Gents,

I have been reviving a long-stored XK8 to operating condition.

Things done:
- Fuel pump & tank replacement (Denso pump, refurbished tank)
- Oil & filter change to 5w30 full synth
- Alternator changed
- Upper radiator hoses changed
- New tires (245/50R17 all 4; I chose to forego the staggered 255's on rear)
- Hydraulic pump confirmed to have the green stuff, topped off
- Timing chain tensioners confirmed to be steel, not plastic

Two days ago I took her on her maiden voyage, only to be faced with a quickly climbing temperature gauge. Immediately pulled off to a stop, and had it towed home.

Pulled the thermostat, which tested fine in the pot on the stove, opening just prior to boiling. I then pulled the water pump, which seemed OK to sight but, to be thorough I ordered a new replacement (they are inexpensive in any case.)

With these reinstalled, I put coolant system cleaner + distilled water, burped the line as best I could, then ran the engine to a middle-gauge normal reading. Idled for 10-15 minutes or so, no issues.

But then, I held 2K RPM for about 100 seconds or so, at which point the needle started to jitter and climb slowly to the red zone. So, the same symptom as before.

At this point, I obtained a laser temperature gauge, and checked the actual engine surfaces. Here is what I was reading, when the needle climbed to the Red zone:

175-178 degrees at the brass temp sensor located at the front-top of the engine.
160 degrees at the thermostat housing
145 degrees on the upper hose
101-104 degrees on the lower hose

With the needle sitting in the normal / middle of gauge, the readings were a bit lower all around, most noticeably at the temp sensor, it read 160-164 degrees.

So my question is, could it be a faulty temp sensor this whole time? What readings should I be getting there? If the T-stat is designed to open at 180, shouldn't the temp sensor read 180-190 before the cluster gauge would read middle / normal? I have also noted that the radiator fans tend to kick on earlier than I would think; this being my first Jaguar, I thought perhaps this is just typical. But in fact, if the ECU *thinks* the car is hot, when it really isn't, then perhaps the overly-aggressive cooling fans + the weep hole in the thermostat is enough to keep it down to a false "good" reading of actually 160 degrees or so... but, I am unsure.

Thanks in advance for any insight as to what I should normally be seeing, and if this does in fact sound like a faulty temp sensor....

 

Laser temp pointers are not that accurate. And in any case only tell you the temperature of what you pointing at, not the fluid inside, which can and does vary widely.

my solution would be to first examine the coolant temp with a OBD-II reader under actual driving conditions . If that reads very hot then test or change out the sensor before doing anything else.

Z

 

This is great advice, thank you!

Cold, not started, laser says: 90F. Computer says, 90F.

Hot, laser says: 161F. Computer says: 245F.

I decided to test the laser itself, so I did same on the wife's Sonata.

Hot, laser says: 172F. Computer says: 183F.

So strange that a temp sensor could be correct when cold, but wrong when hot? ...

I'm not sure what more I can do to troubleshoot or test, other than some tricky stovetop tests involving sensors, pots of water, thermometers, and angry wives... So I decided to go ahead and put in an order for a sensor. Sometimes waiting is the hardest part

 

Are the cooling fans working?

 

Change the sensor ..make sure it has a green top there is another temp sensor often supplied in mistake.
Personally I would take out the thermostat and if it does not overheat fit a 84 degC one .Mine runs at 88 degC unless I boot it but I have the fans on half speed when the ignition is on ( I have written it up ..only one wire to earth).
PS why are US still using deg F ? You cannot teach science in school unless you use deg C. the change has been a long time coming. My 10 year old grandchild does not know what deg F is

 

Done; NTK brand sensor. Temp readings significantly improved, but I am still able to run it fairly hot (3/4 on the gauge, 230F/110C via OBD reading) holding 2K RPM at a stop (however it does recover fairly quickly when I let off.) Perhaps this is not a realistic test, so I shall take it on a short road trip later today.

Did you mean test without a thermostat? I had not thought of that; may do if it is still getting hot on the road test.

