False low oil pressure reading, need help troubleshooting

For a long time now my XJ has been running 40-45psi at speed and barely 13psi at idle. I wasn't happy, but I figured, "well it does have 200K and it is within spec after all." Then today it dropped to ZERO! I had just changed the plugs so I thought this was an awfully big coincidence. Stopped, popped the hood, jiggled the wiring harness (that I had bumped quite a bit changing the back plug) and it jumped back up somewhat, however it was still on the low side and very inconsistent. Drove it 1/8 mile home and jumped in my truck and went to Autozone. Purchased a Sunpro mechanical gauge for $14 and hooked it up. 35psi at idle and ~60-65psi at speed!

So now I'm trouble shooting the wiring/ gauge/ sender and have some questions: With the plug unplugged from the sensor it should read 80psi, right? Mine reads ZERO. What does this mean? What is my next step? There appear to be 2 wires going to the sensor. I feel much better knowing my pressure is actually okay (assuming I can trust the mechanical gauge....thoughts?), but now I need to fix the factory gauge/ circuit, and I'm a little lost. Oh and this is on a 98 XJ with the 4.0. Thanks in advance.

Factory gauges are rarely accurate. Buy a new sending unit and try that. When unplugged it should read zero, if you ground the wire at the harness it should read max psi.

I've replaced the sender once (a while back when I first noticed the low pressure... didn't really do anything then) but that doesn't mean that it's not the problem.

What should happen if I jump the 2 wires going to the plug to one another?

I got out the meter, and here's what I've got:

One of the wires is a ground. With my meter set to the"resistance 200K" setting (whatever that means) it reads a consistent 7.2 even when I jiggle the harness. One lead in the plug, the other on the neg terminal of the battery. For reference, the hood bolts in the fender and the distributor housing each read 1.2ish. All of the above produce a beep on the audible continuity setting.

The other wire is a hot lead. With the engine off (but the key on) it reads ~5 volts. I didn't check it with the engine running.

With the plug just hanging there, the gauge reads zero.

With the 2 wires on the plug jumped to one another, the gauge pegs at 80.

Does this rule out the wiring and the gauge?

If it does, this means that the problem lies in either the sender or something interfering with the sender. My only reluctance to think it is the sender is because I have now determined that my pressure is significantly higher than the gauge ever indicated (this sender or the original sender). Is the factory system that inaccurate (even with functional senders the gauge still reads low)? Or is my Sunpro gauge that inaccurate? I have a hard time believing that both senders were faulty and read equally low until today and this one is now even more faulty and is reading even lower. However, I am currently leaning towards the sender anyway.

I am very open to suggestions, and if anyone can interpret my meter readings I'd love to take advantage of your knowledge. Does 5 volts and 7.2(ohms?) sound right?

The volts sound right, the ohms might be right, but you need to read the meter manual to see if it is autoranging, or if 1.0 on the 200K range is 1000 ohms, or 1 ohm. Meters vary in how they display and in how to read them, which is a bit of a pain at times.

Sounds to me like the wiring harness connector pins (female?) are loose and dirty which WOULD cause your symptoms. Try cleaning them, and using some needle nose pliers, try squeezing the female metal part from the outside to make the slot a little tighter so they grip the male pin better. Do the tightening on the small thin width, not the wider width. The outside is probably rubber so it should be easy to do, but if it is plastic it is trickier to tighten as the plastic would break if you try the needle nose from the outside.

If it is plastic, try some tweezers from the inside maybe.

Update:
This morning I confirmed my meter readings in the daylight. 5 volts still consistent, however I was able to get my ohms down somewhat by scratching around (since I could see what I was doing this time around).

I reinstalled the sender and bent the prongs in the plug. One of the prongs appeared to be split. It is a hard plastic plug so I stuck my meter probe in above the prongs and bent them down. The split one may be from me cramming meter probes in there over the years.

