jaguar xk8 random overheating that is not quite overheating
 

i know this is a honda S2000 thread, but some of us have british car experience. Here is the issue
1999 jag xk8 with 4.0 v8. Replaced water pump, coolant pipe, sensor, belt, thermostat about 5,000 miles ago
no problem....then about 2 months ago the expansion tank began to leak....replaced that with a new one as well, it all hooked up right...except.

i have this random problem, i can drive it for 20 min or so and then the temp gauge shoots to high in 10 seconds. i turn it off , it is ok for another 5-10 min and it happens again. I have found several folks have had this issue with various ideas as to what causes it...from a broken water pump, or thermostat, to a hard reset of the electrical system, but nobody seems to know what the issue is....some say bad head gasket. The car also goes into stuck in 2nd gear limp home mode when the over heating, but, but the engine runs fine all the other time, no water in oil, no oil in water, no smoke out tailpipe, it can idle for 10 min smooth as glass, normally it runs fine...smooth...

so the problem is so random....and the engine is not hot, but the fans run for 5 min after i turn it off, which is normal, the heater works excellent....

all this started with i replaced the expansion tank and sensor....i have burped the system repeatedly as well....

so ideas????, its a tough one....several have had this identical issue and nobody has a solution other than to take it to a shop....does anyone know a good jag shop on the penninsula near redwood city????

Have you posted the same on www.Jaguarforums.com?

Check the 2 hoses on top of the reservoir, they are slightly offset to each other, 1 is a little bit higher than the other. Make sure the slightly higher hose gets connected to the hose that goes into the fender well. Make sure the lower hose goes into the radiator. If those 2 are crossed, this exact symptom can happen.

Another thing to do is to make sure you bleed the symptom properly. Air pockets absolutely PLAGUE Jaguar motors of all years after extensive coolant repairs like you had.

If all of that checks out, I'd check the fan relay. It could be that your fan isn't coming on when it's supposed to or is staying on when it's not supposed to. The easiest way to check is to wait until the symptoms show up (i.e. when it overheats) and see if the fan is on.

Thank you all for your advice so far...I know this happens to others, just not a standard cure. I want to fix it myself, and have done all the other work successfully. I am about at wits end.

Maybe probs around inlet manifold gasket area +2 hoses under there also

I would check those 2 hoses that I mentioned before you take it anywhere. It's happened to me more than a few times when working on it. It happens to guys that have worked on jags for a living.

valley hoses, they leak, i know, there is no sign of that ...this car lived inside and most hoses are superb shape, hard to believe, only 45,000 miles even, i know and 20 years old
spare tire was original by the way,, a perelli and it turned to plastic, never used and cracked in half when taken off.

Now I know you said you replaced the thermostat, but just to make sure. You replaced the thermostat itself and not JUST the thermostat housing right? Because if you didn't mention that you replaced anything, the first thing I'd say is the thermostat. It's almost 99% of the time the thermostat failing.

Also, I'm not saying that anything is leaking. I'm saying that the hoses that are attached to the expansion tank sometimes are not routed back the same. Make sure you have the slightly higher hose on the expansion tank routed to the fender well area and the lower hose go into the radiator. People mix these 2 hoses right here. So the slightly higher one in the picture is the one that goes into fender well.

EDIT - added link

I would start by bleeding the system again. If you haven't used this yet, but it and use it.

https://www.amazon.com/Lisle-24610-Spill-Free-Funnel/dp/B001A4EAV0

6,000 miles of so ago, replaced thermostat, water pump, coolant pipe, sensor and seals, coolant....now expansion tank with sensor. hoses hooked up right....

this all started with expansion tank float coming loose from age, replaced that with new tank, cap....then this happened. i see many have complained about the exact same thing but no solution other than burping which i have done, i can try that again as it would be a logical solution for sure.