06 XJ8 overheats. I drove it for a few miles without coolant reservoir cap (forgot to place it back), resulting in coolant spilled out. Now it overheats, heater doesn't work too. I am not sure if the issue was pre-existed or came after I drove it without the cap since it was my first drive on it after I bought it from auction. Just tried the basic burping through the top hose but no success. Looks like the coolant flow is blocked, it keeps bubbling and coming to top when cap is off and starts boiling (overheating) as cap is on. Engine sounds good. Any info on how to bleed it properly or where is the bleed valve located on this model? Possibility of thermostat or water pump or head gasket? Thanks

 

Do not let it overheat.These engines do not respond well to boiling over. It happens quick.Boat anchor.
Air in system can cause this, perform a search here for bleeding procedure. Also there is a proper orientation for thermostat if it was changed recently. The small check valve has to be at the top to allow air bleeding.
If you have a scanner check coolant temperature after start up. Shut off engine if you see over 95C.
Since new to you it is difficult to diagnose but bleed system first. Stuck thermostat can be found by feeling the hoses. Same with water pump. If hot enough to boil over all hoses should be hot.

 

How many miles were driven without the coolant cap ? I suspect one or both of your head gaskets has gone, the bubbling being combustion gases. You need to get the coolant checked for presence of combustion chamber products. Unfortunately, previous posts on this subject have rarely been positive, with distortion of heads and block commonplace, making the engine scrap.

 

As Fraser asked, how long did it get driven without the cap? Did the coolant temp gauge indicate overheating?

 

I do not get how the coolant (expansion) tank cap causes overheating. Coolant should be good to at least 105C and I never see over 95C in 100+ Days and stop and go traffic.
In fact it was recommended that if I wanted to drive car to get repaired to pull the cap to reduce flow from leaking valley hose. I am not adventurous, a trailer was only 60.00 from U-Haul.
Long term is something else.

 

It was driven about 5 miles all on the highway, and during that time it didn't overheat. When I stopped I noticed the fans were on, coolant leaking and realized the cap was missing. Overheating started later on after I found and placed the cap back with coolant filled and started driving. Had to make 4-5 stops to cool it down to bring it back to my place. Coolant seems clean. Will check again and update.

 

5 miles on highway. Started overheating later on after placing the cap back, yes gauge was going high.

 

A knowledgeable Jaguar tech told me that driving mine thirty miles with cap removed will not hurt anything. The leak spraying under the hood and possibility of further hose damage made the decision easy.
But five miles without cap did not cause the initial boil over. The OEM thermostat is 190F. The 50/50 coolant should be good to 220F. I have never seen over 95C monitoring it. That is a hot day with a 20 mile run on freeway and then heavy traffic.
This car most likely was damaged then auctioned off.

 

If it didn't overheat on the initial drive with the cap off, there's a chance you just have air stuck in the system. You loosened the bleed screw when refilling the system? Also possible that bleed hose is clogged (not likely but easy to check).

I agree with Panelhead... driving 5 miles with the cap off and no overheat condition would not have killed the engine. I've driven mine several miles with the cap off. It might spill coolant everywhere, but it is not an immediate death sentence. Either you have air trapped in the system or there was prior head gasket damage. Or, enough coolant was lost/boiled out of the engine that damage was caused.

 

It didn't overheat on the initial test drive. My friend tried to bleed it through the bleed screw. He opened bleed screw completely, bubbles were coming out from bleed screw hole for long time, at the same time he was pouring water too in the reservoir. Then he placed the cap on and after a while water started boiling and coming out with pressure from the bleed screw too, fans also turned on, but still no heater. We had to stop, but will be looking it again. In the middle at one point heater also started working lightly for a while then it stopped again at the end. Temp gauge was on the middle (normal), and it took long time to reach to the middle, I mean it didn't go up quickly. He said if it's head gasket it shouldn't take that long to overheat. Will try again, if it doesn't work then what could be the next option to go for, thermostat or water pump?
To Panelhead: it came from insurance auction with little accident/body damage so it was a genuine sale, if it was a regular auction where people sell vehicles with issues then I would be sure that issue was pre existed.

 

Could still be air in it.

 

Could try raising the front end, if you believe air could be trapped.

 

Some of these cars have a lot of trouble getting air out--no idea why. It should be fairly straightforward--engine off, loosen bleed screw, fill reservoir. Tighten bleed screw, start engine, add more coolant, done. If you're not getting heat (or intermittent heat) I would most definitely suspect air in the system. Water pump impellers are not known to fail on these.

With the engine running, are you seeing coolant flowing out of the bleed hose into the reservoir? (don't try and remove this hose from the reservoir, the nipple will break easily). You should be able to see through the housing if coolant is flowing. If it's not, this would cause issues with the air not purging from the system. Had this happen on another vehicle, the tiny orifice in the reservoir actually clogged solid.

If really becomes an issue, you might look into a vacuum coolant filler. Or take the car to a shop that can do this.

Slow warm up would be a thermostat housing issue, could cause overheating on long drives, but doesn't match the symptoms you've described. The plastic thermostat housing on these cars only last 60k miles. Internally falls apart and the thermostat no longer regulates + allows hot water to bypass the radiator.




Thermostat should be one assembly with plastic inlet.

 

I will check the coolant flow into the reservoir and get back. Also will try to bleed it more.
Sorry I forgot to mention when I got the car it has engine light on with 2 codes P0128 and P050B. I turned them off and they the codes didn't come back.
Also I don't know how to test the thermostat so can I totally bypass/take out the thermostat and then check the car if it still overheats? That's to isolate the issue from thermostat so I can look further.

 

I'd say that P0128 answers the question: the thermostat is not regulating correctly. That's the cause of your slow warm up right there. It may also be the cause of your overheating.

No, you can't remove the thermostat and run the car. That will cause overheating. In this design the tstat directs both hot and cold coolant in/out of the engine. Without the thermostat in place the flow to the radiator will be screwed up and hot coolant will recirculate back to the engine block. Just order a new tstat housing, it comes with everything (thermostat, ECT sensor, gaskets).

 

I would also put in a new auxiliary water pump

 

I have ordered the thermostat, will install as get it. Tried again today to bleed, air was coming out till water started boiling and coming like fountain from the bleed hose after the temp gauge reached middle, had to stop bleeding. Noticed one thing more that water on the reservoir side was normal (just little warm) but on the bleed hose side it was boiling hot. Water flow from bleed hose to reservoir seemed good. Heater still didn't start working.


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You shouldn't open the bleed screw while the engine is running. That is only used when for filling the reservoir after a drain.

It is normal for the purge line to have hotter water, as that is directly out of the engine. Sounds to me like your thermostat is not opening. Again, I would replace that thermostat assembly and go from there.

The heater may be a separate issue altogether. The cores have been known to clog on cars where the coolant wasn't maintained, or it could be a failed aux pump as stg424 suggested.

 

So that's not how to bleed the air? Can you share how to bleed it properly?
A friend of mine tried to bleed it that way, with engine running he opened the bleed screw and bubbles were coming out.
I tried to check in the forums how to bleed this XJ8 but couldn't find it any info.

 

Hi Alex,
The service manual is available on this forum, look at the top, one of the sticky posts has a link to it.
The relevant section is 303-03A.
I have snipped the relevant section for you.




Pete M