Fsher21

My xj takes over 5 minutes of driving, or about 3-4 miles to get close to warming up. It rises very slow and when it finally makes it close to 210 it drops all the way down to the ~160 line and slowly rises. It never sits at 210 or hardly makes it there before the tstat opens, and it takes about 15 miles of driving to get the temp a little more steady but still fluctuating.

My brother has a 98' and his gets right up to temp and stays there in just a couple mins and has lava coming from his heat vents about 3x faster than mine. I would assume my tstat was stuck open with that info, but if that is the case i dont see how the temp gauge still shows the rapid drop as it cycles. Can it be stuck partially open so it never closes but can still open more?

From there I ask, can a stuck tstat affect the pressure in my system after running the vehicle for about half a mile and shutting it off? I have had an ongoing problem with coolant leaking from the path of least resistance (old control valve/loose hose connections/and now the plastic radiator seams). There is a huge timeline of issues with my cooling system, but I have a feeling it is all cause and effect from one problem hopefully related to the above.

Thanks!

Edit: Could this be a bad head gasket? I have no coolant in my oil and my exhaust looks fine. Sometimes when starting it up it isn't as smooth as normal but only is noticeable for about a second or less before I let off the key. The three or so separate times I fixed something that was leaking, i went problem free for 2-6 weeks before a new area started leaking.

cruiser54

Get a new stat from the dealer 83501426.

Whenever you stop, the residual heat builds up the pressure in your cooling system. Ditch the HCV and replace it with straight pipe before it blows apart leaving you stranded. Good time to do that is when you're installing that new t-stat.

djb383

Is your cooling system full of coolant (no air)? The moment the motor starts, coolant should be flowing from the block, up thru the head, out the bypass in the t-stat cover (beside the t-stat) and back to pump to start the loop over again. If there's a slug of air in the system, the temp gauge could be doing wild swings as hot coolant flows across the temp sending unit, then a slug of air. The wild swings could also be due to a bad sending unit/electrical connection. The t-stat does not seal air or water tight or pressure tight.......it simply restricts or allows coolant to flow to the radiator to be cooled. If the motor is running, a small amount of coolant will pass thru the t-stat even if the motor is cold.

Fsher21

The HCV was eliminated about a month ago due to the small leaking. There is no air in my system, hoses feel normal, rad is full and the overflow bottle fluctuates in level. The gauge moves smooth as the temp rises, just when it gets close to temp it drops to just above that only line between 0 and 210. It cycles in a tighter range the longer I drive. Takes about 2 seconds to drop in temp, and about 15 seconds to go back up. Again the longer I drive, the less of a drop the gauge takes. Hope that explains it a bit better.

99 Purple XJ

Replace your thermostat with a 195

Fsher21

adding on to my reply above, the tstat is closed for the most part and coolant is not cycling through my rad correct? How come the radiator weeps coolant only after accelerating a couple times in under a half mile and the coolant is cold to the touch after? Technically that part of the system is not in use or have enough temp to increase pressure. It does not leak if I let it idle for longer/hotter. Really pushing me towards a bad head letting pressure leak into my system.

djb383

In the 1st post there is reference to "a huge timeline of cooling system issues" so I presume some of the issues are leaks. Leaks usually occur as heat/pressure builds in the cooling system and finds it's way out the hole(s)/ crack(s)......air gets sucked in at the leak point(s) when the motor cools. Check for bad electrical connections at the temp sending unit. Think about it, the temp sending units are at or near where the hot coolant (+/-210F) exits the motor.......why would all of the sudden cool/cold coolant (+/-160F) be flowing by the temp sending unit?.......it's not.

Fsher21

Yeah the path of least resistance changed as I fixed them. Tightened two loose clamps that were leaking, the other two slightly tighter started leaking cause they were next in line. Now all clamps are good, hcv is gone, the next weakest place is the radiator seams on both sides of the plastic tanks. The system is open so it should purge those tiny ~10ml pockets of air that may of gotten in if the system wasn't replacing it with coolant from the bottle instead.

I will take a look at the sending unit connection tomorrow, and will research how to clean it up tonight.

djb383

FWIW, our ScanGauge only indicates maybe a 5 degree (or less) drop in actual coolant temp when the initial t-stat opening occurs from a cold start-up. This time of year, after initial warm-up, coolant temp fluctuates maybe 10 degrees or less, above the t-stat temp, depending on ambient temp, heater use, vehicle speed, etc.

Fsher21

I have noticed a drop in fuel mpg as well. I assumed it was due to my filter that rusted out and was leaking. Used to get about 180 miles to my tiny tank, and noticed the last few fill ups I have gotten about 150-160 even with adding the usual 12 gallons or so.

My main worry right now is still the pressure getting into my system, even if I fix my temp problem i am still weeping coolant out of my rad after a very short cold drive with increased rpm. That looks like a head gasket issue right?

cruiser54

How much pressure is there? They're supposed to run from 13 to 16 PSI. That's what the caps are rated at. You need a radiator.

djb383

^^X2, good advice.

XJ RYAN

I have noticed after i upgraded to a larger radiator that when my stat first opens my temp falls way off like you have stated,just getting a big gulp of cool water, Then heats up again then not so big of a drop,and so on. takes me a few cycles to reach 210. Thinking of blocking part of the rad for winter

Fsher21

it is a new 16psi cap, and when it is leaking I can open the cap with no coolant blowing out while it is full to the top, and stick my finger in the cold coolant. If it is the radiator, why did it leak from everything else first, then after all those clamps were tightened and the valve eliminated, did it finally leak from the seams on both sides of the rad, a month after my last issue? This is really messing with my mind!

A radiator is leaking when technically no coolant should be flowing through it, and pressure makes things leak but I can take the cap off and nothing happens, and when there is actually pressure in the system from the heat it does not leak a drop.

cruiser54

The pressure is throughout the whole cooling system...........