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94 4.0L six HIGH oil pressure


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  • 94 4.0L six HIGH oil pressure

    94 4.0L six with 31,000 miles, oil and filter changed twice a year. Last summer on five day, 800 mile freeway trip, after one hour of driving on final day, oil pressure went from 40# where it has been for 12 years to 60# in ten minutes or so, then swung wildly pegging at 80# when I let off on the accelerator. Let it sit 1.5 hours, had oil pressure sensor replaced and pressure back to 40. After an hour or so it crept up to 60 and is there all the time except 40 at idle. Changed oil filter, still at 60# when driving. Should I replace high pressure relief valve? Anyone had this or know someone who did and they lived happily ever after? Any known causes (that are bad?) Thanks

  • #2
    This is from Daless2 on JeepBBS:

    How-to “Tune-up” the TJ Oil Pressure and Fuel Gauges

    ***** Disconnect Battery before performing any of this work!******

    For those of you with “wondering gauges” you know what I mean. The Oil Pressure is sitting nice around 50 PSI and then all of a sudden, it takes a dive to the left. Stays there for a bit as you play with the throttle and them seems to come back to the right all on it’s own. And your heart is still beating erratically.

    Or the fuel gauge that rarely, if ever works with any kind of accuracy dances all around the arena when it’s not reading “Empty”..

    If you have an hour or so and are willing to run to the store for a rattle can of Electric Motor Cleaner (Ace Hardware, Radio Shack, most Auto Stores like O’Rilley’s or AutoZone) you can have a good chance of resolving these problems.

    How the Sending Units Work

    Both the Oil Pressure Sending unit and the Fuel Level Sending Unit on the TJ are “variable resisters”.

    As the oil pressure increases or decreases, or the fuel level raises or falls the resistance in the sending unit changes. These changes are sent to the PCM (Computer) and then sent on to the CCD microchip, via the data bus, located in the instrument cluster of the Jeep.

    The resistance for both the Oil Pressure and Fuel Level send units works in inverse order, meaning as resistance goes UP, the gauge reading goes down.

    For example:

    When oil pressure in the engine falls, the resistance recorded at the sending unit increases; this higher resistance is sent to the PCM, Bus and Microchip and is then interpreted by you as a drop on oil pressure on the gauge as it points to the left.

    The Fuel Level sending unit works the same way.

    As the fuel level in the tank drops, the float follows the drop in level, which causes the Resistance of the sending unit to Increase. This increased resistance is sent to the PCM, Bus, and Microchip where it is then displayed on the fuel gauge as it too moves to the left, showing a decrease in fuel level.

    The key here is this, an Increased Resistance causes a Decrease in the Gauge Reading in the instrument pod, for both the Oil Pressure and the Fuel Level.

    And, it doesn’t matter where the Increase in Resistance comes from!

    Over the last year or so I have performed these simple steps to resolve issues with false reading on both the Oil Pressure and Fuel Level gauges on more then a dozen Jeeps. If you suspect you are getting false reading on either your Fuel Level or Oil Pressure gauge you might want to give them a try.

    What you need to do

    As I said above, any increase in resistance in the circuit will cause the gauge to read lower. If the resistance is NOT coming from the sending unit, but still in the circuit, the gauge will read low. This can and does happen when inline electrical connectors become dirty.

    In my playing around I have found several electrical connector on the TJ Jeep that get incredibly dirty and will cause false reading on these two gauges.

    On all but 1 out of 17 Jeeps, cleaning these connectors resolved these problems.

    Go Buy some Electric Motor Cleaner and Dielectric Grease

    While you could use Electric Contact Cleaner or even Tuner Cleaner, both available at Radio Shack, I prefer to use Electric Motor Cleaner. I like this because is a bit stronger, uses no silicon, and leaves absolutely no residue or coating once evaporated.

    Here is what I use.

    As I said before you can pick up Electric Motor Cleaner at Ace Hardware, Radio Shack and most auto parts stores for about $4 a can. You will only use a faction of one can.

    While you’re out pick up a tube of Dielectric Grease too. (About $3.00)

    One you clean these connectors you are going to want to fill them with Dielectric grease to keep dirt and grime and pollutants out of the pins of these connectors.

    Oil Pressure Sending Unit

    The Oil Pressure Sending unit is located just above the oilfilter. It looks like a small black cylinder about 1 inch in diameter and about 1.5-inches long pointing to the left.

    Attached to the end of this is a two-wire plug. Here is a picture.

    You need to unplug this connector.

