99 XJ Sudden temp rise and sudden drop shortly after

[carves]

As mentioned in the other posts ...

Pressure loss ... generally a dodgy radiator cap ... and a system not self burping itself ( no jiggle pin in the thermostat ) will be the common causes ... but not nescessarily the cause.

Some other causes are listed in the FSM ..... In fact they top the list of overheating causes.

Hot weather,
Steep grades,
Towing,
Highway speeds .....

Quiet, flat country lanes, driven slowly in winter, would seem to be the optimum driving conditions for a stock XJ according to the factory.

 

[sfxjdent]

Hi All,
I'm hoping for help on a problem with my 99XJ that's got me and my mechanic stumped.
I recently had my auto transmission flushed and filled -routine maintenance. I took a camping trip last weekend and had the car packed with gear and people in hot (~90s) weather.
Going up hill on the freeway, my check gauges light went on and my temp gauge jumped from normal to one notch below max in a flash. I pulled over and checked my fans and coolant level. All fans were on (including aux) and coolant was ok.
After a couple of minutes, the temperature dropped as quickly as it rose in the beginning and all was good.

Later on I was going slower up a steeper hill. The same things happened except this time my oil pressure dropped to about 15 and I was leaking ATF. I checked my trans fluid level and it was well above max. Just like the first time, after a few minutes temp dropped to normal and all was good.

Any idea on what's going on?

I had the exact same problem on my 98, i changed the radiator and hasn't happened again. Also put in a new thermostat, there was a big glob of built up gunk on the old t-stat- I'm sure the old radiator was full of that stuff.

 

[fyrfytr1717]

Just to confirm, this isn't due to the torque converter unlocking and generating lots of extra heat from the tranny right?

Honestly, I couldn't tell you for certain. Nothing feels or sounds any different than usual. Is the torque converter typically locked at high RPM's, or low RPM's? Tranny fluid looks and smells fine, no evidence of overheating or burning of the fluid.

Regardless, it seems to me that this isn't the issue. I would think that if the transmission fluid running through the radiator was overheating the coolant, it would cause an actual overheat of the entire cooling system, not the sudden rise and fall in temperature that I'm experiencing.

 

[sjx40250]

Interesting discussion. I had the same problem last week but found that the aux fan was not plugged in from when the front end was repainted due to an accident.

My only experience with overheating was with my 90 which is a totally different system. I found two problems with it. First was that the radiator was 30% plugged. Second solution happened when the head was sent in for a complete valve job. Haven't had a problem since over 75K miles ago.

Comment on radiators. Plastic tanks are not the best option, get brass. Don't be fooled by the number of rows of tubes. What counts is the cross sectional area for water flow. Sometimes the tube cross section for a three row is less than a two row, which is does not allow for good fluid flow. The other consideration is that three rows may not be great due to the third row being blocked by the other two rows and the air flow over the 3rd tube will be restricted. You want optimum coolant flow and air flow. That doesn't even take into account the condenser and tranny cooler in front of the radiator!

My 90 had no problem pulling a 19' travel trailer all over Oregon when it was new. I think that if you maintain the cooling system, like new, there shouldn't be a problem. The problem usually occurs with higher mileage.

Trany over heating sounds like a plausable contributor but doesn't satisfy me as a contributor to the rapid rise and fall of the temp guage. Steam would though. I don't believe there is a t-stat for the tranny fluid line that would account for sudden temp changes!

In my experience last week, the temp rose and fell rapidly, less than a 2 min., but not suddenly (1 second).

 

[fyrfytr1717]

Good news and bad news...

Good news is that my cooling system maintenance seems to have solved my "overheating" issue. I did everything I mentioned above with the exception of the Water Wetter. I ran the grade up to Shaver Lake at a respectable pace with the A/C going full blast. The temp stayed in the 210-225 range for the duration of the climb. No temperature spikes, no coolant leaks, all was good.

Bad news is that due to replacing everything at once, I don't know exactly what it was that solved my problem. For all I know, just replacing the hose clamps may have been all I needed to do to eliminate my temperature spiking issue. The other bad news was that my under-hood temps were high enough to melt one of the airlines for my ARB's. The only other time I have managed to do this was after about the fifth temperature spike cycle during the uphill trailer pulling incident I mentioned in an earlier post. At least this confirms my engine was keeping it's cool despite extreme under-hood temps...

 

[joe_peters]

If you are getting that hot under the hood, maybe some hood vents are in your future.

As these beasties get older, things really deteriorate under the hood from the temperatures.

 

[FoMoCo]

Just a thought..Is the temp gauge an actual gauge or more of a glorified idiot light like Ford has?

all obd II gauges are basically idiot lights with hands

 

[joe_peters]

all obd II gauges are basically idiot lights with hands

LOL! Yeah, your "gauge" is indicating what the PCM wants it too.

 

[heyhar]

Big fan here of auxiliary tranny coolers. I started installing them almost 40 years ago, after the tranny in my first ride, a '63 Catalina took a massive dump. The old timer that fixed it explained, in German accented broken English, how heat was breaking down the clutches. 'Das ist sheiss' It was so toasted that they had to get another whole tranny, as the innards of mine literally poured into the pan. Ever since, any automatic-equipped vehicle we acquire gets a cooler bought for it on the way home. And, knock on wood, things have been good ever since.