oil pressure is normal. When I give the engine a little gas, or put the transmission in park (both of which raise the RPM's), the oil gauge returns to normal. Both in going up and down, the gauge moves very rapidly. Other than the oil gauge, there are no symptoms of this problem.
Some other things that may or may not help you with a diagnosis:
This truck is a Jeep Grand Cherokee '97 with the 5.2L V8. It has 152,000 miles.
The coolant in the engine now is very very nasty looking, and is in dire need of flushing (my next project)
The engine burns coolant (just a little, but has increased lately I think)
The engine leaks oil a little (but the quality and quantity of the oil is fine right now)
My best guess is that there is something mildly wrong with the oil pump. This is not good, because in order to get to it, I'll need to remove the drain pan, and the exhaust pipes and front axle are both in the way of doing that.
My questions to you:
What is your opinion of what is causing this?
Is it worthwhile to fix it, or should I just let a sleeping dog lie? (Or should I bump up the idle speed to prevent the low RPM's?)
Well it could be something wrong with the pump, or it could be something wrong with the oil pressure sensor. I know the wire that guys to my sensor once got to close to the headers, and melted it shorting it out, and my gauge didn't work properly until I put a new wire on. I'd say either ask a real mechanic, or replace the sensor before you replace the pump, because sensors are A. cheaper, and B. easier. Whenever I am faced with a car problem I figure out what the possible causes could be, then examine those possible causes by starting with the cheapest, easiest fix and working up.
Im not an exper but I Think the oil pressure gauge just malfunctions when you are in an traffic jam. If it wasn't like this before it could be that the oil gauge is broken or something (otherwise a design/production error (very unlikely)). It is possible that the oil pressure gauge problem can get worse and give complete false readings when youre not in an traffic which can result that one day you dont have any oil pressure. And you'l be destroying youre engine.
Your best bet is too look (as said by coolclay) too look at the wire and replace it. The cause is most likeley that when the wire gets very hot the oil pressure gauge malfunctiones*, if you get the cooling work better it might help too. And you dont have to replace the wire at all.
*hot metal has a bigger Electrical resistance and combined with the fact that the singal going to the oil gauge is very small the Electrical resistance can ''block'' the current and then the gauge drops to zero.
good luck with your car.
It could be your main or big end bearings on their way out , you have done a fair few miles .
it could also be the oil pump , it it still runs fine and is not too noisy I would just live with it till it gets a bit worse.
Perhaps you may just trade it in and get something different before it gets past the point of getting rid of it.
I dug around a little and got this:
| a person on the internet wrote: |
| I think I would check the engine for sludge. You say the oil is clean...I assume that means looking at the dip stick. I think the first thing I would do is drain the oil and look for sludge in the bottom of the pan. I have seen these engines with so much sludge in the pan that the oil pump pick up is burried. In your case it may be still able to draw oil with RPM, but not at idle. You could also pull the valve cover to check for sludge. |
Seems reasonable, because it explains why the pressure is OK sometimes but not at other times. You could also try wiring a separate oil guage to make sure that your in-dash guage isn't simply malfunctioning. Since you change your own oil, maybe you've developed a bit of sludge that doesn't drain from the pan. Maybe it would be a good idea to flush the oil when you're doing the coolant.
I'm guessing that 500 rpm isn't enough to bring the oil out of the pan. Maybe the rpm goes that low when the car is heating up a bit to prevent more overheating. Changing your coolant might help avoid the problem by allowing the car to run faster on idle.
While you're stopped, try giving the motor a bit of fuel to pick up the rpms. If the oil guage snaps back into working when the engine speed picks up, that would support this idea. The sensor/wire idea is worth looking at.
The sludge thing is also a possibility.
On the next oil change use a bottle of engine cleaner.
You dump it in the crankcase and run the motor for 5-10min.
Then drain the oil and pull the filter.
Before you put the plug back in
pour about half a quart of oil through the motor
let it drain out and flush out remaining sludge.