    What you will find is this is a water resistant connector. However, in this case I think all that means is that it takes water and dirt and grime a bit longer to get in, yet once in there there is no escape for it.

    Clean It

    If the pins are dirty on the inside of this connector and/or the sending unit, the resistance coming from the Oil Pressure Sending unit will be considerably higher then what it should be. This will give you a false (lower or significantly fluctuating) Oil Pressure Readings on the dashboard gauge!

    Clean the connector pins and the pressure sending unit pins, where the connector plugs onto it, using the Electric Motor Cleaner. Spray both well and then plug the connector onto the Sending Unit.

    Let it sit for a minute and then unplug it again. Re-spray and plug it back in.

    One more time un-plug it and spray liberally. This plugging and unplugging helps to clean the pins between the connector and the sending unit.

    After you have cleaned and sprayed both a third time, blow the connector and sending unit dry with compressed air if you have it, else just let them air dry.

    Dielectric Grease It

    Once dry, fill the connector “female” pins with dielectric grease.

    It is ok to get carried away here. Fill it up completely.

    This is what dielectric grease is designed for. To fill all voids in a connector so that foreign “stuff” and air and pollutants can’t get in and corrode (increase resistance again) so easily in the future.

    Start it Up

    Once filled, re-plug the connector to the oil pressure-sending unit and start your Jeep. You should see an improvement in oil pressure and or behavior of the oil pressure gauge.

    In reality there will be no improvement (Increase) in oil pressure at all, but rather a more accurate reading of the engine oil pressure that you have always had.

    As I said, in 16 of the 17 Jeeps I did this to saw a 10 to 15 PSI improvement in the dash pod gauge and far less fluctuation in that gauge. At a minimum at idle (600 rpm) you should read 13 PSI. At 1,600 rpm the reading can be anywhere between 37 and 75 PSI.

    Most important, there should be no sudden drops in oil pressure to scare you half to death when your driving down the road, cause by a dirty connector on the sending unit.

    The one Jeep that did not respond to this simple procedure had a bad oil pressure-sending unit.. Once replace all was well with it too.

    Fuel Gauge Tune-Up

    If your Fuel Level Gauge fluctuates all over the place, or even worse doesn’t work at all, you may get some benefit from doing this.

    There is an incredibly difficult to reach connector located on top of the fuel tank. This connector is located on the driver’s side, near the front of the fuel tank, on top of the tank.

    How to Find Connector

    If you lay on your back and slid under the Jeep till you have your drivers side shock to the right of your head and look up, you will see the wiring harness that goes to this connector at the front edge/corner of the fuel tank.

    If you reach up there and follow the harness you will feel the connector.

    If you are lucky you will find about ½-inch slack in the harness and will be able to pull on it to barely see the connector.

    This is by far, in my opinion, the worst piece of engineering in my otherwise very nicely engineered Jeep. Not only is it difficult to get to, it is in a location where just about everything gets thrown up on it. And why this is not a weatherproof connector is beyond my ability to reason.

    The Potential (But Highly Likely) Problem

    The odds are this connector is full of “crud” and the connector pins are dirty. This will increase the resistance through the connector, and in some cases completely break the circuit between the wires going through this connector, giving you false readings on the Fuel Level Gauge.

    This is a 6-pin connector, but only four pins are used.

    Two are for the Fuel Pump and two are for the Fuel Level Sending Unit.

    Here is a rather poor picture of this connector, after I disconnected the wiring harness from the connector attached to the fuel tank.

    What you see is only the connector plug attached to the wiring harness. The other end, attached to the fuel tank can’t be seen. (It’s on top of the fuel tank.)

    Unplug It

    Do you see that little bit of Red in the picture?

    That is a locking key. In order to disconnect this connector you need to slide the Red Locking Key to the left. This will then allow you to separate the two half’s of the plug. You do this with the fingernail on your thumb.

    Don’t worry if you can’t see it in the picture. You wouldn’t be able to see it in real life either. Basically you need to do this using the “brail system”. Keep at it, it can be done and I believe it is worth the effort.

    Please be sure to wear safety glasses or a full-face shield when you do this, cause the limited amount of dirt and crud that didn’t manage to get inside this connector will fall on your face when you do this.

    Clean It

    Remember that full-face shield I just mentioned, it will be very helpful here. DO NOT let the Electric Motor Cleaner get in your Eyes, Nose, Mouth or Face in general.

    Put the little “straw-tube” on the end of the spray head for the Electric Motor Cleaner and figure out some convoluted way to get the can up there so you can spay clean both ends of this connector. It can be done but it always takes me a few tries to find the right position.

