1991Jeep_Man

Hey guys,

Just got a couple of questions regarding my '91 XJ. As many of your are probably aware, Fall is around the corner, with cold weather trailing behind. I've had a rather wierd issue with my XJ the last couple of weeks as well as last winter after I did a t-stat swap. When the weather is cold, my XJ will barely rise above the second white tick mark on the gauge (Im guessing this is about 160-170*). 2 summers ago I had installed a new 2-row all metal radiator from radiator barn as well as new hoses, new t-stat and housing, new heater control valve, the whole nine yards minus both the ECU temp sensor and gauge sending unit. Everything was great until last winter when I had to replace my t-stat and gasket due to a leak. Now, during cold weather, I never get past 160-170*. But during the summer on the hottest days, after warm up the jeep will idle at roughly 208-210 and under load around 200, which is good. What's with the winter though? Is is just the cooling system working TOO well? Or could a lower degree t-stat have been accidentally given to me at AutoZone? Wierd sending unit junk goin on?

Second question, I run Mobil 1 High Mileage in my XJ with a Mopar filter. Last oil change, to try and combat a little oil burning as well as some slight engine noise (not knocking), I switched from 10w30 to 10w40, with FANTASTIC results. However, with the colder weather coming, I'm concerned with cold weather starts. What is the best "easy start flow" additive to use? Now, before anyone screams 'Snake Oil' or anything, I'm not looking for a miracle in a bottle. I'm just looking for one of those "Easy Cold Start" additives for a little easier cold starting on the Jeep. Also, before anyone chimes in on running 5w20 or even 0w20, please keep it to yourselves. I'm not saying its a bad thing, and I've given a glance to bobtheoilguy, but I know my 4.0 with 210K+ miles on it, and I know there were notable improvements in engine noise, performance, and oil consumption with the 10w40.

Thanks for any help guys!

Zj heep

What kind of t stat did you get?
Going from 10w30 to 10w40 you won't tell a diffrence in starting.

djb383

Only way to tell for sure on t-stat temp is pull it out and look at the temp stamp on the center part (that fits in the head). Most are stamped 195, or 180, or 160 which represents the open temp. Here's a 180.

1991Jeep_Man

I ordered the 195 t-stat, but out of blind ignorance I never checked if I actually got what I wanted. But, even if I was given a lower t-stat, wouldn't the engine have ran cooler during the summer?

Also, here's a another quick question; anyone running 5w-40 in their XJ?

xjforfishing

I agree on the t-stat...pull it out and look....I am running 5w-30 in my 95 xj and it seems to be fine

Firehawk068

If you are running Mobil-1 Synthetic, you shouldnt have any cold-start issues with 10W40.
Cold flow should be just as good as the 10W30

I am running a 195 degree t-stat in my XJ, and on cold mornings, and freeway driving in cooler temps, it seems to run just above the 2nd mark on the gauge.
During the summer it ran around 210 or so around town, and around 230 or so when slow trail crawling up a steep hill.

I have since flushed my whole cooling system, and ran a cleaner in it twice for about a week.
It now runs about 10 degrees cooler everywhere.

djb383

With a properly functioning 195 t-stat and properly functioning gauge/sending unit, shouldn't the needle be at minimum very close to the 210 mark (+/-)?

.......and any 0WXX or 5WXX will flow quicker to the top of the motor than 10WXX.

1991Jeep_Man

I would think so. As you've stated many times before (I was one of those students who took notes ), the t-stat controls minimum coolant temp. If that's the case, then the coolant should be staying in the block until it's warm enough to open the t-stat and allowed to flow through the rad. My 4.0 seems to follow this principle during the summer, but then in colder weather it's like someone snuck in overnight and swapped my t-stat to one of those crazy 160* ones.

As for the oil, again I took notes on one of the MANY MANY oil threads on this forum, and that's why I wanted to run a 5w40 oil. However, all the 5w40's I've found are diesel oils, and I remember Dukie mentioning that the stronger detergents can have some ill-effect IIRC. I may be wrong though. But, what I do know is that with the current 10w40, when the engine is hot, oil pressure is great, drivability is wonderful, and the chatter stays down; a bit better than the 10w30 did. Cold is a slightly different story.

djb383

Can u verify coolant temp with a infrared temp gun? Remember, during the winter your heater core is removing heat. In winter, u should be close to the t-stat temp, seldom above it......not the case in the summer.

If an oil is good enough for a 25:1 compression turbo diesel motor, surely it's good enough for the 4.0 and the 5WXX gives it better flow when temp is below normal.

1991Jeep_Man

Wish I had an infrared gun. I'm going soley on the factory gauge. I think for poots and giggles, I'll replace the thermostat, or at least pull it to make sure it's the correct one, change the sending unit, and maybe even swap the ECU temp sensor.

In regards to running 5w40, I've heard mixed reviews on my usual oil of Mobil1 when it comes to their 5w40. I would either use Rotella T6 or Mobil1. What would you suggest?

djb383

I'd suggest 0WXX or 5WXX and I'll leave it at that, or it will turn into an oil thread. LOL

When u get a chance, get yourself one of those temp guns.....handy as a pocket on a shirt.

1991Jeep_Man

It's only a true full blown oil thread when Dukie and Diesel get into it lol . I'll look into it more on my own research, because you are right... I'd rather not have this turn into a 9 page long boxing match on motor oils, where someone then suggests using water instead of oil, and another person chimes in on using 50w0MG oil and wondering why they have chatter.

As for the temp gun, I think I've seen them fairly cheap at lowes. I'll have to buy when I get the chance. Thanks for the help!