Hi, I have been a Jeep owner for the first time in my life for about a year now. We bought a 99 Jeep Grand Cherokee Laredo, from my neighbor for 1k. It had been sitting for a bit. Since we have owned it , we have changed out the bad hoses (from sitting), put on new belts, Installed new headlight assemblies, put in a new radiator, on top of regular maintenance such as oil changes. Before it was sold to us we were told it had a tune-up since we knew the people assumed they were honest.

So on to some Questions.

High Oil Pressure: last month our oil pressure gauge started reading high pressure up to 80 while we are driving 60 while idling. We've installed a new oil filter and had an oil change done, to no effect. Mechanic says there is a very slow seep of oil but nothing that should effect the pressure in this way.

Tune up: I'm going to go ahead and take her in for a tune-up this month. What are some parts that will get me better performance?

 

What size engine?
The high oil pressure is more then likely the oil pressure sending unit going bad.

 

6 cyl 4.0l (Thank you!)

 

I agree high oil pressure is probably the sending unit going bad. Mine at start is 80 while gas is applied and 60 at idle also, until it warms up then its 40/60 I believe. I've been meaning to replace mine.

 

Oh and as for tune up stick with the champion copper plugs.

 

Yes stick with the champions. I bought e3 plugs and they were horrible then i changed them back and evrything is good

 

What all do you consider a tune up? Is it like plugs, wires, cap and rotor, or are you doing fluids also ex: trans, transfer case, differentials, coolant, etc?

 

I second this. Champion 438's are OEM spec'd in our '01 Sport and they work great. The best "high performance" parts we have used, were all OEM factory parts. They are expensive, and as with most things, you get what you pay for. Insist the shop use only OEM Jeep parts for your tune-up and you will be fine. (Most use the cheapest junk they can find, so put your foot down on this one.)

Hopefully, you didn't use a Fram filter when you changed the oil. Ask around...they're pretty bad. Congrats on your new/used Jeep - have fun! That 4.0L engine is a gem and will serve you well.

 

Don't change the granny fluid/ filter unless its absolutely necessary and the fluid is burning. The new fluid will break loose the old fluids buildup and can clog your system...so if you do change it, be aware

 

1999 WJ 4.0L is still distributor land. For tuneup hardware, a brass distributor cap & rotor is a good thing. Wires you can go with OEM, Belden, NGK, or Accel (No experience with Accel but people here are happy). In terms of plugs you can run Champion RC12ECC or NGK ZFR5N coppers gapped to 0.035".

It does indeed sound like your oil pressure sending unit is toast. If it just suddenly jumped one day, your sensor could have croaked (very common) or your filter may have collapsed internally. If you get high oil pressure without the engine running (key in RUN but engine off) it's definitely the sensor. If you have a cheapie FRAM orange thing on there, or a similar low end filter, put on a real one like a WIX or NAPA Gold.

Urban legend.

Powerflushing in some transmissions can knock crud loose due to the pressure. A simple drain & refill of the correct fluid (I believe all WJs have Chrysler autos, so ATF+4) is part of regular maintenance. All of the fluids wear out over time. Not changing your fluid makes as much sense as not changing your underwear. Do you wait for a hole or for crud to build up? lol Automatic transmissions are essentially sealed. Unlike the crankcase the only contaminants are going to be clutch material and bits of metal from the gears. The fluid's additives and chemical properties are changed over time as a result of heat and mechanical shearing. Regular fluid changes are essential to maintaining proper operation.