I'll look this up, another good idea, thank you

Agreed. In fact, the schools are changing plenty of curricula here in the States, except for curricula that actually would make sense.

 

What is the ambient air temp. My car got up to 234 F / 112 C a few times in 2021 when traveling in Texas. But the air temp was 119 F / 48.3 C, and I was stuck in traffic. Once moving it went down to 205-210 F.

Z

 

Yesterday was roughly 70F / 21C; today is quite a bit cooler actually, will be interesting to see what the road test reveals (if the rain would just stop, already!)

 

Should I ask again?
Does the fans work?
Both?
Half or full speed?

 

Yes, in my original post, when I said "overly aggressive cooling fans" what I meant was, they both kick on at high speed earlier than any car I've owned before.

Today's update, the rain kept me parked, and I figured since the new T-stat arrives tomorrow, I'll just hold off on any more flushing until I have the new coolant and new T-stat. I'll also boil them both and compare opening times, they are both supposed to be 180F T-stats so it'll be interesting to verify.

 

Fans controlled by your faulty temp sensor so aggressive fans not relevant. they always run when the air con is on.
Running at 2000 when stationary is a stupid test ...all that will happen is that the temp will go up until the fans cut in and the fans will hold it steady.
Sort the sensor and get a 180 deg thermostat.

 

I replaced these, as well as the upper / lower rad hoses, new water pump, flushed including chemical treatment, drained and filled with 50/50 distilled + HOAT (yellow) coolant. Topped off, burped the lines as best I could. Ran the heater when topping off, and the heater blows hot so (I think) that lets me know I've routed the coolant thru the heater core.

Test drive last night was marginally better, yet when climbing roughly 1/4 mile uphill, it went to 7/8 on the gauge again - although not as quickly as before. Briefly illuminated the red warning, less than a second, and at idle cooled down pretty quickly. So the cooling system is working better, but still not well enough under load, it seems.

I must have overlooked something... Perhaps the radiator IS actually clogged, despite the appearance of the old coolant and despite the chemical flush. When I performed a fill, it took 5 litres with burping the lines - I don't know that this indicates anything one way or the other, but all signs I have seen do not indicate a clog. But I don't know what else could be going on - unless, perhaps one of the fans goes out intermittently? That does not seem likely either.

I'll scratch my head for awhile and see if I can figure out what I've missed... I have not yet implemented Pistnbroke's always-on fan suggestion, may give that a try if I can't think of anything else. Thank you all again for the advice!

 

I think you can only get half the water out of the engine....sounds suspiciously like rad is blocked.
Putting the fans on will only give you a buffer .
I would take the rad out lay it on its back and put some drain unblocker in it with boiling water ( caustic soda). agitate it back and forth every 30 min then reverse flush it after a few hours

 

This is embarrassing. Air bubbles.

I went to start it for more diagnosis, and it read Low Coolant - first I'd seen this since the flush. Quite odd.

I put maybe a teacup more coolant in, to bring it up to the cap, that's all - and it held solid temp. Using the OBDII reader, I couldn't break 203F even with my (admittedly silly) 2000RPM driveway test.

Took it on a road test, ran it up to 5.5KRPM and couldn't break 203F. Cruising, it remained around 190-195.

Also, whereas the heater "worked" before, now it truly blasted quite hot. So I believe, maybe the air was hiding in the heater core or something. I don't know.

Anyway, egg on my face, but this seems to have been caused by improper bleeding after the parts change + flushing.

Thanks, all!

 

Just a few tips.
You must fill the expansion tank right to the top.
There is also an oveflow tank behind the plastic (rear) in the right wheel well .
It must be at least half full or the coolant will be pumped into it and you will keep getting low water warnings

 

Whenever I have to pump coolant out of the cooling system in order to effect whatever repair I need to do, after completing the repair and refilling the coolant I always check the coolant level in the reservoir first thing each morning for the next three to four days. It typically takes that long to top up the coolant level to where it needs to be. Works for me....

 

needs to be right at the top where you left it . It wont work right if its 1/2 in down