jaydlogan

Oil question regarding Jeep TJ 4.0L (rear main seal leakage)

5 posts in this topic

 Hey, Moses! Great to be on your forum. I have just replaced my rear main seal, timing chain cover gasket and seal this week on my 1999 Jeep TJ 4.0L and is ready for oil.  I am the original owner and the engine currently has 120,000 km on it. I am now only using the Jeep in the summer.

 

I have only used Mopar 10w30 and Mopar filter. Over the years, I have had two rear main seals replaced with no success by the dealer and an independent mechanic.

  

The last oil change I replaced the oil with Royal Purple 10w30 synthetic and still no success. So... once again for the third time, I decided to replace the rear main seal myself!  Can you suggest a good oil and BRAND to use to help combat my issue with oil leaks. Should I be using synthetic? Also, is the 10w30 Mopar oil synthetic now?  In the mean time, I will be looking at the installation of the rear main seal, oil pressure and PCV to see why I have had this issue.—Jason

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Jason, I use 5W-30 in the winter on our 4.0L Cherokee. It will drip, not much, livable and not warranting the rear main seal change. In summer, the use of 10W-40 has eliminated seepage to this point. Rear main leaks have been an AMC/Jeep inline six challenge six the first 232 and 258 sixes. The 4.0L is no exception, despite seal design changes to combat this issue.

 

The Jeep six rear main seals will not tolerate 5W- very well. Use of a block heater (our Kat freeze plug heater, inexpensive, has worked well for years) is a regular ritual in temps below 40-degrees F. I could run 10W-40 year round with block heating, although we travel and park away from an electrical source at times, where the engine has to start without benefit of the block heater.

 

As for oil brands, Chevron’s Iso-Syn formulated oil works as well as many synthetics. This is inexpensive, labeled as “Chevron Supreme” and sold at box stores and Walmart. Unless you have always run synthetic motor oil, the Chevron oil works fine when changed at proper intervals. I, too, use Mopar filters. See my Vlog on oil and filters for more details. It's a four-part HD video series on filtration and oil types. Scroll down the 4WD Vlog and 'Live Tech' Q&A Channel playlist, you'll find it!

 

Regarding the end of rear main seal leaks, do check the PCV/crankcase ventilation system. Oil pressure has no effect, and all Jeep/AMC design engines run high oil pressure. Install the main seal "by the book", using the right sealant at the correct points only, and you should get it this time.—Moses

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Since the OEM specs specify 10W30 for warmer climates, Is there any ill effects using 10W40 in the same conditions (temperature)?   Isn't 10W40 engine oil used in higher mileage engines and is my engine suitable?   I am just being cautious!.......and curious!

 

Thanks,  Jason

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10W-40 will simply provide the same protection as a 40 weight oil at high engine operating temperatures.  It does not thicken, as many assume.  So in my view, your higher mileage engine would be grateful for 10W-40 weight oil...I have run it summertime in our 4.0L XJ Cherokee since 94,000 miles, when we purchased the 1999 vehicle.  If we were in a warmer climate with low winter temperatures above 32-degrees F, I'd go with 10W-40 year round at the current mileage...

 

As a footnote, contrary to myths, you will not increase the engine's bearing clearances by using 10W-40 nor will you "spin a bearing" during cold start in temperatures above 32-degrees F. Bearing clearances are certainly sufficient for 10W-40 above 32-degrees F.  Cold start flow and viscosity is the issue, so the concern is really the 10W cold pour rating of the oil, not the 40-weight protection when hot.  For start-up temps below freezing, follow the Jeep guidelines for viscosity...Moses

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