npk

i have a 95 xj 4.0 that won't reach operating temp unless it is parked idling on a warm day. i can let it warm up sitting still (takes over 20 minutes on a 60 degree morning) and as soon as i drive down the road and get a little air running through the radiator it almost instantly drops to 150 degrees. i have changed the thermostat twice and changed the thermostat housing.

the reason i am concerned is that the oil pressure stays pegged out until temp reaches about 180. i had a past jeep that had pegged oil pressure at low temps as well and would blow the oil filter gasket if you didn't let it warm up before touching the accelerator. this will leave you stranded unless you are comfortable running your engine without oil. i do not want this to happen again.

i haven't changed the sending unit. i have verified it with an infrared thermometer on the thermostat housing. the oil pressure gauge confirms it as well. if i let it idle and rise to 210 degrees the oil pressure will drop to 40 and not peg out when the engine is revved. if i then drive it 20 miles the temp will drop to 150 and the oil pressure will rise to 80 while driving and about 65 while parked idling.

i have also been told that gas mileage is worse at temps below 185. i only get about 12 miles per gallon down the highway.

what in the world could be causing this? the xj runs great.

npk

i should add that the heater works well and i am running 5w30 mobile synthetic oil

djb383

Doesn't it seem odd that the heater would work well if actual coolant temp was 150-160? Have u tested the stats before installing them? Have u verified oil pressure with a mechanical test gauge?

npk

the heat works well enough when water temp is 150. it will flat burn you out if parked idling at 210 water temp. my point was that i highly doubt the heater core is stopped up. i have had that happen on other vehicles.

i replaced the oil pressure sending unit when i bought the xj. the new one reads the same as the old one. i haven't tested with a mechanical gauge. the fact that the pressure increases and decreases with the water temp tells me the engine is running cooler than it should while driving.

i didn't test the two new thermostats, but what are the odds of 3 bad stats in a row?

i should add that i have done tests with the grill covered to restrict airflow through the radiator. i can raise the temp as high as i want with the grill covered, but haven't reached boil over.

i admit that it sounds like a bad tstat stuck open and a very efficient radiator. at 60 mph on a 100 degree day the temp wouldn't break 165. the two stats i have installed were different brands (both 195 degrees). is there a "premium" brand i should look for that has better quality control?

Rockville

thermostat stuck open or you live in Alaska.
Replace thermostat or move to AZ.

npk

never been to Alaska. live in Tennessee. i will replace the tstat one more time. and i will test the new one before i install it. i also bought a sending unit. but if it is the sending unit it won't fix the high oil pressure. not that high oil pressure is a problem, it's just my previous experience that makes me leery of it.

is it possible the water pump produces so much pressure that it forces coolant through the tstat? i bought the xj already built is there a chance there is some type of beefed up water pump on it?

hankthetank

...and/or high flow oil pump? i would take the oil filter adapter off and clear any obstructions if there are any...

Rockville

no

npk

i suppose that is possible but it wouldn't explain my cool running temp. i mentioned the high powered water pump as a possible cause of premature coolant flow. i have never heard of this happening, just throwing one out there.

high flow oil pumps have a pressure regulating spring valve inside them just like a stock oil pump. they maybe set at a higher pressure but they should also function to keep pressure below a certain limit. i figure an eighty psi gauge pegged out is probably above that limit.

npk

thank you for your direct answer Rockville. i know it sounded ludicrous but i have been dealing with this since i bought the xj. it just bugs me. i have spoke to several mechanics and they all say they have no idea what it could be. but they are willing to try and figure it out if i pay them. i know that is completely fair but i would rather hear them say it could be this or that or this other thing....not you have done all the right things and i don't know what else to do.

djb383

There ain't a rad or water pump made, that will fit the XJ, that will cool to actual 165 on a 100* day. Again, suspect faulty gauge readings. I have yet to see a heater work good at 150-160 actual coolant temp.

playbass

Word to that. My XJ runs a bit cool, and it takes about 20 minutes to get up to 180*, which is when my heater starts to feel warm on these cold Utah mornings...

I have a failsafe 195* t-stat, thinking about replacing it in case it's stuck open.

djb383

Interesting read, fwiw. Failed t-stat discussed toward the end.

http://www.stant.com/Consumer-Produc...of-Thermostats

DFlintstone

After the T-stat is ruled out.....what's your mileage? Are you sure of which injectors you are running, and has the 02 sensor been changed?

I mention that because I was running too cold, too much with injectors supplying too much AND my 02 stone..cold ...dead. (all that fuel evaporating removes heat from the system).

That doesn't match your description very well, but something to ponder...

milodajeep

Have you looked at the engine temp sensor?
It may be sending the wrong temp signal to your PCM causing the module to think the engine is colder than what it is which in turn will be overfueling causing bad fuel economy.