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Oil Sender

25-cent Oil Sender Fix


In May 2001, I was sent from Fort Collins to a weeklong meeting at Hewlett-Packard in Boise. I decided take a few extra days vacation and drove up there in my 96 Cherokee. Since I was short on time, I used the services of my local Greasy Simian chain to do an oil change; you'll see why that became a problem shortly. The drive up was uneventful, stopping over in Jackon Hole for the night.

The last day in Boise, I went through the engine bay and made sure I was good on fluids before setting off for home. Just before the last Boise exit, I realized there was a cloud of smoke following me. The next stop was many miles down the road I pulled off to check it out. There was a small hemorrhage of oil coming out, which is never good... Since I was at a truck stop I let the engine cool a bit while I made some phone calls to try to find a place to stay for the night. It wasn't long before I learned that the hotels around town were booked solid, and the parts stores were closing.

To make a long story short, I realized the oil sender was cracked, and leaking freely out the wire connector end. Since the oil sender was now useless, I needed to find a bolt that would plug the oil sender hole, but the truckstop mechanics looked through their boxes and couldn't come up with anything that small and metric, so I ended up buying a tube of JB Weld.

I left the sender wire disconnected and brought the sender into the cab to clean it off. The mechanics and I determined there is apparently a membrane in the body of the sender that had ruptured, allowing oil to flow through at high pressure. Accoring to them, this damage is consistent with what could happen if the sender was struck, and failure is not immediate. After cleaning the oil off, I applied JB Weld to a quarter onto the end of the sender and liberally applied more JB Weld as a fillet along the end, sealing it. It set overnight while I dozed in the Jeep, and by the next morning I was able to top off the oil and drive off with my newly-repaired "plug". The oil pressure showed 'max' while I drove, but I was more concerned that there were no more leaks, since the oil flowing out of the sender travelled back along the block and coated my exhaust, causing the smoke cloud. My first stop was at the Jeep dealer where I bought a replacement sender, but I still ended up driving more than 3000 miles with the repaired one.

While a smoke cloud is one thing, I'm more concerned with what it coul hav been. I've heard of too many Jeeps that caught fire after leaking for awhile, and looking at my old catalytic converter I feel I know why - the cat operates at hgh temperatures (reports are as high as 1500*F). It doesn't take a rocket scientist to see that oil or oil mist hitting a surface at that temperature may cause a fire. As a matter of fact, one Jeeper I know in Canada had exactly this problem, and as a result his Jeep was roasted. Fortunately, he and his family were able to escape, and he won a partial settlement in Court against the dealership as they failed to prove that a failed oil pressure sender couldn't cause a fire.

You can see the repaired sender after 3000+ miles, next to the replacement unit
You can see the repaired sender after 3000+ miles, next to the replacement unit

Note that a disconnected sender on the 4.0L HO engine, or one with a bad ground will report high (max) oil pressure.

The oil pressure sender needs to be unplugged to remove the filter located below it. Although the filter can be made to fit without removing the sender wire, the filter can crack the oil sender during installation. I'm pretty sure this is what the Greasy Simian did, which is why I normally prefer to perform all my own maintenance.

One new addition is a change in the oil filter. There have been a few filters used for the 4.0L engine since it was introduced, but I found that I could add a larger filter to mine so it has twice the filtered area and adds a half-quart to the oil in the system. According to the manual, the Jeep 4.0L HO motor uses a small 1/2-quart filter. The earlier Renix-motored 87-90 uses a small metric-threaded filter as well, but I don't know of a larger replacement for that. The 1991 4.0L HO uses a 3/4"-16 thread, which allows you to use a more commonly available 1-quart filter.

The new filter is longer, but more importantly it has a larger filter area, although you need to remember to add another 1/2-quart of oil at every change to account for the added capacity. Fitting it in over the starter does take a bit more juggling, but in my book the added protection is worth it. I highly recommend disconnecting the negative (-) battery cable before changing the filter though, as a dropped filter may short across the starter solenoid.

e-mail Jim
created: March 10, 2003
Modified October 12, 2005