Plugged it up and started the jeep.... 45-50psi cold idle (revs up to 60psi or so). Let it run a while, drove it down the road, got it up to temperature and then drove it some more to be sure the oil was up to temp. 45-50psi at cruising speed, fluctuating 20-25psi at idle. Never even dropped below 20, has idled at 10-13 (by the gauge) for the last 2 years.

Still... my factory gauge readings are consistently 10-15psi lower than my mechanical readings. Is this inaccuracy just inherent to the system? Is there anything I can do about it?

The engine runs like a top and there are no oddball noises so I'm going to trust my data and say it is purely a reading problem. Unless some internal problem magically fixed itself or I need to drive it around more for it to surface.

IMO, the factory gauge is just an idiot light without the light. It can tell you that something is different than normal, but I don't trust it for absolute readings unless you compare it to a more accurate gauge. Fortunately, mine are close enough.

As for your reading, 200K means the maximum reading for the selected range (200,000 ohms). You didn't say if it was digital or analog. With an analog meter (one with the needle), the max is at full deflection of the needle but still on the scale. The needle can still move more but it would be off the scale and unreadable. So you click the dial another notch to increase the range.

So on a 200K ohms range, 7.2 would read 7.2K ohms.

It's a cheap Wal-Mart digital (not exactly a Fluke, but it seems to serve my purpose, I wouldn't stand a chance with an analog meter- too many scales on one display)... and thanks.

Lol... I have a cheapo digital as well plus an old Radio Shack Analog (must be 30 years old now). I prefer the digital.

So are these good pressure readings you posted from the Jeep dash gauge or the mechanical gauge? Is the dash gauge still reading 10 to 15 psi lower than the mechanical gauge, or did the contact cleaning solve the problem?

Gauge needles can move on the gauge stem they are mounted on. When the needle is slamed against a stop like at >80 psi, the needle can move some thus throwing the gauge out of calibration. Also many have reported that non OEM pressure sensor replacements are not calibrated to Jeep gauges.

If either is the case, it may be possible to calibrate the jeep gauge /needle. Most gauges have a trim screw on them somewhere (front or back) that allows you to re-zero the needle so that it reads zero at zero pressure, with the power on the circuit completed. If the trim screw can not zero the gauge then the needle has moved, or moved too much, in which case you can try to reset, reinstall the needle itself (sometimes this works, sometimes it does not, meaning the needle my break as it is very delicate).You reinstall the needle by pulling it straight off and then pressing gently back straight on so that it reads the proper current pressure. So if it reads 10 when you pull the needle, and it should be reading 20 then reinstall it at the 20 mark.

The other option is to get a new gauge at that point and replace the bad one, or just do the math on the dash gauge error once you know that say 10 lbs on the dash gauge is really 20 lbs of real oil pressure. Then just add 10 lbs to what ever the dash gauge reads (20 - 10 = 10 lbs error).

Lastly, if that was truely a ground wire reading 7.2 K ohms (7200 ohms) then he has a real major ground problem. Even 7.2 ohms is too high for a ground wire, that is why I questioned what his meter was displaying on the 200K scale. I assumed he was using a digital meter, as few people except old time electronic guys like me even have analog meters anymore, and we know how to read them. I grew up with analog, so its kinda funny that I sometimes have trouble with the display differences on the digital meters as they do vary, and I have about 10 versions of them, so it gets confusing.

tnxj said:

Update:
Plugged it up and started the jeep.... 45-50psi cold idle (revs up to 60psi or so). Let it run a while, drove it down the road, got it up to temperature and then drove it some more to be sure the oil was up to temp. 45-50psi at cruising speed, fluctuating 20-25psi at idle. Never even dropped below 20, has idled at 10-13 (by the gauge) for the last 2 years.

Still... my factory gauge readings are consistently 10-15psi lower than my mechanical readings. Is this inaccuracy just inherent to the system? Is there anything I can do about it?

Click to expand...

