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98 Explorer oil pressure


PFA
12-05-2011, 10:05 AM
I just bought a 98 Explorer for winter use. I am having an issue with the oil pressure gauge. While cold it shows good pressure. After it is warmed up, I only have pressure while pressing the gas pedal. It's fine while driving but when you come to a stop, it drops off. I did buy it this way, thinking it was not a big deal. I didn't pay much for the truck so, I wasn't concerned much about having to work on it. But, it looks like the 4.0sohc is not a fun job to change the oil pump...
Any insite is appreciated.

thanks

shorod
12-05-2011, 01:34 PM
The oil pressure "gauge" on the Explorer is not really a gauge. It uses a switch and if the pressure is above x psi, the gauge reads in the middle. If it is below that it reads low. First thing to check is that the oil is in good shape and at the correct level. Next would be to use a mechanical gauge to confirm what the pressure really is, or if you don't have a mechanical gauge, you could just replace the switch and see if that fixes the issue. It's not uncommon for oil pressure switches to fail.

-Rod

PFA
12-05-2011, 08:47 PM
Thanks. The oil was just changed. I put in 15w40 Rotella, hoping to keep it thicker. The pressure is good at about 2000rpm and above or the first minute or so then I have issues. Where would I hook up a mechanical gauge??


The oil pressure "gauge" on the Explorer is not really a gauge. It uses a switch and if the pressure is above x psi, the gauge reads in the middle. If it is below that it reads low. First thing to check is that the oil is in good shape and at the correct level. Next would be to use a mechanical gauge to confirm what the pressure really is, or if you don't have a mechanical gauge, you could just replace the switch and see if that fixes the issue. It's not uncommon for oil pressure switches to fail.

-Rod

shorod
12-05-2011, 10:52 PM
Hmmm, I wouldn't suggest the 15W40, especially if you suspect a possible oil pressure issue. Thicker oil will not equal higher pressure.

You would put the mechanical gauge in place of the oil pressure switch temporarily.

-Rod

PFA
12-06-2011, 09:02 PM
Thanks
I was hoping that the thicker oil will hold the pressure lower in the rpm band.
I will try to get a gauge together and put it in.

I haven't looked yet, but I assume that the sensor is near the oil filter?

shorod
12-07-2011, 07:44 AM
I was hoping that the thicker oil will hold the pressure lower in the rpm band.

I used to think that too. Then one day I noticed on Dad's truck that the oil pressure gauge read higher after an oil change which didn't make sense to me. I asked him about this and he explained that the gauge reads how much pressure the oil is being pumped with and fresh, "thinner" oil is easier to pump, therefore the pump can put more pressure behind it. It's a bit like using a straw. Think about trying to suck up chocolate milk through a straw and how quickly you can fill your mouth. If you suck really hard, the milk will enter your mouth under so much pressure it may cause you to cough and choke. Then try to suck up maple syrup, the syrup will fill your mouth at a much slower pace, and it will enter your mouth under a lot less pressure.

I haven't looked yet, but I assume that the sensor is near the oil filter?

It's probably not too far from the oil filter. According to the factory service manual, all three engines (you don't mention which engine is in your Explorer) have the switch located "front of engine, near power steering pump".

-Rod

PFA
12-07-2011, 08:56 AM
Thanks.
I have the 4.0 SOHC in it.

I guess in your analogy, I can see what you are saying, but thinking that this is a pump pushing rather than sucking, I would think that the thicker oil would build more pressure because it is thicker/harder to push, creating more pressure. :banghead:

shorod
12-07-2011, 01:15 PM
The thicker oil will create a greater load on the pump, but will not yield a higher pressure in the oiling system. The pump would need to pump harder to build the same pressure, but since the pump is driven off the engine, it is limited on the amount of work it can due. So basically you have the same pump volume, same pump speed, but higher resistance in the fluid. I would say the analogy still applies because the units are the same. Sucking (vacuum, in psi) is still a measure of pressure. In our atmosphere a vacuum is created by pumping the air elsewhere. In an ideal world the amount of vacuum would be equal to the amount of air being pumped out. You could seal the lid to the cups of milk and syrup, insert a second straw in each cup, then blow in to both at the same time (same pressure applied to both fluids) and watch the force at which the milk exits it's cup compared with how much force the syrup exits. Or borrow your kid's Super Soaker, fill it once with water and measure how far it sprays, then fill it with syrup and measure how far it squirts out. Of course you'll need to pull the trigger back with the same force.

-Rod

PFA
12-21-2011, 02:14 PM
Well, I guess I'm not fully convinced. Trying to understand why at startup when cold it has pressure until the oil warms and thins, then lose pressure.

Anyway, I put a mech. gauge on it. Definitely losing pressure at idle, so not a bad switch.

Now to weigh my options on what to do...

billn2
12-21-2011, 03:25 PM
If you're losing oil pressure as it warms up, sounds like bearings not an oil pump. I would pull the oil pan and get some plasti gauge and check your crank bearings.

DeltaP
12-21-2011, 06:18 PM
Stop using the 15W40 Rotella. Thats a diesel engine motor oil. It's not formulated for gasoline engines!
Besides, multi weights don't increase viscosity. 15W-40 flows like a 15 weight with a film strength of a 40W. Thats all.

PFA
12-21-2011, 06:50 PM
I figured that if I did get to the oil pump, I'd change out the bearings too. BUT, the big deal is getting that deep into the engine from below. The SOHC is not a fun job from what I can tell. It might be easier to just pull the engine. Haven't read an easy way yet.


So what oil would be correct for a high mileage motor?




Thanks.

If you're losing oil pressure as it warms up, sounds like bearings not an oil pump. I would pull the oil pan and get some plasti gauge and check your crank bearings.

DeltaP
12-21-2011, 07:20 PM
I know, I own an '02 4.0L. There ain't much easy on that mill! At least nothing I had to fix. If its a 4WD then ya got a balance shaft thrown into the mix also. Although its no permanent fix and I dont know the climate where you are, you could try a straight weight oil like 30W HD. But like billn2 says ya probably got excessive bearing clearances. Thats where the first pressure drop occurs at the main bearings. Tolerances increase as the metal warms up. And if ya got that much wear on the engine youre lookin overhaul time. Ya cant do that work in the frame. Its gotta come out.

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