    Get carried away in cleaning it. Use the entire can of cleaner if you have to. You are not going to want to do this job again any time soon.

    If you can plug and unplug the connector a few times to add some friction to the cleaning process.

    Dielectric Grease
    When you are satisfied you have it as clean as you can get it, fill the wiring harness end of the connector up with Dielectric grease and then plug it into the end attached to the fuel tank. Again, using the brail system.

    Make sure you push it in all the way. Once in, the red Locking tab will slide to the right, locking the connectors together.

    To give you some idea how sensitive this connector is to increases in resistance think about the sensitivity of the Fuel Level Sending unit. When the tank is Full, the sending unit registers 20-ohms. When the tank is empty, the sending unit registers 220-ohms. This is only a 200-ohms difference, and that folks isn’t much at all. The slightest amount of dirt or crud in this connector can easily add several hundred ohms to the circuit. And this is what causes the fuel gauge to act so erratically in many cases.

    Secondary Connector

    The fuel Level sending unit signal has to pass through one additional connector prior to reaching the PCM. This is connector C103 that is located in the engine compartment.

    C103 is a 16-pin connector located on the firewall.

    C103 is Gary and is almost always wrapped in a 1/8-inch covering of foam. (Why? I have no clue?)

    Find the PCM on the firewall.

    See that big fat main wiring harness on top of the PCM?

    Follow it to the right (Toward the driver side).

    See the first set of wires heading down from the wiring harness.

    See the GRAY, 16 pins, wrapped in 1/8-foam connector attached to these wires headed south?

    That’s the connector you want.

    Take it apart and clean it up like you have done with all the other connectors. Grease it up with Dielectric Grease and put it all back together.

    Start the Jeep and take it for a drive. While this will not solve every problem with an erratic fuel level gauge, in my experience it will solve many of them.


    I have done this simple, but frustrating procedure on Nine (9) TJ’s. On seven (7) of the Nine (9) this solved the problem.

    One is still having problems and I have no clue why.

    The last was my own Jeep, which had a bad fuel level-sending unit. I replaced the fuel pump assembly to resolve the issue.

    I also replaced the Mickey Mouse connector on top of the fuel tank too, with a waterproof connector.

    Bottom line

    Dirty sending unit wire connectors will increase resistance and will cause inaccurate (low or fluctuating, and in some cases no) indicators on the TJ gauges for oil pressure and fuel level.

    I do not know nor do I think this will solve all problems related to the Oil Pressure or Fuel Level gauges, but I do know from first hand experience that cleaning these connectors will indeed remove unwanted resistance from the circuits. And this will correct many of the problems folks are having with these gauges.

    I hope some will find this useful and take the time to give it a try. I believe it will be time and effort well spent for many.

    From Daless2 at JeepBBS
    Ya Savvy?

    Motech Performance


    • #3
      Since were on the topic of guages and stuff could ya tell me why my battery light is allways on when i put the keys in? Does this mean that the battery is fine or that the battery is low?
      B R E T T
      87 XJ 3" lift, 31s-thats all thats worth mentioning


      • #4
        I don't know about the battery thing (sorry!) but about a month ago, I notice the oil pressure guage in my jeep pegged to the right, and not moving. It had been a while since I'd had it into the mechanic (yes, I'm a terrible mother), so I immediately pulled over and called my dad. He came and picked it up and we realized it did't have to do with my oil being kinda low, so I drove it to my mechanic. By the time I had gotten there the oil pressure had returned to normal. My mechanic flushed the system and and such. He checked my guages and concluded that they were all accurate. Anyway since then, I let it sit for a few days, then went again and flushed the system with a fresh oil change and it's been grooven ever since. Keep in mind, it's an '89 with 81K miles. Sorry if this is no help, and I'll ask my pops what he thinks was up and I'll keep you posted! He probably has a lot better info than I- I apologize!
        one happy jeep lovin' kid


        • #5
          Oh yeah- what I meant to say, thanks for posting that article Chris!
          one happy jeep lovin' kid


          • #6
            Try replacing the PVC valve. If it clogs, then the crankcase pressure will spike, and cause your oil pressure to read high, and eventually cause leaks in places like valve cover gaskets (as I recall, the 4.0L doesn't actually use a gasket on the valve cover, there is a channel on the mating suface where it is intended that a bead of high-temp silicone sealant is applied). In any event, I find it useful to always try the least expensive, and easiest fix first...