You may even be able to actually see globs of sludge plop out.
If you do see big gobs of sludge
take a piece of wire like a coat hanger and bend ~4" leg in it
and try to scrape the inside bottom of the pan.
Have you ever replaced the fuel filter yet?
At 152K it's about time for a new fuel filter.
An old fuel filter can cause weird symptoms.
They only run about $10.
You'll probably find it along the inside of the drivers side body frame rail.
And it's usually just a little bit behind the drivers seat.
But, under the vehicle of course.
Now for the bad news,
you have a head gasket leak.
An engine burns coolant when it first starts
when coolant that has collected in the cylinders overnight burns off as white smoke.
Coolant can also get into the cylinder during the intake stroke.
The crummy looking dark coolant is from motor oil leaking into the coolant.
Again at the area of damage in the head gasket head gasket.
An easy confirmation of a gross head gasket leak.
While the motor is cold and stopped,
take off the radiator cap
start the engine
look for bubbles in the radiator.
The next test you need to do is a compression test.
Ideally the cylinders will all have about the same compression pressure.
I suspect that you will 1 or 2 cylinders noticeably lower than the average of the rest of them.
That will tell you which side of the motor needs a new head gasket.
Many things can cause a low compression.
But, given your symptoms, odds are it's the head gasket.
Do these last 2 tests before flushing the radiator.
Try to confirm/rule out a head gasket leak before flushing the radiator.
Other wise you could be wasting your time and money on antifreeze.
| jwellsy wrote: |
Now for the bad news,
you have a head gasket leak.
|
I know, but it isn't a bad one, and doesn't interfere with the engine enough to lower performance (it still performs better than factory specs), so it isn't worth the considerable expense of having it fixed. (At least not yet.)
I'll look into the faulty gauge possibility, and check for sludge, thanks for the suggestions there; I'm also planning on replacing the fuel filter (sooner or later) anyway.
I was also thinking that the high temperature might change the viscosity of the oil enough to make the aging pump have a difficult time pumping it. I may try a higher viscosity oil next time around, because it never gets very cold around here.That low oil pressure could be valid.
Right now you can probably find a local mechanic
to change out the head gasket on one side for about $250.
If you wait, and that loss of oil pressure is valid,
then you will probably eventually trash both heads and all the bearings.
That will be a $1500 job.
That head gasket leak can easily cause the loss of oil press at low rpm.
You can hook up a second oil press pressure gauge.
I would do what ever it took to verify the validity of the low oil pressure reading.
If it is valid, I would use the compression test results to pick the side to replace.
And I would spend the $250 to fix that side.
You may even be able to barter building the guy a web site or something, for his labor.
That would be a heck of a lot cheaper than just ignoring it.
It is not going to get any better by itself.
And, I hate being broke down along side the road.
The fuel filter thing would be an attempt at helping keep the rpm's up.
BTW, all the engine additive marketing hype is mainly just BS.
^Well, even if the low oil pressure reading is valid, it only happens rarely, and I know how to avoid it happening now. (Put the transmission in park whenever stopped while the engine is overheating.) Also, I hope that flushing the coolant will make it far less likely to overheat, possibly making this problem go away.
And yes, I know I should fix that head gasket, but it has been driving for at least 50,000 miles that way, and it doesn't seem to be harming anything. (It is a very small leak, after all.) Unless I notice that I start loosing coolant faster than usual (which is about the amount the reservoir holds per tank of gas), I'll just hope it'll keep on running till when I'm planning to replace the truck (at 200,000 miles).
Flushing the coolant could even make the problem worse.
The particulate matter in the coolant may be keeping the leak somewhat plugged up.
^True, but the flush does really need to be done: it is causing the overheating (I think), the coolant is extremely dirty, and I suspect that in all the truck's 10 years and 150K miles (Including a good deal of hard, off-road use) it has never been changed before.