Ecomike said:

I assumed he was using a digital meter, as few people except old time electronic guys like me even have analog meters anymore, and we know how to read them.

Click to expand...

Hey! Who you callin' "OLD"?

Ecomike said:

Lastly, if that was truely a ground wire reading 7.2 K ohms (7200 ohms) then he has a real major ground problem. Even 7.2 ohms is too high for a ground wire,

Click to expand...

Hmmm... somehow I missed that point when I replied earlier.

TNXJ, try it again on the lowest resistance scale. My cheapo Vellman has a 200 setting (not to be confused with 200K which it also has).

Okay, on the lowest resistance setting (200) it reads 00.3 This is also the setting that emits a tone when there is continuity, and it does emit an unbroken tone. Apparently the 7.2 was a product of my inability to simultaneously position the leads on the contacts, turn my head to the display (which was resting on the valve cover in the path of my flashlight which was resting on the radiator support), and of course.... hold my mouth right. 00.3 is also the reading it displays from one spot on the battery terminal to another, so I'm assuming that's good.

How do ya'll feel about putting dielectric grease on the plug (I've got a little of my 98 cent pack left over from my plug swap)?

Well, that sounds much better. Make sure the contacts are clean. I put a little on all of my connectors when I end up having to clean them.

0.3 ohms is excellent for the ground!

Old, is a relative term. My relatives are old!:laugh3:

Update:
Today the gauge dropped again, not to zero, but idle at ~10 and cruising at ~30. Pulled into a parking lot, popped the hatch, grabbed my mechanical gauge out of the back, and swapped them out. As suspected, 25psi idle 62.5psi under throttle. So I decided to replace the sending unit.

Drove to Napa and picked one up ($40).... 20+psi at idle, 50+ under throttle. All readings are immediately following the original sender readings (well, a minute or two after to account for unplugging and swapping senders) so oil temperature is the same.

The original sender was a cheap one from AutoZone or Advance and read 10-13psi idle its whole 2 year life. The Napa one is at least much closer to being accurate, although it doesn't read exactly the same as the mechanical.

The moral of the story: Always check with a mechanical gauge before you freak out (unless its overheating or knocking of course). And go ahead and spend the extra $12 to get the sender from a real parts house.

Thanks to everyone for their help.

tnxj said:

Update:

The moral of the story: Always check with a mechanical gauge before you freak out (unless its overheating or knocking of course). And go ahead and spend the extra $12 to get the sender from a real parts house.

Click to expand...

Worked there. Yup.

A-zone specializes in the soccer mom/Walmart automotive buyer. Originally that family's money started it, used the same merchandising platform, and it still operate the same way. It's all about spiffs on this week's special items, not special order or in stock quality items - which are hidden on the back shelves for commerical buyers.

The operating method is to sell the cheapest part possible and beat the competition on price.

TiRod said:

Worked there. Yup.

A-zone specializes in the soccer mom/Walmart automotive buyer. Originally that family's money started it, used the same merchandising platform, and it still operate the same way. It's all about spiffs on this week's special items, not special order or in stock quality items - which are hidden on the back shelves for commerical buyers.

The operating method is to sell the cheapest part possible and beat the competition on price.

Click to expand...

I starting buying from AZ the day they opened. Only problem I have really ever had with them was their not having a part (very rare), or not having one to offer at all (NA). Then I would go to O'Rileys to get it, or someone else like the dealer for hard to find parts.

I have seen no real trend towards quality only or junk only at any of the different auto parts stores I have used. Seems to vary with the knid of part you are looking for. I will say I have always bought the high end, lifetime warranty parts at AZ, and never the low end parts whenever I had a choice. And I have almost never had gotten a bad, or had one fail prematurely. The only ones I even recall was a warped brand new brake drum, and an igntion switch that fell apart in less than 12 months (but I may have bought the bad ign switch at O'Rileys, just don't recall).

I think the trick is to buy the best brand name / quality part they have to offer and not the cheapest.